multiple receptacles on 240V circuit

[rszimm]

I'm building a shop and I don't yet know exactly where things are going to go. I'm planning putting a 120V outlet every 4' or so on a couple 20A circuits, so that takes care of that. Can you do that same sort of thing with 240V? I was thinking about running a couple 240V 30A circuits and putting maybe 4-5 receptacles on each circuit. That way I could plug in a 10A band saw, or a 20A table saw, or even a 30A milling machine wherever I wanted. However, in reading the code I'm not certain this would be allowed. It appears that NEC wants me to special purpose each circuit and size it according to the piece of equipment it attaches to. So a 20A table saw would get a separate 240V 20A circuit. That's obviously problematic because I don't know where I I'm going to put the equipment, not to mention the fact that I don't even own much of yet, so I have no idea what power it's going to require.

 

[suemarkp]

See 210.23. You can have multiple receptacles on a 30A 240V circuit, but no one item is allowed to draw more than 80% of the circuit rating in that case. Good luck having anyone enforcing that unless the items are there when the inspection occurs. One other problem is that a given appliance is limited as to how large its supplying circuit can be (NEC 422.11(E), but it doesn't apply to single motor operated appliances). Typically, the limit is 150% of nameplate. So if you have a 15 amp appliance, you could put a 30A plug on it if you wanted to (150% of 15 is 22.5). But you could not run it on a 40A branch circuit.

The floor for circuit ratings in the NEC is 20A in most cases (NEC 422.11(E)(2)) -- just about anything can plug into a 20A circuit because 15A receptacles are permitted on 20A circuits. So wiring your shop with a bunch of T slotted 20A 240V receptacles could be useful. But if you have mostly larger things with 30A plugs, that less useful. You can still put multiple receptacles in there so you have choices as to where things go, but you can't always just run everything off of 30A circuits.

With single motor operated tools, you may ave more flexibility since motor circuits can have overcurrent up to 250% of the nameplate (and even higher in some cases). See NEC 430 Part III. But I've not seen overloads in many portable tools, but maybe the motors are thermally protected. Trying to comply with the motor rules with things like bandsaws and drill presses can be difficult if you have a motor with not much on the nameplate and no obvious overload devices in the tool. But take a look at 430.42(C) - Cord and Plug connected motors on General Purpose Branch Circuits.

 

[Little Bill]

@rszimm
What is your occupation?

 

[rszimm]

@little-bill
Occupation: Ostensibly a EE, but it's been a while. Used to design factory floor automation equipment so NFPA 79 was more of interest than NFPA 70, but I've always had a working knowledge of the NEC. Now I'm building my own house and just want to make everything to code even if the AHJ won't be sticklers.

@suemarkp
Where do you see the 250% of the nameplate requirement? I looked at my table saw and it seems to have a over-current protection device in the switch, but it's not "integral to the motor", so I'm not sure it applies? If it does, does that mean I can basically connect this to any amperage circuit? My lathe seems to have a thermal overload device built into the motor, so I'm guessing this also means it can be connected to multi-motor branch circuits at any amperage? (incidentally, it's an "auto-reset" device, which seems insanely dangerous to me, but that's a different subject)

 

[suemarkp]

2014 NEC table 430.52 Inverse Time Breaker column. Motor rules are tricky and article 430 is long. It even harder when you don't know if your tool is an appliance or a motor.

 

[Little Bill]

@little-bill
Occupation: Ostensibly a EE, but it's been a while. Used to design factory floor automation equipment so NFPA 79 was more of interest than NFPA 70, but I've always had a working knowledge of the NEC. Now I'm building my own house and just want to make everything to code even if the AHJ won't be sticklers.

Then I'm very sorry that we can't help any further on this since it is against the forum rules. I take no joy in doing this. With that said,
I am closing this thread in accordance with forum rules. This forum is intended for use by electricians and electrical contractors, inspectors, engineers, and vendors with their job-related duties. We are not allowed to provide how-to advice to persons who are not employed in this industry, or persons who do not perform electrical installation or maintenance as part of their jobs. Engineers are welcome to participate, but only if their questions do not involve attempts to perform their own electrical installation, troubleshooting, or maintenance work.

If I have misread the situation, if for example you have an electrical contractor on board to perform the installation work, then send me a PM to explain the circumstances.