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Joined: Jul 2004
Posts: 10,041 Likes: 37
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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. ![[Linked Image from gfretwell.com]](http://gfretwell.com/electrical/red%20leg%20transformers.jpg) 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.
Greg Fretwell
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Joined: Jan 2005
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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.
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Joined: Jul 2004
Posts: 10,041 Likes: 37
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That is why delta is the best choice. You can run your big loads 240/3p and your 120/240 panel stays the same.
Greg Fretwell
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Joined: Jul 2001
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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.
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Joined: Jul 2004
Posts: 10,041 Likes: 37
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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.
Greg Fretwell
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Joined: Jul 2001
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My understanding is that 240 delta with one side center tapped for 120/240 had a limitation that the single phase loads could be no more than 5% to 10% of the total transformer capacity. It's certainly not a choice for buildings that have all or nearly all single phase loads, but so much load that the PoCo puts them in the "must use 3 phase" category. 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.
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