Three phase Service Used For Single phase

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360Youth

Senior Member
Location
Newport, NC
A customer is building a new sewer plant for a small condominium complex and wants to use the existing three phase service (from the existing plant that is being torn down) for the new plant which is entirely single phase. How much, if at all, does this setup affect utility service that is designed for three phase use when all loads will be on only two of the legs? I have not looked at wires run from the transformer to see if they were sized based on 3-phase load calculations or if they were simply based on a 200 amp service. Thanks.
 

480sparky

Senior Member
Location
Iowegia
What effect it will have depends on the load(s). If it's small compared to the total, no biggy. But if it's significant, then there could be problems.

Why not just install a 3-ph panel and stagger the loads across the 3 lines?
 

360Youth

Senior Member
Location
Newport, NC
What effect it will have depends on the load(s). If it's small compared to the total, no biggy. But if it's significant, then there could be problems.

Why not just install a 3-ph panel and stagger the loads across the 3 lines?

I was leaning that direction. But the suggestion made me think about issues I never really had to before. Go figure. :roll::)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Just beware of "high-leg" systems. 480sparky's idea is still valid but a bit more tricky for high-leg including selection of gear and breakers.
 

360Youth

Senior Member
Location
Newport, NC
Just beware of "high-leg" systems. 480sparky's idea is still valid but a bit more tricky for high-leg including selection of gear and breakers.

Yea, just mulling that over. I think what plant operator wants is a single phase disconnect fed off the initial 3-phase power to then feed his MDP, using the 120-volt legs of the system, leaving the high leg unused. If a 3-phase panel was used and the loads were distributed across the lines, it would take no time for someone to install a 120 volt circuit on the high leg. Not that it should, but it would.
 

sameguy

Senior Member
Location
New York
Occupation
Master Elec./JW retired
call the power co.. if I recall a 3p xfmr will lose ~10% of out put capacity if single phased.
 

WirenutNH

Member
Location
NH
I would be carefull if service is 120/208. We do a lot of sewer station work and one diffrent design we ran into was contractor wanted to install a sewer pump to be able to pump out holding tank to leach field at a large condo complex. The problem we ran into was the service that we planned on feeding new control cabinet was a 120/208 3 phase service with invidual meters that where 120/208 single phase. So when we went to look into getting single phase 208v sewer pump none existed and all manuf required that the voltage be in the 240v range so we had to install a large boost transformer to get our 240v 1 ph.
 
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