Headlight relay Wire size - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 11 (permalink) Old Mar 26th, 19, 3:37 PM Thread Starter
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Dave
 
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Headlight relay Wire size

Guys, I purchased the relays, now I am trying to get a straight answer on wire size for each terminal.

Battery to relay = what ga?
all other wiring from relay= what ga?

I have some saying 8ga from the battery, some say 12 ga, big difference in price.

Can you help me out

1966 Chevelle.
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post #2 of 11 (permalink) Old Mar 26th, 19, 3:46 PM
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Bill
 
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Re: Headlight relay Wire size

14 ga is 35 amps, 12 ga is 40 amps, 10 ga is 55 amps, 8 ga is 80 amps, 6 ga is 105 amps.
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post #3 of 11 (permalink) Old Mar 26th, 19, 3:50 PM
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Jim
 
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Re: Headlight relay Wire size

Hi Dave -

I just did this conversion a couple of months ago. I don't remember the gauge wire used, but I know it wasn't 8 - in fact it might have been 18. And looking at Mad's wiring kit, I believe they're using 18. I'm running both headlights on each side on a 10 amp fuse, and when I checked the load it was between 6-7 amps. 18 gauge is rated for 10 amps, so that should be sufficient - if you really wanted to be sure, 12 would be plenty.

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post #4 of 11 (permalink) Old Mar 26th, 19, 5:05 PM
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Steve
 
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Re: Headlight relay Wire size

Ideally you want to use 12 awg for the battery power to the relay and from the relay to the headlight itself.

For the trigger input line, you can use 16-18 awg.

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post #5 of 11 (permalink) Old Apr 3rd, 19, 10:38 PM
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Frank
 
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Re: Headlight relay Wire size

Figure the amount of power the lights and relays will use, plus the lengths of wire and the operating temperatures (moderate temp in this case behind the radiator support). I would recommend THHN or XHHWinsulated wire good to 170F temps.

Headlights on low pull between 35w to 45w (3.2A) each bulb, high beams about 65w (4.8A) each. Two low beam bulbs would draw 6.5A. Two high beam bulbs pull right about 10A.
My recommendation would be to pull power from the horn relay and not the battery. When the engine is running, the alternator provides power directly to the horn relay over 10g wire, then to the battery through a second 10g wire (on the 1970 model car).
Battery to high beam relay: draw of 10A for two high beam or 20A for four high beams. I would recommend wire for the battery to high beam relay of 10g or 12g up to 10 feet in length.
Battery to low beam relay: 6.5A for two low beams. Recommend 12g or 14g wire.
Relays to each headlight lead, low or high beam: 14g wire. This size wire will support the 5A each bulb can draw. The factory wire to each low beam and high beam connection in 1970 was 16g wire, smaller than my proposal of 14g.

Between the power source and relay, for circuit overload protection I suggest a self-resetting circuit breaker instead of a fuse. Low beam relay can be protected with a 20A circuit breaker such as Littelfuse 20 Amp Circuit Breaker UCB20BP ($5), while the high beam could use a 30A breaker Littelfuse 30 Amp Circuit Breaker UCB30BP ($5).
Headlight sockets could be replaced with Dorman Conduct-Tite Headlight Socket 85810 ($3) or equivalent.

I did this headlight relay conversion in 2007 and have been happy with its performance.
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post #6 of 11 (permalink) Old Apr 4th, 19, 3:38 PM
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Re: Headlight relay Wire size

I would use 14 gauge from the horn relay to the relay contacts and from the relay contacts to the headlights and 18 gauge on the relay coil.

Louis
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post #7 of 11 (permalink) Old Apr 4th, 19, 4:07 PM
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Eddie
 
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Re: Headlight relay Wire size

12gauge for battery to relay (fused), relay to headlights and ground, trigger wire 14 or 16gauge. This is how I've done on several cars and I haven't had any issues. Wires are not getting warm and lights are working fine without flickering.
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post #8 of 11 (permalink) Old Apr 4th, 19, 8:33 PM
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Re: Headlight relay Wire size

Ideally, you will use copper bars as wide as the car is. In reality, 12 gauge or bigger (lower number) will do perfectly fine.

'71 chevelle, 468, TH400, 12 bolt posi w/3.08's, drinks a lot of 87 octane.
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post #9 of 11 (permalink) Old Apr 4th, 19, 8:59 PM
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Re: Headlight relay Wire size

Building wire such as THHN or XHHW has fewer thicker strands, is less flexible and THHN coating will harden and chip off over time when exposed to heat cycles. And less flexible means it could vibrate out of things.

Search for marine wire on eBay. They sell lengths in packs, pick your needed colors. Much better flexible high strand wire for auto use.

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post #10 of 11 (permalink) Old Apr 4th, 19, 9:48 PM
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post #11 of 11 (permalink) Old Apr 5th, 19, 12:39 AM
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Scott
 
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Re: Headlight relay Wire size

Quote:
Originally Posted by paul bell View Post
Building wire such as THHN or XHHW has fewer thicker strands, is less flexible and THHN coating will harden and chip off over time when exposed to heat cycles. And less flexible means it could vibrate out of things.

Search for marine wire on eBay. They sell lengths in packs, pick your needed colors. Much better flexible high strand wire for auto use.
Agreed. I like to use MTW wire or GXL as it's readily available on EBay in any color/gauge you want.

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