selecting the proper fuse

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tilt2468
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Joined: 05/27/2009
Points: 76

I am wiring some lights to a battery and I would like to install an in-line fuse to protect the battery from the possibility of a short in the lights or wiring.

The maximum draw for the lights is 25 watts-- is there a formula for what ah rating fuse I should use?

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tilt

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e-doggies
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Joined: 12/30/2007
Points: 280
Re: selecting the proper fuse

Divide the watts by the voltage and you will get the current in amps. If you have a 24V power source, the lights would draw about 1 Amp. A 1 1/2A or 2A fuse would be my choice.

Fuses are rated in Amps, not AmpHours (Ah).

marylandbob
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Joined: 06/22/2009
Points: 520
Re: selecting the proper fuse

Yes, there are formulas, as stated previously, but what you should protect is the vehicle! To protect the vehicle, and get reliable performance, the size of the WIRE and FUSE are BOTH important, and related! Wire size should be chosen to give minimal voltage drop, typically 5% drop or less (0.6 volts at 12 volts)Wire should also be rated to carry the number of amperes drawn by the load or more. EXAMPLE: You want to add a 55 watt, 12 volt halogen headlight. At 55 watts, current draw will be a little less than 5 amperes (Ohms Law formula), so the wiring must be able to sustain this current, with minimal voltage drop, to avoid loosing brightness. I would use 14 guage or larger stranded wire for a run of 10 feet or less, as it is good for at least 15 amperes, is flexible, and mechanically durable. I would install a 7.5 or 10 ampere fuse, very near the battery end of the cable-installation of the fuse near the supply, or battery end, is done to protect the wiring, so that if the wiring gets pinched or overloaded, it does not ignite, and cause a fire! For most vehicle circuits, use of 18 guage or bigger stranded wire is advised, for reasons of physical durability, even if the current is very low. (as the guage number gets higher, wire size gets smaller-10 guage wire is much bigger than 18 guage wire) If the length of wire used is very long, it often is advisable to increase the wire size, to reduce voltage drop, especially in wiring used for loads like motors, car stereo amplifiers, or 2 way radios! Most automotive fuses will allow DOUBLE the rated current to flow, for at least 30 seconds before blowing, so take this into consideration when designing! The proper fuse is selected based upon current characteristics of the load, need for wirijng protection, and reliability requirements. Generally, if wire is chosen to accomodate at least TWICE the power demanded by the load, and a fuse rated at 150% of the load is used, safe operation can be attained, if wire lengths are 10 feet or less on 12 volt systems. (double the length allowed to 20 feet on 24 volt and higher systems)--Bob
PS:To directly answer your question, use at least 18 guage stranded wire, (larger 16 or 14 guage is even better) and a 3 to 5 amp fuse.(for 12 volts)-if using 24 volts, use fuses 1/2 the rating.

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Robert M. Curry

tilt2468
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Joined: 05/27/2009
Points: 76
Re: selecting the proper fuse

This is a great help-- I can do the wiring, but I clearly lack the background knowledge to get the job done right.

I took the bike apart the other day and discovered that I will be able to feed directly from the terminal posts where power is transferred from the battery to the bike-- this should be a plus in terms of having a solid connection for the lighting.

My plan is to run a pair of lead wires (18 gauge w/ a fuse near the positive terminal) to a waterproof junction box mounted to the front rack. I will make all my connections inside waterproof boxes to avoid water problems-- there will be a second box in the rear for the rear lighting connections. The headlights are rated at less than 5 watts combined, there are a pair of led strands at less than 2 total watts, a couple of flashers less than 2 watts and a pair of xenon strobes at less than 4 watts combined. Waterproof switches for the lights will be mounted on the front junction box. All the lights are for a 12v system, so I will be running them in paralell series with an identical device.

One possible problem that I see is that the leads from some of the lights are a different size wire than 18 gauge-- is it ok to use two sizes of wire in the run to the lights? I know that is a bad idea when running speaker wire.

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tilt

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