Motor rating and circuit

[David C. Roseman]

If you have a shop tool that is 110v or 220 the only advantage is you can use smaller wire on the 220v. Some think you will use less electricity or have more power, that is not true.
If you have a 20 amp circuit on 110v you have a hot, neutral, and ground so you have all 20 amps through the hot and neutral. If you have a 220v 20amp you have 2 hot wires a neutral and a ground. So you have 10amps on each hot wire not 20. So all you need is 12/3 wire with ground and a 30amp breaker. Take this from a guy who wired his shop with 10/3 and found out he didn't need it.
Mark Walden

Agree with this as to wire sizing and current draw. Watts are watts, and we pay for electricity in watts (expressed in kilowatts). HOWEVER, I hope the breaker size is a typo. It is a huge mistake to protect a 12/3 wire with a 30A breaker. NEC requires a maximum of 20A overcurrent protection on that size wire for a reason.

David

[John M Wilson]

Watts are watts, and we pay for electricity in watts (expressed in kilowatts).

Actually, we pay for electricity in kiloWatt-hours, (kWh) which is not actually power, but energy, and could be measured in Joules.

But, I still call it the "power bill" even though our utility reminds us that they are in the energy business. ;-)

[Michael W. Clark]

If you have a 220v 20amp you have 2 hot wires a neutral and a ground. So you have 10amps on each hot wire not 20. So all you need is 12/3 wire with ground and a 30amp breaker. Mark Walden

I thought a 20A 220V circuit is rated for 20A and you would have 20A per hot leg and the wire/breaker sized accordingly, correct?

At the home shop level, probably not a savings in going 220V as far as the circuit goes. Most people run at least 12/2 to give them 20A which will run the majority of 3HP motors. Builds in some capacity for future larger tools.
Mike

[Chris Friesen]

Agree with this as to wire sizing and current draw. Watts are watts, and we pay for electricity in watts (expressed in kilowatts). HOWEVER, I hope the breaker size is a typo. It is a huge mistake to protect a 12/3 wire with a 30A breaker. NEC requires a maximum of 20A overcurrent protection on that size wire for a reason.

Minor quibble....on a motor circuit the NEC allows the breaker to be oversized as long as the tool has its own overload protection. Most woodworking equipment could reasonably fall into this category (just because it's connected with a plug doesn't mean it can't be a motor circuit). In this case the tool's overload protection prevents long-term minor overload, while the breaker still protects against shorts in the wall wiring.

[David C. Roseman]

I thought a 20A 220V circuit is rated for 20A and you would have 20A per hot leg and the wire/breaker sized accordingly, correct?

At the home shop level, probably not a savings in going 220V as far as the circuit goes. Most people run at least 12/2 to give them 20A which will run the majority of 3HP motors. Builds in some capacity for future larger tools.
Mike

Mike, I agree, and I think you, Mark and I are all saying the same thing, but in a different way. Here is my understanding: Each hot leg of the 220V circuit rated for 20A supplies 110V and is supposed to be capable of carrying a full 20A. So each leg requires at least no. 12 cable, and must be protected by 20A overcurrent protection. So a double-pole, double-throw 20A breaker is used at the service panel, with the two poles being on different legs of the incoming line power from the utility. They must be on different legs so that they are out of phase, yielding 220V potential across the poles. A 220V breaker does this by design, with one side connecting to the right-side hot busbar, the other side connecting to the left.

I think Mark is speaking about this from the appliance side of the circuit, i.e. actual current draw, rather than circuit capacity. If a 220V tool is drawing 20A nominally, each hot leg needs only to supply 10A when it is running, whereas with a 110V tool drawing 20A, the single hot leg must supply the full 20A.

If a motor in a home shop is capable of operating on either 220V or 110V, running it on 220V does not use less power (expressed in kilowatts, billed in kilowatt-hours, as John Wilson correctly points out). What it does do is allow for the use of smaller conductors, which is a cost-savings, and for a given conductor, it can reduce voltage drop. The reduction in voltage drop can result in better performance of the tool, especially noticeable with a long cable run.

There are electrical engineers and journeyman electricians on this board who know way more about this than I do, but I think I've stated it correctly.

David

[Art Mann]

If you have a 220v 20amp you have 2 hot wires a neutral and a ground. So you have 10amps on each hot wire not 20. So all you need is 12/3 wire with ground and a 30amp breaker. Take this from a guy who wired his shop with 10/3 and found out he didn't need it.
Mark Walden

Your statement about a 240VAC circuit is very much in error. A 240VAC, 20A circuit carries 20A on both hot legs. It is, in fact, the very same current in both legs. The two legs are connected directly to each other through the windings of the motor and the circuit is continuous. A neutral wire isn't even required for most 240VAC machines. The exception is when that machine also needs 120VAC for some reason or other. The comment about the 30A breaker protecting a 12 AWG wire is also wrong and following that advice would be a severe safety hazard.

