[MikeK]

Originally Posted by JackA

Whoa! 20 amp fuse in a 15 amp circuit is asking for a fire. IMHO

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.