Why Example D1(b) in Annex D in NEC is Calculated using Amperes Instead of VA

[Mike Lima]

In example D1(b) One-Family Dwelling, Informative Annex D Examples NEC, the calculated service rating = 110A Ungrounded and 84A Grounded. However, if I do the calculation using VA instead of Amperes, I would get a service rating = 101A Ungrounded and 77A Grounded . Why was the calculation in Example D1(b) done that way using Amperes a table format instead of the way I did it below?


Example D1(b) calculated using VA


Ungrounded Conductors

Amperes from Example D1(a) = 18720 VA
One 230-V air conditioner = 1440 VA
One 115-V air conditioner = 1440 VA
One 120-V dishwasher = 1200 VA
One 115-V disposer = 960 VA
25% of largest motor (see 430.24) (1200 x 0.25) = 360 VA
= 24120 VA

24120VA / 240V = 101A


Grounded Conductor

Amperes from Example D1(a) = 14550 VA
One 115-V air conditioner = 1440 VA
One 120-V dishwasher = 1200 VA
One 115-V disposer = 960 VA
25% of largest motor (see 430.24) (1440 x 0.25) = 360 VA
= 18510 VA

18510VA / 240V = 77A

 

[Dennis Alwon]

Not sure why they went to amperes but it should make no difference. They probably figured 1(a) did the calculation in va so to save space they went to amp.

I think you have some numbers that are incorrect-- 18,720 ??? should be 18,600 va. Also you combined the loads of phases A and B and they separated it out in a specific manner

 

[Mike Lima]

Not sure why they went to amperes but it should make no difference. They probably figured 1(a) did the calculation in va so to save space they went to amp.

I think you have some numbers that are incorrect-- 18,720 ??? should be 18,600 va. Also you combined the loads of phases A and B and they separated it out in a specific manner

Your Right. Sorry, 18,720 was a typo. So using 18,600VA:

Ungrounded Conductors

Amperes from Example D1(a) = 18600 VA
One 230-V air conditioner = 1440 VA
One 115-V air conditioner = 1440 VA
One 120-V dishwasher = 1200 VA
One 115-V disposer = 960 VA
25% of largest motor (see 430.24) (1200 x 0.25) = 360 VA
= 24000 VA

24000VA / 240V = 100A

 

[Smart $]

It was done in amperes because if you did it in VA the current would be wrong for the line-to-line loads. For example, the 230V air conditioner at 1440VA divided by 120V yields 12A... when it is only 6A.

You can do it in VA, but you have to use 720VA (i.e. 1440VA/2).

PS: In case you didn't realize it, it was done that way because the loads are unbalanced. The service needs to be designed to support the line conductor with the highest current.