H4 Headlights - what amp fuse?

[viczeva]

Gents,

Ive updated my headlights to H4 and installed the relays. What amperage should the fuse i use be? Currently running a 30Amp fuse, but afraid its too much and will cause damage. I tried 15a and they lasted 5 min.

Should they be:
a). 15a (which mean i might have a short somewhere and this is the reason why they blow)
b). 25a
c.) 30a which is what i'm currently using

[spuggy]

DC amps to watts calculation formula
The power P in watts (W) is equal to the current I in amps (A), times the voltage V in volts (V):

P(W) = I(A) × V(V)

So watts are equal to amps times volts:

watt = amp × volt

or

W = A × V

So, to get amps, use watts/volts. To get volts, watts/amps.

For a 55w high beam filament, you need a 55/12 = 4.58 amp fuse. But fuse to protect the lowest-rated cable in the circuit, and a little headroom so they don't blow in normal use.

The stock fuse rating is 8A for each of the low/high beams either side - so 4x 8A fuses.

You need to explain how you've wired your lighting circuit. A 15A fuse is good for something over 200W with the car running (eg 15A * 14V = 210W) - so if you're blowing 15A fuses, it would seem that there's something quite wrong.

[kanadary]

When I switched to h4, I didn’t change the fuses at all. I already had the relay kit with my h5s

[viczeva]

I’ve switched back to 15a after fixing up the wiring. Maybe there was a short somewhere... so far so good.

[viczeva]

Nope, 15a didn’t cut it. Lasted about 5 min while driving. Went back to 30a and it works.


Does any body have any insight on the actual cause? Even though it works, I’d preferred if it worked properly. Before a bigger issue arises. Thanks!

[HarryD]

You need to disconnect each headlight from the lighting circuit and measure the amperage to each one (high and low) with an ammeter.

How are the relays wired? Do you have two relays (one relay for high beam, one for low beam) or four relays (one for each beam)? What is the wattage of the bulbs?

If you have one relay for the high beams and one for the lows, and standard bulbs, your current draw is as follows:

Low beam (55 watts each). total draw of two bulbs 110 watts --> 110/12 = 9.2 amps
High Beam (60 watts each). total draw of 2 bulbs 120 watts --> 120/12 = 10 amps

If you have 100 watt bulbs, total draw is 200 watts --> 200/12 = 16.7 amps.

Please let us know what you have installed.

[Quickstep192]

What size wire do you have for your lights?

Fuses should be sized according to wire size. If the load exceeds the capacity of the wire by enough, it will heat up, the insulation will melt and big trouble will ensue. If the original fuse was properly sized at 15a, the wire likely can’t handle a 30 amp load.

Here’s a chart showing wire size for given loads.