What size breaker and wire for a 220v 18 amp motor

[JAMES SAKLAS]

I am installing a table saw that has a 220v, 18 amp motor. Does that mean that the motor draws 9 amps from each of the two hot wires? If so, then should I be able to use a double pole 20amp breaker and 14/3 wire since 14 gauge wire is sufficient for up to 15 amps.

This new circuit will have no other equipment.

[Steve Juhasz]

This circuit should be 12/2 wire and 2 pole 20A breaker. There are no allowable circumstances for 20A breaker to be used with 14 gauge wire.

[David Masters]

My table saw has a 220 18A motor as well. I decided to use 10 gage wire and a 30A breaker. I'm not an electrician, but the code called for no more than 80% load through a circuit, which on a 20A circuit would be 16A.

[Allen Grenz]

My suggestion is to go with David's advice... The extra price of the 10ga wire and 30amp breaker is minimal.. Then you will always have plenty of power when you need it down the road... I just went through this and put in 4 separate 220V 30amp circuits and am glad I did.. welders can be plugged in anyplace now too... And no, 14ga won't cut it..

[Art Mann]

Your initial assumption that each hot leg carries 9 amps is wrong. The two legs are connected to each other through the windings of the motor and both carry the same 18A. It is difficult to understand what is going on unless you have a fundamental understanding of AC power. 12 AWG wire is rated for 20A and is perfectly acceptible so long as the wire length isn't too long. Having said that, I would probably run 10 AWG anyway just to keep line losses low.

[David L Morse]

Your initial assumption that each hot leg carries 9 amps is wrong. The two legs are connected to each other through the windings of the motor and both carry the same 18A. It is difficult to understand what is going on unless you have a fundamental understanding of AC power. 12 AWG wire is rated for 20A and is perfectly acceptible so long as the wire length isn't too long. Having said that, I would probably run 10 AWG anyway just to keep line losses low.

Exactly right, do what Art said plus use a 30A breaker as Allen and David suggested.

[JAMES SAKLAS]

Thanks to all you that responded.

[Jim Neeley]

James,

At 240V, your motor is almost certainly an induction motor. My statements below will also assume you don't have a variable speed drive or other advanced "very soft start" circuitry attached.

Motor loads are very different from lighting or electric heating loads in that there is a surge of electricity that flows for a very short period of time (a tiny fraction of a second). This surge is typically on the order of 6x the nameplate rating. The good thing is our circuit breakers follow an "inverse-time curve" for these "small" [much less than short circuit] loads.

Nevertheless, this is the reason why the NEC treats motors in a special way, typically permitting larger circuit breakers on motors than you would use for other large loads.

Will it run on 18A? Perhaps, if it doesn't trip on startup but good engineering practice tells us to go with a 30A breaker and #10 wire to be pretty certain it will plug and play.

Because some motor/loads are so hard to start, the NEC permits an 18A motor to size the breaker UP TO: (full load = 18A * 225%) and then round up to the next size, which would be 50A (and require large wire).

Don't do that though, as the go on to say you should use nothing larger than "necessary" and require the motor to have its own thermal overload protection. Additionally, this would require using a giant 50A plug on the tool, likely replace the wire from the motor to the plug, and perhaps more.

FWIW, my Unisaw draws 20A and Delta recommends a 40A breaker.

Read more here: http://en.wikipedia.org/wiki/Circuit_breaker

Just my $0.02.. YMMV..

Jim in Alaska
(Also EE)

[Wade Lippman]

FWIW, my Unisaw draws 20A and Delta recommends a 40A breaker.

My cyclone draws 22a (it actually does, I thought they were exaggerating, but I measured it). Grizzly recommends a 40a breaker.
I used #10 and a 30a breaker after finding out that everyone else does that and verifying the startup current is well within the trip curve of the breaker.
Beats me why they say 40a; maybe to cover defective breakers?

[Keith Hankins]

Ok first check your manual to your saw! It should tell you the size circuit. Match the plug, outlet receptacle, wiring, and breaker to match that load. Not knowing the brand motor, distance to your panel from the saw, a blanket statement is hard to make. However, at a minimum I'd do 12/2 w/20amp breaker. Assuming not running other piece of equip at same time i.e. dust collector.

Now if you are looking to upgrade later to a 5hp motor and that would take a 10 awg min (assuming a lot here) then you could same a little money down the road by putting in the 10 wiring now and not have to pull out the old to upgrade that later. It all depends. I doubt I'd use a 15A wire that is a throw away. Heck that pops a breaker when using hand sanders. Seems most houses built since the 90's that was the craze to save a nickle. Anyway go by what the manual says. Good luck!

[Rick Christopherson]

You should be fine with either a 20 or 30 amp circuit. With a human-fed machine like a tablesaw, there is really no meaning to Full Load Amps, because the operator has control of how much load is given, including the ability to drive it past its full load rating. If you put a meter on the motor, you would see that it virtually never reaches 18 amps during operation. The majority of the time the saw is running, it will be at 1/2 (idle) to 3/4 full load amperage.

Personally, I would stick with a 20 amp circuit. Working with #10 wire can be a pain, plus you will then be required to use a NEMA 6-30 receptacle and plug, which are much more expensive and much larger than the 15 or 20 amp versions.

Leviton-061-5372-0-rw-163417-228148.jpg

[Anthony Whitesell]

Your initial assumption that each hot leg carries 9 amps is wrong. The two legs are connected to each other through the windings of the motor and both carry the same 18A. It is difficult to understand what is going on unless you have a fundamental understanding of AC power. 12 AWG wire is rated for 20A and is perfectly acceptible so long as the wire length isn't too long. Having said that, I would probably run 10 AWG anyway just to keep line losses low.

Don't forget to add in the extra for start-up in rush current. Definitely install 10ga with a 30A breaker.

[Adrian Anguiano]

I installed 10GAUGE line througout my garage, then i install either 15, 20, or 30 amp receptacles/breakers to match the power tool. I always match my breaker/receptacle to my tool, just incase my motor decides to go nutso on me the breaker will switch before it goes kaboom.

[Art Mann]

I installed 10amp line througout my garage, then i install either 15, 20, or 30 amp receptacles/breakers to match the power tool. I always match my breaker/receptacle to my tool, just incase my motor decides to go nutso on me the breaker will switch before it goes kaboom.

I think you meant to say you installed 10 gauge wire rather than 10 amp wire. While there is nothing unsafe about running oversized wire or undersized breakers, the chances of you protecting your equipment by doing so are almost zero. The subject is much more complicated than it appears at first. If you look at the trip curves of standard breakers for house wiring, you will find that they will sustain twice the rated current for 30 seconds or so and 10 times rated current for 3 or 4 seconds. They are designed that way on purpose due to the high startup current of motors and some other devices. If you have a 20A table saw motor that develops a fault and is drawing 40A for half a minute or 200A for 3 seconds, it is probably long gone before the breaker ever trips. If the condition is a real short circuit, a 30A breaker will trip as quickly as a 15A one. Good electric motors have internal thermal overload protection that will shut off the voltage long before the breaker trips. That very thing happened to me yesterday when a small wooden cutoff wedge stalled my band saw. The thermal overload opened up before I could hit the off button.

The important thing to remember is that breakers are there to protect the housie wiring and receptacles. It is up to your equipment manufacturer and ultimately you to protect what you plug in.

[Adrian Anguiano]

While there is nothing unsafe about running oversized wire or undersized breakers, the chances of you protecting your equipment by doing so are almost zero.

I think the same thing about seat belts on air planes, but it cant make things any worse.