multiple outlets on one 240-volt circuit?

[Bill Bryant]

A 240-volt line runs between the floor joists directly above my basement shop on the way to an outlet upstairs behind the stove. I have a gas stove, so I could re-route the line entirely and put the outlet down in the shop. But it got me curious. What about two or three outlets on the same 240-volt line? I'm fairly familiar with 120-volt wiring conventions (I recently finished 1300 sq. ft. of basement and ran multiple circuits without a hitch and passed inspection) but I don't know what's kosher with 240.

[Tom Veatch]

... What about two or three outlets on the same 240-volt line? ...

There seems to be some sentiment that there can only be one outlet on a 240 circuit. That is not the case. A 240 circuit can feed multiple outlets just as a 120 circuit can feed multiple outlets. The only differences are that there is a 240 volt potential between the two conductors instead of a 120 volts and that both conductors instead of just one have a 120 volt potential relative to ground.

My shop, wired by a licensed electrician and inspected by the local code enforcement inspectors, has 4 separate general purpose circuits, all of which are 240 volt and each of which feeds multiple outlets. The wiring is 3-wire plus ground and the outlets are dual-voltage duplex receptacles - one 120 and one 240 outlet in each receptacle. If I hadn't wanted the 120 available, it could have been wired with 2 + ground and with multiple 240 volt outlets.

[Rob Will]

A 240-volt line runs between the floor joists directly above my basement shop on the way to an outlet upstairs behind the stove. I have a gas stove, so I could re-route the line entirely and put the outlet down in the shop. But it got me curious. What about two or three outlets on the same 240-volt line? I'm fairly familiar with 120-volt wiring conventions (I recently finished 1300 sq. ft. of basement and ran multiple circuits without a hitch and passed inspection) but I don't know what's kosher with 240.

Bill, if the stove wire is 4-wire you could use it to feed a sub panel in the basement. That would be an easy way to terminate the existing wire and provide a good take off point for extra circuits.

If the stove wire happens to be 6/4 romex, I would install a sub panel with a 50 amp breaker as a main disconnect. The main problem with using your existing wire alone is that the breaker in the panel upstairs is also probably 50 amp. For anything short of a welder plug or a range plug, that is too big. You will want to step down to smaller wire for most of your tool outlets. These wires must be individually protected with the proper size breaker, hence the sub-panel.

Rob

[Ray Knight]

I just checked on this. You can absolutely run more than one 240 receptacle on one breaker. The issue is the wire might need to be big enough to accommodate both receptacles being used simulataneously, something we home hobbyists don't tend to do, and our argument we use when we push code. I just finished my rough in, 3 240 circuits, one dedicated to air conditioner/heat pump (code requires dedicated), and two 240 receptacle circuits, one with two receptacle and one with three. Used the bigger wire, so I could use them simulaneously, but keeping breakers at 20A for now, to discourage that (since my feed is only 50A)Ray Knight

[Brad Townsend]

I have two 240v circuits in my shop, each with multiple outlets. My logic being that I am only one person and can only run one machine at a time. Having said that, I had a friend over a couple of weeks ago and we were running my Griz 8" jointer and Unisaw at the same time off the same circuit and I didn't even think about it until later. No problem whatsoever.

[Gary Herrmann]

I'm running two outlets off each 220 off each 220 breaker in my shop.

[Rick Christopherson]

I think Rob addressed this situation the best. Because this circuit is probably a 50-amp circuit, you will need to step it down to more reasonable size. You cannot install any other size outlet on a 50-amp circuit besides 40-amp or 50-amp, and these can be very expensive.

Install the 50-amp subpanel and then you can run your circuits how ever you choose down in the shop.

[Bill Bryant]

I just looked the situation over.

The circuit going to my stove is on a 40-amp breaker and uses 8/3 with ground.

Does this change anybody's thinking?

[Rob Will]

I just looked the situation over.

The circuit going to my stove is on a 40-amp breaker and uses 8/3 with ground.

Does this change anybody's thinking?

Nope, I would install a small sub panel in the basement with a 40amp 2 pole breaker as a main disconnect. (Since this is a home shop and not a production shop, the chance of multiple machines running at the same time is small). I think you will be fine with about four 20amp 230v circuits. Just make sure you use 12 ga wire. Personally, I would avoid 30amp branch circuits in this setup and also avoid things that turn on automaticaly like a big air compressor.

BTW: It is normal for the total capacity of the branch circuits to exceed the capacity of the main. For example, the 200 amp panel in my house has branch circuits totaling 635 amps.

Rob

[Tom Veatch]

... The issue is the wire might need to be big enough to accommodate both receptacles being used simulataneously...

Any particular idea as to why this might be a requirement and, more to the point, how would it be implemented. Seems like there are some inherent contradictions in that requirement as stated.

Say you have 3 20 amp outlets on the circuit. Simultaneous use could draw current up to 60 amps. Therefore you would need wire with a 60 amp capacity. But without a 60 amp breaker, you couldn't use the 3 outlets simultaneously at their full capacity. So, adding a 60 amp breaker makes it a 60 amp circuit. But now you have 20 amp outlets on a 60 amp circuit....? So, increase the receptacles to 60 amp capacity. Now you're looking a a possible simultaneous draw of up to 180 amps ....?

There's got to be more to it than just what you stated because it seems to me that way lies madness.

[Mike Cutler]

Any particular idea as to why this might be a requirement and, more to the point, how would it be implemented. Seems like there are some inherent contradictions in that requirement as stated.

