Three wire service entrance cable for three phase panel

[3phaseninja]

I have three phase 400a panel 240/480. There are only 3 wires coming into the panel from the pole, black, red, blue. I have 240v on "a" phase and "c" phase. "B" phase has no voltage. My objective was to use one of the circuits leaving the panel for a single phase sub panel (120/240) and I'm not sure how I can pick up a neutral.

 

[LEO2854]

I have three phase 400a panel 240/480. There are only 3 wires coming into the panel from the pole, black, red, blue. I have 240v on "a" phase and "c" phase. "B" phase has no voltage. My objective was to use one of the circuits leaving the panel for a single phase sub panel (120/240) and I'm not sure how I can pick up a neutral.

You can't ,,you need one coming in.

 

[GoldDigger]

You can't ,,you need one coming in.

If you measure 240 to ground from A and C, what do you measure from A to C? If A and C are two phases of a three phase circuit, then you have 240/360 with the B phase grounded, not 240/480, yes?

 

[Dennis Alwon]

Is this an delta system? How do they get 120V in the rest of the building. Surely they have it-- you may need a trany

 

[hardworkingstiff]

I have 240v on "a" phase and "c" phase. "B" phase has no voltage.

Where are you placing your meter leads when you get these voltages?

A to B = ?
A to C = ?
B to C = ?

A to gnd = ?
B to gnd = ?
C to gnd = ?

 

[3phaseninja]

I only checked all three phases to ground. Will have to check phase to phase in the morning. Once I realized we were piping into the wrong panel I just started the process of rerouting. Still curious though, career first for me.

 

[hardworkingstiff]

I only checked all three phases to ground. Will have to check phase to phase in the morning. Once I realized we were piping into the wrong panel I just started the process of rerouting. Still curious though, career first for me.

If you have 240v phase to phase on all 3 combinations (which matches the phase to ground voltage), then I believe you have a corner grounded Delta.

 

[kwired]

If you have 240v phase to phase on all 3 combinations (which matches the phase to ground voltage), then I believe you have a corner grounded Delta.

Bingo.

Only way to get 120 volts is with a transformer. Up to you if you want secondary with single phase 120 volt only, 120/240 single phase, or you can even have a three phase 208/120, or a three phase delta 120/240 with high leg, but you must have a conversion device of some kind.

I guess there is solid state conversion options, but probably pricey unless necessary output is not very high capacity.

 

[ActionDave]

If you have 240v phase to phase on all 3 combinations (which matches the phase to ground voltage), then I believe you have a corner grounded Delta.

Corner grounded almost certainly. More likely 360V phase to phase.

 

[GoldDigger]

Corner grounded almost certainly. More likely 360V phase to phase.

If it is 360 phase to phase, and one phase is at ground, I do not see how you can get 240 volts to ground out of any of the phases. Unless the "B phase" is not actually a phase line at all but is just a ground/neutral and only two phases are present of a symmetric delta or wye. That would be consistent with 360 phase to phase.
In any case it sounds like the OP has realized that the panel did not actually contain what he needed to connect to.

 

[kwired]

Corner grounded almost certainly. More likely 360V phase to phase.

If it is 360 phase to phase, and one phase is at ground, I do not see how you can get 240 volts to ground out of any of the phases. Unless the "B phase" is not actually a phase line at all but is just a ground/neutral and only two phases are present of a symmetric delta or wye. That would be consistent with 360 phase to phase.
In any case it sounds like the OP has realized that the panel did not actually contain what he needed to connect to.

I'm putting more money on the corner grounded system.