3 phase main breaker calculation question

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rjn72

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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
 
rjn72 said:
I think we should take the highest leg of the unbalanced load and that would determine our service size.
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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
 
rjn72 said:
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
Click to expand...
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.
 
Thank you gentlemen. After a good nights sleep it all made better sense this morning.
 
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