My 2003 Opel Agila has 0.85 mm2 wiring for the entire headlamp circuits. I want to install higher wattage bulbs, up to 110 W. It takes H4 bulbs so the high and low beams are combined into one bulb.

I have included the wiring diagram for reference:

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However, when I googled, I found some stories of the stock wiring melting when using higher powered bulbs. This would obviously be a disaster!

As they've used 15 A fuses, I assume the maximum power from each side is 15 A * 12 V = 180 W for each side, so I assume it'd be okay, but I don't really know.

Note there is one fuse for each side, so each side can draw 180 W.

Will my wiring be able to cope with this sort of power?

  • 7
    Installing more than 55W of lighting is illegal in many jurisdictions. And without careful adjustment of the headlights, you will end up blinding all oncoming traffic. This is a bad idea for road use.
    – Hobbes
    Dec 18, 2016 at 16:03

3 Answers 3


TL;DR: You're on the edge, instead of pushing it, use the existing circuit to control a set of relays and supply the bulbs with a short run of larger wire. Use at least 3.3 mm² (12 AWG) wire, which you can fuse at 20 A. Use wire rated for "under hood" (high temperature) applications such as type GXL or TXL.

You didn't ask, but I think it worth noting that the traditional means of specifying lamps is by the amount power they consume, not by how effectively they light the road or even by the light output. This almost made sense when there was only one common bulb technology, but it makes no effort to take into account the efficiency of the bulb or the suitability of the bulb for a particular fixture. You may find that your lamp housing aren't designed to run 110 Watt bulbs and that the light pattern produced may be poorly controlled – the result is that you don't get a better picture of the road ahead of you and oncoming drivers get blinded by your lights. Because of that, in the US at least, bulbs above 55 Watts aren't legal.

The proper wire for the circuit depends on a number of factors:

  • The expected current in the circuit.
  • The allowable voltage drop (how much voltage will be lost due to resistance in the wire and connections).
  • The total length of the wire run. This includes both the hot and ground wires and is the full path from the battery through the fuse panel to the switch and then to the light and back to ground.
  • The way the wire is routed – if it runs in a bundle with other wires allowance needs to be made for the reduction in cooling that comes from proximity and the heating of the other wires.
  • The expected temperature range where the wire is installed.

You can find calculators online that will provide sizing recommendations based on these factors. The one provided by Wire Barn seems good and easy to use. The calculator shows that with a 3 meter (10') run and a 5% voltage drop your existing wiring would be marginally adequate. However, a larger voltage drop means less light output a 1% or 2% drop is a better design standard and for that your existing wiring is inadequate.

A common strategy when upgrading lighting is to use the car's original wiring to control relays that carry the actual power for the lights. The relays are installed close to the battery and the wire run to the lights is kept short and made with wire sized for a low voltage drop. New fuses are added to protect the load circuits through the relays. You can take a look at my answer to this question for a longer discussion about fuses, but the bottom line is that common practice is to size the fuse to protect the wiring.

The wiring diagram below, from this question shows the changes necessary to upgrade to relays. The diagram is for a Ford truck, but concept is right. The parts you're interested in are the blue changes, the green are unrelated modifications to the headlight circuit. Also note that on this truck there is one relay per side (high and low) whereas your car would be wired with one per beam. Also your car is switching the ground side of the circuit rather than the hot (+12V) side.

Wiring diagram, showing relay upgrade

Since the lamps you're planning on installing consume significantly more power than the original one there is the distinct possibility that the fixtures themselves will not be up to dissipating that much heat. In particular, be concerned about any plastic parts. At the very least you probably have plastic housings for the bulb terminals, it would be prudent to consider upgrading to ceramic ones. They are commonly available and not too expensive. I was able to find these on Amazon by searching for "h4 ceramic headlight connector".

Ceramic H4 lamp connector

You can also find bare ceramic ones (without the orange plastic covers).


0.85 mm² copper wires have a resistance of about 0.02 ohm/m, and when a current of about 10 A is driven through this wire, it dissipates about 2 W/m. This isn't soo much and should not be a problem.

But there are other weak points: Junctions and switches might not be designed for this current. And the bulb itself becomes really hot, since it generates 100 W instead of 55-60 W of heat. This could melt the headlight and/or the socket, or even start a fire.

Finally, a fuse rated for 15 A doesn't mean the circuit is capable of a permanent current of 15 A. First, fuses will blow after a while, even if the current is just a little below the rating. Second, bulbs have a high inrush current, e.g. they may sink a 3-5x higher current for the first half second after switching on. Fuses typically allow a current much higher than their rating for a very short time, but it can yet wear them out, and they will blow up one day, though there was no real over-current.

For these reasons, fuse ratings are typically chosen about 30-100% above the expected current.

  • The fuse should be able to carry its rated current forever. Fuses are designed to blow at currents slightly over their rating. The reason that fuses blow is due to heating, so a fuse that is installed in a hotter than expected location may blow at a lower than expected current. This can also happen if there is corrosion in the fuse holder that results in heating.
    – dlu
    Dec 18, 2016 at 16:55
  • @dlu: Indeed, they should. I've seen fuses in a well ventilated, temperature controlled environment failing after some weeks at 85-90% of their rated current. Thus, never assume a fuse will be fine with 15.000A and will blow at 15.001A. See here
    – sweber
    Dec 18, 2016 at 19:56
  • That would be a very expensive fuse and fuse holder if you could get it to do that. On the other hand a fuse that is rated at 10 A with a 10% margin between the nominal rating and the long term carry current in its specs is arguably defective. I think you choice of the word fail there is exactly right. Not that an engineer shouldn't be characterizing the components and basing her/his designs on actual behavior. But it should be happening on both sides.
    – dlu
    Dec 18, 2016 at 20:07

You may or may not get away with it - the answer from Sweber is correct.

So you can use the existing wiring to drive a relay(s) fed via a fuse directly from the battery - a neat solution and no problems.

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