Black Fuses

You most likely have have the glass cylinder fuses. They have two ratings. The one everyone is familiar with is the current rating. That must be matched to what is listed on the box. 10 and 20 amp fuses are common and are the same length. Oddball values will be shorter so you can't put one in that has too large a current rating. A 9 amp fuse is a good example of that. They're about half as long as the 10 and 20 amp ones. Basically that's all you have to worry about is putting a 20 amp fuse where it's called for and a 10 amp if it's called for.

The other rating is the voltage. That is insignificant in cars. The automotive fuses you buy from an auto parts store or hardware store will have at least a 32 volt rating and that's more than enough for cars. All that rating means is when the fuse blows, there will be system voltage appearing between the two burned ends, and there will be a momentary arc which is the flash you might see. With a 32 volt rating, the manufacturer guarantees the glass cylinder won't explode or shatter when it's used to protect a circuit powered by up to 32 volts. Your car uses a 12 volt battery and electrical system, so 32 volt fuses are fine. In electronics, we use fuses that are rated at 250 volts for things like tvs and vcrs that are powered by 120 volt house current. A 32 volt fuse would still protect the circuit but the glass could shatter if the fuse blows. Those fuses also are available in quarter amp increments from less than one amp up to 7 or 8 amps. That many choices aren't necessary for automotive applications.

All you have to concern yourself with is the current rating. If someone tries to give you a line about using the wrong voltage, you'll know better. Also, rather than buying lots of fuses, it's common to visit a salvage yard, especially if you have one of the many pick-your-own-parts yards nearby. One of our local yards sells small plastic buckets full of fuses, (like butter or yogurt containers), for ten bucks. You get about 3,000 fuses. That's really for shops that use a lot of them. That beats two or three bucks for a package of five new ones.

Two things to watch for with used fuses is there's no corrosion on the metal end caps, or you must scrub them clean, and those caps must be tight. Corrosion adds resistance to electrical flow and that results in heat buildup. Besides limiting how well that circuit works, heat is what causes a fuse to blow. If the fuse is already hot from that corrosion, it will blow easier than intended. That heat can also melt the glue that holds the caps tightly to the glass cylinder. Being loose doesn't mean the fuse is bad, but if you have a choice, pick the fuse with tight end caps.