Question for Jim Wier (or other electrical guru)

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.

"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 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.

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.

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.

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.

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.

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.

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:

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.