Question for Jim Wier (or other electrical guru)

[Mike Rapoport]

In my 1974 Helio Courier, the avionics bus is powered by a contactor that is
activated by the avionics master switch. There is no circuit protection
between the contactor and the avionics bus. The wiring between the
contactor and the bus consists of two 12ga wires. There also doesn't appear
to be any circuit protection in the field circuit for the avionics
contactor. It seems to me that since there is no breaker between the
contactor and the bus, the wire size should be sufficient to supply the
rating on all the avionics CBs (32.5A total), but I don't know what the
absolute maximium permissible temperature rise is before the avionics CBs
pop. Would one 12ga wire be sufficient between the contactor and the
avionics bus? Also, am I looking at the problem correctly, sizing the wire
considering all the avionics CBs to reach their limits simultaneously?

Mike
MU-2
Helio Courier

[kontiki]

Your should use the biggest wire you can comfortable use so it will contribute
the least voltage drop under the full load of all avionics and electrical
systems. 12Ga wire is typically rated for 20amps continuous. I would try to
go with al least 10GA or even 8 if you can work with it. Basically, in
that section of the circuit (before the breakers) the bigger the better,
within reason of course.

Mike Rapoport wrote:
> In my 1974 Helio Courier, the avionics bus is powered by a contactor that is
> activated by the avionics master switch. There is no circuit protection
> between the contactor and the avionics bus. The wiring between the
> contactor and the bus consists of two 12ga wires. There also doesn't appear
> to be any circuit protection in the field circuit for the avionics
> contactor. It seems to me that since there is no breaker between the
> contactor and the bus, the wire size should be sufficient to supply the
> rating on all the avionics CBs (32.5A total), but I don't know what the
> absolute maximium permissible temperature rise is before the avionics CBs
> pop. Would one 12ga wire be sufficient between the contactor and the
> avionics bus? Also, am I looking at the problem correctly, sizing the wire
> considering all the avionics CBs to reach their limits simultaneously?
>
> Mike
> MU-2
> Helio Courier
>
>

[G.R. Patterson III]

Mike Rapoport wrote:
>
> In my 1974 Helio Courier, the avionics bus is powered by a contactor that is
> activated by the avionics master switch. There is no circuit protection
> between the contactor and the avionics bus. The wiring between the
> contactor and the bus consists of two 12ga wires. There also doesn't appear
> to be any circuit protection in the field circuit for the avionics
> contactor. It seems to me that since there is no breaker between the
> contactor and the bus, the wire size should be sufficient to supply the
> rating on all the avionics CBs (32.5A total), but I don't know what the
> absolute maximium permissible temperature rise is before the avionics CBs
> pop. Would one 12ga wire be sufficient between the contactor and the
> avionics bus? Also, am I looking at the problem correctly, sizing the wire
> considering all the avionics CBs to reach their limits simultaneously?

Two 12ga wires should be adequate; one is not. As kontiki said, the maximum
steady load for a 12ga wire is 20 amps.

In the absence of a breaker on the supply, you are looking at the problem
correctly, IMO. You could total the actual maximum current draw of the avionics
and add an appropriate safety factor, but this will be very close to the total
of the avionics CBs anyway.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Mike Rapoport]

"G.R. Patterson III" > wrote in message
...
>
>
> Mike Rapoport wrote:
>>
>> In my 1974 Helio Courier, the avionics bus is powered by a contactor that
>> is
>> activated by the avionics master switch. There is no circuit protection
>> between the contactor and the avionics bus. The wiring between the
>> contactor and the bus consists of two 12ga wires. There also doesn't
>> appear
>> to be any circuit protection in the field circuit for the avionics
>> contactor. It seems to me that since there is no breaker between the
>> contactor and the bus, the wire size should be sufficient to supply the
>> rating on all the avionics CBs (32.5A total), but I don't know what the
>> absolute maximium permissible temperature rise is before the avionics CBs
>> pop. Would one 12ga wire be sufficient between the contactor and the
>> avionics bus? Also, am I looking at the problem correctly, sizing the
>> wire
>> considering all the avionics CBs to reach their limits simultaneously?
>
> Two 12ga wires should be adequate; one is not. As kontiki said, the
> maximum
> steady load for a 12ga wire is 20 amps.
>
> In the absence of a breaker on the supply, you are looking at the problem
> correctly, IMO. You could total the actual maximum current draw of the
> avionics
> and add an appropriate safety factor, but this will be very close to the
> total
> of the avionics CBs anyway.
>
> George Patterson
> The desire for safety stands against every great and noble enterprise.

The maximium draw of the avionics (speaker on, transmitting on both coms) is
20A, the continous draw (not transmitting) is under 15A. I realize that the
max for *continous* use is 22A with 12ga, but how much overload is
permissible for short term use (long enough for the avionics CBs to pop.
Starter cables are often overloaded to 250% of their continous rating..

Mike
MU-2
Helio Courier

[Mike Rapoport]

The length of the wire is less than two feet and the avionics draw less than
15A when not transmitting so voltage loss isn't really the big issue. I
assume that they used two 12ga wires for flexibility compared to a single
8ga wire.

