neutral wire size

[Bigrhamr]

I am preparing to install an electric powder coating oven. It needs to be fed 3 phase which runs the elements. It also has a fractional hp fan motor, light and the control box which are single phase so they tap one of the hots and also need a neutral wire. That is all factory wired.

My service is 120/208 y 3 ph.
The oven draws 80 amps.
#2 copper THHN is in place for the three hot legs, and a #6 copper ground.

I'm confused on sizing the neutral wire. Is it sized based on the 80 amp circuit?
(I'll guess yes) Or just the small single phase load?

Do I have all the info needed here?

Thanks!

 

[rockwithjason]

does your instruction manual say anything about it? what size is the lug for the neutral wire? my first instinct is that to be code compliant the neutral will need to be #2 but if your manual says different then follow that.

 

[wyliesdiesels]

On a 208Y service, the neutral carries the imbalance between the phases(not that there would be though with a 3Ø heating element), so I imagine it would need to be sized the same as the ungrounded conductors. I will have to check my code book when I get to work!

 

[mrjaw14]

The neutral needs to be the same size as the "Hot" legs. because it carries the imbalalce from the hots it counts as a current carrying conductor and must be sized accordingly

 

[Speedy Petey]

I know it's crazy, but the neutral needs to be the same size.

Typically in these situations the manufacturer puts in a small transformer to get the 120V from the 208vY.

 

[Speedy Petey]

On a 208Y service, the neutral carries the imbalance between the phases(not that there would be though with a 3Ø heating element), so I imagine it would need to be sized the same as the ungrounded conductors. I will have to check my code book when I get to work!

For a service or feeder the neutral can usually be reduced, although typically not for a 3-phase Y system.
See 220.61(C)

 

[robertwhite]

Where would you connect a #2 neutral wire at the panel side? It certainly won't fit into the neutral/ground bus bar.

 

[mrjaw14]

They make reducer lugs where you can terminate a large cable on a smaller panel lug, but you'll have to check and make sure that'll fly where you live

 

[Speedy Petey]

They make reducer lugs where you can terminate a large cable on a smaller panel lug, but you'll have to check and make sure that'll fly where you live

Exactly, but I'm not sure why you say it might not fly. A split lug fitting the panel is completely complaint.

 

[Speedy Petey]

See post #36: http://www.garagejournal.com/forum/showthread.php?p=2634447

 

[wyliesdiesels]

For a service or feeder the neutral can usually be reduced, although typically not for a 3-phase Y system.
See 220.61(C)

Yeah, I started to read that whole art. when I got to work @ 8 and then I got a power out call so I had to go. I'll look again tomorrow, but when I skimmed over it, I didn't see a section dealing specifically with 3Ø heating appliances. I'll look again......

I know it's crazy, but the neutral needs to be the same size.

Typically in these situations the manufacturer puts in a small transformer to get the 120V from the 208vY.

But if there's a built in transformer, then the device/appliance wouldn't need a neutral!

 

[Bigrhamr]

Thanks for the help guys.

Out of curiosity are there code issues with having 2 circuits supplying one appliance or machine? I know I've see large 3 ph machine tools with a seperate 120v circuit for lights and my 30 KW boiler is also wired like that with a 15 amp 120v circuit to the control and circulators instead of another pinky sized chunk of copper for a neutral wire.

 

[Speedy Petey]

But if there's a built in transformer, then the device/appliance wouldn't need a neutral!

Yeah, that was really vague, wasn't it? Sorry.

I meant they use a transformer to avoid having to run a neutral to a 3-phase machine just for a timer or light bulb.