8N Headlight Fuse?

[John P.]

Can anyone tell me what fuse is needed to fill the empty fuse socket on my late 1950 8N light switch?
It's never been there as long as I've owned it.

 

[Dave G9N]

(quoted from post at 15:52:32 08/13/23) Can anyone tell me what fuse is needed to fill the empty fuse socket on my late 1950 8N light switch?
It's never been there as long as I've owned it.

o, because I don't know the wire size or how many lights or what the wattage of the lights is.

The math boils down to dividing the total wattage of the lights by the battery voltage to get the number of amps used by the lights. The lights should be labeled with the wattage, and if not, it is easy enough to look up.

If your battery is 6V and the headlights are both 40W, the total wattage is 80 and the current draw is 13.33 amps. You would need a minimum 15 amp fuse. The fuse is to keep the wires from overheating, so the maximum fuse size is whatever the wires can handle. If you need a larger fuse than the wire can handle, you should replace the wire with a heavier gauge.

The ampacity of single THHN wires in open air is:
[code:1:ad813b5a53]AWG Amperes
18 18
16 24
14 35
12 40
10 55[/code:1:ad813b5a53](Page 144 of Glover's Pocket Ref 4th ed.)
THHN is one of the higher temperature wires, the next group down will take about 5 amps less. If you need a larger fuse for the number of lights than the wire gauge is rated for, the wires will run hotter than they should. Not so bad in the winter, but on a hot day you might melt the insulation.

The math:
Volts = current times resistance (E = IR)
Watts = IR
IR = I x IR
Volts = 6 so IR = 6
Watts = I(=6) x I
Two 40W headlamps = 80W
80 = 6 x I
I = 80/6
80/6 = 13.33 amps, so a 15 amp fuse is enough.
If you have a 12V conversion, the amperage is cut in half (IR = 12)

 

[John P.]

Hi thank you for the info.
I found the data for the fuse in an old Ford parts book.
It is supposed to be a 3AG-20.