New circuit for my garage...

[Toller]

My garage had a single 15a circuit, which it shared with the mud room next to it. That didn't leave much capacity for using any tools so I added a 20a circuit.

I got into turning and had to put the lathe in the garage as I didn't have room in my shop. I ran the 20a circuit over to it.

I just upgraded my shop to a cyclone, so I have a 2hp DC left over; oddly I can't sell it for even a third of it's retail value, so I am going to keep it use it with the lathe.

Obviously I don't have enough power in the garage and have to run a new circuit, but what...

1) I originally figured it would be a 240v as the DC is 240v only, but didn't see that a 240v circuit in the garage would be real useful in the future, for me or the next owner. 2) Playing with the motor wiring, I thought it could be converted to 120v. Tech support confirms that it can, but they call it 240v only because of concern over voltage drop at 16a. I could install a new 120v circuit, about 50' long and 16a; there would be some voltage drop, but not too bad. 3) I have a roll of 12/3; I could put in a one outlet multiwire circuit for the lathe and DC. Since they would always be running together, the voltage drop would be driven down some. But I have been a little paranoid about multiwire circuits after I found one in my bedroom was installed incorrectly, and 12/3 is harder to work with than 12/2.

All three options are reasonable, none are perfect; which would you do?

[Edwin Pawlowski]

Best option is a 60A sub panel.

[Chris Friesen]

240V is always useful. Welder, big saw, DC, bigger lathe, electric heater, etc.

16A is the limit for general-use 20A circuits.

Nothing wrong with a multiwire circuit...especially if you use a double-pole breaker.

Personally, I'd convert the existing circuit to 240V and use your wire to run a pair of 20A 120V circuits.

Chris

[Doug Miller]

This is incorrect.

16A is the limit for a continuous load (explicitly defined by the NEC as maximum current for 3 hours or more) on a 20A circuit.

16A is also the limit for any *single* cord-and-plug-connected load on a 20A circuit.

The limit for noncontinuous loads on 20A circuits is 20A.

[Chris Friesen]

Thanks for the clarifications.

In this particular case it seems likely that a 2HP DC is cord-and-plug-connected, no?

Chris

[Doug Miller]

Not necessarily; it could be direct-wired.

[bud--]

As far as I can find out, this is because most fuses or circuit breakers may trip at less than 100% if the load is over 3 hours, but will not trip at 80% or less. I believe the reason is heat buildup. Far as I know it is not a safety limitation.

There were multiple proposals to change the relevant NEC section several code cycles ago. One of the arguments is that UL allows equipment (like hairdryers??) to have a 20A plug and have a load of over 16A (or it might have been the equivalent 15A plug with load over 12A). Under this code section it is a code violation to use that UL listed equipment. The code panel response - 'we're right, they're wrong'.

(Another rationale to change the code was this is a restriction on how the wiring is used after installation and inspection - there aren't many of those restrictions in the NEC, and it is essentially unenforceable. The 3rd argument was that 80% is applied elsewhere in the code to continuous loads, as above. The code panel's response was 'the code is the way we want it.')

[To dpb - I was not impressed with the action of the code making panel on this; an aberration?.]

(I agree with everything you said Doug.)

-- bud--