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Summary of the situation:

I have a 1975 Ford truck that suffered from the "Ford flashing headlight problem".

The factory wiring uses 18ga wires over a very long path and through an under-rated switch to power the headlights. When someone "upgrades" the headlights to halogens, the increased load overheats the switch causing its internal breaker to trip.

The fix is insert a relay and use heavier wires (14ga) to carry the power to the headlights over a shorter trip. You can read about it here: http://www.ford-trucks.com/article/idx/0/039/article/Whiter_Whites_Brighter_Brights.html

Original wiring:

Revised wiring:

Is 20A fuse correct here? It seems too high for 14ga wire.

Should there be a fuse on the control line for the relay (contact #85)?

(A couple months ago my first Garage SE question would have been "why do the headlights on my truck flash?")

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2 Answers

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As far as the fuses are concerned, it's never a bad idea to add a fuse to a particular section of a circuit that you're concerned about. For the high-beam fuses, in particular, I would start conservatively. Your average high beam (for a normal headlight) is 55 or 65 watts. In a 12 V circuit, that's less than 6 amps nominally. If you put in a 10 amp fuse, you should be ok. I have looked at some amperage vs wire diameter charts and 20 amps seems a bit too close to the 24 amp maximum for my comfort levels. To reiterate, it's never an issue to start conservatively on these things, and I think 10 amps will suit you just fine.

Wire Gauge vs amperage diagram

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You can safely use a 20A fuse on 14ga wiring. The chart you reference is concerned with sizing the wire for voltage drop over runs of differing length. When it comes to fusing for overcurrent protection, the length of the run isn't a factor, just the size of the wire. –  mac Nov 16 '12 at 16:34
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20 amps is more than enough for each circuit - you can probably run a 10 or 15amp fuse without issues. You're not going to cook the wiring with 20 amps...

And you should definitely fuse your control wiring as well (Contact 85) - That would only require a 3 amp fuse at most.

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