Nardie

Member
I am reviewing a new panel schedule design that has a three-phase 208V 30A ckt breaker. From my understanding each leg can handle 24A with the 80% rule from the NEC, that is why we installed a 30A breaker. So what should the VA load be on the panel schedule for each phase? A single-phase calculation (at 2880 VA per phase) or three-phase calculation (at 8636 VA per phase) ? I should know this, but having a brain freeze. I think 2880 VA per phase. So any help to unlock my mind would be appreciated!

Xptpcrewx

Senior Member
I am reviewing a new panel schedule design that has a three-phase 208V 30A ckt breaker. From my understanding each leg can handle 24A with the 80% rule from the NEC, that is why we installed a 30A breaker. So what should the VA load be on the panel schedule for each phase? A single-phase calculation (at 2880 VA per phase) or three-phase calculation (at 8636 VA per phase) ? I should know this, but having a brain freeze. I think 2880 VA per phase. So any help to unlock my mind would be appreciated!
Three phase calculation at 2.88-kVA per phase. 8.64-kVA total.

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vw55

Member
Can you explain to me the "80% rule from the NEC" being referred to?

infinity

Moderator
Staff member
The 80% rule is the amount of continuous load that the OCPD can be loaded to. If it's non-continuous you can load the OCPD to 100%.

WA_Sparky

Member
Now you all are making me second guess my math...
How would it be 2.88 per phase?
I would have said its 2.88/3 seen per phase..

Correct me if I'm wrong..
If you have a 30A breaker, regardless of how many poles it is, it only protects the equipment up to the combined amperage seen across all the poles its connected to. Your 2.88kva should be you max across all three phases.
208V*30*0.8/1.732= 2.882 kva for the given breaker
each phase would see 960.73VA

iceworm

Curmudgeon still using printed IEEE Color Books
WA -
you don't want to divide by sqrt(3)
VA = V * I* sqrt(3)
VA = 208 * 24* 1.732 = 8646 VA

iceworm

Curmudgeon still using printed IEEE Color Books
WA -
get rid of "amps per leg", or "amps per phase". Those terms don't mean anything.

For example, when one says the current is 24A, this implies the current is fairly well balanced and average is 24A. An actual clamp reading might be A: 23A, B: 25A, C: 24A. yep, 24A

If the current is not balanced, Then list the currents, for example: A - 15A, B - 25A, C - 10A, N - 9A.

If you want to calculate the load single phase do that. You will end up with a VA by phase. Then calculate each phase amperage.
IF you want to calculate the 3ph load, do that. You end up with a 3ph VA. Calculate the 3ph current.

Note for later discussion: 208V single phase loads can get a bit tricky to account for the VA by phase

ron

Senior Member
If the loads can be derated in some fashion (based on demand) from the Article 220 calculation, then the VA total from the panel schedule may exceed the panel rating. It is the demand that can't exceed the panel rating.

vw55

Member
The 80% rule is the amount of continuous load that the OCPD can be loaded to. If it's non-continuous you can load the OCPD to 100%.
My understanding is that if your calculation includes 125% of all continuous load on the circuit + 100% of all non-continuous load on that same circuit, as long as your final load doesn't exceed the rating of the OCPD, you comply with the requirements. Now if the only load on the circuit is continuous, then, yes, you are only allowed to use 80% of the OCPD rating, but there is nothing that states that you can't load an OCPD up to only 80% of it's rating, correct?

charlie b

Moderator
Staff member
. . . but there is nothing that states that you can't load an OCPD up to only 80% of it's rating, correct?
I don't think you asked that in the way you intended. But the answer is that you can certainly load an OCPD to 100% of its rating, if all of that load is non-continuous.

charlie b

Moderator
Staff member
Would this be a good time to mention that not one of the previous responses has correctly answered the original question? :happyno:
So what should the VA load be on the panel schedule for each phase?
The correct answer is that each phase should be assigned one third of the total VA load of whatever components are powered by that circuit. If, for example, the circuit supplied a 150 gallon water heater for which the manufacturer called for a 30 amp circuit, and that drew 21 amps at full heating capacity, then the panel schedule would be assigned 2520 VA on each phase.

