System with Grounded Conductor

DOE-HDBK-1092-98

Sections 250-50(a), (b), Ex. and 250-60 for exceptions to this basic rule. NEC Section 250-23(a)

requires the grounded conductor to be connected as follows:

1. The grounded conductor shall be connected to the grounded (neutral) service conductor.

2. The connection shall be at an accessible point.

3. That accessible point can be anywhere from the load end of the service drop or service lateral

to and including the neutral bar in the service disconnecting means or service switchboard.

The NEC allows the grounded conductor to be terminated and connected to ground at a multitude of

locations on the supply side of the service equipment. These locations are as follows:

1. Service equipment

2. Meter base

3. Current transformer (CT) can

4. Metal gutter or wireway containing service entrance conductors.

See Figure 4-6 for the rules concerning the use of the grounded conductor.

NEC Section 250-23(b) lists the rules for sizing the grounded conductor where it is not used as a

grounded neutral circuit. NEC Section 220-22 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:

1. The basic rule is to select the size directly from Table 250-94 when the size of the service-

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

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

aluminum, the grounded conductor shall be 12 1/2% of the largest phase conductor.

3. 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 250-94 250 kcmil requires #2 copper

Answer:

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

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

a total kcmil rating of 2400 per phase? (All conductors are THWN copper.)

4-12