Question for Jim Wier (or other electrical guru)

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

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 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.

"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

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" 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.

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 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, 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

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