Maker Pro
Maker Pro

Wire rating for HID headlight ballast

john2k

Jun 13, 2012
188
Joined
Jun 13, 2012
Messages
188
I have a 40A 12V relay which I am using to provide a direct +12v feed from car battery to power 2x HID xenon bulbs. The wire from the battery will go from the battery to a waterproof blade type fuse-holder, then it will go to pin 30 of the relay. The output on pin 87 I then need to take to 2x ballasts that will power the bulbs.

Now my question is, what fuse should I put in there firstly? 5 or 10A? then, what kind of cable should I use? I have option of:
  • 0.75mm² 14 Amp 18AWG
  • 1.0mm² 16.5 Amp 17AWG
  • 1.5mm² 21 Amp 16AWG
Should I take two different cables from the fuse-holder, or can I take one cable and splice of it for the first bulb?

Any advice would be appreciated.

Thanks
 

kpatz

Feb 24, 2014
334
Joined
Feb 24, 2014
Messages
334
What's the wattage of the bulbs and the current draw of the ballast? HIDs put out more light per watt compared to halogen so if you're using the existing halogen wiring it should be adequate. It should say on the ballast, or in the documentation how much current it draws. Note that HIDs require more power to start up than to stay lit, so take that into consideration when sizing the wiring and fuse.

Unless these are super duper bright lights they probably draw less than 10A each, probably under 5A, and the 14 amp wiring and a 10A fuse should be adequate.

I'd run a separate wire from the relay to each ballast, and fuse each separately, that way it's less likely you'd lose both lights if something fails or a fuse blows.

How long are the wire runs? You'll want to use thicker wire if the runs are long to reduce resistance.
 

john2k

Jun 13, 2012
188
Joined
Jun 13, 2012
Messages
188
It's a D1S bulb which I believe is a 35W bulb. The original wiring running to inside the headlamp is made for a 55W halogen bulb. I'm not sure what the current draw for the ballast is. The wire to the first bulb will probably be something like 40 to 50cm and the second will be more like 200cm.
 

dave9

Mar 5, 2017
1,188
Joined
Mar 5, 2017
Messages
1,188
Incan bulbs are more sensitive to a voltage drop because they are a direct resistance and output goes down (yet lifespan goes up remarkably) a lot with a voltage droop, but HID shouldn't have this issue, should boost and regulate output regardless of whether the input voltage is a little lower.

If the HIDs don't use more power than the stock bulbs they're replacing then there is no need to do any of this. The existing headlight wiring, fuse and relay should work, but if it is worn/degraded and needs replaced, then considering that you don't have the same penny pinching constrains as an automotive manufacturer, I'd go ahead and use the lower gauge wire and at least a 10A fuse or separate fuses for each as kpatz mentioned so you have some redundancy to help keep one of the two headlights operational in case of a fault on the other.

If you are planning to keep your existing headlights and add an additional two more, HIDs, consult your regional laws as this may be illegal to run 4 headlights.
 

Kiwi

Jan 28, 2013
467
Joined
Jan 28, 2013
Messages
467
Auto sparky here.
I would use 2mm² 14AWG wire. Larger wire adds mechanical strength, as well as reducing the chance of any possible voltage drop.
20Amp fuse between battery and relay. Fuse close to battery.
As suggested, separate wires to each ballast with 5Amp fuses.
2mm² earth wire to clean chassis grounding point near each ballast.
DON'T connect to any bolt that has a single large E on its head. These are factory installed grounding points for the vehicle electronics. Weird things can happen if you mess with these bolts.
 
Top