Mike
MU-2
Helio Courier

"kontiki" > wrote in message
...
> Your should use the biggest wire you can comfortable use so it will
> contribute
> the least voltage drop under the full load of all avionics and electrical
> systems. 12Ga wire is typically rated for 20amps continuous. I would try
> to
> go with al least 10GA or even 8 if you can work with it. Basically, in
> that section of the circuit (before the breakers) the bigger the better,
> within reason of course.
>
> Mike Rapoport wrote:
>> In my 1974 Helio Courier, the avionics bus is powered by a contactor that
>> is activated by the avionics master switch. There is no circuit
>> protection between the contactor and the avionics bus. The wiring
>> between the contactor and the bus consists of two 12ga wires. There also
>> doesn't appear to be any circuit protection in the field circuit for the
>> avionics contactor. It seems to me that since there is no breaker
>> between the contactor and the bus, the wire size should be sufficient to
>> supply the rating on all the avionics CBs (32.5A total), but I don't know
>> what the absolute maximium permissible temperature rise is before the
>> avionics CBs pop. Would one 12ga wire be sufficient between the
>> contactor and the avionics bus? Also, am I looking at the problem
>> correctly, sizing the wire considering all the avionics CBs to reach
>> their limits simultaneously?
>>
>> Mike
>> MU-2
>> Helio Courier
>

[TaxSrv]

"G.R. Patterson III" wrote:
> You could total the actual maximum current draw of the avionics
> and add an appropriate safety factor, but this will be very close to
the total
> of the avionics CBs anyway.
>
Not necessarily. The factory that built my plane wired avionics boxes
drawing less than 1A to 5A breakers. Subsequently a shop wired a
low-draw item to a 5A breaker, I guess just because there was one
unused CB already there. I would determine the total draw, before
messing around replacing wire which may not be necessary.

Fred F.

[kontiki]

At less than two feet I wouldn't worry about it. There will be more
potential resistance in the connections at the ends of the wirer than
in the wire itself.

Mike Rapoport wrote:
> The length of the wire is less than two feet and the avionics draw less than
> 15A when not transmitting so voltage loss isn't really the big issue. I
> assume that they used two 12ga wires for flexibility compared to a single
> 8ga wire.
>
> Mike
> MU-2
> Helio Courier
>
> "kontiki" > wrote in message
> ...
>
>>Your should use the biggest wire you can comfortable use so it will
>>contribute
>>the least voltage drop under the full load of all avionics and electrical
>>systems. 12Ga wire is typically rated for 20amps continuous. I would try
>>to
>>go with al least 10GA or even 8 if you can work with it. Basically, in
>>that section of the circuit (before the breakers) the bigger the better,
>>within reason of course.
>>
>>Mike Rapoport wrote:
>>
>>>In my 1974 Helio Courier, the avionics bus is powered by a contactor that
>>>is activated by the avionics master switch. There is no circuit
>>>protection between the contactor and the avionics bus. The wiring
>>>between the contactor and the bus consists of two 12ga wires. There also
>>>doesn't appear to be any circuit protection in the field circuit for the
>>>avionics contactor. It seems to me that since there is no breaker
>>>between the contactor and the bus, the wire size should be sufficient to
>>>supply the rating on all the avionics CBs (32.5A total), but I don't know
>>>what the absolute maximium permissible temperature rise is before the
>>>avionics CBs pop. Would one 12ga wire be sufficient between the
>>>contactor and the avionics bus? Also, am I looking at the problem
>>>correctly, sizing the wire considering all the avionics CBs to reach
>>>their limits simultaneously?
>>>
>>>Mike
>>>MU-2
>>>Helio Courier
>>
>
>

[G.R. Patterson III]

Mike Rapoport wrote:
>
> The maximium draw of the avionics (speaker on, transmitting on both coms) is
> 20A, the continous draw (not transmitting) is under 15A. I realize that the
> max for *continous* use is 22A with 12ga, but how much overload is
> permissible for short term use (long enough for the avionics CBs to pop.

You should design the system such that the maximum load that continues for more
than a second or two is no more than 80% of the breaker capacity. This is
intended to prevent deterioration of the breaker due to frequent operation at
temperatures close to the trip point.

Design for wire is a different matter, since the heat will not cause the wire to
deteriorate until you get it very hot. The risk with wire is that an overload
will cause a fire by melting the insulation. I don't know how far you can go and
still avoid this, but it should be well above 20 amps. I know that "delayed
action" breakers in home wiring situations allow temporary surges of up to 25%
of the rated capacity.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Mike Rapoport]

Thanks, but the question remains: should the wire size be based on the
expected continous load (15A) or the combined rating of the circuit
breakers. As an example, suppose the expected load is 15A and the circuit
breakers total 30A. A single 12ga wire would easily handle the 15A expected
load and would momentarily be over loaded by 36% if all the avionics shorted
simultaneously. Is this OK? My understanding from reading the
Aero-Electric Connection book and AC 43.13-1B is that the 22A rating on a
12ga wire is the *continous* load that produces a 35C increase in
tempertaure and that some amount of overloading is permitted for short
durations. The Aero-Electric Connection says that in some cases (short
length, well ventilated wire) it is OK to run 10A through 22ga wire for
short durations (a 100% overload).

