selecting the proper fuse
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?
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).
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.