Wiring and plug for a 3 hp cabinet saw

[David Todtman]

I have to wire for a 3hp cabinet saw. The motor is a Baldor and the tag on the motor housing says it is 12.5 amps at 230V. (It also says it will operate on a minimum of 203v at 13.7amps. It has an over-heat shutoff.)

Does this mean I can use a 20amp double pole breaker? What difference would it make if I used a 30amp? I think I'll use #10/3 wire (romex-type).

I need to wire a plug-lead from the saw's mag switch. I reckon a 20amp/230v male plug with recepticle to match would be correct. Again: what difference would it make if I used a 30amp setup?

Oh yeah, something else: I have two (unused) 15amp breakers. If I gang them with a tie bar, does this make the setup function as a double pole

30amp breaker?

I will appreciate comments and recommendations.

Best, David

[Greg O]

No problems with anything until you came to the two 15 amp breakers, you need a double 30 amp breaker for this setup. Myself, I would go with the 20 amp breaker 12 gauge wire, and an appropriate

20 amp plug, socket, and cord, but if you want to up it all to 30 amp breaker, 10 gauge wire and 30 amp socket, plug and cord I would not have a problem with that either. Assuming the length of run of the circuit is not excessive. The breaker is there to protect the wire, not necessarily what is connected to the circuit. Greg

[Toller]

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

[Leon]

Because, it is designed to protect the wiring. If you want to protect what is connected you should buy the tool with it's won built in protection.

If the breaker was designed to protect the tool, you would only be able to plug "1" tool into a circuit at a time. Many tools only pull 7 amps. Are you going to put in 7 amp breakers and what if you want to plug in 2, 7 amp tools into the same circuit? What if you want to run a dust collector and a TS on the same circuit? You can't have a breaker that will protect both and not trip when both are on.

[DJ Delorie]

Yes. Doesn't the saw's manual tell you what size breaker to use?

You can't use a bigger breaker without ensuring that everything after it is also rated for 30 amps, at least the hard-wired stuff (wire and outlet). The purpose of a breaker is to protect the house from wire insulation failing from heat, and starting a fire. The breaker is NOT intended to protect your tools, although it can.

You can use bigger wire with a smaller breaker, to get the protection you need yet reduce the resistance of the wire. You can't use a larger breaker with a smaller wire, though.

Unless you're really far from the breaker, a larger wire won't make that much difference. Larger wire reduces the resistive losses somewhat, but the shorter the wire, the less an impact this will have.

A larger breaker won't do squat for you.

Larger wire is, however, more expensive and harder to work with.

[Toller]

I know what it designed to protect, but why the heck not put a 15a breaker on the circuit instead of a 20 or 30? Doesn't cost anything so you pick up protection for free. I am not saying it required, or even necessary, but that doesn't mean it isn't preferable.

If he wanted to run a DC and a TS on the same circuit he would wouldn't have asked about wiring a 12a TS. And if he did want to run both, the 20a circuit wouldn't be adequate either!

[DJ Delorie]

Even if that were legal and wise, it would still only give you a double pole 15 amp breaker, not 30 amp.

[DJ Delorie]

Consider a 20 amp circuit. Beyond 20 amps, the wire may melt and cause fire. Below 20 amps, all is safe.

Consider what you plug into a 20 amp circuit. It could have *any* current rating, from a tiny lamp cord to a power tool. The breaker can't protect what's plugged in unless everything you plug in is capable of carrying 20 amps safely.

So, devices plugged into a circuit which desire overcurrent protection must provide a *suitable* protection themselves.

Note that in the UK every device has a suitable fuse in the plug, which protects the device.

To recap, a circuit breaker...

• CAN protect the hardwired circuit

• MAY protect some plugged-in devices

• CANNOT protect ALL plugged-in devices

[DJ Delorie]

In the case of power tools, the rated current is usually less than the startup current, so you don't want the circuit to be *too* close to the rated current, or you risk occasionally tripping the breaker when you start the tool.

