Ok Net
I'm not going to try to play expert{Has been that can't cum) on this subject,
but I read some incomplete information and it made me get my text book out. A
simple circuit contains a source(battery, alternator), load device
(motor,light,etc) and a conductor (wire). To that you can add a control device
(switch) and circuit protection (fuse, circuit breaker). There are two
formulas that I required all students to learn, and they used the following
terms and symbols: E (electromotive force measured in volts); I ( current
measured in amperes); R ( resistance measured in ohms); and finally P (power
measured in watts). The formulas are EIR where E = I times R and PIE where P
= I times E. With these two formulas you can compute all that you need to know
about your circuits. In addition to this information you must know that with
direct current voltage the size of the wire must be increased as the wire gets
longer, because the resistance in wire increases with length. Anyway there is a
chart to help you select the correct size. To correctly select a fuse or
circuit breakers you must consider the amperage in the circuit. Your
computation must include the power consumed by the load device (motor) for
example a 12 watt motor in a 12 volt system uses 1 amp. A 100 watt landing
light in a 13.5 volt system pulls a current of 7.4 amps and could use a 7.5 amp
fuse or CB if you don't consider wire length. Once you know the circuit
amperage you can go to the chart and pick a wire which can carry the current
over the distance intended. In a fuse it is the heat in a circuit that caused
the fuse to melt and in a CB it is a magnetic field that causes it to open.
conductor (wire) size is picked to carry the current to the load with out
overheating. you pick the smallest wire to get the job done so as not to add
too much weight.
This is a big subject and it is not my intent to do an online lecture or bore
anyone. Tony B's book is good and the FAA Advisory Circular AC43-13, is an
excellent resource on the how to's of aviation.
Orma L. Robbins
AP/IA
aka AviationMech
19 Years flying the KR-2
