The purpose of the fuse is not to protect the device, but instead to protect
the wire. You don't want the wire to burn. As a general rule, if the device
draws X amps at operating voltage, the fuse should be about a X+1/3 rating
and the wire should be about X+1/2 or higher capacity. Authorities disagree
somewhat as to what amperage wires can safely handle. Sometimes minimizing
voltage drop due to resistance in the wire run is the limiting factor rather
than capacity. But for wires with 150 Centigrade rated insulation enclosed
in bundles in a 70 Centigrade under-hood ambient temperature the following
continuous amperage carrying capacities are a good rule of thumb. Some argue
that the fuse can be equal or a little higher than the wire's continuous
current capacity because most circuit failures involve dead shorts that
greatly exceed operating currents and will cause the fuse to blow quickly
before the wire heats up to much. This is true only if dead shorts are your
predominant circuit failure mode but would not be appropriate for a load
device, like say a dual filament headlamp where the circuit design
(unlikely) could allow both filaments to be powered by a wire intended to
power only one filament.
Authorities differ as to their wire capacity limits. I provide two well
respected authorities limits below. Which is correct depends on the
individual situation, how hot is ambient? how continuous is continuous? what
heating is occurring in the other wires in the bundle when this device is
powered? is the ground return within the bundle? are there surge loads? are
the connections high or low resistance? One easy solution is to rate wires
conservatively.
18 gauge = 9.3 amps capacity, 7.5 amp fuse, 6 amps load OR 18
gauge = 11 amps capacity, 10 amp fuse, 7 amps load.
16 gauge = 11.1 amps capacity, 10 amp fuse, 7 amps load OR 16
gauge = 15 amps capacity, 15 amp fuse, 10 amps load.
14 gauge = 15 amps capacity, 10 or 15 amp fuse, 10 amps load OR 14 gauge
= 22 amps capacity, 20 amp fuse, 15 amps load.
12 gauge = 20 amps capacity, 15 or 20 amp fuse, 15 amps load OR 12 gauge
= 29 amps capacity, 30 amp fuse, 19 amps load
10 gauge = 27 amps capacity, 25 amp fuse, 18 amps load OR 10
gauge = 39 amps capacity, 35 to 40 amp fuse, 26 amps load.
8 gauge = 37 amps capacity, 35 amp fuse, 25 amps load OR 8
gauge =51 amps capacity 50 amp fuse, 34 amps load.
The fuse should be the lowest value that can handle the device draw but not
higher than the wire's capacity. For example if you use an 18 gauge wire to
power a 2.5 amp load, use a 3 amp fuse even though the wire could handle
much more.
Sometimes another practical issue dictates the minimum wire capacity. All
the wires in a circuit protected by a single fuse need to be of equal or
greater capacity than the fuse. Do not add an 18 gauge wire to a fused
circuit fused for 16 gauge wires.
Hap Polk
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