isolated phase arrangement

Status
Not open for further replies.
B

Bill W

Member
I have a question regarding a parallel underground installation coming soon. I would like to pull these 120/208 feeds as isolated phases for ease of termination. My question is: do the grounded conductors also constitute an independent phase, and therefore require their own conduit, or can one neutral be pulled in each conduit with the isolated phases, thereby relieving me of 300.20(B)?
 
Each conduit should contain an A phase conductor, a B phase conductor, a C phase conductor, a grounded conductor, and in certain cases a grounding conductor.

What you don't want to do is put all of A phase in one conduit, B in another, etc. Neutral or not. Not no how, not no way.
 
al hildenbrand said:
What about 300.3(B)(1) exception ?
Click to expand...
That's the exception I would like to use. However, there are three runs, so it's either A,B,C,N, or AAAN, BBBN, CCCN. I'm not sure the latter is permissible. I've seen an isolated phase arrangement before, but there were four conduits penetrating a slab under switch gear and one conduits had all the neutrals...I think.
 
I don't know of anything that would prohibit, or any reason for not installing the grounded conductors in this manner, it will save you a conduit but, it will not relieve you of complying with 300.20(3)(B)

Think of phase to phase loads.

Roger
 
Last edited:
It seems 300.20(B) refers to single conductors (3-Aphases being a single conductor) if the grounded conductor is added in each conduit then the magnetic field should be balanced. I'm starting to think I'm reaching too far for this thing, but it would make the terminations a lot cleaner. Is this really an isolated arrangement if the neutrals are included with the phases, arranged: AAA, or ABC
 
Last edited:
Bill, if at any time the loads are just phase to phase, there will be no cancellation of the current flow and these conductors will introduce inductive heating where they pass through the magnetic enclosure and locknut.

Being that you are talking a Wye, more times than not you would be able to measure current flowing in any conduit for An, Bn, Cn, if you put a clamp on ammeter around the all the conductors in any one conduit.

Thus you will have inductive heating.

Roger
 
roger said:
Bill, if at any time the loads are just phase to phase, there will be no cancellation of the current flow and these conductors will introduce inductive heating where they pass through the magnetic enclosure and locknut.

Being that you are talking a Wye, more times than not you would be able to measure current flowing in any conduit for An, Bn, Cn, if you put a clamp on ammeter around the all the conductors in any one conduit.

Thus you will have inductive heating.

Roger
Click to expand...
Thanks , Roger that's the piece of the puzzle I was missing. I don't think it's going to be possible to cut slots in the tub between the conduits.
 
Bill, personally I would try hard to cut the slots, it really does make the terminations easier and neater if you can do it.

Roger
 
roger said:
Bill, personally I would try hard to cut the slots, it really does make the terminations easier and neater if you can do it.

Roger
Click to expand...
Roger, What about the lock-nuts?
 
Bill, I know there are aluminum locknuts available.

The few times I have done this has been with open bottom switchboards, so I can't say I have actually encountered the cutting slots and locknut issues, but I wouldn't hesitate to do so either.


Roger
 
roger said:
Bill, I know there are aluminum locknuts available.

The few times I have done this has been with open bottom switchboards, so I can't say I have actually encountered the cutting slots and locknut issues, but I wouldn't hesitate to do so either.


Roger
Click to expand...
Something makes me want to say aluminum is not suceptible to hysteresis heating, but is still suceptible to inductive heating...negligible, I'm sure. This job is being done in a neighboring state w/ reciprocal licensing. Might need the inspector's approval before going ahead.
 
2005 NEC 300.20(B) FPN:
Because aluminum is not a magnetic metal, there will be no heating due to hysteresis; however, induced currents will be present. They will not be of sufficient magnitude to require grouping of conductors or special treatment in passing conductors through aluminum wall sections.
Click to expand...
 
While the fine print note says "wall sections", and does not mention fittings, the common sense to me is that the lock nut has less volume than the wall section. Therefore, the locknut will have even lower induced current.
 
Bill W said:
It seems 300.20(B) refers to single conductors (3-Aphases being a single conductor) if the grounded conductor is added in each conduit then the magnetic field should be balanced. I'm starting to think I'm reaching too far for this thing, but it would make the terminations a lot cleaner. Is this really an isolated arrangement if the neutrals are included with the phases, arranged: AAA, or ABC
Click to expand...
300.20(B) does not mention the grounded conductor or that its presence
will eliminate inductive heating. The way inductive heating is eliminated is to
have all 3 phases in the conduit so that the magnetic fields cancel. You can use PVC conduit but what about the 90 degree sweep at the end of the conduit run. Is it not metal?

Bill W said:
Originally Posted by al hildenbrand
So, Bill, are your raceways nonmetallic?

c'mon now!
Click to expand...
Thats a good question. You have not answered it.

Bill W said:
Something makes me want to say aluminum is not suceptible to hysteresis heating, but is still suceptible to inductive heating...negligible, I'm sure. This job is being done in a neighboring state w/ reciprocal licensing. Might need the inspector's approval before going ahead
Click to expand...

FPN at the end of 300.20 speaks to that.
 
Last edited:
Status
Not open for further replies.
Top