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Old 12-30-2013, 04:10 PM   #1
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Use of neutral in high-amp 240V applications


Dad is considering adding a couple of new 240V circuits to his workshop. The most common applications for those circuits would be a welder in the 50A range and an electric heater in the 30A range. I've suggested to him that he should at least be prepared to size his breaker to the device, and that it's not generally a problem for short (20-30') distances to have larger-than-required wires to allow for future changes.

This suggestion was based on him buying some form of Romex or other bundle where he couldn't effectively change it after-the-fact once the sheetrock goes up, so I had suggested 6/3 with ground so that he could run 50A and have a neutral available if he ever had a device that requires neutral.

Now he's talking about putting in some conduit, which we've done for other places, and he's also talking about more than one circuit, pulling in both wire for 50A and wire for 30A. Trouble is, three #6, three #10 for current, and one #10 for ground would go over the 3/4" conduit allowable fill.

Is there any in-code solution that would allow one neutral to function for this? I'm not thinking that there is even though I expect that such a neutral would only serve low-amperage control circuits on a machine, but even still, I don't think it'll work that way.

I've suggested that he install his conduit and install only a pair of #6 THHN for 240V 50A and a single ground, and that later we can pull more wire in if he needs to change it, even with the walls finished.

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Old 12-30-2013, 06:12 PM   #2
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Use of neutral in high-amp 240V applications


There is no way to share neutrals between the circuits, or to undersize the neutrals. Larger conduit would be the solution. Either that, or run two romex cables. The best solution may be to place a small subpanel in the shop. A 100A 12-circuit panel is pretty handy to have available. That way he can install shop circuits of whatever sizes and configurations are necessary and change them around easily. It may cost more, but not a huge amount more since you can use aluminum wire (SE-R cable is handy) and small panels are cheap.

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Old 12-30-2013, 06:22 PM   #3
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Use of neutral in high-amp 240V applications


Yeah, I didn't figure that it would be possible to share Neutral, just wanted to be sure.

As a detached shop that we built, it's already got a subpanel with three or four 120V 20A circuits for receptacles, a couple 120V 15A circuits for lighting, a 240V 20A circuit for the air compressor, and it'll probably need at least one more 120V 20A circuit for the garage door openers, so I'm already concerned about space in the subpanel. He's planning on adding a vehicle lift which'll take another two positions, in addition to the two positions for this receptacle for a welder. If I remember correctly we used tandem breakers for the 120V stuff where possible, so he might have two positions left when this is all said and done.
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Old 12-30-2013, 06:30 PM   #4
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Use of neutral in high-amp 240V applications


They make conduit larger than 3/4".

What is the capacity of your sub panel? You are adding quite a bit of load.
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Old 12-30-2013, 08:08 PM   #5
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Use of neutral in high-amp 240V applications


Yeah, there is bigger than 3/4", but when we hard-piped the compressor with 3/4" EMT it was hard enough, and the walls of the shop are 2x4s, so drilling for thicker than 3/4" EMT through the top of the wall doesn't leave a lot of material left.

We ran 2AWG for the subpanel, with a 100A breaker in the main panel for it, if I'm remember what we did correctly. It's exceedingly unlikely that he'll be running multiple 240V devices at the same time, working alone. Closest he'd probably ever come would be if he bought a plasma cutter and ran it in concert with the air compressor. There's no major HVAC in the shop, only a 120V swamp cooler motor...
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Old 12-30-2013, 08:33 PM   #6
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Use of neutral in high-amp 240V applications


Did you change over to some sort of flex for your final connection to the compressor?
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Old 12-30-2013, 08:43 PM   #7
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Use of neutral in high-amp 240V applications


Yes. Terminated in a doublegang box, with a NEMA6-20 receptacle, and the cord on the compressor has a matching plug.
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Old 12-30-2013, 09:04 PM   #8
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Use of neutral in high-amp 240V applications


Often in a shop environment conduit is run as surface mounted.
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Old 12-30-2013, 09:09 PM   #9
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Use of neutral in high-amp 240V applications


Yeah, I know. Since it's a new shop and getting sheetrocked, we figured we'd at least start out with all of the infrastructure in the walls and ceiling. Eventual add-ons might require conduit strapped to strut, but we'll see how long we can go before that becomes necessary.

We'll probably also install some empty 3/4" EMT at least up the wall to where it meets the ceiling, so that when the lift is installed the conduit can be finished to where power is needed and can be filled.
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Old 12-30-2013, 09:45 PM   #10
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Use of neutral in high-amp 240V applications


Just got off the phone with him, he's apparently drilled some holes for the conduit and ended up using a 1-1/8" spade bit to make them, so I've asked him to buy some 1" conduit to test. He'll pick up a pre-bent elbow section and see if he can make it fit.

After our conversation I think the plan now is to install empty conduit to where the receptacle for a potential welder will go, and to install empty conduit that stubs up into the ceiling to eventually be finished when a lift is installed, and to not pull any wire at this time. He looked up the lift that he's interested in and it uses 240V 30A service, so when it's time we can get away with 10AWG for that circuit, and if we need one or two circuits over at the wall for a welder or space heater then we can do that when it's time.

Thanks everyone.
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Old 12-30-2013, 11:44 PM   #11
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Use of neutral in high-amp 240V applications


I believe you said this was a separate shop. If this is the case that you can't have 2 different feeders supplying it from the main panel. You will need to upsize the subpanel in the garage as one of the appropriate feeders to it to allow for everything you want to run there.
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Old 12-30-2013, 11:54 PM   #12
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Use of neutral in high-amp 240V applications


Quote:
Originally Posted by Msradell View Post
I believe you said this was a separate shop. If this is the case that you can't have 2 different feeders supplying it from the main panel. You will need to upsize the subpanel in the garage as one of the appropriate feeders to it to allow for everything you want to run there.
?

Every circuit in the shop, existing and prospective, comes from the subpanel in the shop.

To add a circuit from the main panel we'd have to pull almost 90 feet of wire through the conduit buried in the dirt...
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Old 12-31-2013, 11:41 PM   #13
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Use of neutral in high-amp 240V applications


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Originally Posted by TWX View Post
?

Every circuit in the shop, existing and prospective, comes from the subpanel in the shop.

To add a circuit from the main panel we'd have to pull almost 90 feet of wire through the conduit buried in the dirt...
With all the talk you were having about conduit and worrying about how much wire to put in it it certainly sounds like you were running another feeder. Most of us don't even worry about conduit, like you are, inside the building. Just wanted to make sure you weren't considering running a 2nd feeder because it something a lot of people think they can do.
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Old 01-01-2014, 09:58 PM   #14
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Use of neutral in high-amp 240V applications


All of the 120V stuff is romex, and all of the 240V stuff is in conduit. It's much more likely that the 240V stuff would need later modification once the walls are up than the 120V stuff...

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