Technical What Gauge Wire Do I Need?

I've got to upgrade to brighter headlights. found some sealed beams with FoMoCo pressed into the lens using H4 bulbs (They look traditional) BUT, I need to lengthen the run of wire from the new relays to the headlight sockets. What gauge wire do I need for my splices? Thanks.

 

Kind of depends on a lot of things....#14 would be playing it real safe. And you might want to replace the entire run from relay to headlight with one wire, although you'll probably end up splicing near the headlight because it's hard to find the correct terminals to crimp on the end of the wire and fit the socket housing, as they are wider than normal spade terminals.

 

Thanks, I've got 14 and 16 but it might be fun splicing 14 and 18 or whatever the small stuff in the relay kit is

 

for headlights and such i would use 14 on 12v and 12 gauge on 6 volt, depending on amperages. the more amps your headlights can move the brighter they will be.

remember this chart for figuring out amperage

 

Dont forget the wire size needs to be enough to blow the fuse not just take the load of the bulb.

 

Good point about the fuse, but also consider a circuit breaker, which was common in headlight switches in American cars in the old days.

 

Even though it's not traditional, if you install relays, you will be happy in the long run. Some Fords such as a 72 F100 have a circuit breaker built in to the light switch. The load of a halogen bulb caused the circuit to break. The relays took care of that. It's all about personal choice but as we age and can't see after dark, headlights have a new importance.

 

circuit breakers also weaken with time.

 

I would use relays and have them between battery and headlights. 2 small #18 gauge wires then go to the hi beam switch. You need 2 relays, one for low beams and one for high beams. Battery to relays and relays to headlights should be at least 14 gauge. Make sure to use relays rated high enough for two high beam bulbs and don't forget to fuse each battery to relay wire.

 

You can't use too large of wire. So to speak. I use 12ga on most everything that does not need 10ga. I use 8ga on that.

Ben

 

This makes my head hurt

 

convert the watts to amps, then google what gauge wire you need for the given amperage.

 

there are two things at play here...one is current, which requires a certain size wire to keep the wire from overheating. The other is voltage drop, which you want to minimize to keep the lights bright.

There is no exact mathematical answer, you kind of have to understand what the trade-offs are, and what will work best as far as safety, efficiency, and ease of installation. #14 or #16 probably would be best, #18 will handle the current but you start losing significant voltage to the lights.

 

I used an NORS relay on my panel truck with newer insert bulb headlights. I used 10g from the battery to the relay (through a fusible link) and used the new harness the rest of the way (14g). I've also hidden 2 Bosch type relays inside of a OEM voltage relay. Big thing (opening my can-o-worms) is to make sure that the new housings have a good spread pattern (has similar lines on it like the old bulbs) so the light won't be just a flood.

 

smaller is worse, bigger is better until it gets too expensive, typically.

 

I have been using that exact relay in my '63 for many years. This past weekend I was on a run that went well after dark, something I try to plan not to happen. Best laid plans and all that ... Anyway, on the way back to the hotel, about 30 miles out, the fuse (30 amps) inside the relay housing for the high beams took a dump. Actually I think it melted rather than blew from heat build up inside the cover of the relay. That part really doesn't matter, what matters is the lights went out. It's a good thing the highs and lows are fused separately. I quickly found the dimmer switch and got a little (very little) light back on the road. With the lights out that road was darker than the inside of a cow.

So, I'm replacing that old electro-mechanical relay with 2 high quality 40 amp Bosch style (Cole Hersey) relays and 2 new circuit breakers. The wiring and all else is ok, it was upgraded years ago when I put in the high output headlights. I'm not wanting to go down that dark road again.

 

That's no fun, I keep spare fuses in the vehicles just in case as I drive A LOT at night.

The headlights I have now are a little more amps then the Silver Star sealed beams I had in there before. It's just the fact the sealed beams lasted 3-5 months @ $28-30 a bulb and now the insert bulbs last 4 to 5 times as long and cost about the same for 2. The first kit I got was kinda pricey @ $100 then the bottom dropped and the kits are $19 if you can still order them (2 housings, 2 bulbs, 2 relays and a harness).....only downside it the lenses are flat instead of curved.

 

There's an argument for fusing the left and right headlight separately. A bit more complicated wiring than separating high & low, but it reduces the risk of total blackout.

It's absolutely a huge compromise. If you look at things like the headlight wiring in a modern car (or any wiring in it really) the wires are very skinny compared to what any good DIYer would use. The DIYer usually focuses on getting the best function, auto manufacturers factor in the cost of bigger wires in production runs of thousands of cars, the extra weight it would add to the cars and the impact that has on wear and fuel economy, and how high voltage they really need to reach the lamp to make it work well enough without reducing lamp life too much - higher voltage gives more light but reduces life.

Auto manufacturers show that you can get away with pretty thin wires, DIYers show that thicker, preferably shorter relayed wires can improve light output quite a bit. What compromise works best is up to the person making it to decide.

 

first mod i do on all my daily drivers is upgrade to relays, and even then my cars sealed beams are brighter...

 

You might want to consider the OT LED lamps to use in your new headlights. They've come a long ways with color ranges and today you can buy LED's that don't look too blue, or too white and have a warm light that look like old sealed beams.
I'm running halogen adapted headlamps, but chose an LED lamp that looks more yellowish like an old sealed beam. They give great light, but draw less amperage than halogen or even original sealed beams draw.

 

Just an update. Added the new harness and still no lights. Took an electronics guy quite a spell to find the OEM wire/connector splice that had corroded with age and couldn't supply the needed power. All good now and way less than the cost of hitting something in the dark. Darn connection couldn't last more than 58 years. What was Henry thinking?

 

Pretty much all too small IMO....

Keep voltage drop down in any given circuit to 3% or less, the amps will take care of themselves.
Voltage drop is also additive; if you have a 5% drop between points in a circuit, you can still end up with too-low voltage. Say you have 13V at the battery. Lose 5% at the panel, it's now down to 12.35V. Panel to light switch, another 5%. Now you have 11.73V. Switch to dimmer switch same, now it's 11.14V. Dimmer to lights same drop, 10.58V. That's almost a 20% drop at the light... You WILL see that.

 

Resistance in wire is rated per 1000 feet.

 

I don't think I've ever in my life bought a roll of 18g wire.

 

They're very pretty

 

Not saying I'd do it, but if W/V=A, then a 100W H4 bulb only pulls 8.5 amps and 18 gauge wire could handle that