Isaiah

Senior Member
Would this be a good time to mention that not one of the previous responses has correctly answered the original question? :happyno: The correct answer is that each phase should be assigned one third of the total VA load of whatever components are powered by that circuit. If, for example, the circuit supplied a 150 gallon water heater for which the manufacturer called for a 30 amp circuit, and that drew 21 amps at full heating capacity, then the panel schedule would be assigned 2520 VA on each phase.
Hi Charlie, I have a similar scenario. I am checking a panel schedule, 480V three phase where they used 'Amps' instead of 'Voltamps' for the loading. For a 30kVA Xfmr (480V primary side) across all three phases, A, B and C they're showing 36A. In reality the loading should be 12A for each phase - is this correct?
Isaiah

charlie b

Moderator
Staff member
In reality the loading should be 12A for each phase - is this correct?
No it is not correct, for two reasons. First of all, the 36 amps that leaves the source on Phase A will return to the source on Phases B and C. Simply put, it's the same amps. You can't see 12 amps on each of the three phases and add them up to get a "total" of 36 amps. In this context, the word "total" has no meaning.

More importantly, however, it was not correct for the person creating the panel schedule to assign load in units of amps. It is not impossible for that method to end up with the correct result. But it is also not impossible (and is even more likely) that that method will end up with an incorrect result. If all loads are single phase or balanced three phase, I believe the final totals would be correct. But if you interject a few single phase 277 volt loads or some imbalanced three phase loads, all bets are off. The basic reason for this is (brace yourself!) "Amps plus Amps will not (necessarily) equal Amps." The three phases do not each reach their peak values at the same moment. For a balanced 3-phase system, when Phase A is at a positive peak, Phases B and C are both negative, both equal to each other, and half way back from (or heading towards) their negative peaks. Look on-line for a graph of three phase current, and you will be able to see what I am trying to describe.

On the other hand, KVA plus KVA does equal KVA. You can be confident in getting correct results.

Isaiah

Senior Member
No it is not correct, for two reasons. First of all, the 36 amps that leaves the source on Phase A will return to the source on Phases B and C. Simply put, it's the same amps. You can't see 12 amps on each of the three phases and add them up to get a "total" of 36 amps. In this context, the word "total" has no meaning.

More importantly, however, it was not correct for the person creating the panel schedule to assign load in units of amps. It is not impossible for that method to end up with the correct result. But it is also not impossible (and is even more likely) that that method will end up with an incorrect result. If all loads are single phase or balanced three phase, I believe the final totals would be correct. But if you interject a few single phase 277 volt loads or some imbalanced three phase loads, all bets are off. The basic reason for this is (brace yourself!) "Amps plus Amps will not (necessarily) equal Amps." The three phases do not each reach their peak values at the same moment. For a balanced 3-phase system, when Phase A is at a positive peak, Phases B and C are both negative, both equal to each other, and half way back from (or heading towards) their negative peaks. Look on-line for a graph of three phase current, and you will be able to see what I am trying to describe.

On the other hand, KVA plus KVA does equal KVA. You can be confident in getting correct results.
Perfect. Great explanation, I am going to push to have everything changed to VA for the reasons you have noted. So in other words, if the panel had been loaded using 1/3 of total VA, i.e. 36A x 480V = 17280/3 = 5760VA for each phase would be correct for this particular load?
thanks again Charlie.

charlie b

Moderator
Staff member
So in other words, if the panel had been loaded using 1/3 of total VA. . .
That part is correct.

. . . i.e. 36A x 480V = 17280/3 = 5760VA for each phase would be correct for this particular load?
That part is not correct. You need to be aware that anytime you do a calculation that involves three phases, the factor of "the square root of 3," or about 1.73, is going to come into the equation at some location.

For your example, VA = (A) times (V) times (1.73). So the total VA is 36 x 480 x 1.73, or about 29,900 VA. Each phase would be assigned one third of that value, or 9,967 VA.

Isaiah

Senior Member
That part is correct.

That part is not correct. You need to be aware that anytime you do a calculation that involves three phases, the factor of "the square root of 3," or about 1.73, is going to come into the equation at some location.

For your example, VA = (A) times (V) times (1.73). So the total VA is 36 x 480 x 1.73, or about 29,900 VA. Each phase would be assigned one third of that value, or 9,967 VA.
Got it - thanks Charlie