3HP 220 V motors - 20 amp or 30 amp circuits?

My new shop is finally under construction http://www.sawmillcreek.org/images/icons/icon7.gif, and the electrician should be showing up in about a week to start the wiring. I have a question on the wiring and circuit breaker size for 3 HP motors.

I will have individual 220V circuits for all of the heavier machines (air compressor, dust collector, lathe, tablesaw, bandsaw and planer). Each of these will be 3 HP motors.

None of the wiring runs will exceed 50 feet from the 200 amp sub-panel.

The best information I can find says I should size the circuit based on a 3 HP motor drawing 17 amps, and sizing the circuit for 125% of the load. That would mean each of these circuits should be sized for 21.2 amps. This means a 30 amp circuit and 10 gauge wiring.

Does that sound right? My gut feeling is a 20 amp circuit with 12 gauge wire would be ok. Are there any of the above machines that should definitely be on a 30 amp circuit?

Thanks, Mike

I would like to point out that there may be an advantage in having at least one circuit that will handle a heavier load, this way if you ever up-grade to a more powerful machine, you will be ready for it.

The 3 HP cabinet saws that I have had experience with were wired with #12 wire on double 20 amp breakers.

Worked fine on these saws....:)

Mike.
I'm sure you'll recieve a lot of replies and opinions. In the end you'll install the breaker that the inspector wants to see there. I know that's a "crappy" answer, but there it is
The local licsense should know what the inspector wants to see.
A 20 amp breaker wit 12/2 will do it, but I'll be surprised if you don't end up running #10awg, and installing a 30 amp breaker.
The "heaviest loads" I see in your list is the dust collector, and the planer. A bandsaw resawing draws significant current also. They all have 3 hp motors, so I would expect them to all be treated equally.

After having two shops wired and two differ electricans, my answer would be #12 wire and 20amp breakers.
Rob

My 3hp Cabinet Saw was run on a 30amp breaker in the old house, can't recall the wiring, think it was 10 gauge.

Just wired up the new shop and my electrician buddy told me 10 gauge. So, now the 3hp Cabinet Saw and ShopFox 1741 jointer are on 10 gauge runs, both 30 amp breakers.

A 20 amp breaker is all you'd ever need for a 220v 3HP motor. It wouldn't hurt anything to run 30amp circuits if you so choose, but it's overkill. 12 ga 2 conductor with ground would be my choice for wire unless there's an extraordinalily long wire run from the box to the outlet.

Unless you're going to hardwire them as as a dedicated circuit, you'll run the wire to one (NEMA 6-20) or more wall outlets (NEMA 6-20 or NEMA 6-15) and the inspector would never see what's plugged into the outlet(s). And since these aren't continuous duty applications, the 125% upsizing isn't required.

I can't imagine requiring more than a 20 amp circuit to satisify a 3HP intermittent load (appx 12 - 15 FLA and much less than that on average). But, if you do decide to overkill it and go with 30 amp branch circuits, even for the 3HP machines, you'll need NEMA 6-30 outlets, and, in any case, a matching plug on the equipment.

I would suggest that you do use 30 amp/10 ga circuits for the Dust Collector and Air Compressor circuits. If you're going to upsize any of the things you mentioned to 5HP, my guess is those will be the ones. Unless you're looking toward becoming a serious production shop, 3HP woodworking machines are plenty big enough.

All the above, notwithstanding, my advice is to tell your electrician the intended use of the circuits and let him/her install what he/she knows is appropriate and will pass inspection.

My 3HP 220V motor is rated for 18 amps. So a 20 Amp breaker and 12 AWG wire is all that is needed.

My 5hp saw draws 19 amps. General of Canada told me when I bought it that the motor simply would not trip even a 20-amp circuit breaker. So, what does that tell you?

Gary Curtis

Mike,

I ran 10 gauge wire and installed 30 amp breakers on all 8 of my 220 circuits. The difference in wire cost is negligible compared to the problems running it later. The breaker is really protecting the wire not the machine.
Thus if you run 12 gauge copper, use a 20 amp breaker.

Your electrician should know what will meet your local inspector's expectations.

When I built my shop, I wanted no electrical limitations for what I could potentially have in the future. There is no difference in cost between 20amp and 30 amp breakers. The only difference you would see is the relatively minor difference in cost between 10 gauge and 12 gauge wire. At the time of building the shop, 20 amp circuits would have satisfied my needs at the time. Now, 17 years later with several machine upgrades, I would have had to rewire my shop for larger circuits. Glad I went with 30 amp circuits.
Steve

Mike
Would you give us a little more info on your new shop?

1. Is this a separate new structure and if so has the concrete slab been poured?
2. Are you running the wiring in romex in the walls?

The reason I ask the questions is that if you haven't poured the slab yet you have a number of options open to you. If you are running the wiring in the walls then at least run 10 ga. and for now I would put them on a 20 amp breaker. I would only put a 30 amp breaker on these circuits if the manufacturer recommends this.
These suggestions come from being in the electrical construction industry for close to 30 yrs. and also from someone who has had to redo their own shop.
I hope this helps
DAVE

While 20a wiring MIGHT work. 30a WILL work for sure, plus you'd have room to bump up to a bigger tool later.

The compressor's your highest amp drawing tool, for sure. The dust collector might pull some heavy amps when you have it moving it's maximum air. The planer might pull some amps when you're taking a deep cut on wide stuff. But most of the time, none of the other tools will draw full amperage under normal use like the compressor will.

Startup current is going to be your biggest draw, for sure. Especially on that compressor. Those things can pull a TON of amps for a full second or more (beyond the slow-blow threshold sometimes).

Wire for more than you need. You'll be glad you did the day a tool gets upgraded. You'll kick yourself if your 20a breakers pop in the winter when that compressor's cold.

Second option: run 10ga wire but put in 20a breakers and upgrade if need be. That's a little more cost effective. It's easy to upgrade a breaker. Not so much the wire.

Here we go again!!

I am new to this forum and I very much like reading all the posts - many, have been a great help to me as I try to put my new shop together.

But, I must say that I have read too many posts regarding electrical issues that are not accurate - or better said not quite accurate.

From a safety "point of view" there is no direct problem with installing #10 or even #8 wire and then installing a breaker with a lower Amp rating -- say 20amp. We would not allow this in our factory on a rewire.

What some people do not understand is that 30A 240V is a lot of power- and it can kill you -- as well as allow a situation to develope that can very easily create a fire.

If a tool only requires a 20 amp line -- install a good 20amp breaker - you gain nothing having a 30 amp breaker on that tool.


sorry to preach

You'll get mixed replies on this one. 3HP saws seem to generally recommend 20a and 12ga. Grizzly recommends a 30a for their 3HP (18a) jointer (via email) and Penn State recommends the same for their 3HP DC.

I ran 10ga to my 20a positions. That way if I want to go higher, I just change the breaker and outlet.

P.s. Tom, preach away ;-)