i just googled a question and found an answer.

i've been wondering about the watts vs amps vs wire size because i'm going to put my headlights and my off road lights on relays.

what i found out is:

to get the amp draw you divide watts by volts.

i plan on running 2 off road lights and 1 high beam on each relay and both low beams off one relay as well.

my 2 100 watt off road lights and my 65 watt high beam will draw 265 watts.
so i divide 265 watts by 12 volts (battery) and i get 20 amps. or if i divide 265 watts by 14 volts (battery with the engine running) then i get 18.92 amps.

so that's good, my relays are 40 amp and i'll put a 25 amp fuse inline since the high beams are going to draw 19-20 amps when they are on.

the low beams relay will be drawing 90 watts (7.5 amps with the engine off and 6.42 amps with the engine running). so that relay will get a 15 amp fuse and the 40 amp relay is overkill.

now for wire size vs amp draw, i found a table for wires used in automotive applications:

for wire length, you need to measure the wire from the battery, to the accessory and back to ground.



i planned on using 16 gauge wires to go from the relay to each light and i now know that its overkill.
the 100 watt (off road), 65 watt (high beam) and 45 watt (low beam) lights only need a 18 gauge wire each.

i planned on using a 12 gauge wire to go from the battery to the relay but i only need a 18 gauge also.

i figure less than 3 feet from the battery to the relays and less than 7 feet from the relays to the lights, so that is how i got the 18 gauge figure from the chart.

cool, now i know that my lights won't be drawing too much power and as soon as it warms up a little (or i warm up a little, soco ftw) then i'll be putting the new wires and relays in my truck and i'll be able to see a little better.

the main reason i want to do this is because i like running my off road lights on back country roads but i hate having to reach down to the knee blocker to flip them off every time a car passes. its going to be nice to have all the lights on the front of my truck operated by my multifunction switch and the headlight switch.

 

loving the bumper lights and the lights upfront. thats a cool chart too! never knew some of that

 

thanks, if you can't tell, i like lights. i think i'll finally be happy when it looks like daylight coming off the front of my truck.

i didn't know that stuff either, that's why i summoned the almighty Google.

i was curious because i didn't want to take the chance of melting any wires or relays due to improper wire or fuse sizes. its a good thing too, i would have put a 15 amp fuse in the inline fuse holder and popped it because the high beams will be drawing 20 amps.