using only 2 phases of 3 phase system??

[ver]

A commercial (church classrooms)building has a 3 phase service. For some reason the general contractor wants to use only 2 legs. In addition to lighting and convinience receptacles, there will be 4 A/C units and an elevator.
Utility issues? Code issues? Electrical bill costs? Other thoughts?

 

[bthielen]

Re: using only 2 phases of 3 phase system??

Why???

Does this contractor have some special reason for not balancing the load among all phases?

Bob

 

[bennie]

Re: using only 2 phases of 3 phase system??

Get the utility engineer involved immediately.

 

[ver]

Re: using only 2 phases of 3 phase system??

Thanks guys,
I will contact the utility and see what they have to say.

 

[hbiss]

Re: using only 2 phases of 3 phase system??

For some reason the general contractor wants to use only 2 legs.

Who the heck is he? Why would he be qualified to make such a judgement? Do you tell him how to build his buildings? A GC has no business getting involved in such matters.

-Hal

 

[bphgravity]

Re: using only 2 phases of 3 phase system??

Out of curiosity, do you mean that the GC only wants you to use two legs period, meaning on say A/B legs only and not use C at all?

Or, do you mean that only two phases will be used for any given installation, meaning A/B for one load, B/C for another, and so on?

 

[luke warmwater]

Re: using only 2 phases of 3 phase system??

Is it wye or delta service?

 

[richwaskowitz]

Re: using only 2 phases of 3 phase system??

If it's a delta system, maybe he had a bad experience once (or twice) with someone unqualified incorrectly using the high leg for a 120 volt circuit????? Just a quess.

 

[laengineer]

Re: using only 2 phases of 3 phase system??

If an elevator is involved, it will usually require 3 phase power. I haven't seen a single phase elevator in a commercial building. You are the professional of record and your stamp is on the drawings. Need to remind the GC who is the professional and who is the contractor.

 

[caosesvida]

Re: using only 2 phases of 3 phase system??

other than three phase panels being more expensive, why would you not use it and increase the available load? I've rarely seen a service end up being grossly undersized after time goes by.

 

[rbalex]

Re: using only 2 phases of 3 phase system??

It?s been a while since I?ve seen it, but it was fairly common at one time to use this arrangement where the total single-phase load was very large compared to the three-phase load. (It used to be a common test question, in fact)

Essentially the system is operating in an ?open-delta? with the single-phase load fed from the larger transformer (center ? tapped for 120/240). A much smaller transformer was then used to accommodate the smaller three-phase load (240V). There was a modest economic benefit since the high-leg service conductor doesn?t have to be as large as the other two legs and 240V conductors would (in theory) be smaller than the alternative 208V. BTW 110.15 and 230.56 would apply here.

Obviously, this would only apply if the three-phase load is 240V.

Now days its usually pretty sound to simply use a 208-120V three-phase system.

 

[rbalex]

Re: using only 2 phases of 3 phase system??

I should have added this was a common service connection, before other overcurrent protection issues made it less economical.