Question for Jim Wier (or other electrical guru)

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.