Re: Wiring and plug for a 3 hp cabinet saw

Toller wrote:
The breaker is there to protect the wire, not necessarily what is connected
to the circuit.

Everyone says that, but why shouldn't it also protect what is connected?


Four reasons:
1. The circuit breaker has no way of knowing what is plugged into it, might be a 5 watt night light, might be toaster oven, might be an arcwelder. Generally there are multiple sockets tapped off each branch, with who knows what plugged into them. You don't want the circuit breaker to pop until the current is enough to overheat the wires.
2. Electric motors draw enormous amounts of current getting started. To avoid popping circuit breakers, you want to furnish the motor plenty of current to start with. The starting load only lasts a few seconds while the motor is coming up to speed. Your 12 amp motor might draw 30-40 amps at startup, and then drop down to a couple of amps after to getting going. If you put it on a 12 amp breaker, it will pop that breaker on every startup.
3. The house circuit breaker is supposed to prevent house fires. Without protection, a short circuit will heat the wire, running thru the wooden studs of your wall, up to red hot. Then the studs catch fire and your house burns down. Circuit breaker size is set by the branch wire size. Rule of thumb: 14 gauge wire = 15 amp breaker, 12 gauge wire = 20 amp breaker, 10 gauge wire = 30 amp breaker. Doesn't matter what you plug into the branch circuit, choose a breaker that's right for the wire.
4. Stationary saw motors always have a thermal protection switch built into them that shuts the motor down if it gets too hot. They don't need any more protection. Speaking of which, should the thermal overload pop, you want to be aware that it might spontaniously reset, starting the saw up again. You want to keep your hands clear of the blade until you unplug the tool.

Was it me, I'd run your 220 volt saw on a 12 gauge branch circuit protected with a pair of 20 amp breakers, one in each hot lead. Use two conductor plus a ground wire cable. I'd make sure the iron frame of the saw was connected to the green ground wire. Back at the terminal box make sure the green ground wire goes to ground.

David Starr
.

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