Three phase to single phase question

[storeytime]

Customer provided transformer, 150 kVA, 3 PH, 480V. They're only using the secondary to provide power for two 200A, 1PH load centers. The actual amperage requirements of the two load centers combined is not more than 100A.

Secondary is 208/120 and feeds through a gutter and then out to the two load centers.

How would we hook up two phases off the secondary for the single phase load centers and maintain balance?

 

[Dennis Alwon]

You really can't. One phase will be common to both panels so the best you can do is avoid loading that particular phase more than necessary.

 

[iwire]

You really can't. One phase will be common to both panels so the best you can do is avoid loading that particular phase more than necessary.

In the real world I agree with Dennis 100%.

On the other hand it seems on paper you could intentionally load the single phase panels out of balance so that the load would be balanced on the three phase supply. I think in the real world this would be difficult to maintain.

 

[Electric-Light]

You'll have to use an electronic converter or a motor-generator set.

 

[Smart $]

Customer provided transformer, 150 kVA, 3 PH, 480V. They're only using the secondary to provide power for two 200A, 1PH load centers. The actual amperage requirements of the two load centers combined is not more than 100A.

Secondary is 208/120 and feeds through a gutter and then out to the two load centers.

How would we hook up two phases off the secondary for the single phase load centers and maintain balance?

Thinking outside the box here, but if the tranny has voltage taps or other means to isolate and short one primary winding, just use two primary-secondary winding pairs.

My other reply is tell the customer to get 208/120 panels...

 

[storeytime]

Thanks for all the replies. Not sure which way we're going to go yet. Appreciate the help.

 

[Besoeker]

Thinking outside the box here, but if the tranny has voltage taps or other means to isolate and short one primary winding, just use two primary-secondary winding pairs.

Don't think shorting any winding,even if isolated, is a smart thing to do.

 

[storeytime]

Thinking outside the box here, but if the tranny has voltage taps or other means to isolate and short one primary winding, just use two primary-secondary winding pairs.

Would that not still be putting all the load on two phases thereby making an unbalanced situation upstream from the transformer? That unbalance upstream is the main thing we're concerned with.

 

[billsnuff]

piggy back on IWIRE

piggy back on IWIRE

install three panels
ab
ac
bc

 

[Smart $]

Would that not still be putting all the load on two phases thereby making an unbalanced situation upstream from the transformer? That unbalance upstream is the main thing we're concerned with.

Yes, and it would be unbalanced going upstream... but possibly easier to balance upstream.

 

[Smart $]

Don't think shorting any winding,even if isolated, is a smart thing to do.

Why not? ...and what would be best practice if not using a winding pair?

 

[Besoeker]

Why not? ...and what would be best practice if not using a winding pair?

Not using it is not the same as shorting it. (This isn't a current transformer.)
Energise any winding and it will induce voltages on all other windings on the transformer. Short out any one and high currents are likely to ensue - subsequently followed by an acrid smell.

 

[tryinghard]

Customer provided transformer, 150 kVA, 3 PH, 480V. They're only using the secondary to provide power for two 200A, 1PH load centers. The actual amperage requirements of the two load centers combined is not more than 100A.

Secondary is 208/120 and feeds through a gutter and then out to the two load centers.

How would we hook up two phases off the secondary for the single phase load centers and maintain balance?

Take your best shot at balancing the secondary by arranging the single phase loads accordingly. Then monitor the results with a recording amp meter at the transformer secondary and rebalance from results, even this will change per seasons or usage. It?s just a mismatch system you might explore changing the sub panels to 3ph.

I wonder what reasonable limits of unbalance can occur maybe ask the transformer manufacture.

 

[tryinghard]

I understand 10% is the rule of thumb and even more if not fully loaded

 

[beanland]

Scott-T Connection

Scott-T Connection

The Scott-T transformer connection allows converting 3-phase to 2-phases 90 degrees apart. This might achieve the balance you seek. Google it.