Equipment Grounding Conductor

DOE-HDBK-1092-2004

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)

Step 1:

Service exceeding 1100 kcmil - NEC Table 250.66, 2400 kcmil x 0.125 = 300

kcmil

Answer:

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

conductor.

Note: NEC Table 250.66 is used only if the service conductors are rated less than 1100 kcmil

for copper or 1750 kcmil for aluminum.

4.10 EQUIPMENT GROUNDING CONDUCTOR

Equipment grounding conductors for ac systems, where used, should be run with the

conductors of each circuit per NEC 250.119, and 250.134.

Earth and the structural metal frame of a building may be used for supplemental equipment

bonding, but they shall not be used as the sole equipment grounding conductor for ac systems.

For circuits having paralleled conductors in multiple metal raceways, an equipment grounding

conductor shall be run in each raceway. Each paralleled equipment grounding conductor must

be full size based on the circuit overcurrent protection. (See NEC 250.122)

4.10.1 SIZING THE EQUIPMENT GROUNDING CONDUCTOR

NEC 250.122 lists the requirements for calculating the size of the equipment grounding

conductors in an electrical circuit. There are basically five steps to be applied in sizing,

selecting, and routing the equipment grounding conductors:

This method is used where the service entrance conductors are over 1100 kcmil copper or

1750 kcmil aluminum. NEC Table 250.66 cannot be used for sizing the grounded conductor.

The grounded conductor is required to be not less than 12 percent of the cross-sectional area

of the largest phase conductor.

NEC Table 250.122 shall be used to size the equipment grounding conductor.

When conductors are run in parallel in more than one raceway, the equipment grounding

conductor is also run in parallel.

Where more than one circuit is installed in a single raceway, one equipment grounding

conductor may be installed in the raceway. However, it must be sized for the largest

overcurrent device protecting conductors in the raceway.

When conductors are adjusted in size to compensate for voltage drop, the equipment

grounding conductor shall also be adjusted in size.

5. The equipment grounding conductor is never required to be larger than the circuit

conductors.

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