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H4 conversion wiring question?

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Can anyone advise what minimum wire size is required to successfully install a high output H4 conversion kit?

I will be using 100/55W bulbs, and more or less following the how-to in the FAQ.

I have sourced some nice 4 conductor 14AWG wire ...... but Susquahanna Motorsports (www.rallylights.com) recommends 12AWG for all of their installations.

In my wiring layout, the maximum run is approx. 6ft., and only one lamp will be powered by each wire.

Do I need to use 12AWG for this application?

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On early cars, the headlights aren't even fused! On later roundies, the low beams are, but not the high beams. Squarelights at least have the headlights fused, but not all have relays.

Here's what I'd do at a minimum, regardless of year.

Use relays, especially for those 100 watt high beam filaments. Use 12 gauge wire to feed the relays, and from the relay to the lamp. If your high beams aren't fused in the current setup, you can buy relays with built in fuses. And make sure the grounds at both headlights (the brown wires) are tight and clean.

You're probably OK with your existing wiring for the low beams, as that's what normal H4 low beams use--I've been running 'em for years on my '73 with no problem, even without a relay.

An alternative: Install two 55 watt driving lights and use normal wattage high beams--you'll get more light (4 projectors vs 2) with the same wattage; you can wire the driving lights thru a relay with heavy wire that's independent of your current wiring harness. And...you won't have to mess with your existing harness. You can wire the driving lights thru a switch so you can have 'em go on when you hit the high beams, or off all the time...easy wiring scheme.

cheers

mike

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I should have been clearer that the original /existing wiring is only being re-used to trigger the operation of the new relays that I will be installing as part of this project.

So essentially the question boils down to whether 14AWG wire is sufficient to handle the current required to operate a single 100/55W H4 lamp.

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electrical fires and/or melted wiring, go with 12 gauge wire. Ya can't use wiring that's too large, only too small! I wired my 55 watt driving lights with 10 gauge wire, 'cause I had some around but didn't have any 12 gauge.

cheers and happy wiring

mike

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From http://www.danielsternlighting.com/tech/relays/relays.html :

SELECTING WIRE SIZES FOR USE IN LAMP CIRCUITS

Use only stranded wire, never solid (household type) wire, in automotive applications.

Wire gauge selection is crucial to the success of a circuit upgrade. Wire that is too small will create the voltage drop we are trying to avoid. On the other hand, wire that is of too large a gauge can cause mechanical difficulties due to its stiffness, particularly in pop-up ("hidden") headlamp systems. The headlamp power circuit ought to use no less than 14-gauge (2.5 mm2) wire, with 12-gauge (4.0 mm2) being preferable. 10-gauge (5.2 mm2) can be used if bulbs of extremely high wattage are to be used, but it's usually overkill. Be sure to pick a kind that flexes easily if yours is a hidden-headlamp system. Do not fail to use the large wire size on both sides of the headlamp circuit! Voltage drop occurs due to inadequate grounding, too! you will only sabotage your efforts if you run nice, big wires to the feed side of each headlamp, and leave the weepy little factory ground wires in place. Most factory headlamp circuits run the too-thin ground wires to the car body. This is an acceptable ground--barely--on a new car. As a car ages, corrosion and dirt build up and dramatically increase resistance between the car body and the ground side of the vehicle's electrical system. It takes little extra effort to run the new, large ground wires directly to the battery Negative (-) terminal or to the metal housing of the alternator, and this assures proper ground.

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Thanks for the link...

In my heart I know 12AWG is superior, but I was hoping an engineering type would tell me that 14AWG was adequate to avoid having to find a way to dress up a nice looking harness from basic bulk 12AWG wire.

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2002lightswiring005.jpg

Here's a pic of my wiring harness I made up for my H4s on my 72.

Basically, I built a little black box with two relays and two blade fuse holders. One for the highs & one for the lows. I got a couple of used headlight connectors and fab'd up a harness to go to each light. The whole shebang is controlled off of the stock wiring. The control end of the harness plugs into the existing headlight connector. I just had to run a power lead from the battery and a ground wire to the relays.

No cut, just plug and play. Unplug the power and ground, disconnect the original headlight connectors from the harness and you are back to bone stock.

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Radio Shack project box. I cut out one side of the plastic for the wires to come out. The whole thig gets stuck to the fender/inner well with some 3M marine velcro(the plastic stuff).

It's simple, but I did need to draw it out just to be sure I had it right when I wired it up.

Definitley gives you a piece of mind even if you run lower wattage lamps.

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Serious resurrection here!

I'm building this plug and play "box", and may try and reconfigure a few things to include my fog relays into it as well.. then this is going into the FAQ construction zone for those that search this type of project out later. This is a great solution.

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