Mike
MU-2


"kontiki" > wrote in message
...
> At less than two feet I wouldn't worry about it. There will be more
> potential resistance in the connections at the ends of the wirer than
> in the wire itself.
>
> Mike Rapoport wrote:
>> The length of the wire is less than two feet and the avionics draw less
>> than 15A when not transmitting so voltage loss isn't really the big
>> issue. I assume that they used two 12ga wires for flexibility compared
>> to a single 8ga wire.
>>
>> Mike
>> MU-2
>> Helio Courier
>>
>> "kontiki" > wrote in message
>> ...
>>
>>>Your should use the biggest wire you can comfortable use so it will
>>>contribute
>>>the least voltage drop under the full load of all avionics and electrical
>>>systems. 12Ga wire is typically rated for 20amps continuous. I would try
>>>to
>>>go with al least 10GA or even 8 if you can work with it. Basically, in
>>>that section of the circuit (before the breakers) the bigger the better,
>>>within reason of course.
>>>
>>>Mike Rapoport wrote:
>>>
>>>>In my 1974 Helio Courier, the avionics bus is powered by a contactor
>>>>that is activated by the avionics master switch. There is no circuit
>>>>protection between the contactor and the avionics bus. The wiring
>>>>between the contactor and the bus consists of two 12ga wires. There
>>>>also doesn't appear to be any circuit protection in the field circuit
>>>>for the avionics contactor. It seems to me that since there is no
>>>>breaker between the contactor and the bus, the wire size should be
>>>>sufficient to supply the rating on all the avionics CBs (32.5A total),
>>>>but I don't know what the absolute maximium permissible temperature rise
>>>>is before the avionics CBs pop. Would one 12ga wire be sufficient
>>>>between the contactor and the avionics bus? Also, am I looking at the
>>>>problem correctly, sizing the wire considering all the avionics CBs to
>>>>reach their limits simultaneously?
>>>>
>>>>Mike
>>>>MU-2
>>>>Helio Courier
>>>
>>
>>
>

[kontiki]

The circuit breaker has a special element the heats on overload within
a specific time period that trips the breaker. If the wire feeding
the breaker is not rated for current draw then it can end up dissipating
some of the heat energy required by the breaker. You now have this
total load divided up by many smaller breakers so the feeding wire has
to be rated for at least the total maximum normal current load, plus the
additional load of the highest breaker current.

As I said before, you cannot err on the high side in the instance.
Use the heaviest wire you can reasonable deal with that meets the
above criteria.

Mike Rapoport wrote:

> Thanks, but the question remains: should the wire size be based on the
> expected continous load (15A) or the combined rating of the circuit
> breakers. As an example, suppose the expected load is 15A and the circuit
> breakers total 30A. A single 12ga wire would easily handle the 15A expected
> load and would momentarily be over loaded by 36% if all the avionics shorted
> simultaneously. Is this OK? My understanding from reading the
> Aero-Electric Connection book and AC 43.13-1B is that the 22A rating on a
> 12ga wire is the *continous* load that produces a 35C increase in
> tempertaure and that some amount of overloading is permitted for short
> durations. The Aero-Electric Connection says that in some cases (short
> length, well ventilated wire) it is OK to run 10A through 22ga wire for
> short durations (a 100% overload).
>
> Mike
> MU-2
>
>
> "kontiki" > wrote in message
> ...
>
>>At less than two feet I wouldn't worry about it. There will be more
>>potential resistance in the connections at the ends of the wirer than
>>in the wire itself.
>>
>>Mike Rapoport wrote:
>>
>>>The length of the wire is less than two feet and the avionics draw less
>>>than 15A when not transmitting so voltage loss isn't really the big
>>>issue. I assume that they used two 12ga wires for flexibility compared
>>>to a single 8ga wire.
>>>
>>>Mike
>>>MU-2
>>>Helio Courier
>>>
>>>"kontiki" > wrote in message
...
>>>
>>>
>>>>Your should use the biggest wire you can comfortable use so it will
>>>>contribute
>>>>the least voltage drop under the full load of all avionics and electrical
>>>>systems. 12Ga wire is typically rated for 20amps continuous. I would try
>>>>to
>>>>go with al least 10GA or even 8 if you can work with it. Basically, in
>>>>that section of the circuit (before the breakers) the bigger the better,
>>>>within reason of course.
>>>>
>>>>Mike Rapoport wrote:
>>>>
>>>>
>>>>>In my 1974 Helio Courier, the avionics bus is powered by a contactor
>>>>>that is activated by the avionics master switch. There is no circuit
>>>>>protection between the contactor and the avionics bus. The wiring
>>>>>between the contactor and the bus consists of two 12ga wires. There
>>>>>also doesn't appear to be any circuit protection in the field circuit
>>>>>for the avionics contactor. It seems to me that since there is no
>>>>>breaker between the contactor and the bus, the wire size should be
>>>>>sufficient to supply the rating on all the avionics CBs (32.5A total),
>>>>>but I don't know what the absolute maximium permissible temperature rise
>>>>>is before the avionics CBs pop. Would one 12ga wire be sufficient
>>>>>between the contactor and the avionics bus? Also, am I looking at the
>>>>>problem correctly, sizing the wire considering all the avionics CBs to
>>>>>reach their limits simultaneously?
>>>>>
>>>>>Mike
>>>>>MU-2
>>>>>Helio Courier
>>>>
>>>
>
>

[Mike Rapoport]

"G.R. Patterson III" > wrote in message
...
>
>
> Mike Rapoport wrote:
>>
>> The maximium draw of the avionics (speaker on, transmitting on both coms)
>> is
>> 20A, the continous draw (not transmitting) is under 15A. I realize that
>> the
>> max for *continous* use is 22A with 12ga, but how much overload is
>> permissible for short term use (long enough for the avionics CBs to pop.
>
> You should design the system such that the maximum load that continues for
> more
> than a second or two is no more than 80% of the breaker capacity. This is
> intended to prevent deterioration of the breaker due to frequent operation
> at
> temperatures close to the trip point.
>
> Design for wire is a different matter, since the heat will not cause the
> wire to
> deteriorate until you get it very hot. The risk with wire is that an
> overload
> will cause a fire by melting the insulation. I don't know how far you can
> go and
> still avoid this, but it should be well above 20 amps. I know that
> "delayed
> action" breakers in home wiring situations allow temporary surges of up to
> 25%
> of the rated capacity.
>
> George Patterson
> The desire for safety stands against every great and noble enterprise.

