Ask Me Help Desk - 3-phase AC Panel Load Calc

Hello,

I am having trouble figuring out the attached AC panel power numbers. Looking at circuit 1, if P=VI, and P=1440, V=120, then I=12A, is that was why a 20A breaker was selected? Now to circuits 24/26 where a 2-pole 30A breaker was installed, P=2496 per leg, V=120 per leg, so I=20.8A per leg, is that why a 30A dual pole was used?
I am looking to install three (3) new 30A dual pole breakers in this panel and I am trying to run the numbers to see if it will be over loaded. Each breaker will be feeding a rectifier unit for a DC power plant. To get the VA power value on each leg do I just take the worst case scenario power draw of 30Ax120V to get 3600VA? Should I use the manufacturer's nominal current draw of 12-14A for the power calc instead? If you need me to provide more information please let me know. Thank you in advance for your time.

After I input the worse case scenario numbers, C-phase is slightly higher than A-B. Will this be an issue.Attachment 46376

-Guy

You have conflicting information. Main breaker rating 225 amps. Feeder rating 150 amps. If the main breaker is rated 225 amps it should be supplied by wires rated for at least 225 amps. If you put an ammeter on each phase and record through a normal day cycle you will probably see that the total computed amps are somewhat smaller than the values on the panel schedule. That being said if you actually do have 180 amps on a 225 circuit breaker, you will be running right at the maximum for a 225 amp breaker. The main breaker must be sized 125% above the load. 1.25 X180 amps = 225 amps based on your values. (180 amps was the largest phase load you listed) You always try to balance 3 phase loads, but in practicality it becomes quite difficult due to the changing nature of the connected loads. You are about 8% difference on your theoretical phases, which should be fine, but lower would be better. The unbalanced load of the 3 phases shows up on the neutral wire from the panel to the source(transformer). The 3 phase transformer is really where you need to check for system balance if you are concerned enough. You may be pulling heavily on one or two phases and little on the other phase or maybe one of several combinations if you have more than one panel connected to the transformer.

I notice that you have a lot of loads that could vary- "furniture loads", for example may not carry anywhere near the allotted amperage, or any of the other loads. If I were concerned I would try to get a 3 phase recording ammeter and watch the loads for a few days during normal operation to get a better understanding of what each phase is actually drawing. In theory you are within workable limits.
I would bet that you aren't going to be pushing the 180 amps, but I don't know your system. The 8% current imbalance could be less or greater in reality. Balancing can be a great challenge and sometimes almost impossible because the load isn't static very often. I guess it all boils down to how important these issues are to you and to what lengths you are willing to go. I hope this answers your question and gives you some insight as to why.

Quote:

You have conflicting information. Main breaker rating 225 amps. Feeder rating 150 amps. If the main breaker is rated 225 amps it should be supplied by wires rated for at least 225 amps.

This is not true at all. In fact the opposite is true.
The feeder should be rated at no more than the breaker rating, not at least the breaker rating. The main breaker/lugs is just the maximum rating of the panel. The feeder can definitely be smaller if that is what is wanted.

Really, I'm just curious but is this from an actual job site or are these copies from a textbook work assignment?

Also, with respect to figure 1, CBs # 1 and 2.

L1 is supplying an aggregate load of 2596 VA. This load is sourcing CB #1 @12 amps. and CB #2 @ 9.6 amps Both breakers are 20 amp breakers. See figure #2 for a breakdown of the loads.

Look at figure #2. Note that both breakers 24 and 26 have the same VA rating, 2496 VA / 120 = 20.6 amps.

Now look t breakers 23 and 25, their VA load differs and that's why you see the difference. You have to look at the load being serviced by the breaker, bot on the total line VA.