Question for Jim Wier (or other electrical guru)

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

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

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.

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.