The main purpose of the grounded conductor is to carry unbalanced neutral current or fault current in the event that one phase should go to ground.

Note: The grounded conductor does not always have to be a neutral conductor. It is a phase conductor when used in a corner grounded delta system.

In solidly grounded service-supplied systems, the equipment grounding conductors shall be bonded to the system-grounded conductor and the grounding electrode conductor at the service equipment. The grounded conductor may be used to ground the noncurrent-carrying metal parts of equipment on the supply side of the service disconnecting means per NEC Section 250-61(a). The grounded conductor can also serve as the ground-fault current return path from the service equipment to the transformer that supplies the service.

The grounded conductor shall not be used to ground the metal parts of enclosures enclosing conductors and components on the load side of the service per NEC Section 250-61(b). See NEC

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 serviceentrance 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.)

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.

Step 1:Service exceeding 1100 kcmil-NEC Table 250-94 2400 kcmil x 0.125 = 300 kcmil

Answer: The grounded conductor is required to be at least a #300 kcmil, THWN copper conductor.

This method is used where the service entrance conductors are over 1100-kcmil copper or 1750-kcmil aluminum. NEC Table 250-94 cannot be used for sizing the grounded conductor. The grounded conductor is required to be not less than 12 1/2% of the cross-sectional area of the largest phase conductor.

Note: NEC Table 250-94 is used only if the service conductors are rated less than 1100 kcmil for copper or 1750 kcmil for aluminum.