Fuses, shorts and overloads
A fuse tells you what kind of problem you have by how it blows. Read it before you replace it.
The short version
- Blows the instant the switch goes on: that's a short to ground. Nothing was running yet.
- Blows after a while, or only under load: that's an overload, a tired motor, or a connection heating up.
- Unplug the load and try a new fuse. If it holds, the fault is in the wiring, not the device.
- With the load unplugged, ohm the positive wire to the frame. Anything under a few hundred ohms is your short. Follow the wire to find where it rubs.
- Never go up a size. The fuse protects the wire, and the wire gauge sets the fuse: 14 AWG gets 15 A, 12 AWG gets 20 A, 10 AWG gets 30 A.
Numbers
- 14 AWG: 15 A fuse max
- 12 AWG: 20 A fuse max
- 10 AWG: 30 A fuse max
- Wire to ground, load unplugged: O.L is good; ohms is a short
Shop tip
Chafe points are where wires pass through steel: floor penetrations, frame edges, slide rooms. Look there first.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.