[David C. Roseman]

Your statement about a 240VAC circuit is very much in error. A 240VAC, 20A circuit carries 20A on both hot legs. It is, in fact, the very same current in both legs. The two legs are connected directly to each other through the windings of the motor and the circuit is continuous. A neutral wire isn't even required for most 240VAC machines. The exception is when that machine also needs 120VAC for some reason or other. The comment about the 30A breaker protecting a 12 AWG wire is also wrong and following that advice would be a severe safety hazard.

Aha! Art, this highlights a distinction I was failing to make in the second paragraph of my own description of current flow. If a machine is drawing 20A when running on 220/240VAC, you're saying an ammeter will actually measure 20A on each of the two hot legs of the circuit. That makes sense. What I, and perhaps Mark, was confusing is the current draw of a dual-voltage machine that can be changed to run on either 110/120VAC or 220/240VAC by selecting different terminals on the motor. In that case, a machine drawing 20A at 110/120VAC will only require 10A when selected for 220/240VAC . So we would then measure 10A on each of the hot legs. Do I have that right now?

David

[Duane Meadows]

Yes, David That is correct.

[Curt Harms]

The discussion has been interesting. I just want to spin up the cutter (no planing) to be sure everything is working. I agree that the start-up current is the main problem and may trigger the breaker.

The actual planer is a 12" Powermatic 100 with a 3" cutter head. I would be happy with a 3 HP motor, but it was retrofitted with a 5 HP motor (with a maximum draw of 21.5 A--does that make it a 4 HP motor in real life?) I think the only way I could bog it down with a 5 HP motor would be planing old railroad ties. The motor is rated close to a 20 A load, which makes me want to cheat, but it is also a good excuse to power up the garage.

If I do try it on a 20 A circuit, I will let you know the result.

Scott

As long as the wire is protected by the approriately sized breaker, I don't see an issue. I'll bet you could plane on a 20 amp circuit too as long as you weren't trying to remove 1/4" from a 12" wide oak plank. A 20 amp breaker will carry a LOT more than 20 amps for a second or maybe two to allow for start surge. I believe there are, or at least used to be breakers that would carry an overload longer (but not long enough to overheat the wire) without tripping. I doubt a 12 ga. wire carrying 40 amps will immediately burst into flame. I expect it'd get pretty warm after a minute though.

[John Lanciani]

As long as the wire is protected by the approriately sized breaker, I don't see an issue. I'll bet you could plane on a 20 amp circuit too as long as you weren't trying to remove 1/4" from a 12" wide oak plank. A 20 amp breaker will carry a LOT more than 20 amps for a second or maybe two to allow for start surge. I believe there are, or at least used to be breakers that would carry an overload longer (but not long enough to overheat the wire) without tripping. I doubt a 12 ga. wire carrying 40 amps will immediately burst into flame. I expect it'd get pretty warm after a minute though.

I've posted this before, but this Square-D PDF is loaded with great information; http://static.schneider-electric.us/...0600DB0105.pdf Take a good long look at the example time-current curve on page 2 in the PDF, it shows just how long a breaker really takes to trip and at what current levels. For example a typical molded case residential breaker will take upwards of 10,000 seconds to trip at 125% of rated current.

[Scott Rychnovsky]

I took your advice and just tried it on a 20 A, 220 V circuit. I did turn the magnetic switch overload protection to about 19-20 A, which is only slightly below the motor rating of about 21A. It turns on and off fine with no problems with overlaid or breaker on the circuit. I switched out the blades to new ones and have been trying some light planing. It works fine and does not show any signs of overload (magnetic switch or breaker shut downs.) It looks like it will work fine under normal circumstances, so I will keep it set up this way until I run into problems.

Thank you for the advice.

Scott

[Wade Lippman]

Great. I guess I was wrong.

[Art Mann]

Breakers are set to trip at around 80% of rated load. So a 20 amp will get you about 16 amps.

Actually, quite the opposite is true. A breaker will never trip at the rated current and will take several hours to trip at 125% of rated current. Commonly available breakers that are used for domestic purposes will sustain twice the rated current for 30 seconds to a minute. Go download and study the trip curves from any of the standard household breaker manufacturers and you will see what I mean.

[Cody Kemble]

Sorry it has been a few years since I worked in the electrical trade. I did look at some curves. Regardless a circuit should not be overloaded for safety reasons. The 80% is meant for continuous load admittedly (3 hrs), but it is meant to allow the 125% trip that you mentioned. Take a look at what gauge wire and temperature your cables are rated for as you will be generating some heat.

[John Lanciani]

Breakers are set to trip at around 80% of rated load. So a 20 amp will get you about 16 amps.

I guess you didn't look at the Square D PDF I posted the link to, you're not even close. at 150% of rated load a thermal/magnetic breaker will take upwards of 5 minutes to trip. Even at 500% load it can take as long 12 seconds to trip.