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#78773 - 10/23/01 09:33 PM 3 phase delta
wolfdog Offline
Member

Registered: 10/24/01
Posts: 135
Loc: Dallas,TX
Is there a code reference regarding single phase load on the high leg of 240v 3ph delta?
I was asked to defend my stand and could not quote or find anything in the NEC

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#78774 - 10/23/01 10:55 PM Re: 3 phase delta
glenn35 Offline
Member

Registered: 07/21/01
Posts: 48
Are you asking if the single Ø 208-grnd is usable? I have never seen any 240V 4 wire that has used a hi-leg by its self. I suppose its possible but what would it be used for?

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#78775 - 10/24/01 03:49 AM Re: 3 phase delta
electure Offline

Member

Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
The situation you'd run into with the 208V to Neutral would be that standard 1 pole C/B's would not be rated for the voltage, as they're rated at 120V
If you were to use a C/B with a higher rating (277V), or use a fuse with a 250V rating, then I don't think that there's anything to prohibit it in the Code.

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#78776 - 10/24/01 05:51 AM Re: 3 phase delta
wolfdog Offline
Member

Registered: 10/24/01
Posts: 135
Loc: Dallas,TX
Thanks for the replies. I was not as clear as I could have been. I am asking about single phase 240 using the high leg with either 'A' or 'C'. I was aware of the 208 and breaker rating mentioned.
In my area it is an 'understood rule' that only 3ph. goes on the high leg. Transformers and services are sized accordingly. Is it a code issue or just a local thing?

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#78777 - 10/24/01 06:31 AM Re: 3 phase delta
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
If you think about it, a three phase load is made up of three single phase loads that work together to make advantage of the 120 degree phasing of the source. Each load of a three phase load, taken by itself, is indistinguishable from a single phase load.

Given that phase B is the high leg, adding 240 volt 2-wire loads to A-B and B-C will shift load off the A-C 120/240 side of the delta.

Nothing in the NEC encourages or discourages this placement of load on a 120/240 volt 4-wire delta.

The first thing I would wonder is what the local PoCo's method is for sizing the transformer(s) that supply phase B, the high leg? Do they put the smallest possible can(s) on the pole?

Al
_________________________
Al Hildenbrand

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#78778 - 10/24/01 06:31 AM Re: 3 phase delta
therain4 Offline
Member

Registered: 12/18/00
Posts: 37
Loc: Dunmore,Pa. U>S.A.
I am not sure if this addresses you're Question specificly, atricle 240-85 mentions breakers with a slash designation 120/240 cannot be used between the high leg and another phase (a-b)you need a breaker with a straight 240v rating.

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#78779 - 10/24/01 06:49 AM Re: 3 phase delta
George Corron Offline
Member

Registered: 05/16/01
Posts: 728
Loc: Lorton, Va USA
Since sometimes people take code rules and put them where they don't belong (Ya know, I remember the code sez....) someone may be confusing 455-9 for phase convertors where it says single phase loads are not to be put on the derived phase, but this has nothing to do with delta high legs (which are also getting pretty rare).

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#78780 - 10/24/01 06:58 AM Re: 3 phase delta
wolfdog Offline
Member

Registered: 10/24/01
Posts: 135
Loc: Dallas,TX
To ElectricAl
Power Co. sizes 'B' for three phase only.

To therain4
That jogs something in my foggy brain. I remembered it was something indirect. 240 delta is still very common here. Is this uncommon in other areas?

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#78781 - 10/24/01 07:20 AM Re: 3 phase delta
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
PoCo here, covers several states, refuses all 120/240 4-wire 3-phase requests since the mid '90s. 120/208 only.

As I understand it, they're trying to shift their generator load balancing down to the consumer.

Is the service you are considering old and has 3-phase load been shed, freeing transformer capacity?
_________________________
Al Hildenbrand

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#78782 - 10/24/01 08:05 AM Re: 3 phase delta
wolfdog Offline
Member

Registered: 10/24/01
Posts: 135
Loc: Dallas,TX
Existing service. Someone has added single phase 240 load to high leg and now I am questioning my understanding.

[This message has been edited by wolfdog (edited 10-24-2001).]

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