Thanks. I was hoping for a rule like: "It is permissible to load Tefzel
wire over its (35C) continous rating by XX% for up to XX seconds". It seems
that no such "rule" exists.

BTW I am not going to rewire my certified airplane but I was under the panel
lubricating some bowden cables for the cabin heat and noticed two wires with
the same origin and destination points which got me to thinking "Why". I am
also going to be building a Moose and so I am interested in all this kind of
stuff.
I appreciate the information.

Mike
MU-2
Helio Courier

[Mike Rapoport]

OK this makes sense. I had not considered that the breakers might not trip
because of the resistance of the wire feeding the bus during the overload.
I suppose that there is also the tolerances of the breakers to be
considered. Thanks!

Mike
MU-2
Helio Courier


"kontiki" > wrote in message
...
> The circuit breaker has a special element the heats on overload within
> a specific time period that trips the breaker. If the wire feeding
> the breaker is not rated for current draw then it can end up dissipating
> some of the heat energy required by the breaker. You now have this
> total load divided up by many smaller breakers so the feeding wire has
> to be rated for at least the total maximum normal current load, plus the
> additional load of the highest breaker current.
>
> As I said before, you cannot err on the high side in the instance.
> Use the heaviest wire you can reasonable deal with that meets the
> above criteria.
>
> Mike Rapoport wrote:
>
>> Thanks, but the question remains: should the wire size be based on the
>> expected continous load (15A) or the combined rating of the circuit
>> breakers. As an example, suppose the expected load is 15A and the
>> circuit breakers total 30A. A single 12ga wire would easily handle the
>> 15A expected load and would momentarily be over loaded by 36% if all the
>> avionics shorted simultaneously. Is this OK? My understanding from
>> reading the Aero-Electric Connection book and AC 43.13-1B is that the 22A
>> rating on a 12ga wire is the *continous* load that produces a 35C
>> increase in tempertaure and that some amount of overloading is permitted
>> for short durations. The Aero-Electric Connection says that in some
>> cases (short length, well ventilated wire) it is OK to run 10A through
>> 22ga wire for short durations (a 100% overload).
>>
>> Mike
>> MU-2
>>
>>
>> "kontiki" > wrote in message
>> ...
>>
>>>At less than two feet I wouldn't worry about it. There will be more
>>>potential resistance in the connections at the ends of the wirer than
>>>in the wire itself.
>>>
>>>Mike Rapoport wrote:
>>>
>>>>The length of the wire is less than two feet and the avionics draw less
>>>>than 15A when not transmitting so voltage loss isn't really the big
>>>>issue. I assume that they used two 12ga wires for flexibility compared
>>>>to a single 8ga wire.
>>>>
>>>>Mike
>>>>MU-2
>>>>Helio Courier
>>>>
>>>>"kontiki" > wrote in message
...
>>>>
>>>>
>>>>>Your should use the biggest wire you can comfortable use so it will
>>>>>contribute
>>>>>the least voltage drop under the full load of all avionics and
>>>>>electrical
>>>>>systems. 12Ga wire is typically rated for 20amps continuous. I would
>>>>>try to
>>>>>go with al least 10GA or even 8 if you can work with it. Basically, in
>>>>>that section of the circuit (before the breakers) the bigger the
>>>>>better,
>>>>>within reason of course.
>>>>>
>>>>>Mike Rapoport wrote:
>>>>>
>>>>>
>>>>>>In my 1974 Helio Courier, the avionics bus is powered by a contactor
>>>>>>that is activated by the avionics master switch. There is no circuit
>>>>>>protection between the contactor and the avionics bus. The wiring
>>>>>>between the contactor and the bus consists of two 12ga wires. There
>>>>>>also doesn't appear to be any circuit protection in the field circuit
>>>>>>for the avionics contactor. It seems to me that since there is no
>>>>>>breaker between the contactor and the bus, the wire size should be
>>>>>>sufficient to supply the rating on all the avionics CBs (32.5A total),
>>>>>>but I don't know what the absolute maximium permissible temperature
>>>>>>rise is before the avionics CBs pop. Would one 12ga wire be
>>>>>>sufficient between the contactor and the avionics bus? Also, am I
>>>>>>looking at the problem correctly, sizing the wire considering all the
>>>>>>avionics CBs to reach their limits simultaneously?
>>>>>>
>>>>>>Mike
>>>>>>MU-2
>>>>>>Helio Courier
>>>>>
>>>>
>>
>>
>

[G.R. Patterson III]

Mike Rapoport wrote:
>
> Thanks. I was hoping for a rule like: "It is permissible to load Tefzel
> wire over its (35C) continous rating by XX% for up to XX seconds". It seems
> that no such "rule" exists.

Not exactly true. My copy of AC 43.13-1A (I should've looked there first)
contains a chart that can be used to determine the minimum wire size for both
continuous and intermittent loads. If you're going to be building an aircraft,
best pick up a copy. This chart is used to size wire to prevent excessive
voltage drop or to prevent overheating.

