NEC in the Facility, August 9, 2010

Annex D3(a) shows you how to solve this problem, beginning with how to determine the calculated load

As we noted in our previous issue, Annex D3(a) begins with a description of an industrial multi-building application case. Then, it poses this problem: “Determine the overcurrent protection and conductor size for the feeders in the common raceway, assuming the use of XXHW-2 insulation (90°C).”

Annex D3(a) shows you how to solve this problem, beginning with how to determine the calculated load. Then, using the values you determined for the noncontinuous and continuous loads, you determine the total VA. Convert this figure to 3-phase AC current, and you have the minimum size overcurrent protective device.

Next, Annex D3(a) illustrates how to determine the feeder conductor sizes. You do some straightforward calculations, using numbers from the 75°C column of Table 310.16. However, there is a twist. Per 310.15(B)(4)(c), you must count the neutral conductors as current-carrying conductors because the discharge “has a substantial nonlinear content.”

This brings us to an even trickier twist. The example says to use the 90°C column, not the 75°C column, for this neutral conductor. Why would that be?

If you go back to the application case description, you see that 150 ft of the raceway for this circuit passes through an access corridor along with process steam. So, as Annex D3(a) shows, you select the conductors for that portion of the run from the 90°C column to satisfy worst case conditions. Why not run the entire circuit based on 90°C column? Because, as stated in the description, all equipment terminations are rated at 75°C (see 110-14(C) for why this matters).

As you work through Annex D3(a), you see that some common assumptions about conductor sizing aren’t true.

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