what size fuse for the headlight wiring upgrade?

Joined

·

[external_link_head]

1,907 Posts

Discussion Starter · #1 · Nov 16, 2001

What size fuse do i use with this?

i still don’t uderstand why i can’t take the power from the battery . . . it seems like it’s just taken power from a different section of the wire when you take it form the battery, since it’s all connected anyways.

http://lighting.mbz.org/tech/how_to/relays/

Joined

·

15,848 Posts

[external_link offset=1]

Re: what size fuse for the headlight wiring upgrade? (Rippen Raymen)

The relays you are using are probably 15A or 30A relays, so find something along those lines. Fuse each relay individually. I’m using 30A inline fuses with my headlights, and I’ve never blown a fuse, even with using 100W H3 bulbs, and 130W/100W H4 bulbs…although I have pulled enough juice through the fuses to melt the contacts on one of them…the fuse didn’t blow, but the fuse holder just melted away. They were the most HD fuse holders I could find at the time, but I think I was looking in the wrong places. I think next time, I’ll try a local car stereo place. They always sell those huge inline fuses for amplifiers, so I’ll use those for the headlight.

Oh, and you can take power from the alternator or the battery. The idea from taking it from the alt is to escape any voltage lost in the thick wire from the alternator to the battery. I used 4ga wire from my alt to my battery and then from the battery to the starter, and from the starter to the alt, so I have a triangle of wire that shouldn’t see much voltage loss at all

what size fuse for the headlight wiring upgrade?

Broke

Joined

·

503 Posts

Re: what size fuse for the headlight wiring upgrade? (Rippen Raymen)

Size of fuse depends on the wattage of lights.

Watts/Volts = Amps

Ex: 400 Watts / 12 Volts = 33.33 Amps

Be sure to use this calculation to insure that your relays are big enough for the job, if they are not, you may need to double up on the relays. In my example, a single 30A relay would burn out (in time) due to the high current demand. But if I put two 30 amp relays in parallel, each relay would only experience a 16.66 amp load.

Joined

·

5,657 Posts

[external_link offset=2]

Re: what size fuse for the headlight wiring upgrade? (jtmoe)

quote:[HR][/HR]Size of fuse depends on the wattage of lights.

Watts/Volts = Amps

Ex: 400 Watts / 12 Volts = 33.33 Amps[HR][/HR]​

REMEMBER THIS FORMULA!!!

Keep in mind that you would be crazy to run 400 watts on one relay, if you have quad rounds run a relay for the inners and the outers.

another thing to remember is that the voltage on a car isn’t 12, the alternator puts out 13.5 volts (or was it 14.5?), but this creates lower amperage. I always calculate with 10 volts since this makes higher amperage and puts in a margin of saftey. And when the car isn’t running I think it registers at around 10 or 11 volts, so if you had your high beams on while it wasn’t on….

Joined

·

17,550 Posts

Re: what size fuse for the headlight wiring upgrade? (88Jetta)

Generally a 30A relay are common is can be easily used for two bulb filaments (e.g.: one relay for outer lows, another for outer highs, and another for inner highs (if you have a quad-lamp system)) and 30A in a relay for two bulb filaments is plenty. Fuse wise, I’m using 10A per relay, which is fine with the basic 55/60 H4s and 55W H3s I’m using. 10W is just enough to power the bulbs and any more amps through the wiring would blow the fuse (very safe). Use the calculation above to figure out what you’d need to do for upgraded bulbs if you go higher. I guess my philosophy is use the lowest possible fuse to support what you’re running so it’ll blow ASAP is something goes awry. Not sure if that’s right or wrong, but it works for me.

what size fuse for the headlight wiring upgrade?

[external_footer]