Normally you take the load in amps, length of wire, and voltage and read off the
size. Working backwards, 12ga wire shorter than 24' long will carry an
intermittent load of about 45 amps at 28 volts (the scale is not very precise).
The charts don't go down to 2' long pieces of wire.

Disclaimer: I'm not used to working with this chart. Hope I'm reading this
correctly.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Jon Woellhaf]

Breaker rating is based on the size and type of the wire downstream of it.
The current drawn by the load is immaterial.

"TaxSrv" > wrote in message
...
> "G.R. Patterson III" wrote:
> > You could total the actual maximum current draw of the avionics
> > and add an appropriate safety factor, but this will be very close to
> the total
> > of the avionics CBs anyway.
> >
> Not necessarily. The factory that built my plane wired avionics boxes
> drawing less than 1A to 5A breakers. Subsequently a shop wired a
> low-draw item to a 5A breaker, I guess just because there was one
> unused CB already there. I would determine the total draw, before
> messing around replacing wire which may not be necessary.
>
> Fred F.

[RST Engineering]

You've gotten several excellent answers, so I won't belabor the issue.

However, if you want an excel spreadsheet showing temperature rise versus
wire gauge and current, go to the Jim's Engineering Data page on
www.rstengineering.com and download the Wire Table spreadsheet.

Enjoy.

Jim


"Mike Rapoport" > wrote in message
ink.net...
> In my 1974 Helio Courier, the avionics bus

[Aaron Coolidge]

Mike Rapoport > wrote:
: OK this makes sense. I had not considered that the breakers might not trip
: because of the resistance of the wire feeding the bus during the overload.
: I suppose that there is also the tolerances of the breakers to be
: considered. Thanks!

Hi Mike. These breakers that we have have a fairly complex trip curve.
Potter & Brumfield make the majority of breakers, looking at their web site
will probably turn up a document on how the breakers actually work.

In general, a breaker MUST carry about 110% rated without tripping. At 140%
rated current the trip time us usually measured in hours. 200% rated gets
you into minutes before the breaker trips. 500% rated it should trip in
milliseconds.
--
Aaron C.

[[email protected]]

The circuit breaker is there only to protect the wire it is connected to.
Manufacturers generally estimate the breaker size to trip when the
appliance is drawing enough current that it is toast. (pun intended)


Aaron Coolidge wrote:
> Mike Rapoport > wrote:
> : OK this makes sense. I had not considered that the breakers might not trip
> : because of the resistance of the wire feeding the bus during the overload.
> : I suppose that there is also the tolerances of the breakers to be
> : considered. Thanks!
>
> Hi Mike. These breakers that we have have a fairly complex trip curve.
> Potter & Brumfield make the majority of breakers, looking at their web site
> will probably turn up a document on how the breakers actually work.
>
> In general, a breaker MUST carry about 110% rated without tripping. At 140%
> rated current the trip time us usually measured in hours. 200% rated gets
> you into minutes before the breaker trips. 500% rated it should trip in
> milliseconds.

[G.R. Patterson III]

Jon Woellhaf wrote:
>
> Breaker rating is based on the size and type of the wire downstream of it.
> The current drawn by the load is immaterial.

Every time I installed something or had something installed in my aircraft, the
breaker has been sized to be the minimum that will supply the appliance based on
current load. This has not always been true of the wire size; some of my supply
wires are oversized.

George Patterson
The desire for safety stands against every great and noble enterprise.

[David Lesher]

"G.R. Patterson III" > writes:




>Two 12ga wires should be adequate; one is not. As kontiki said, the maximum
>steady load for a 12ga wire is 20 amps.

FYI: as a rule-of-thumb; 2 parallel 'n' gauge wires are roughly
equal to one n-3 wire.

In other words, two #12's ~= one #9.


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Jon A.]

The breaker protects the wire. The wire needs to be able to handle
the draw. You'll do no good by using a beefier breaker. If this were
your house, I'd say use the biggest baddest wire going, but if this is
an airplane, use the wire appropriate for the load, be it 10, 18, 0r
even 24 gauge.

On Thu, 06 Jan 2005 20:30:39 GMT, "Mike Rapoport"
> wrote:

>Thanks, but the question remains: should the wire size be based on the
>expected continous load (15A) or the combined rating of the circuit
>breakers. As an example, suppose the expected load is 15A and the circuit
>breakers total 30A. A single 12ga wire would easily handle the 15A expected
>load and would momentarily be over loaded by 36% if all the avionics shorted
>simultaneously. Is this OK? My understanding from reading the
>Aero-Electric Connection book and AC 43.13-1B is that the 22A rating on a
>12ga wire is the *continous* load that produces a 35C increase in
>tempertaure and that some amount of overloading is permitted for short
>durations. The Aero-Electric Connection says that in some cases (short
>length, well ventilated wire) it is OK to run 10A through 22ga wire for
>short durations (a 100% overload).
>
>Mike
>MU-2
>

[Jon A.]

