120/120/208 Confused

Nem066

A to grnd = 120v
B to grnd = 120v
c to grnd = 208v
A + B = 240v
A + C = 240v
B + C = 240v

Hooked up new 240v 1ph A/C system in building that has what I believe is 240v 3ph delta wiring.
Both air handler and condenser are on 2 pole breakers utilizing legs A and B, X13 DC drive motor overheated very quickly in air handler, as soon as contactor in condenser pulled in, it threw the main breaker. Why?
Swapped legs A and B in the panel and now everything works perfectly. Why?
This makes no sense to me as both legs are 120v and 1ph equipment is not phase specific. What am I missing?

stickboy1375

That's a high leg delta service, generally phase "B" is the wildleg, You may have an open delta high leg and not a full delta, not sure if this is related to your issue or not. But the "kicker" transformer could be maxed out.

Was the air handler strictly 240v as well?

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Nem066

Equipment is a residential 5 ton carrier split system 240v 1ph

stickboy1375

So was both the outdoor unit and indoor unit using the high leg? And then you moved the breakers so the high leg wasn't being used, correct?

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Nem066

No, was using both 120v legs in both instances, only thing changed polarity of legs A and B to equipment.

L1=A L2=B-----not working
L1=B L2=A-----working
C- not used in either scenario and is the 208v high leg.

Swapped A,B polarity multiple times and results always the same, just don't understand why.
Air handler uses a DC drive motor that has an integrated rectifier pack, in first scenario motor overheated quickly, in second scenario motor functions normally.

stickboy1375

Well that is a strange scenario you're describing, technically speaking, it should work either way, even if the high leg was included in the description.

What made you reverse the conductors on the breaker? Just out of curiosity....

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AllanJ

All I can think of is that 120 volt loads in your building and also in neighboring buildings, combined, were unbalanced in a manner that when you swapped the two 120 volt hot to neutral lines (A and B in your case) you balanced the 120 volt loads (yours and your neighbors') better.

It would have been interesting to find out what the voltage the air conditioning units got at the time they overheated. Including hot to neutral on those lines.

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stickboy1375

Allan, he's wiring a 240v appliance, the amperage drawn will be exactly the same on T1 and T2.

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Nem066

Shot in the dark mostly, I knew it wouldn't work when I did it, except it did.

stickboy1375

Funny what you will try when you get desperate to make something that should work, work.

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AllanJ

Before: Most of the neighbor's 120 volt loads on phase A, most of your 120 vikr loads on phase A, adding the AC overloads phase A.

After swapping A and B in the panel: Most of the neighbors 120 volt load on phase A, most of your 120 volts loads on phase B, adding the AC (same additional draw on A and B) works OK.

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stickboy1375

I'm not sure what your trying to say here?

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