amp rating for fog light wires

I have a set of fog lights on my truck that have H3 55W bulbs. The manufacturer said the wire is 18 gauge.

I'd like to upgrade to 100W bulbs, which, at 12V, will be 8.3A. Is this too much for 18 gauge wire?

Power & Ground Cable Specs
Cable Size Wire Gauge	Current Capacity Amperage (amps)
1/0	350
2	225
4	150
8	100
10	60
12	40
14	25
16	15

The chart on this site does not go down to 18 Ga.  But doing the math, it will be very close.  If you do try it, fuse it at 7.5  amps (I know that 7.5 is below the 8.X amps you figured up) and turn the lights on for a minute or two and monitor the heat buildup in the wire.

The way I tackle it is that a 0.5V drop between source (alternator else battery) and the bulb is to much - especially for halogen.
That includes the ground path - ie, if battery +ve to -ve terminal is is 12.5V (with lamps on), then less than 12.0V between H3 +12V & its body ground is too low.

But that is MY ROT for illumination reasons; HIDs and tungsten lamps are different.
Others tolerate much higher voltage drops. (And without relays, higher drops are almost guaranteed.)

Thanks. I think I'll play it safe and run 12 or 14 gauge.

Would you recomend fusing each light seperately with a 10A fuse?

I have my 55W HID high beams powered by a realy and fused seperately with a 25A fuse. I have HIDs in my low beams also but they are flickering a little bit. I have a relay harness built for them with 20A fuses.

Fogs may not be that critical, so one fuse may be ok.

I fuse individually, but I only have the 4 headlights - each with their own relay.
Spots I fuse separately, but they are usually 100W or more each.


Even when I have a common upstream fuse (eg, left & right side, or upper & lower beams), I'll usually fuse separately to try to have any fault effect the faulty lamp only.


And though I say fuse, I really mean auto-resetting circuit breaker - at least for the common power feed(s) if not the individual lamps (unless separate feeds...).   

Thank you.

On a related note I am planning on putting HIDs in my wife's 2003 Kia Sedona van. I would like to do the 4-HI mod so that the lows stay on with the highs. I know on my truck the ground is shared between the lows and highs...that is how the lows are shut off when the highs are on. I made a seperate ground for the lows so they stay on. I suppose I could do the same with the van...by using a voltmeter to discover if the + or - is removed from the lows when the highs turn on then providing a seperate + or -.

If they are common ground, then it must be +12 to power them.


My car's outer beams are hi-lo or lo selectable. (The inners are high only.)

Although standard with outer dual-filament hi/lo, I converted to full-time low.
Just move the low beam from the dip switch to BEFORE the dip switch - ie, to the "BEAM" part of the circuit.
That means whenever the "BEAM" is on, low beams are on.
The dip switch then becomes a mere on-off switch for the high beams.

The other mod that may be required is to connect the high beam to the low beam via a diode so that when flashing (hi-beam), the (outer) lows also come on. However that depends on legalities and is probably not desirable when using HIDs for low beams.   
(IN4004 diodes etc assuming relays are used.)