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)
Service exceeding 1100 kcmil - NEC Table 250.66, 2400 kcmil x 0.125 = 300
The grounded conductor is required to be at least a #300 kcmil, THWN copper
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 12½ 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