Say you have 3 20 amp outlets on the circuit. Simultaneous use could draw current up to 60 amps. Therefore you would need wire with a 60 amp capacity. But without a 60 amp breaker, you couldn't use the 3 outlets simultaneously at their full capacity. So, adding a 60 amp breaker makes it a 60 amp circuit. But now you have 20 amp outlets on a 60 amp circuit....? So, increase the receptacles to 60 amp capacity. Now you're looking a a possible simultaneous draw of up to 180 amps ....?

There's got to be more to it than just what you stated because it seems to me that way lies madness.

There are two things that need protection on a motor circuit. Whether there is a single motor, or multiple motors.
The motor(s) needs to be protected. This is accomplished by determining the correct size wiring to carry the current.
The wires, or conductors, or branch circuit need to be protected. These are protected by selecting the correct breaker size.
The particular section of the NEC that deals with motors is Article 430. At the beginning of article 430 there is a flow diagram that breaks down the article into the parts and the sections of the code that apply to each part, and the reference to the correct article, not contained in 430, that apply.

NEC 430.24 addresses multiple motors and branch circuit sizing requirements;
125% of the FLC of the largest motor plus the sum of FLC of the other motors . to determine branch circuit sizing and breaker protection.
My copy of the 2005 code is at work, so I'm kinda running off memory right now. Dangerous,I know.
If someone has an NEC copy, perhaps they can work through Article 430 to answer the question more completely.

[Rob Russell]

There are two things that need protection on a motor circuit. Whether there is a single motor, or multiple motors.
The motor(s) needs to be protected. This is accomplished by determining the correct size wiring to carry the current.
The wires, or conductors, or branch circuit need to be protected. These are protected by selecting the correct breaker size.
The particular section of the NEC that deals with motors is Article 430. At the beginning of article 430 there is a flow diagram that breaks down the article into the parts and the sections of the code that apply to each part, and the reference to the correct article, not contained in 430, that apply.

NEC 430.24 addresses multiple motors and branch circuit sizing requirements;
125% of the FLC of the largest motor plus the sum of FLC of the other motors . to determine branch circuit sizing and breaker protection.
My copy of the 2005 code is at work, so I'm kinda running off memory right now. Dangerous,I know.
If someone has an NEC copy, perhaps they can work through Article 430 to answer the question more completely.

Mike,

I generally agree with what you posted, but there is another factor with motors - overload protection for the motors. To run multiple motors on the same circuit, one would need to ensure that the individual motors have appropriate overload protection. The circuit breaker protecting the wiring could well be too large to protect the motor. In that case, a separate overload is required to protect individual motors.

Rob

[Mike Cutler]

Mike,

I generally agree with what you posted, but there is another factor with motors - overload protection for the motors. To run multiple motors on the same circuit, one would need to ensure that the individual motors have appropriate overload protection. The circuit breaker protecting the wiring could well be too large to protect the motor. In that case, a separate overload is required to protect individual motors.

Rob

Absolutely agree Rob. This is also covered in 430. I was just trying to Keep things a little simple(r).
Sorry for the omission.

[Tom Veatch]

Is there a presumption in the code that if the circuit is 240 volts, it is a "motor" circuit? If the circuit feeds wall outlets, then anything, not necessarily motors, can be plugged into those outlets. And, even if it is a motor, it's impossible to determine in advance what the FLA of those motors might be. How then can a 240v circuit that feeds wall receptacles be sized per Article 430.

If it is assumed that the FLA won't be in excess of the current rating of the outlet, and there are multiple outlets, does not the circumstances of my previous post then come into play. Or does the code allow, for example, 20 amp receptacles in circuits protected by 60 amp breakers?

[Mike Cutler]

Is there a presumption in the code that if the circuit is 240 volts, it is a "motor" circuit? If the circuit feeds wall outlets, then anything, not necessarily motors, can be plugged into those outlets. And, even if it is a motor, it's impossible to determine in advance what the FLA of those motors might be. How then can a 240v circuit that feeds wall receptacles be sized per Article 430.

If it is assumed that the FLA won't be in excess of the current rating of the outlet, and there are multiple outlets, does not the circumstances of my previous post then come into play. Or does the code allow, for example, 20 amp receptacles in circuits protected by 60 amp breakers?

1st question

Article 430 deals only with motors. Receptacles for dryers, ranges, hottubs, AC units, evaporators, are covered in other sections. The sizing requirements are in other articles. 210, 220,250, etc.There are a lot of them.
This would be a huge discussion to get into, so I'm going to stick with motors as the point of discussion.
The FLA is marked on the motor by the manufacturer. Most, if not all motors have a UL approval. In absence of manufacturers name plate data there are Tables 247-251 at the back of 430 based on HP ratings.
Most od the motors in a shop will run at a current draw significantally less than FLA. The biggest load, in terms of continuous current draw while in operatoin is most likely the dust collector.
You want the motor and the wiring protected

2nd question

In this example the answer would be no. The code would not allow this much downsizing. Additionally there are max breaker sizing requirements. Add to this that there are manufaturers specified breaker requirements also.
Remember we want to not challenge the wiring, or the motor. We want that breaker to trip on a high current condition. Supplying a 20 amp load with potentially 60 amps is asking for trouble.

While it is fun to discuss these subjects, and a good learning experience. I hope that folks are taking away from threads like these that this work is really best left up to a liscensed electricians.
There is a lot to the code.(NEC)