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Sizing Feeders, Part 3

One reason Annex D3(a) has a more complicated example of feeder sizing is the circuit is being divided up into different runs with different temperature requirements. But another reason is the neutral calculations are complicated by the fact that the load isn't calculated on the basis of VA per square meter or square foot. Thus, 210.11(B) does not apply and we cannot assume the load is evenly distributed. Before we look at the implications of this, let's see how dividing up the circuit affects the feeder sizing for the ungrounded conductors.

First of all, dividing the circuit this way means you essentially have two sets of feeder conductors supplied by one breaker but you're treating each set as if it's a different circuit. Each will meet its own set of requirements for the conditions it's in. Because the one set has terminations, its temperature rating is limited by the ratings of its terminations. Because the other set has its terminations outside the high temperature area, its temperature rating isn't strictly limited by that of its terminations.

The upshot for the high temperature set is that, instead of using a 3/0 conductor from the 60°C column, you can use a 1/0 conductor from the 75°C column (Table 310.15(B)(16). That's a significant savings labor and materials.

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