3 phase main breaker calculation question

[rjn72]

Its been a really long day guys and I am second guessing an engineers calculations. He has phase a 17864 watts, phase b 30119 watts, and phase c 30214 watts. For his load calc he added all three together for 86460 watts and calculated out to 240 amps necassary and is now calling for a 400 amp where there is an existing 200 amp service.

I think we should take the highest leg of the unbalanced load and that would determine our service size.

Am I crazy? or just really tired? 120/208 3 phase service

 

[wwhitney]

I think we should take the highest leg of the unbalanced load and that would determine our service size.

Sure, unless you could rebalance the 86460 VA to be equally split on all 3 legs.

But using the unbalanced loading you listed, the most loaded leg is 30214 VA, which is 30214 VA /120V = 252A. So not a huge difference from the 240A figure, and not a different service size.

Cheers, Wayne

 

[kwired]

Its been a really long day guys and I am second guessing an engineers calculations. He has phase a 17864 watts, phase b 30119 watts, and phase c 30214 watts. For his load calc he added all three together for 86460 watts and calculated out to 240 amps necassary and is now calling for a 400 amp where there is an existing 200 amp service.

I think we should take the highest leg of the unbalanced load and that would determine our service size.

Am I crazy? or just really tired? 120/208 3 phase service

Which gives you about 252 amps on the highest loaded line.

You will be installing gear with 400 amp bussing even if you only put a 300 amp OCPD on it. So why not go with a little bigger conductor and get the full 400 amp capacity of the base gear components? will allow for some future expansion as well.

 

[rjn72]

Thank you gentlemen. After a good nights sleep it all made better sense this morning.