Three phase transformer single phase panelboard secondary


Senior Member
I have three phase 45kva transformer delta primary to wye secondary. Primary breaker is sized 100A and secondary has single phase panelboard with main breaker of 200A.
Primary three phase conductors are brought with EGC and secondary two phase conductors plus neutral plus EGC is brought.
to single phase panelboard. Primary voltage 480V secondary is 208/120V. I have 208V loads to be clear.

It is kind if like this post I posted previously:

It is correct three phase transformer is used on secondary panelboard which is single phase.

Questions that were not asked on above link:

1. How does grounding and bonding work? Same way as one would do if secondary had three phase panelboard or different? Sizing of GEC, SBJ, SSBJ sizing etc. I have two phase going to secondary single phase Panelboard.

2. How would the delta loads work on primary two phases see x kva and third phase 0 kva primary side? Just trying to figure out how to do calculation demand and connected panelboard transformer primary side each phase.

3. It was kinda of not clear above link how does one size primary and secondary breaker? Its not normal in this case

4. Would 240.21(c) apply in above configuration or not? Taps 10 feet or 25 feet its indoor non industrial setting
Last edited:


Staff member
State Electrical Inspector
1. Same as if you were feeding a 3 phase panel.
2. Look back at the earlier thread .. same scenario but 50 kva
3. Primary breaker would be the same. In your case with a 45kva and a 100 amp primary breaker (almost 200%, you will need 125% protection on the secondary.. next size up can't be used so your 200 amp secondary breaker would not be acceptable
4. 240.21(c) rules apply.