Calculation question please confirm

[Matt Mckenzie]

Building has central heating unit of 150kw, 480volt, 3 phase fixed electric heat. What is minimum THHN branch circuit conductor required? I get 2/0 copper. Is this correct?

 

[texie]

Building has central heating unit of 150kw, 480volt, 3 phase fixed electric heat. What is minimum THHN branch circuit conductor required? I get 2/0 copper. Is this correct?

424.3(B) requires this load to be considered continuous load. That would require 4/0.

Edit to add-hope this is not a homework question as I shouldn't give a direct answer.

 

[augie47]

agree 4/0 and that's assuming the 150kw is evenly divided and there is no or very limited other load such as a blower.

 

[JoeStillman]

It looks like you picked 2/0 copper based on 90? ampacity. It is almost never possible to use the 90? column in table 310.16. See 110.14(C).

Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

The load is categorized as continuous in 424.3(B).

Branch-Circuit Sizing. Fixed electric space-heating equipment shall be considered continuous load.