Single Phase from 3 Phase

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rookie4now

Senior Member
Can someone tell me where my thinking is wrong here?

The existing service is 3 phase to a meter/main that feeds a three phase panel and a couple of single phase panels.

We are adding a three phase panel in another part of the building. My boss asked how I thought we should do it.

I said I would run 4 #2 wires (3 phases plus neutral) in 1-1/4 EMT to the room where we are adding the three phase panel. I would install 4 3-pole breakers to service the 4 pieces of equipment. I would use a 2-pole breaker and a multi-wire branch circuit to feed the lighting and general receptacles for the room.

He smiled and said "keep thinking."

Am I missing something or is he messing with me? What would a better way be?

Thanks.
 
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benaround

Senior Member
Location
Arizona
Re: Single Phase from 3 Phase

rookie4now,

What is the 3Phase voltage at starting point,What is needed for new equipment?

frank
 
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rookie4now

Senior Member
Re: Single Phase from 3 Phase

The new sub panel is a 100amp. The machines are all 220 3 phase. The 120 is only for lighting/duplex receptacles. The total run to the sub is only 90 feet.

[ July 25, 2005, 09:40 PM: Message edited by: rookie4now ]
 
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benaround

Senior Member
Location
Arizona
Re: Single Phase from 3 Phase

rookie4now,

I was trying to find out if the supply voltage to the building was possibly 480/277 or an open delta 240/120 with a high leg? So in main electrical room there are no transformers,correct?

frank
 
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redfish

Senior Member
Re: Single Phase from 3 Phase

Originally posted by rookie4now, Hmmm, won't the EMT work for that?
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There I go again, not reading the post. By the way , for a rookie, you sure know how to post like a pro. :)
 
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GG

Senior Member
Location
Ft.Worth, T.X.
Re: Single Phase from 3 Phase

#2 is too small for a 100A panel - 110.14(C)(1) states that for equipment 100Amps or less you must use conductors rated 60 deg. C. That would put you at #1 with 110A. Your neutral doesnt need to be #2, but a #8 - 215.2(A)(1).
 
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rookie4now

Senior Member
Re: Single Phase from 3 Phase

Originally posted by GG:
#2 is too small for a 100A panel - 110.14(C)(1) states that for equipment 100Amps or less you must use conductors rated 60 deg. C. That would put you at #1 with 110A. Your neutral doesnt need to be #2, but a #8 - 215.2(A)(1).
Click to expand...
I have read 110.14(c)(1) 4 times and I still don't know what it says :eek:
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Re: Single Phase from 3 Phase

#2 is too small for a 100A panel - 110.14(C)(1) states that for equipment 100Amps or less you must use conductors rated 60 deg. C. That would put you at #1 with 110A. Your neutral doesnt need to be #2, but a #8 - 215.2(A)(1).
Click to expand...
Actually you could use #3 THHN (rated 100 amps/75 degree column-table 310.16) providing that a major portion of the load at the subpanel was linear. This would mean that the neutral is not counted as a current carrying conductor so derating wouldn't apply to your feeder.

Regarding 110.14(C)(1)(a), if the terminations at both ends of the conductor, (in this case #3 THHN) are rated for 75 degrees than the 75 degree column of table 310.16 maybe used. See 110.12(C)(1)(a)(3). Most CB's and lugs are now rated for 75 degree terminations.

As stated, your 1 1/4" EMT may serve as your equipment grounding conductor. If you chose to install one than it would be sized according to table 250.122. Maybe your boss wanted you to figure using #3 THHN instead of the #2 to save him a few bucks.

[ July 26, 2005, 05:22 AM: Message edited by: infinity ]
 
sceepe

sceepe

Senior Member
Re: Single Phase from 3 Phase

Maybe I'm missing something but why use the
2-pole breaker and a multi-wire branch circuit to feed the lighting and general receptacles for the room.
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The Multiwire branch is fine for the receptacles but I question the use of the 2 pole breaker. Why not 2 single pole 20's. Use one for the lighting and one for the recpts. You do not have to use a two pole breaker on a multiwire branch circuit.

If this is a 240 Delta service with a high leg you can't use that phase (usually B) for 120V loads. Was that what your boss was talking about
 
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rookie4now

Senior Member
Re: Single Phase from 3 Phase

Wow. This is great. Thanks for all the answers. I'll find out today what it was and let you know.
 
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rookie4now

Senior Member
Re: Single Phase from 3 Phase

Well, the verdict is in. And he was actually thinking about part of the job that I didn't mention. I said I would run #2 to the sub panel, but I didn't mention in my original question that off of the gutter I had a 100a disconnect then fed the panel 90 feet away.

What he wanted to do was to use a load center instead of a disconnect as this would give them readily available 3 phase for future use in that area of the bldg. Makes sense to me :D


Redfish: - "For a rookie you post like a pro"

Thanks. I can post without any problem...computers I understand. It's this NEC thing that confuses me ;)
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Re: Single Phase from 3 Phase

Infinity - 215.2A in my 2002 code book didn't seem to mention anything about this. I'm not sure why the neutral could be #8. The only thing I could think of is that the PRESENT situation only has 2 30a circuits that use the neutral. If they were on opposite phases, I could see this working, but what if someone adds other loads later? I'm a little confused about this one.
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Actually this was originally part of the post posted by GG. I don't think that it will apply directly to your installation as I remember feeder neutral calculations. I will check it out and get back to you. For now you can take a look at 220.61(B).


Trevor
 
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redfish

Senior Member
Re: Single Phase from 3 Phase

I am trying to figure out the #8 neutral also, perhaps if there is only (2) 1500 VA branch circuits the standard formula applies, 3000VA/120V=50A neutral load. :)
 
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