ECN Forum Forums Code Related Discussion NEC & other Code issues Switching to three phase

They can size the 120/240 center tapped transformer to what your expected load is. Typically it is a 37 or 50 in the installations I see and they put in something smaller for the 3p loads.


Where we got in trouble was some of the DP equipment had 3P line cords but contained a significant amount of single phase load. Tape controllers were the worst because the satellite drives could be plugged into any one of 8 receptacles in the controller. As long as you plugged them in in sequence or had a "full boat" they were fairly well balanced but if someone only had a couple drives and they weren't careful to balance them you could corrupt the red leg balance.

I still want to avoid three phase power at home. So I guess the thing to do is make sure I don't build a mega mansion. OTOH, if I were that rich, I could afford to bring in 480/277 and waste power transforming it to whatever I wanted

Actually, If I had that kind of money, I'd more likely be doing something off-grid, anyway.

That is why delta is the best choice. You can run your big loads 240/3p and your 120/240 panel stays the same.

Be careful with your words, not all deltas are centered tapped.
I was at a facility, in October, that had both a 240V delta grounded B-phase 3PH 3W and a 120/240 1Ph 3W services.

I assumed we were talking center tapped delta.
There are also times I could make a case for corner delta.
If your equipment is good for 208, then wye makes sense but you can have harmonic problems on the L/N loads.
All of these have good points and bad points depending on your customer. That's why the PoCo still sells them all.

The 5% loading restriction only exists on center-tapped delta transformers that share a common core. In recent years several manufacturers have starting building some center-tapped single phase units that do not have this 5% restriction.

Transformer banks created by using multiple single phase units may be sized to handle any mix of 3-phase and 1-phase loads.