Imbalance on a three phase system 208/120v an issue for this case?

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Cartoon1

Cartoon1

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Florida
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Electrical Engineer
I have a commercial space that has an 800A 208/120v, three phase system. This system was mostly balanced. The owner recently wanted to add a single phase 200A, 2 pole requiment load for a pool they are building (which the 800A service can handle, as their load is currently low). My question is: since 2 of the 3 hot legs will probably be pulling close to 200A, would this cause a serious imbalance issue? Should I worry about it?

Thank you
 
Cartoon1 said:
I have a commercial space that has an 800A 208/120v, three phase system. This system was mostly balanced. The owner recently wanted to add a single phase 200A, 2 pole requiment load for a pool they are building (which the 800A service can handle, as their load is currently low). My question is: since 2 of the 3 hot legs will probably be pulling close to 200A, would this cause a serious imbalance issue? Should I worry about it?

Thank you
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Based on how large that service is and how low actual loads typically are compared to calcs or what we think they will be, I wouldn't worry about it.
 
augie47 said:
I agree that I would not let it worry me but I would sure try to convince the owner to utilize the 3 phase power available.
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Apperantly this particular connection in only a single phase and they install it at homes only. But in this case they want to install it for this commercial space. They say they dont have a 3 phase setup for it and it only takes a single phase. I agree! I find it hard to believe.... but they give me no other information.
 
When does loading imbalance becomes a problem? Lets assume for this case that you have 700Amps on phase A and B, and 500A on phase C on an 800Amps service for about 3 hours. will this cause serious increased heating, and potential issues with the power distribution network?? or dose it take high imbalance numbers to start seeing problems?
 
Cartoon1 said:
Apperantly this particular connection in only a single phase and they install it at homes only. But in this case they want to install it for this commercial space. They say they dont have a 3 phase setup for it and it only takes a single phase. I agree! I find it hard to believe.... but they give me no other information.
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Probably thinks two more phases will run up his electric bill
 
Unless he's installing electric pool heat I'd say the actual won't be significant. In any event I would not fret.
 
augie47 said:
Unless he's installing electric pool heat I'd say the actual won't be significant. In any event I would not fret.
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Probably be actually a little cheaper. Cheaper motors, maybe a little less I2R heating on the branch circuit.

But believe me, I have heard all kinds of ridicules excuses from business owners to NOT install 3 phase. Lots of them think that the motors are more expensive, or they think it will be a problem powering the single phase equipment they already own. I've even gone places where they have 3 phase service and stuff all powered by capacitor motors because they don't know any better
 
Cartoon1 said:
When does loading imbalance becomes a problem? Lets assume for this case that you have 700Amps on phase A and B, and 500A on phase C on an 800Amps service for about 3 hours. will this cause serious increased heating, and potential issues with the power distribution network?? or dose it take high imbalance numbers to start seeing problems?
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Load imbalance can potentially impact different parts of the system.

The service equipment will probably be rated for continuous use at 80% of its nominal rating on each of the phases individually or in combination. So the service equipment is probably fine with extreme imbalance.

Neutrals might be reduced in size with the expectation of L-L loading and balance. So the absolute worst case imbalance of 700A on phase A with 0 on B and C might overload the neutral. However that is far more extreme then what you are asking about, because you are considering a L-L single phase load which doesn't hit the neutral at all.

Unbalanced loading will cause unbalanced voltage drop on the various service components. This unbalanced supply voltage can increase heating in three phase motors. The effect of unbalanced loading on the over-all system depends on how 'stiff' the supply is. Say your supply system has an expected 2% voltage drop at full load; a heavily unbalanced load might cause 2% voltage imbalance. Ideally the supply to a motor is kept to less than 1% imbalance. So if this hypothetical system is supplying motors that care about voltage balance, you could reasonably tolerate imbalanced loading of 50% full load. A different system with 5% voltage drop at full load could only tolerate imbalanced loading of 20% full load.

-Jon
 
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