Commercial Load Calculations, Part 5

In our previous examination of Example D3 in the NEC, we looked at how many circuits you’d probably need in order to carry the 44A lighting load. You’d need at least four 15A breakers.

How would you size the conductor and breaker for the feeder? To work this sample problem, you first find a conductor with sufficient ampacity to carry the 177A load.

As noted earlier in Example D3, you can use the 90 DegrC column of Table 310.15(B)(16). But why is this true? After all, the terminations are rated at only 75 DegrC, thus limiting you to the 75 DegrC column [110.14(C)].

But 110.14(C) also says you can use conductors with higher temperature ratings than the ratings of the terminations for ampacity adjustment or correction (or both). Using that column, you find you need a 2/0 AWG conductor.

An over current protective device (OCPD) must always provide over current protection for the feeders based on the ampacity of the feeder [240.4]. The one exception to this is the feeder for a fire pump [695.4].

Apply the temperature adjustment to the feeder, and you get 131A. Since that’s not a standard OCPD size, you use the next size up [240.6(A)]. That means your OCPD needs to be 150A.

The last section of Example D3 deals with the feeder neutral conductor. Example D3 presents several issues to address. In our next part of this series, we will look at what those are and why they are solved the way they are.

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