On Thu, 06 Jan 2005 20:47:13 GMT, "Mike Rapoport"
> wrote:

>
>"G.R. Patterson III" > wrote in message
...
>>
>>
>> Mike Rapoport wrote:
>>>
>>> The maximium draw of the avionics (speaker on, transmitting on both coms)
>>> is
>>> 20A, the continous draw (not transmitting) is under 15A. I realize that
>>> the
>>> max for *continous* use is 22A with 12ga, but how much overload is
>>> permissible for short term use (long enough for the avionics CBs to pop.
>>
>> You should design the system such that the maximum load that continues for
>> more
>> than a second or two is no more than 80% of the breaker capacity. This is
>> intended to prevent deterioration of the breaker due to frequent operation
>> at
>> temperatures close to the trip point.
>>
>> Design for wire is a different matter, since the heat will not cause the
>> wire to
>> deteriorate until you get it very hot. The risk with wire is that an
>> overload
>> will cause a fire by melting the insulation. I don't know how far you can
>> go and
>> still avoid this, but it should be well above 20 amps. I know that
>> "delayed
>> action" breakers in home wiring situations allow temporary surges of up to
>> 25%
>> of the rated capacity.
>>
>> George Patterson
>> The desire for safety stands against every great and noble enterprise.
>
>Thanks. I was hoping for a rule like: "It is permissible to load Tefzel
>wire over its (35C) continous rating by XX% for up to XX seconds". It seems
>that no such "rule" exists.
>
>BTW I am not going to rewire my certified airplane but I was under the panel
>lubricating some bowden cables for the cabin heat and noticed two wires with
>the same origin and destination points which got me to thinking "Why". I am
>also going to be building a Moose and so I am interested in all this kind of
>stuff.
>I appreciate the information.
>
>Mike
>MU-2
>Helio Courier
That's for redundancy. One breaks, the other handles it. My current
avionics panel was wired professionally some years back. When I
upgraded the avionics, I found this beautiful job of double wiring,
both sides. I wish I would have repeated it, but I didn't. If you're
building a Moose, go for it, but there's no need to do a double feed
to the buss, especially when you're getting up there in the 12 gauge
range.
>

[RST Engineering]

You do understand, don't you, that when you bottom post a five line answer
to fifty lines of repeated drivel that we've all read several times, that
your pearls of wisdom are generally just trashed before we get down to them.

Please, people, SNIP.

Jim



"Jon A." > wrote in message
...
> On Thu, 06 Jan 2005 20:47:13 GMT, "Mike Rapoport"
> > wrote:
>
>>
>>"G.R. Patterson III" > wrote in message
...
>>>
>>>
>>> Mike Rapoport wrote:

[Mike Rapoport]

Can you give me the figure number that you are looking at?
Thanks

Mike
MU-2
Helio Courier

"G.R. Patterson III" > wrote in message
...
>
>
> Mike Rapoport wrote:
>>
>> Thanks. I was hoping for a rule like: "It is permissible to load Tefzel
>> wire over its (35C) continous rating by XX% for up to XX seconds". It
>> seems
>> that no such "rule" exists.
>
> Not exactly true. My copy of AC 43.13-1A (I should've looked there first)
> contains a chart that can be used to determine the minimum wire size for
> both
> continuous and intermittent loads. If you're going to be building an
> aircraft,
> best pick up a copy. This chart is used to size wire to prevent excessive
> voltage drop or to prevent overheating.
>
> Normally you take the load in amps, length of wire, and voltage and read
> off the
> size. Working backwards, 12ga wire shorter than 24' long will carry an
> intermittent load of about 45 amps at 28 volts (the scale is not very
> precise).
> The charts don't go down to 2' long pieces of wire.
>
> Disclaimer: I'm not used to working with this chart. Hope I'm reading this
> correctly.
>
> George Patterson
> The desire for safety stands against every great and noble enterprise.

[Victor J. Osborne, Jr.]

Thank you for finally saying (?) this. Will you folks please snip or just
refrain from copying message text entirely?

Thank you, {|;-)
Victor J. (Jim) Osborne, Jr.

[Mike Rapoport]

"Jon A." > wrote in message
...
>>BTW I am not going to rewire my certified airplane but I was under the
>>panel
>>lubricating some bowden cables for the cabin heat and noticed two wires
>>with
>>the same origin and destination points which got me to thinking "Why". I
>>am
>>also going to be building a Moose and so I am interested in all this kind
>>of
>>stuff.
>>I appreciate the information.
>>
>>Mike
>>MU-2
>>Helio Courier
> That's for redundancy. One breaks, the other handles it. My current
> avionics panel was wired professionally some years back. When I
> upgraded the avionics, I found this beautiful job of double wiring,
> both sides. I wish I would have repeated it, but I didn't. If you're
> building a Moose, go for it, but there's no need to do a double feed
> to the buss, especially when you're getting up there in the 12 gauge
> range.
>>
>

I'd be surprised if it was for redundancy since if one breaks and shorts it
will require shutting off all the avionics or else it will start a fire
since it is unprotected. The current goes from the battery to the master
contactor to the avionics contactor to the avionics bus where the first
circuit breakers appear. It was professionally wired too.

Mike
MU-2
Helio Courier

[Jon A.]

If one breaks and shorts, the beaker pops and the circuit is dead. If
one is loose or breaks and doesn't short because it well bundled, then
it's done the job.

And let's not forget to either snip or top post, or the feds will
arrive at your hanger to help you.

