Wire sizing

[Ralph Baker]

There are three considerations when picking wire and breaker sizes.
1- Load amperes.
2- Voltage drop.
3- Breaker size to protect the wire.

Load amperes is either calculated from known watts & volts (W=AxVolts). Or
given by the manufacturer. The minimum wire size is the amp rating of the
wire. This is influenced by the wire insulation type. (We are all using
Tefzel insulation aren't we?) The basic NAPA pvc insulated auto wire is
not good enough in my opinion.

Voltage drop is created by the inherent resistance of the wire. Larger
wire has less resistance and less voltage drop over a given circuit length.
AC43-13 requires no more than 1 volt drop for intermittent circuits and
1/2 volt drop for continuous circuits. ( I think I remember "continuous"
is defined as longer than 1 minute but don't take that to the bank) Also,
lighting brightness is greatly influenced by voltage. Also, remember the
circuit length is the round trip distance the current has to travel, not
just the one way distance to the load.

Breakers are sized to protect the continuous rating of the wire. Therefore
you may correctly use a smaller circuit breaker than the wire rating as
long as it covers the load draw. An example of this is a landing light
(continuous) that may require #10 copper conductor tefzel insulated due to
circuit length even though the actual current draw is only 8.5 amps.

As a final comment, the extra cost for aircraft quality copper conductor,
Tefzel insulated wire and crimp connectors is cheap insurance. Electrical
problems can be life threatening at worst and a pain to trouble shoot at
best. As my mother used to tell me "the best way out of trouble is to stay
out".

These comments are my opinion and as always, you are responsible for all
your building decisions.
Ralph Baker
Elite 611E
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