Code Calculations

Jun 23, 2004 2:17 PM, By Mike Holt

The grounding of the grounded (neutral) conductor to earth at service equipment is intended to help the utility limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines by shunting potentially dangerous energy into the earth. In addition, grounding of the grounded (neutral) conductor to earth helps the electric utility clear high-voltage ground faults when they occur. Because the grounded (neutral) service conductor is required to serve as the effective ground-fault current path, it shall be sized so that it can safely carry the maximum ground-fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. Do you know how to properly size this conductor? Let’s check your skills with an example problem.

Q. What's the minimum size grounded (neutral) service conductor required for a 400A, 3-phase, 480V service where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A?

A. You size the grounded (neutral) conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor [250.24(B)(1)]. In addition, you must make sure the grounded (neutral) conductors have the capacity to carry the maximum unbalanced neutral current in accordance with 220.22.

In this example, the unbalanced load requires the grounded (neutral) conductor to be 3 AWG, which is rated 100A at 75°C in accordance with Table 310.16. However, Table 250.66 requires the grounded (neutral) service conductor to be sized no smaller than 1/0 AWG to accommodate the maximum possible fault current likely to be imposed on it. Therefore, the correct answer is 1/0 AWG rather than 3 AWG.