right wire gauge for headlights?

C

cfordman

Member
my A came with king bee chrome headlights with 12v halogen sealed beams. they are not hooked up. the A has a 6v system which is what im sticking with. im thinking the pigtail wiring currently in the headlights is too small to carry the 6v lights. what is the proper gauge wire needed? I am not using the original A light switch but rather a foot dimmer and light switch.
 
Volts x amps = watts. If you have typical 50 watt 6V halogens (watts rated @ 7.2V) you have ~7 amps for each lamp. Individual lamp wires and their grounds are OK at typical automotive 16 ga. Where they join to go to the switch, and the feed to that switch, you need 14 ga for the 14 amp load. If you fuse the circuit, use a 20A fuse.
 
JackA said:
Whoa! 20 amp fuse in a 15 amp circuit is asking for a fire. IMHO
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No, it is not, IMHO. This is not a house wiring circuit where variable cumulative loads may push the sustained amperage past the heat dissipation capacity of 14ga wires buried in walls or conduit. In this case the load is 14+ amps and not subject to increase unless there is a short, which will quickly blow a 20A fuse. If a 15A fuse were used with a 14+A load the fuse element would run extremely hot and likely fail a lot. The last thing you want on a headlight circuit is any possibility of losing your lights while driving for no good reason.

While it is prudent to fuse or protect a circuit as tightly as possible, there are variables: The voltage may fluctuate, changing amperage draw. Fuses & breakers never are exact in their rating, and often blow or trip at a lower load if held very close to their rating for an extended time. The actual ampacity of a wire is a function of its heat dissipation ability, not a set rating from a chart.

If you want to look at from the aspect of the NEC, as house wiring, the situation is different. In no case could you divide the individual lamp wires down to 16ga (an oddity in non-automotive and line voltage apps) even if the circuit was fused @ 15A.

If you're concerned about electrical fire, Model A's had no fuses and many still run that way, despite high output alternators!! Yes, that scares me too! I also don't want to advise someone into a situation where their lights may unexpectedly fail. I'd use a 20A fuse for two 6V 50W (watts rated @7.2V) halogens, IMHO.
 
The correct fuse for the 14 Gauge wire is 15 amps.

You are protecting the wire from causing a fire when selecting the fuse. You select the gauge of the wire for the total load it will see, the fuse is set to protect the wire.

There are also different kinds of fuses like slo-blo to handle short high current loads. If you use a breaker instead of a fuse the size of the breaker may be different then a fuse.

Fuse to wire gauge chart link
 
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