circuit breaker size of 3 HP motor

[Ken Vick]

I am setting up a 1953 Dewalt GA 14" RAS that I recently acquired.  I have to run a circuit to it and need some guidance about the size breaker and wire that I should use.  I am running other 3 HP tools (Unisaw, sander) at 230 volts with #12 wire and a 20 A circuit breaker.  They run fine on this and no thrown circuit breakers.  The motor plate on the RAS says it pulls 18 A at 230 V.  This seems high to me and caused me to wonder about whether I need to go to a 30 A breaker and 10 gauge wire.  The 20 A,  12 gauge wire option would be much easier since I have a nearby circuit I can tap into without having to run a circuit all the way back to the sub-panel.  I will add that this is a really heavy, large motor which may correlate with what seems to me to be higher amperage draw than I would have expected.  Thanks.  Ken

[TDKPE]

Newer motors of 3 hp and up (typically, but not a hard and fast rule) use a run capacitor, which lowers the full-load current (the nameplate value) significantly, for reasons you probably don't care about.*  My 3 hp Unisaw motor, made by Marathon, in the 90's, has a FLA value of 12.4A, and no-load current of only 3-4A (don't remember exactly).  The same motor without a run capacitor would be more like 15-16 FLA, and maybe half that with no load.

So your big and old RAS motor, which I rather doubt has a run capacitor, will draw more current than a more modern equivalent.  Technically, you should put it on a 25A or larger circuit, (1.25 x 18A = 22.5A).  You can always try it on the existing 20A circuit, to see if it blows the breaker on start-up, and you can control how much load you put on it by just taking it easy (slap an ammeter on the line, and have someone monitor the current as you make a heavy cut).  But if you pull some 10 gauge copper and wire to a 30A breaker, you'll never have to look back.

And while it may run fine on a 20A circuit, the start-up current will be in the 4-8 times FLA range, or 72-144A. Inverse-time breakers, like those in a residential panel, can typicall absorb 6 to 10 times the rated value before the magnetic trip portion trips it out, depending on make and model, there's a strong chance the 20A breaker won't hold.

*The capacitor corrects the low power factor (pf) that would otherwise exist, and which is a measure of the portion of the current that is out of phase with the voltage, and that is magnetizing current only that just bounces back and forth between the motor and the utility but not doing any actual work, and doesn't even register on a residential watt-hour meter.  Improving that from around 80% to the high 90's (mine Unisaw has a pf value of .98, or 98%) goes a long way to cutting the current draw, and the efficiency will typically go up a bit, too, since less current is passing through the windings in the first place.  All resulting in lower current draw for the same output power.

[Herb G]

I agree with Tom here. Before you smoke the windings & release the magic smoke in the motor, spend a little coin & run a 10 AWG 3 wire circuit & buy a 2 pole 30A breaker.

[Ken Vick]

Thanks Tom and Herb.  Another question: is it likely that I could damage the motor by trying the 20 amp circuit?  I would have thought that the most that would happen would be that I threw the breaker.  I have a fairly long length of a 10 gauge 230 V extension cord that I have left over from making up a power cords for some 5 HP tools I have.  The power cord on the RAS needs to be replaced so I was thinking about using some of that to make the new power cord and putting a 20 amp plug on it so I could give it a try on the 20 amp circuit.  What do you think?  I don't mind the expense of running another circuit but I have my hands full with a full kitchen redo this summer and don't want to have to budget the time to run the circuit if I can avoid it.  Thanks again.  Ken

[Roly]

I would try running it on a 20 amp circuit.   As long as it starts up fine it will be ok   The saw will be starting up under a light load so that should help a lot and it is doubtful it will be run at full load for any amount of time.   Roly

[TDKPE]

Another question: is it likely that I could damage the motor by trying the 20 amp circuit?  I would have thought that the most that would happen would be that I threw the breaker. 

No risk of damage.  It might throw the breaker on start-up, but it might not.  It's an intermittent load, that you control, though it's possible to push that motor well beyond the 18A on the nameplate, which is the rated current at full load, if you really horsed it.

I ran my 3 hp Unisaw on a 14-gauge 15A circuit for a year, until I built walls in my basement shop and wired a 20A circuit with 12-gauge copper.  I think it blew the breaker once or twice, but I had retasked an existing 120V receptacle circuit on the basement wall for just the saw, so I could put it to work, and it was a temporary measure. 

But you won't hurt the motor if the breaker doesn't like it and trips on start-up.  It'll just force your hand that much sooner.

[Ken Vick]

Thanks Tom and everyone.  I'm going to give it a try in the next couple days and I'll report back.  Ken