The main purpose of the grounding electrode conductor (GEC) is to connect the electrical system to earth ground. The GEC actually provides three grounding paths to the grounding electrode system. They are as follows:

1. The grounded conductor path

2. The equipment grounding path

3. The bonding path.

In grounded systems, the GEC connects to the neutral bar in the service equipment enclosure. In ungrounded systems, the GEC connects to the grounding terminal bar. It grounds the following items to the grounding electrode system:

1. The grounded conductor, if present

2. The equipment grounding conductor, if present

3. The metal of conduits, if present

4. The metal of enclosures, if present

5. The bonding jumpers bonding together metal enclosures and conduits

6. The metal enclosure of the service equipment.

4.7.1 SIZING THE GROUNDING ELECTRODE CONDUCTOR

NEC Section 250-94 requires the grounding electrode conductor to be sized by the circular mils rating of the largest service entrance conductor or conductors and selected from NEC Table 250-94 based on these values.

For example, the size of the service entrance conductors from a delta, three-phase, four-wire midpoint tap is #250 kcmil, THWN copper for phases A and C, #2/0 for phase B, and #1/0 for the neutral. What size copper GEC is required to ground this system to a metal water pipe?

Step 1:Finding the largest phase-NEC 250-94 #250 kcmil is the largest phase

Step 2:Finding the size GEC-NEC Table 250-94 #250 kcmil requires #2 cu

Answer: The size of grounding electrode conductor (GEC) is at least #2 copper.

Note: NEC Table 250-94 is used to size the grounding electrode conductor for both grounded and ungrounded systems. The table is used where the grounding electrode conductor is connected to a metal water pipe or the metal frame of building steel.

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?

Step 1: Step 2: Answer:

Finding the largest phase-NEC 250-79(d) #250 kcmil is the largest phase Finding the bonding jumper-Table 250-94 #250 kcmil requires #2 copper 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 121/210 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.