Single Phase Breakers, Three Phase Panel 1

[stripedbass]

Our company sometimes builds trailers with some of the equipment being three-phase and single phase motors. We'll take the trailers to a site and have the utility company build us a bank of transformer(s) for power. The power is normally 120/240 3-phase 200A 4-wire delta.

My question is this: suppose we have all single phase equipment (one and two pole breakers) in a three-phase panel. We go to the site and have the utility company connect 3-phase power. The breaker panel is configured so that the 'B' phase does not have any breakers assigned to it. Typically, the 'B' phase is the "high-leg".

Later, if we want to add a 3-phase motor, the power is available and the panel will accept the 3-pole breaker.

Are there any problems with leaving the 'B' phase unoccuppied?

thanks,

SB

 

[DanDel]

I'm assuming by '120/240 3-phase 200A 4-wire delta', you mean a 240V delta with a center tap between 'A' and 'C' grounded(neutral) to provide 120V. In this case your panel is using the 'A' and 'C' phases to neutral for the 120V circuits. You would then have an unusable high voltage between 'B' and neutral, where your panel should definitely not have any single-phase loads connected. This is not an unusual connection, but I believe, to make it safer, you should have a separate panel for the single-phase and the 3-phase loads.

 

[dpc]

The high-leg delta system is generally used when the majority of the load is 3-phase, with a small amount of single-phase. Pump stations are a typical example.

It's not a good choice when most of your loads are single-phase 120V.

 

[stripedbass]

Thanks guys. I should have posted the question a little different: If I have a three-phase panel and it only contains one and two-pole breakers, is it OK to have the panel configured so that NONE of the breakers occupy the 'B' phase?

For example, there are only 5 single pole 120V breakers occupying the A & C phases. The rest of the breakers (30 circuit panel) are 240V 2-pole that occupy phases
C & A. 'B' phase is not occuppied at all in the panel. A & C to ground is 120V. 'B' to ground is about 212V because of the delta.

Is there any concern for an unbalanced utility company transformer bank load since there are three transformers providing the three-phase power?

I realize that one of the poles of the the 240V 2-pole breaker could occupy the 'B' phase. I was just wondering what would happen if 'B' phase was MT.

thanks again,

SB

 

[busbar]

To appease the masses, one could install ±two three-pole breakers with the requisite installed/visible 3 conductors on each set of breaker-load terminals, but with one of the three leads surreptitiously dead-ended in the utilization-equipment terminal boxes.

Near-ultimate fruition and authenticity could then be realized by serving one 2-wire 240-volt load Aø-to-Bø, and another Bø-to-Cø.

 

[DanDel]

The load is then definitely unbalanced, since all the used power is coming from one single-phase transformer, or one winding of a 3-phase transformer(the one connected between 'A' and 'C'. Let the utility worry about the unbalanced load. As long as your connected load on that one winding doesn't approach the rating of the overcurrent device(the utility always uses smaller-than-necessary transformers), and you don't have a low voltage condition, you should be ok.

 

[jbartos]

Suggestion: Normally, utilities have forms that the customer is supposed to fill in. Then, the Utility designs power supply including the transformer size that is correct for your load. Also, there is a need for correct power distribution panels. The above postings are explaining what is behind it. It is better to cooperate closely with the Utility in this respect to avoid additional calls for Utility power supply changes, e.g. pole transformer changes.