Fusing question

[AJMike]

I had 2 BB Lithium batteries and a Renogy DC-DC charger installed. The charger was running hot and faulting so I added a 12 volt muffin fan to help cool it.

The fan is working properly but I want to add a fuse to the fan line just in case there is an issue. The fan is powered from a 12 volt power connection (the old cigarette lighter connector type) and that, of course, is fused, but I want to add an inline fuse just for the fan, just in case.

The fan uses about 0.6 amps when running and about 0.9 amps when starting and I was wondering what the proper inline fuse size should be. If the max amperage I see is 0.9 amps what is the proper multiple for the fuse? 5 amps seems too high and perhaps 2 amps is too low. I just don't know how the proper fuse size is determined from the running and starting amperage.

[Goodspike]

Isn't fuse size usually determined by the gauge of wire, not the device? For example, my dedicated refrigerator circuit at home is on a 15 amp breaker, but the refrigerator probably never draws even 300 watts.

But if you want to go for a smaller size fuse determined by the device I would at least cover the starting amps, assuming you can accurately measure that somehow. Fuses will often handle short surges (and they make some designed to do so), but why risk the fuse blowing and then the device not performing its job until you discover the blown fuse?

[DavidM]

Originally Posted by Goodspike

Isn't fuse size usually determined by the gauge of wire, not the device? For example, my dedicated refrigerator circuit at home is on a 15 amp breaker, but the refrigerator probably never draws even 300 watts.

But if you want to go for a smaller size fuse determined by the device I would at least cover the starting amps, assuming you can accurately measure that somehow. Fuses will often handle short surges (and they make some designed to do so), but why risk the fuse blowing and then the device not performing its job until you discover the blown fuse?

I suspect the wire is at least 14 gauge and is protected by a 15A fuse. There is no need to protect the device- the fan, with a separate smaller fuse. But follow the advice above if you really want to.

David

[AJMike]

Originally Posted by DavidM

I suspect the wire is at least 14 gauge and is protected by a 15A fuse. There is no need to protect the device- the fan, with a separate smaller fuse. But follow the advice above if you really want to.

David

I don't know enough about wiring to be able to tell by looking but if my wire stripper is any judge it is probably 18 gauge.

[DavidM]

18 gauge would be legal if it is protected by a 15A fuse or smaller as typical 18 gauge wire is good for 20A.

David

[AJMike]

Originally Posted by DavidM

18 gauge would be legal if it is protected by a 15A fuse or smaller as typical 18 gauge wire is good for 20A.

In that case I can just leave it like it is. Thanks for the help.

[Morich]

Wire gauge is almost always printed on the wire but we might need to look at quite a bit of wire to find a part we can read! Fuses are usually sized to meet the wire size to avoid the end item from pulling too much current through the wire and overheating to cause a fire. The end item is often kind of considered expendable from a fuse standpoint, that's somebody else's problem!
In the case of your fan, asking why it needs a fuse may help. Why does a fan get overheated from power, so that it might need to blow a fuse?
Normally this would be caused by a bearing going bad, so is it worth adding a fuse for when a bearing might be bad? Not to me as I would expect to trash the fan rather than change the bearing, so letting a bad bearing go long enough to trip the other fuse/breaker is okay with me as long as I know that increase in power use is not going to overheat the wiring to cause a fire.

[AJMike]

Originally Posted by Morich

Wire gauge is almost always printed on the wire but we might need to look at quite a bit of wire to find a part we can read! Fuses are usually sized to meet the wire size to avoid the end item from pulling too much current through the wire and overheating to cause a fire. The end item is often kind of considered expendable from a fuse standpoint, that's somebody else's problem!
In the case of your fan, asking why it needs a fuse may help. Why does a fan get overheated from power, so that it might need to blow a fuse?
Normally this would be caused by a bearing going bad, so is it worth adding a fuse for when a bearing might be bad? Not to me as I would expect to trash the fan rather than change the bearing, so letting a bad bearing go long enough to trip the other fuse/breaker is okay with me as long as I know that increase in power use is not going to overheat the wiring to cause a fire.

Got some 25X magnifying glasses and found the gauge - it is 22, so not even 18.

The fan is just a simple and inexpensive muffin fan and I have a replacement in case it goes bad. My concern was not burning out the fan but rather a short starting a fire.

What size fuse goes with 22 gauge wire? I guess there must be some table available on the internet so I will look.

UPDATE:

Found a table that said that the max current for a 22 gauge wire is 5 amps so I guess perhaps 2 or 2.5 amps would be the proper fuse size.

[Morich]

With that small gauge wire there are some things that make it safer, even if it did totally burn out. One is how far it goes and what it is next to as a spot to catch fire. If it's only 6-8 inches of wire, there's not a lot of stuff to get hot enough to burn before the wire itself melts down and opens the circuit so that current stopps. But the amount of heat you can get from a 2 amp overheating is just not very much!
To overheat the wire it would take something going wrong with the fan so that it draws more current than normal but doesn't go back enough to just melt down inside the fan and it has to do that long enough for the wire to heat something next to it long enough to catch that item on fire. If you can see the wire inside the fan, there is a coil of tiny wire where it is all piled up and that keeps any heat in that pile and the wire insulation in the fan coil is usually something painted on like shellac, etc, that melts pretty easy. That tiny wire pile is very likely to melt down far before the larger wire outside where the air cools it can even get warm enough to melt the insulation on it.
It could happen but your cell phone charger is far more likely to be a problem. Risk is possible but very low.

[AJMike]

Originally Posted by Morich

It could happen but your cell phone charger is far more likely to be a problem. Risk is possible but very low.

Thanks for the info. That is good to know.