Systemwith Grounded Conductor

DOE-HDBK-1092-98

Step 1:

Finding the largest phase--NEC 250-79(d) #250 kcmil is the largest phase

Step 2:

Finding the bonding jumper--Table 250-94 #250 kcmil requires #2 copper

Answer:

The size of the main bonding jumper (GEC) is at least #2 copper.

Where the kcmil rating of the service entrance conductors exceeds 1100 kcmil for copper or 1750

for aluminum, the bonding jumper shall be at least 12 l/2% of the largest kcmil rating of any one

phase.

For example: What size main copper bonding jumper is required for a service entrance with a makeup

of 2400 kcmil copper conductors per phase?

Step 1:

Finding the largest phase--NEC 250-79(d) 2400 kcmil x 0.125 = 300 kcmil

Step 2:

Finding the main bonding jumper--NEC Table 250-94; Table 8, Ch. 9 requires

300 kcmil

Answer:

The main bonding jumper is required to be at least 300 kcmil copper.

Note: In this case the main bonding jumper is greater in size than the grounding electrode conductor,

which is only required to be #3/0 copper per NEC Table 250-94 based upon the 2400 kcmil copper

conductors.

4.9 SYSTEM WITH GROUNDED CONDUCTOR

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

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