05-13-2012, 10:57 PM
Join Date: Mar 2009
DOT approved marking on the headlight may not be definitive. There's apparently little or no enforcement. The brighter 80/100W headlamps may not be highway-legal. You may want to verify.
Wire size is dictated by the current (Amperes) it carries.
Amperes = Watts/Volts. So your 65W headlights would draw around 65W/12V= 5.5 Amperes each. Your 130W lights would therefore draw just over 10 Amperes each.
Wire size is chosen by either of two limiting factors:
1) Temperature rise, determined by the current (squared) flowing in the wire
2) Allowable voltage drop/loss, which is determined by the wire length and the current.
#14 AWG (American Wire Gauge) wire is generally rated for 15A based on temperature rise. Short, single, un-bundled lengths of wire might be OK for 20A. Similarly,
#12 AWG wire is generally rated for 20A, but short, single un-bundled lengths might be OK for a bit more, say 25A.
In a 12V application, any voltage loss is 10 times larger (in percent) than with domestic 120V at home. For headlights, I would urge that you be very conservative and select slightly heavier wire (lower gauge numbers) than minimally required in order to minimize voltage drop and thus maintain brightness. Use your relays to keep the high-current runs as short and direct as possible. The relay coil-control wires can be any convenient length of skinny wire, as they carry only about 1/8 Ampere, a trivial amount.
Tungsten filament lamps draw a huge cold, inrush surge, possible as much as 10 times the running (hot) current. Select your fuses to protect the wiring, not the load. What you're trying to do is protect against a short circuit/fire, not a simple overload. The surge is no problem as long as your circuit can handle it. So, run your relays around half their rated load current current, say 15A on a 30A relay, and they'll last as long as you need them to. Same with the motor for your air horns, although here you can be less conservative, since the duty cycle is so low (hopefully!). Standard automotive ATO/ATC fuses could be a good choice, as long as you observe a generous de-rating margin. Use a 20A fuse in a circuit that draws no more than 15A. Fuses can and do fatigue over time, causing unexpected failures, so it's best not to stress them unnecessarily. Carry spares.
Last edited by Merlin93; 05-14-2012 at 01:11 AM.