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Discussion Starter · #1 ·
i am planning a remodel of a restaurant with the following service; in the 400 amp fused disconnect there are (3) fuses, (2) 4/0 thhn conductors, (1) #1 thhn conductor. i get 240 volts between the enclosure box and "A" LEG, and leg "C". i get 120 volt between the enclosure box and leg "B" (CENTER). we have (3) 3-phase roof ac units at 60 amp each. all other loads are single phase 120/240 volt. what do i have? 3 wire open delta? corner grounded 3 wire delta? the small center wire concerns me a lot. the electrical plan checker here in houston is saying that i must have (4) wires for a 3 phase service. the previous tenant (subway) was in this building for about (15) years without any electrical issues with more equipment than we want to use. we were not planning to touch any 3 phase stuff. i do not know if the service pole has (2) or (3) transformers. this service pole is serving other service drops for other buildings, so there are many transformers on the pole. there is nothing 208 volt i could find. does that rule out wye? how would i tell that i had a 3 wire ungrounded delta 3 phase for sure? are where special calculations like 87% if (2) transformers are used open delta ?
 

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Discussion Starter · #3 ·
3 wire 3 phase

not in mexico (the land of no grounding). this works, i just do not know why. the worst wiring i have seem in my life was in the philippines at a high end hotel. i thought i was going to die just looking at it and their low voltage is 240. thanks, i was thinking this an experienced electrician would give me a hint. the nec sort of covers it, but the 240 volt part is what quizzels me. maybe it is a "center-tap grounded open delta", because the 3 phase loads are minimal.
 

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I've built many restaurants, all with three phases power. Why do you care if it is a delta or a wye? The only issue I see is that the electrical contractor may have to bring out a fourth conductor, other than that, shouldn't all you care about is enough amps?
 

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Sounds like a delta corner grounded service. The smaller conductor is sized according to the loads imposed on the entire 3 phases. Your description suggests that one of the phases is grounded, producing the corner-grounded voltages you are getting.

The "B" phase may be the center tap of one of the windings, which usually is grounded in delta connected configurations.

Your description of "center-tap grounded open delta" might be better stated "center-tap grounded corner delta"

What are your voltage readings from:

A to B? (I'm guessing 120?)
A to C? (you stated 240)
B to C? (208 perhaps?)

A to ground? (You stated 240)
B to ground? (You stated 120)
C to ground? (You stated 240)
 

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Sounds like a delta corner grounded service. The smaller conductor is sized according to the loads imposed on the entire 3 phases. Your description suggests that one of the phases is grounded, producing the corner-grounded voltages you are getting.

The "B" phase may be the center tap of one of the windings, which usually is grounded in delta connected configurations.

Your description of "center-tap grounded open delta" might be better stated "center-tap grounded corner delta"

What are your voltage readings from:

A to B? (I'm guessing 120?)
A to C? (you stated 240)
B to C? (208 perhaps?)

A to ground? (You stated 240)
B to ground? (You stated 120)
C to ground? (You stated 240)

On a corner grounded system, the grounded phase would read 0 volts to ground. So assuming all the op voltage reading are correct, I dont think it is a corner grounded system. just my .02
 
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