On Fri, 07 Jan 2005 04:54:02 GMT, "Mike Rapoport"
> wrote:

>
>"Jon A." > wrote in message
...
>>>BTW I am not going to rewire my certified airplane but I was under the
>>>panel
>>>lubricating some bowden cables for the cabin heat and noticed two wires
>>>with
>>>the same origin and destination points which got me to thinking "Why". I
>>>am
>>>also going to be building a Moose and so I am interested in all this kind
>>>of
>>>stuff.
>>>I appreciate the information.
>>>
>>>Mike
>>>MU-2
>>>Helio Courier
>> That's for redundancy. One breaks, the other handles it. My current
>> avionics panel was wired professionally some years back. When I
>> upgraded the avionics, I found this beautiful job of double wiring,
>> both sides. I wish I would have repeated it, but I didn't. If you're
>> building a Moose, go for it, but there's no need to do a double feed
>> to the buss, especially when you're getting up there in the 12 gauge
>> range.
>>>
>>
>
>I'd be surprised if it was for redundancy since if one breaks and shorts it
>will require shutting off all the avionics or else it will start a fire
>since it is unprotected. The current goes from the battery to the master
>contactor to the avionics contactor to the avionics bus where the first
>circuit breakers appear. It was professionally wired too.
>
>Mike
>MU-2
>Helio Courier
>

[Mike Rapoport]

There is no breaker protecting the wire going to the bus.

Mike
MU-2


"Jon A." > wrote in message
...
> If one breaks and shorts, the beaker pops and the circuit is dead. If
> one is loose or breaks and doesn't short because it well bundled, then
> it's done the job.
>
> And let's not forget to either snip or top post, or the feds will
> arrive at your hanger to help you.
>
> On Fri, 07 Jan 2005 04:54:02 GMT, "Mike Rapoport"
> > wrote:
>
>>
>>"Jon A." > wrote in message
...
>>>>BTW I am not going to rewire my certified airplane but I was under the
>>>>panel
>>>>lubricating some bowden cables for the cabin heat and noticed two wires
>>>>with
>>>>the same origin and destination points which got me to thinking "Why".
>>>>I
>>>>am
>>>>also going to be building a Moose and so I am interested in all this
>>>>kind
>>>>of
>>>>stuff.
>>>>I appreciate the information.
>>>>
>>>>Mike
>>>>MU-2
>>>>Helio Courier
>>> That's for redundancy. One breaks, the other handles it. My current
>>> avionics panel was wired professionally some years back. When I
>>> upgraded the avionics, I found this beautiful job of double wiring,
>>> both sides. I wish I would have repeated it, but I didn't. If you're
>>> building a Moose, go for it, but there's no need to do a double feed
>>> to the buss, especially when you're getting up there in the 12 gauge
>>> range.
>>>>
>>>
>>
>>I'd be surprised if it was for redundancy since if one breaks and shorts
>>it
>>will require shutting off all the avionics or else it will start a fire
>>since it is unprotected. The current goes from the battery to the master
>>contactor to the avionics contactor to the avionics bus where the first
>>circuit breakers appear. It was professionally wired too.
>>
>>Mike
>>MU-2
>>Helio Courier
>>
>

[Matt Whiting]

Jon A. wrote:
> If one breaks and shorts, the beaker pops and the circuit is dead. If
> one is loose or breaks and doesn't short because it well bundled, then
> it's done the job.
>
> And let's not forget to either snip or top post, or the feds will
> arrive at your hanger to help you.

You meant snip and bottom post.


Matt

[G.R. Patterson III]

Mike Rapoport wrote:
>
> Can you give me the figure number that you are looking at?

Figure 11-7A, entitled "Conductor chart, Intermittent flow". Located in section
3 after paragraph 444 in my copy.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Mike Rapoport]

I have that chart but I am unclear on how it applies to my question. I
think it is only telling me how long a particular guage wire can be if
operated at a given amperage and voltage, given a specified voltage drop and
an operating temp of 20C. I think it says that if I load a 12ga wire with
30A at 14V at 20C the wire can be a maximium of 16 feet long if the voltage
drop is limited to 1V. It also doesn't define intermittent. A 50% duty
cycle could be an hour on and an hour off but I think that would be
considered continous for the purpose of specifyine a wire size.

Mike
MU-2
Helio Courier




"G.R. Patterson III" > wrote in message
...
>
>
> Mike Rapoport wrote:
>>
>> Can you give me the figure number that you are looking at?
>
> Figure 11-7A, entitled "Conductor chart, Intermittent flow". Located in
> section
> 3 after paragraph 444 in my copy.
>
> George Patterson
> The desire for safety stands against every great and noble enterprise.

[Jon A.]

On Fri, 07 Jan 2005 16:50:30 GMT, "Mike Rapoport"
> wrote:

>There is no breaker protecting the wire going to the bus.
>
>Mike
>MU-2
>
There should be - - - somewhere along the line. What about on your AV
master switch?

[[email protected]]

In my Bonanza there are auto reset breakers that protect all sorts of
things. They are behind the panel and you never see them.
Just because breakers aren't visible doesn't mean they aren't there.

Dave

Jon A. wrote:
> On Fri, 07 Jan 2005 16:50:30 GMT, "Mike Rapoport"
> > wrote:
>
>
>>There is no breaker protecting the wire going to the bus.
>>
>>Mike
>>MU-2
>>
>
> There should be - - - somewhere along the line. What about on your AV
> master switch?
>

[Jon A.]

On Sat, 08 Jan 2005 08:35:40 -0600, wrote:

>In my Bonanza there are auto reset breakers that protect all sorts of
>things. They are behind the panel and you never see them.
>Just because breakers aren't visible doesn't mean they aren't there.

