Exceptions to NEC 250-94

DOE-HDBK-1092-98

4.7.2 EXCEPTIONS TO NEC 250-94

There is an exception to the main rule. It has three parts and pertains to specific types of grounding

electrodes. The exception applies to grounded and ungrounded systems.

Exception (a) applies to made electrodes only, such as rod, pipe, or plate electrodes. The grounding

electrode conductor is not required to be larger than #6 copper or #4 aluminum.

Exception (b) to NEC Section 250-94 requires at least a #4 copper conductor to be used as a

grounding electrode conductor to ground the electrical system to a concrete-encased electrode.

Exception (c) requires at least a #2 copper conductor to be used as a grounding electrode conductor

to ground the electrical system to a ground ring. [See NEC Section 250-81(d)]

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 not a complete circuit for fault current and is an invitation

to a dangerous situation.

The main bonding jumper shall connect together the following items:

1. Grounded conductors and grounded terminal

2. Equipment grounding conductors and grounding terminal

3. All metal enclosures enclosing conductors and components.

If supplied, the manufacturers main bonding jumper is the preferred conductor to be used as the main

bonding jumper. NEC Section 250-79(a) requires the main bonding jumper to be a (1) wire, (2)

screw, (3) busbar, or (4) a similar suitable conductor.

NEC Section 250-79(d) 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?

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