Simple calculation question

[Mustwin351]

I believe the answer in my workbook is incorrect so I just wanted verification.

Question is:

what size service and service conductors for a multi family dwelling (paralleled in two raceways)?

load calculation comes out to 575 amps

answer sheet says 600 amp service which is correct but says the service conductors should be 350 kcmil at 310 amps. (2 conductors X 310=620 amps)

Shouldnt the service conductors be 300 kcmil rated at 285 amps because paralleled you get an equivalent of 570 amps which the next breaker size up is 600?

so couldn’t you use 300 kcmil conductors as opposed to 350 kcmil?

 

[infinity]

Although you can up to the next standard size of 600 amps the load calculation cannot exceed the conductor ampacity which at 575 exceeds 570 amps.

 

[Mustwin351]

Although you can up to the next standard size of 600 amps the load calculation cannot exceed the conductor ampacity which at 575 exceeds 570 amps.

got it! I see it now article 220.40. Thanks Rob!

 

[Mustwin351]

What about a multifamily dwelling (two dwelling) with a load calculation (using optional method) of 53,325 VA (240V) = 222A ?

The service size would be 225A but I thought the conductors were sized at 83% of the service rating for a service between 100-400A. 310.15(B)(7)

The book gives an answer of 4/O rated at 230A.

Why is the 83% rule not used and the conductors at 3/O and rated at 200 A

 

[david luchini]

The service size would be 225A but I thought the conductors were sized at 83% of the service rating for a service between 100-400A. 310.15(B)(7)

The book gives an answer of 4/O rated at 230A.

310.15(B)(7) applies to the feeders for the individual units of a two dwellings, not to the combined load.

 

[Mustwin351]

310.15(B)(7) applies to the feeders for the individual units of a two dwellings, not to the combined load.

thank you.