Main Bonding Jumper

DOE-HDBK-1092-2004

4.8

MAIN BONDING JUMPER

The primary function of the main bonding jumper is to connect the grounded circuit conductors

and the equipment grounding conductors at the service equipment. The main bonding jumper

serves as the main link between the system grounded conductors and the grounding electrode

system where metal equipment enclosures and raceways are utilized to enclose conductors and

components. If the main bonding jumper is left out, there is no complete circuit for fault current,

which poses a potentially dangerous situation.

The main bonding jumper shall connect together the following items:

Grounded conductors and grounded terminal

Equipment grounding conductors and grounding terminal

All metal enclosures enclosing conductors and components.

If supplied, the manufacturer's main bonding jumper is the preferred conductor to be used as

the main bonding jumper. NEC requires the main bonding jumper to be a (1) wire, (2) screw, (3)

bus bar, or (4) a similar suitable conductor.

NEC requires the main bonding jumper to be at least the same size as the grounding electrode

conductor where the circular mils rating of the service entrance conductors does not exceed

1100 kcmil for copper or 1750 kcmil for aluminum.

For example: What size main bonding jumper is required to ground the metal enclosure of the

service equipment to the grounding terminal bar where the service entrance is made up of one

#250 kcmil, THWN copper conductor per phase?

Step 1:

Finding the largest phase -- NEC 250.28 #250 kcmil is the largest phase

Step 2:

Finding the bonding jumper -- Table 250.66 #250 kcmil requires #2 copper

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

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 250.28, 2400 kcmil x 0.125 = 300 kcmil

Step 2:

Finding the main bonding jumper -- NEC Table 250.66, 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 250.66 based upon the 2400

kcmil copper conductors.

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