Since Watts= amps X volts, amp= watts/volts----->150w/12v= 12.5 amps so a 15 amp fuse should work
Thats just 1 bulb. Double it for 2 bulbs and you get around 25 amps. I say around because we don't know if the bulbs were rated at 12 volts or at the 13.8 volts they frequently run at. If they were rated at 150 watts at 12 volts then they will use 12.5 amps at 12 volts. However, if you feed the same bulb 13.8 volts it will use around 14.4 amps an put out around 198 watts. These numbers are not exact because of rounding and the lamps voltage/current curve probably isn't strictly linear.
The point here is that because we're uncertain of the actual ratings, circuit components have to be somewhat oversized. Realistically, this circuit could easily draw 29 amps. Yes there are switches out there that are rated at 30 amps maximum. Too close for my comfort. This circuit will definitely need a relay to be safe. If it were me, I would size the relay, wire, and fuse for 40 amps. The additional benefit of using a relay is that you can use almost any switch you want to turn on the relay.
Edit: Try a 30 amp fuse. If it lasts for 10 minutes at full load with at least 2000 engine RPM you should be good to go. Feel the fuse and see if its hot. If it is too hot to touch you're on the verge of blowing it so switch to the 40. Just don't go above the 40 because the relay and wiring aren't sized for it.
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Last edited by Alter Ego; 05-04-2009 at 06:00 AM.