Calculating VA with mixed 1-ph and 3-ph loads

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LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Me again. I think I know how to do this, but I'd appreciate confirmation. Let's say I have a 240/120v service with a high-leg, and the loads are 100a single-phase on A and C, and 25a three-phase all the way around.

Would I subtract the 25 from the 100a, and calculate the 3-ph VA at 25a (25 * 240 * 1.73), and the remaining 75a 1-ph (75 * 240), and then add the two VA's together? This seems logical, but I'd like to be sure.

Thanx as always.
 
Because you have a high leg delta, a single number for loading will be misleading. When you have significant imbalance, you can't describe a load in a single va value without misrepresenting or understating something.

I'd describe it as a high leg service with 24kva of single phase and 10.4 kva of 3 phase load. That would give anyone who is knowledgeable, sufficient information to determine the load on each leg.

Jim T
 
jtester said:
Because you have a high leg delta, a single number for loading will be misleading. When you have significant imbalance, you can't describe a load in a single va value without misrepresenting or understating something.

I'd describe it as a high leg service with 24kva of single phase and 10.4 kva of 3 phase load. That would give anyone who is knowledgeable, sufficient information to determine the load on each leg.

Jim T
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Agreed on the total load. But, let me point out that the single phase load is shared (not equally) by all three transformers.
 
rattus said:
Agreed on the total load. But, let me point out that the single phase load is shared (not equally) by all three transformers.
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If this is a carryover from Larry's other threads of late, it's an open delta service. So the single-phase load would be shared by both of the two transformers... but as you said, not equally.
 
Rocky, thanx for the link. I'm so spacy from working all night last night on this, I'm having trouble concentrating on anything. But, I'll try to explain.

This is a gutted building, except for the existing A/C unit on the roof. The power available is 240/120, 4-wire, open Delta. The only 3-phase load is the A/C.

The reason I'm asking is that 240v breakers need to be special ordered for my supplier, while they stock 120/240v breakers. Either way of landing the 2-pole loads, the VA should be identical, right?

Since my main breaker is a 200a, 3-pole breaker, I can't undersize the high-leg conductor below that of the others even with a smaller load on it, correct?

I can land my 2-pole, non-neutral loads on any pair of lines, which would give me 115a on A and C lines, and 47a on the high-leg, B. Is the demand current simply the highest number, 115a?

Or, I can keep every load except the A/C unit on A and C, which will obviously raise the current on those lines, to 127a and 134a. Do I simply call the demand amperage the highest number (134a).
 
Smart $ said:
If this is a carryover from Larry's other threads of late, it's an open delta service.
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It is indeed. I'm trying to figure out the neutral demand also, although I'm using a 1/0 cu anyway.

I'm having trouble because of the non-balanced loading. If I know the highest line current, do I care about anything else?
 
LarryFine said:
It is indeed. I'm trying to figure out the neutral demand also, although I'm using a 1/0 cu anyway.

I'm having trouble because of the non-balanced loading. If I know the highest line current, do I care about anything else?
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I rarely do neutral calculations. Working commercial and industrial, such has always been engineered... but I've yet to experience a reduced neutral in the field.

From what I gather, you are doing a commercial project so there are no dwelling unit(s) reductions through 220.61(B)(1), and IIRC your service ocpd is 200A so you get no reduction under 220.61(B)(2).
 
Larry,

I am a little confused. If this a 240/120 service from a utility then the effect of unbalanced load and how it is handled by the transformer is not important to your calculations?

In this area we would treat this as a single 120/240 1PH 3-wire serivce for all of our calculations and sizing, then we would add in the (1) three phase load. At the service entrance I would recommend a single meter base, 1-phase panel, and a seperate smaller 3-phase disconnect. Our utility would supply it from an open-delta with the "lighter" transformer greatly larger than the "kicker".

It is projects like this that make me wish for the old days when "delta" circuit breakers were allowed.:rolleyes:
 
Teach me:

Teach me:

Smart $ said:
If this is a carryover from Larry's other threads of late, it's an open delta service. So the single-phase load would be shared by both of the two transformers... but as you said, not equally.
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The term, "High Leg", implies three transformers does it not? Correct me if I am wrong.

Rattus
 
rattus said:
The term, "High Leg", implies three transformers does it not? Correct me if I am wrong.

Rattus
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It does, in general. You were not wrong in your response for "assuming" such as it had not been specified to that point... but you were also not correct. An open delta configuration can have a high leg. It is the secondary Line "a" in the following depiction of an open-delta service:

OpenDeltaforLightandPower.gif
 
Smart $ said:
It does, in general. You were not wrong in your response for "assuming" such as it had not been specified to that point... but you were also not correct. An open delta configuration can have a high leg. It is the secondary Line "a" in the following depiction of an open-delta service:

OpenDeltaforLightandPower.gif
Click to expand...

I get that part OK Smart, but which buzzword specified an open delta? I must have blinked.
 
LarryFine said:
LOL! Honestly, only in the other thread. Sorry. :roll:
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Thanks Larry. I thought I had made a mistake which I never do except for the time I thought I was wrong.
 
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