Sizing Panelboard - Help!

[jobuTupaki]

All,

I am sizing a panelboard (120/208V, 3ph) that has the following loads:

Phase A 25.5 kVA
Phase B 29.9 kVA
Phase C 23.4 kVA

My question is, do I size it based off the sum of the phases or average across the 3 phases:

Phase A+B+C = 78.8 kVA -> 219A load, 300A breaker, 400A frame

OR

(phase A+B+C)/3= 26kVA -> 73A, 90A breaker 100A frame

Most of the loads are single phase. There is only one piece of equipment that is 3 phase at 12kVA, and what is leading me to need clarification. The other problem is I only have access to 200A breakers to feed this.

Any information would be great.

 

[infinity]

I'll let an engineer explain but I would just use the B phase which is the highest.

 

[jobuTupaki]

Thank you. Yeah to me, sizing it across 3 phases does not make sense as for 3 phase loads, you account for the load 3 times. Can't find anything in NFPA 70.

 

[d0nut]

The two calculations you have shown should give you the same result (within rounding error), but you are using the wrong voltage for your per phase calculation in the second example. You should be dividing by 120V rather than 208*sqrt3 volts. If you are calculating per phase, you have to use the L-N voltage rather than the system voltage.

Also, and I mean no offense by this, the questions you have been asking in this forum are fairly basic and fundamental engineering concepts. I may be wrong, but I am assuming you are fairly new to engineering in this industry, perhaps an EIT or something similar working under licensed professional engineers. I would strongly recommend taking your questions to them and discussing these fundamental concepts with them. I think the resulting discussions will provide you with much more benefit then just posting them to this forum as they will be able to drill down into the concepts and help clear up any confusion or misunderstandings you may have with the concepts.

I am not discounting the wealth of knowledge that can be obtained from this forum or its members, but am suggesting that face-to-face conversations may provide you with more value. The limitations of the forum format are in answering the questions that aren't asked. An actual conversation would be a much better venue for discovering the unasked questions so they can be answered and expand your knowledge much faster than by simply posting to this forum.

 

[jobuTupaki]

The two calculations you have shown should give you the same result (within rounding error), but you are using the wrong voltage for your per phase calculation in the second example. You should be dividing by 120V rather than 208*sqrt3 volts. If you are calculating per phase, you have to use the L-N voltage rather than the system voltage.

Also, and I mean no offense by this, the questions you have been asking in this forum are fairly basic and fundamental engineering concepts. I may be wrong, but I am assuming you are fairly new to engineering in this industry, perhaps an EIT or something similar working under licensed professional engineers. I would strongly recommend taking your questions to them and discussing these fundamental concepts with them. I think the resulting discussions will provide you with much more benefit then just posting them to this forum as they will be able to drill down into the concepts and help clear up any confusion or misunderstandings you may have with the concepts.

I am not discounting the wealth of knowledge that can be obtained from this forum or its members, but am suggesting that face-to-face conversations may provide you with more value. The limitations of the forum format are in answering the questions that aren't asked. An actual conversation would be a much better venue for discovering the unasked questions so they can be answered and expand your knowledge much faster than by simply posting to this forum.

I do have face to face conversations with them. Sometimes I like other opinions as it helps me. I did not know there was an experience-criteria for asking a "good question". I know you did not mean offense, but this was pretty discouraging for me to hear and likely will not utilize this platform again.

 

[infinity]

do have face to face conversations with them. Sometimes I like other opinions as it helps me. I did not know there was an experience-criteria for asking a "good question". I know you did not mean offense, but this was pretty discouraging for me to hear and likely will not utilize this platform again.

John,
d0nut was expressing his opinion. Some may share his opinion many others will not. Don't be discouraged by asking what some may view as a simple question. We're all here to lean and share knowledge, there are no stupid questions.

 

[wwhitney]

I am sizing a panelboard (120/208V, 3ph) that has the following loads:

Phase A 25.5 kVA
Phase B 29.9 kVA
Phase C 23.4 kVA

. . .

The other problem is I only have access to 200A breakers to feed this.

Then you are out of luck, as the Phase B load is 29.9 kVA / 120V = 249A.

Cheers, Wayne

 

[d0nut]

I do have face to face conversations with them. Sometimes I like other opinions as it helps me. I did not know there was an experience-criteria for asking a "good question". I know you did not mean offense, but this was pretty discouraging for me to hear and likely will not utilize this platform again.

My comment was not intended to discourage posting your questions to this forum. It is a great resource to tap into a vast wealth of knowledge. I was not trying to put down your questions or get into a credential comparing contest. I know nothing about you personally, and was only drawing some conclusions based on your postings. My intent was to make sure you had access to the best avenues for training and discussion as you gain knowledge in this industry. I didn't want you to discount the value of talking to the mentors and resources available at your employer in favor of posting the questions to this forum.

Keep posting and discussing the same concepts with others in your company and you will go far.

 

[winnie]

The panel needs to supply the total load, so very roughly you use the _total_ kVA that the panel is powering.

However your description is of unbalanced loading, and since one generally selects breakers and conductors assuming balance (usually!) then it is probably safest to take the largest loaded phase * 3 rather than adding up the sums of the phases.

Thank you. Yeah to me, sizing it across 3 phases does not make sense as for 3 phase loads, you account for the load 3 times. Can't find anything in NFPA 70.

This suggests that you are not adding up your loads properly. If you have a three phase load, then only 1/3 of its kVA gets assigned to each phase. Similarly for a line-line single phase load you assign 1/2 of its kVA to each connected phase. That 12 kVA 3 phase load? Count it as 4kVA on each of A, B, and C.

This is a rough and ready approximation that doesn't properly account for the different phase angles of the various connected loads, but is close enough if the various loads are reasonably balanced across the phases.

-Jon