Question for Jim Wier (or other electrical guru)

"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

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:

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.

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.

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
news
In my 1974 Helio Courier, the avionics bus

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.

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.

"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

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