Equipment grounding conductors for ac systems, where used, should be run with the conductors of each circuit per NEC Section 250-57(b).

Earth and the structural metal frame of a building may be used for supplemental equipment bonding, but they shall notbe used as the sole equipment grounding conductor for ac systems perNEC Sections 250-58 and 250-51.

For circuits having paralleled conductors in multiple metal raceways, an equipment grounding conductor shall be run in each raceway. The size of each paralleled equipment grounding conductor is a function of the rating of the circuit overcurrent protection. (See NEC Sections 250-95 and 310-4.)

4.10.1 SIZING THE EQUIPMENT GROUNDING CONDUCTOR

NEC Section 250-95 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:

1. NEC Table 250-95 shall be used to size the equipment grounding conductor.

2. When conductors are run in parallel in more than one raceway, the equipment grounding conductor is also run in parallel.

3. 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.

4. 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.

For example: What size THWN copper equipment grounding conductor is required to be run in a raceway with a 70 A overcurrent protection device protecting the circuit?

Step 1:Finding EGC-NEC Table

250-95 70 A OCPD requires #8 copper

Answer: The equipment grounding conductor is required to be at least #8 THWN copper.

4.10.2 SEPERATE EQUIPMENT GROUNDING CONDUCTORS

The possibility of worker exposure to electric shock can be reduced by the use of seperate equipment grounding conductors within raceways.

The seperate equipment grounding conductors contribute to equalizing the potential between exposed noncurrent-carrying metal parts of the electrical system and adjacent grounded building steel when ground faults occur. The resistance (inductive reactance) of the ground fault circuit normally prevents a significant amount of ground fault current from flowing through the seperate equipment grounding conductors.

Ground fault current flows through the path that provides the lowest ground fault circuit impedance. Fittings and raceway systems have been found that are not tightly connected or are corroded which prevents good continuity. Therefore, the equipment grounding conductor shall be the path for the fault current to travel over and clear the overcurrent protection device protecting the circuit.

NEC Sections 300-3(b) and 250-57(b) require the equipment grounding conductors to be routed in the same raceway, cable, cord, etc., as the circuit conductors. All raceway systems should be supplemented with seperate equipment grounding conductors.

Note: The equipment grounding conductor shall be routed with supply conductors back to the source. Additional equipment grounding may be made to nearby grounded structural members or to grounding grids, but this shall not take the place of the co-routed equipment grounding conductors. Raceway systems should not be used as the grounding conductor.