Proper Amerpage for Fuse (for Fogs, etc)

infernicus · **Posted:** Wed Nov 25, 2009 9:13 pm

Hey, I was just wondering what the proper amp is for auxillary fogs through a relay, should it be 15, or 30? What's the difference, lol, thanks.

VoodooMann · **Posted:** Fri Nov 27, 2009 12:09 am

depends on the lights

I have one set that are 15, & two sets that are 20

infernicus · **Posted:** Fri Nov 27, 2009 1:17 am

Hey, well I have a cheap set of 55w fogs, and a cheap set of 55w spots.. I am also going to add a cheap set of 55w reverse lights.. Should they have 15amp? .. I'm assuming the lower the better because they don't draw much power.. And if any surge happens it will blow and I will be protected.

VoodooMann · **Posted:** Fri Nov 27, 2009 2:52 am

my 55w lights are on a 15.

my 100w lights are on 20s.

depends on the draw of the lamp...

but if they are 55s then a 15 should be fine.

infernicus · **Posted:** Fri Nov 27, 2009 2:59 am

So 15amp would be sufficient, and 30amp, would not actually do anything? lol.

corwyyn · **Posted:** Fri Nov 27, 2009 3:32 am

infernicus wrote:

So 15amp would be sufficient, and 30amp, would not actually do anything? lol.

If you're installing standard 55w lights then as stated above the 15A would be a good choice. Putting a larger fuse on that circuit would be a bad idea since the main purpose of a fuse is to be the weak link in the system, meaning it will fail before more expensive components do. If you put a 30A in the circuit you run the risk of having to replace bulbs if something goes wrong (best case scenario) or possibly causing an electrical fire (worst case scenario).

infernicus · **Posted:** Fri Nov 27, 2009 3:37 am

That's what I thought.. Just wanted to be sure. Haha.
Thanks so much guys.

Boiler · **Posted:** Sun Nov 29, 2009 2:39 am

Fuse sizing is not such a general thing. Here is how to size wiring & fuses.

Step 1: Determine your load.
If you have 2 lights that are each 55 watts on one wire, then you have 110 watts (55x2).

I=P/V (current = power / voltage)

so:

Current in amps = 110 watts / 12 volts (automotive system voltage)

Current in this case equals 110/12 or 9.16 amps.

Step 2: select fuse
9.16 rounded up to the nearest standard fuse size is 10 amps. Also make sure your wiring can handle at least as much amperage as the selected fuse, not the amperage of the load. You want to protect the wiring in case of short, not just in case of full load. So each wire has to be able to carry the entire fuse rating. This includes grounds.

I would use a 10 amp fuse for each of your pairs of 55 watt lights.

CRDMiller · **Posted:** Sun Nov 29, 2009 3:26 am

a pair of 55w lamps on a10 amp fuse it way to close to the fuse rating to be reliable.

12v experiences a large amount of voltage loss over distance especially with smaller sizes. What about when you have a voltage sag? The wattage stays the same, the voltage goes down, the current must go up.

Your going to blow your 10a fuse with 9.1a of current at 12v unless you have a large gauge wire and a short run. Use a 15a and be done with it.

Were not trying to limit the current to the lamps with a fuse, that's a rediculious notion, we are trying to blow the fuse before melting a wire, and or much worse.

So how much current can your wire deal with, is the real issue.

at 11 volts your 55x2 is 10 amps. oops.

Boiler · **Posted:** Sun Nov 29, 2009 11:05 am

You're not going to run at 11 volts for more than an instant, and the fuses don't pop quickly unless at a higher than rated amperage. But yes, if you have long run of wire you'd want to increase wire and fuse sizes.

jhblackwell · **Posted:** Sun Nov 29, 2009 2:08 pm

Both my Hella 55W floods (two lamps) and my Pilot 55W fogs (two lamps) came with 15 amp fuses in the 15' wiring kits. I can only assume that the manufacturer of each kit would recommend 15 amp fuses for similar applications. IMO

Boiler · **Posted:** Sun Nov 29, 2009 11:37 pm

Sounds right for 15' long. I'm not impressed with the hella's wire guage though. I have 10 amps on 3 sets of hella 55 watt lights at the moment and have experienced no problems. I've had them on while driving and on for about 30-45 mins with engine off. I oversized my wiring with intent to go to 100 watt eventually. doubt I will though the 55 are pretty good.

Proper Amerpage for Fuse (for Fogs, etc)

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