Hence, read the manual and do what they recommend.

[Lew Hodgett]

10-2 with ground, a 30A plug and receptacle will make your life easy.

It is legal to use a tie handle most places, but DON'T do it.

A 2P-30A c'bkr with internal tie is not that expensive and offers added protection.

You're on the right track standardizing on 30A circuits.

Have fun.

Lew

[Lew Hodgett]

By definition, a circuit protective device, protect the insulation on the conductors of the distribution system.

Overload devices provide the protection for the device.

About the only thing they have in common is protection, but they of necessity, approach it from different directions.

Lew

[Tom Veatch]

Yes, if the circuit is wired with 12ga or larger wire

(romex-type).

If you use a 30amp breaker, you need 10ga or larger wire. 30 amps is more than you need for this application, and 10ga wire is larger than needed unless you have an excessively long wire run from the breaker to the outlet.

Yes, NEMA 6-20 configuration, if you use a 20 amp breaker

You'll need a 30amp receptacle, NEMA 6-30

NO, it would function as a 15amp breaker.

Tom Veatch Wichita, KS USA

[Lew Hodgett]

Basic rule of thumb for c'bkrs used on motor circuits:

A c'bkr needs to be sized about 2.5-3.0 as large as the FLA of the motor, rounded up to the next std size c'bkr, in order to handle the inrush current of the motor.

Thus, FLA=12.5 x 2.5 = 31.25A And, FLA=12.5 x 3.0 = 37.25A

I'd try a 2P-30A; however, would not be surprised with an ocasional trip.

You could very easily use a 2P-40A for this application and be legal.

Lew

What does the saw mfg spec?

Lew

[dadiOH]

Same as my saw which is on a 20 amp circuit. Starting the saw will occasionally trip the circuit breaker.

[LRod]

Cite, please.

Puhlease. A 3HP saw occasionally tripping a 30A breaker? In what universe do you think that can happen? You'd be unlikely to ever get a trip on a 20A circuit with a 3HP saw. Maybe if he had 200' of feed, but not in a normal run. 40A is more than twice as ridiculous as 30A is in this application.

[Doug Miller]

Yes.

You'll spend more on wire (you may use 12ga wire with a 20A breaker, but you must use 10ga wire with 30A), and you will be required to use 30A receptacles.

Using 10-3 (or 12-3) is pointless -- a 240V motor doesn't have, or use, a neutral. All you need is two hots and a ground. 10-2 (or 12-2) is sufficient.

Again: you'll spend more on wire, and you'll be required to use 30A receptacles. Code does not permit the use of anything but 30A receptacles on a

30A circuit.

No. That will function as a double-pole 15A breaker.

Use a double-pole 20A breaker. Whether you use 12ga or 10ga wire is up to you, but unless the saw is a long way from the breaker box, you're not likely to see any benefit from the 10ga. Either way, though, use a 20A breaker, not a

30A.

[Doug Miller]

Except that it's a Code violation to put the 20A receptacle he intends to use on a 30A circuit...

[Doug Miller]

Because that's not what it's there for. Do you want a 1/2 amp breaker to protect the circuit that you have a 60W table lamp plugged into?

[Doug Miller]

Because that would limit him to using 14ga wire. It will take forever for the saw to spin up. (You're always talking about the importance of avoiding voltage drop -- think it through.)

No, but the slow startup speed for the saw *does* mean that.

Nonsense. 20A would be perfectly adequate for his TS and any DC up to about

2hp.

[Doug Miller]

So according to you, my 14.5A table saw should be on a 40A breaker.

Oddly enough, I've had that saw on a double-pole 30A breaker for five years now, with no trips -- even when ripping 12/4 hard maple. I'd be *very* surprised if the OP ever trips a 30A breaker.

As long as he uses a 40A receptacle, yes.

He could also use a double-pole 30A or 20A and be legal, and still not have to worry about nuisance trips.