Wiring/relays For High Wattage Lights

[Anthony 1,003]

Following on from my topic last week about running high wattage bulbs a couple of weeks ago, I've decided to go the whole hog and properly rewire the headlights so they're performing their best and will be safe for use with high wattage bulbs (say 100W). To do this, I reckon that I'll need uprated wiring, decent earths, and uprated relays.

 

First of all though, I need to work out the current draw of the high wattage bulbs so that I can make sure that's less than the capability of the relays and wiring. This is specified in amps usually, whereas bulbs are specified in watts.

 

The formula is Watts = Volts x Amps and therefore Amps = Watts / Volts

 

Therefore, assuming the normal and worse case scenarios:

 

100W / 14.5 volts = 6.9 amps (engine running)

100W / 12.5 volts = 8 amps (engine off)

100W / 10 volts = 10 amps (worst case)

 

Therefore, as a worse case we're talking about a 10 amp draw per light, and usually significantly less than that at around 7-8 amp during normal running.

 

Obviously all the lights that you'll want the relays to switch are in pairs (left and right headlight, left and right main beam, left and right driving lamp) and therefore you'll need to allow for double the above figures.

 

Assuming you used three relays and three pairs of wiring, you'd need a 20A+ capable relay, 20A+ power feed to the relay, and 10A+ wires to each light.

 

If you combined the main beam and driving lamps into a single relay (given they are both switched at the same time anyway) you'd need a 40A+ capable relay, 40A+ power feed to the relay, and 10A+ wires to each light (assuming four cable runs - 20A+ if the driving lamps are run in series off the back of the main beams)

 

Naturally, the main power feeds to the relays would have to be fused with the appropriate fuses (say about 5-10 amp higher than the expected current draw for the light), and the earth's for each light should be capable of at least 10A current each.

 

Looking on this page it would appear that 2mm thinwall cable is safe for 25amp and therefore should be fine for the relay-light wiring, and the 6mm thinwall cable is safe for 50amp and would therefore be fine for the battery to relay wiring. The single throw 40A relays on this page should be perfectly sufficient for the application I've got in mind.

 

Obviously from a legality point of view high wattage bulbs aren't legal for road use, and there's a limit to the amount of heat the lamp lens will take and current that the alternator can provide. However other than that, is there anything that I've overlooked from a safety/funtionality point of view, given that I'm not a sparky and therefore don't deal with this sort of thing on a regular basis?

 

Cheers

[Rob_the_Sparky 9]

Be careful of the current rating of wires. This is basically down to how hot they get and at what temperature the insulation suffers.

 

The rating will vary significantly depending whether the wire is on its own or in a bundle. Best rule of thumb is make wires much bigger than strictly necessary! Also more finer strands will mean a nice flexible cable that won't break in a hurry. Fewer thicker strands will be stiffer and break more easily when bent.

 

Also note that a bigger wire will drop less voltage at these high currents giving better light output.

 

Rob

 

P.S. A good idea to always use stuff that is overated for what you think you need.

[mrswampy 0]

are the actual swiches going to be able to handle the extra load?

as i know fords have a habit of burning out if you put bigger bulbs in

[Rob_the_Sparky 9]

That is why you use relays. The switches only power the relay coils while the main relay contacts handle the high currents.

 

Personally I'd just fit legal 30 or 50% brighter standard wattage bulbs, its MUCH less hassle.

 

Rob

[Anthony 1,003]

Cheers Rob

 

I did consider the 30% brighter bulbs like Philips Vision etc, but the problem is that I think the earths/contacts/cabling needs attention on my car anyway. The headlights aren't as bright as my previous GTi, but the lamp units themselves look quite new and the reflectors in good condition.

 

Therefore I figures that if I'm going to go to the hassle of cleaning up the earths and contacts, plus checking wiring for large voltage losses, then I might as well go the whole hog and rewire the lights for high-wattage bulbs and then not only will the wiring all be pukka, but I'll have brighter bulbs as well.

 

After all, you can never have enough light IMO, and so long as the headlights are aimed properly it shouldn't dazzle other drivers anymore than a standard 55W setup.

[Ahl 4]

They really don't dazzle any more than standard, providing they've been aimed properly like you said.

As ive said before modern motors with sparkly lights (e.g my mums mazda 323) are brighter with 55/60w bulbs than my 100/80w bulbs in both dip and full beam. And thats before even getting onto HID lights.

 

Theres many a time ive s*it myself thinking its the police as HID projectors seem to flash a lovely piercing deep blue light at you, right before going blue-white and blinding you. I can't see why these lights are legal and 100/80w arent tbh.