Figure 4-6. The grounded (neutral) conductor is used to carry normal neutral current

DOE-HDBK-1092-2004

NEC 250.24(b)

Figure 4-6. The grounded (neutral) conductor is used to carry normal neutral current

or ground fault current in case a ground fault should develop on one of the

ungrounded (hot) phase conductors.

NEC 250.24 lists the rules for sizing the grounded conductor where it is not used as a grounded

neutral circuit. NEC gives the rules for calculating and sizing the grounded conductor when it is

used as a circuit conductor. The minimum size for the grounded conductor is computed as

follows:

The basic rule is to select the size directly from NEC Table 250.66 when the size of the

service-entrance conductors is not larger than 1100 kcmil copper or 1750 kcmil aluminum.

When the service entrance conductors are larger than 1100 kcmil copper or 1750 kcmil

aluminum, the grounded conductor shall be 12 percent of the largest phase conductor.

Where the service phase conductors are paralleled, the size of the grounded conductor

shall be based on the total cross-sectional area of the phase conductors.

For example: What size THWN copper grounded conductor is required for a service having a

total kcmil rating of 250 per phase? (All phase conductors are THWN copper)

Step 1:

Service less than 1100 kcmil - NEC Table 250.66, 250 kcmil requires #2

copper

Answer: The size of the grounded conductor is at least #2 THWN copper.

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