new panel calculations

[mugbug101]

I have a panel that has been installed, and its a 3phase 480v with a 250A main breaker.

I did a VA calculation of all the breakers that I'm going to be putting in it. I would like to know if I did this right.

4-60amp three pole breakers
3-50amp three pole breakers
2-20amp three pole breakers

so I use VA=(number of breakers)times(amps)times(80%)times(1.732)

(4)times(60amps)times(80%)times(1.732)=159,621.12VA
(3)times(50amps)times(80%)times(1.732)=99763.2VA
(2)times(20amps)times(80%)times(1.732)=26603.52VA

(159621.12VA)plus(99763.2VA)plus(26603.52VA)=285987.84VA

(285987.84VA)divided(480V)divided(1.732)=32amps

So does that mean that is have 32amps left over on the 250amp 3phase 480V main panel breaker?

 

[augie47]

I can't say I really follow your calculations, but breaker sizes are not very relevant when determining load primarily do to load diversity.
If your (4) 60s were fully loaded (allowed) then you 250 amp panel would be at 96% of its capacity.
Actual load calculations, not breaker sizes, is the only way to determine the panel load.

 

[JFletcher]

Welcome to the forum. Your math is correct for 3ph power: VA = √3 × A × V_L-L, so 4 60A 480V breakers loaded to 80% would yield the following:

VA = 1.732 x 4 (60A)(.8) x 480V = 159,621VA. I'm dropping the .12 due to rounding error of approximating √3 as 1.732 and significant digits. Your second and third calculations of 99,763VA and 26,603VA are also correct. Adding them all together yields 285,987VA

285,987VA /(480V) = 595.8, which then divided by 1.732 = 344A. Your last answer of 32A is actually what the 2 20A breakers loaded to 80% would use. Not sure how you arrived with a 32A remainder from 250A, or 218A load.

As stated above, breaker size is largely useless for figuring panel loads. If all of your breakers were simultaneously, continuously loaded 80%, such as a warehouse or parking lot lighting panel, you would be 94A above (344-250) what the 250A main breaker could carry. Motors have much larger breakers than what their FLA rating is to allow for starts w/o tripping. Some circuits may be loaded 100% for short times, others could be completely off or at a very low % load.

An average residential panel is 200A, may have nearly 1000A worth of breakers in it. but run an average load of 90A. You would need to do a load calculation to determine the load on the panel.