Are they still using those thermal breakers in the newer airplanes?
>
>Dave
>
>Jon A. wrote:
>> On Fri, 07 Jan 2005 16:50:30 GMT, "Mike Rapoport"
>> > wrote:
>>
>>
>>>There is no breaker protecting the wire going to the bus.
>>>
>>>Mike
>>>MU-2
>>>
>>
>> There should be - - - somewhere along the line. What about on your AV
>> master switch?
>>

[G.R. Patterson III]

Mike Rapoport wrote:
>
> I have that chart but I am unclear on how it applies to my question.

Look at the point at which the 12ga wire size line hits curve 3. That is
approximately 2/3 of the way between the 30 amp line and the 50 amp line. I take
that to mean that that wire size can take that amperage for a short period of
time. If you follow the horizontal line from that point over to the length
scale, you get a maximum length for a 14 volt system of 12 feet. And, yes, the
voltage drop and temperature considerations you mention apply.

> I think it says that if I load a 12ga wire with
> 30A at 14V at 20C the wire can be a maximium of 16 feet long if the voltage
> drop is limited to 1V.

That too.

> It also doesn't define intermittent.

If you take a closer look at the label in the lower right corner, it states
"maximum of two minutes." This is also stated in the text.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Mike Rapoport]

"Jon A." > wrote in message
...
> On Fri, 07 Jan 2005 16:50:30 GMT, "Mike Rapoport"
> > wrote:
>
>>There is no breaker protecting the wire going to the bus.
>>
>>Mike
>>MU-2
>>
> There should be - - - somewhere along the line. What about on your AV
> master switch?
>

Well that is the whole point, it is just two wires with the same origin and
destination points. AFAIK the avioniics master sends current to the field
on avionics contactor which is supplied directly from the master contactor
which recieves its field current from the master switch. Both the master
and avionics switches are simple MS toggle switches, not circuit breaker
type switches.

Mike
MU-2
Helio Courier

[Mike Rapoport]

Do they fit inside a wire? From the avionics contactor to the avionics bus
there is nothing expect two wires. I haven't looked upstream of the
avionics contactor. I will do that this week.

Mike
MU-2


> wrote in message ...
> In my Bonanza there are auto reset breakers that protect all sorts of
> things. They are behind the panel and you never see them.
> Just because breakers aren't visible doesn't mean they aren't there.
>
> Dave
>
> Jon A. wrote:
>> On Fri, 07 Jan 2005 16:50:30 GMT, "Mike Rapoport"
>> > wrote:
>>
>>
>>>There is no breaker protecting the wire going to the bus.
>>>
>>>Mike
>>>MU-2
>>>
>>
>> There should be - - - somewhere along the line. What about on your AV
>> master switch?

[Mike Rapoport]

"G.R. Patterson III" > wrote in message
...
>
>
> Mike Rapoport wrote:
>>
>> I have that chart but I am unclear on how it applies to my question.
>
> Look at the point at which the 12ga wire size line hits curve 3. That is
> approximately 2/3 of the way between the 30 amp line and the 50 amp line.
> I take
> that to mean that that wire size can take that amperage for a short period
> of
> time. If you follow the horizontal line from that point over to the length
> scale, you get a maximum length for a 14 volt system of 12 feet. And, yes,
> the
> voltage drop and temperature considerations you mention apply.
>
>> I think it says that if I load a 12ga wire with
>> 30A at 14V at 20C the wire can be a maximium of 16 feet long if the
>> voltage
>> drop is limited to 1V.
>
> That too.
>
>> It also doesn't define intermittent.
>
> If you take a closer look at the label in the lower right corner, it
> states
> "maximum of two minutes." This is also stated in the text.

I think that your chart might be somewhat different mine certainly doesn't
have that note. I will have to search the text to see if it is there. My
copy is AC 43.13-1B CHG 1 and is dated 9/27/01 which is the most current (I
think)

Mike
MU-2

[Matt Whiting]

Mike Rapoport wrote:

> Do they fit inside a wire? From the avionics contactor to the avionics bus
> there is nothing expect two wires. I haven't looked upstream of the
> avionics contactor. I will do that this week.

Well, there are things called "fusible links" that Chrysler has used for
decades. Many of them look just like wires. I've never seen one on an
airplane, however.


Matt

[G.R. Patterson III]

Mike Rapoport wrote:
>
> I think that your chart might be somewhat different mine certainly doesn't
> have that note. I will have to search the text to see if it is there.

In my copy, paragraph 444 section "d" says "The procedures in 'c' can be used to
find the wire size for any continuous or intermittent operation (maximum 2
minutes) voltage ..."

> My
> copy is AC 43.13-1B CHG 1 and is dated 9/27/01 which is the most current (I
> think)

Ah. Mine is AC 43.13-1A CHG 3. It's dated 1988, though I bought it in the late
'90s.

George Patterson
The desire for safety stands against every great and noble enterprise.

[Jon Woellhaf]

Mike Rapoport wrote that his copy of AC 43.13-1B CHG 1 doesn't have a note
reading, "maximum of two minutes" on the wire chart.

I don't see that note on Figure 11-3 (Conductor chart, intermittent flow)
either, but it's in the text in paragraph 11-68(d): "d. Procedures in
Example No. 1. The procedures in example No. 1, paragraph 11-68c, can be
used to find the wire size for any continuous or intermittent operation
(maximum two minutes). Voltage (e.g. 14 volts, 28 volts, 115 volts, 200
volts) as indicated on the left scale of the wire chart in figures 11-2 and
11-3."

Jon