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This article was posted 06/30/2006 and is most likely outdated.

Grounding vs Bonding: 250.118 Types of Equipment Grounding (Bonding) Conductors

June 30, 2006

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PART VI.

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250.118 Types of Equipment Grounding (Bonding) Conductors.

The equipment grounding (bonding) conductor, which serves as the effective ground-fault current path to the source, must be one or a combination of the following: Figure 250–159

(1) A bare or insulated conductor.

Author’s Comment: The equipment grounding (bonding) conductor can be copper or aluminum and must be sized in accordance with 250.122.

To ensure that it has a low-impedance path, the equipment grounding (bonding) conductors must be installed within the same raceway, cable, or trench with the circuit conductors in accordance with 250.134(B) [300.3(b), 300.5(I), and 300.20(A)].

(2) Rigid metal conduit.
(3) Intermediate metal conduit.
(4) Electrical metallic tubing.
(5) Listed flexible metal conduit meeting the following [348.60]: Figure 250–160

a. The conduit terminates in fittings listed for grounding.
b. The circuit conductors are protected by overcurrent devices rated 20A or less.
c. The combined length of the conduit in the same fault return path doesn’t exceed 6 ft. Figure 250–161
d. Where flexibility is necessary after installation, an equipment grounding (bonding) conductor must be installed in accordance with 250.102(E).

(6) Listed liquidtight flexible metal conduit meeting the following [350.60]: Figure 250–162

a. The conduit terminates in fittings listed for grounding.
b. For 3⁄8 in. through 1⁄2 in., the circuit conductors are protected by overcurrent devices rated 20A or less.
c. For 3⁄4 through 11⁄4 in., the circuit conductors are protected by overcurrent devices rated 60A or less.
d. The combined length of the conduit in the same ground return path doesn’t exceed 6 ft.
e. Where flexibility is necessary after installation, an equipment grounding (bonding) conductor must be installed in accordance with 250.10(E) regardless of the circuit rating or the length of the flexible metal conduit.

(8) Type AC cable as provided in 320.108.

Author’s Comment: Interlocked Type AC cable is manufactured with an internal bonding strip that is in direct contact with the interlocked metal armor. The combination of the bonding strip and the interlocked metal armor makes the cable suitable as an effective ground-fault current path [320.108]. Figure 250–163

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Figure 250–159
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Figure 250–160
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Figure 250–161
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Figure 250–162
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Figure 250–163
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Comments

Mike, very nice presentation in delineating the different conductor grounding methods of 250.118. Is there a possible answer defining the bonding characteristic values associated with the rated interfacing of different conductor types for surface electrical circuit conditions of Class C and Class L Bonds? The White Book does not define any requirements. Can you help in this area where dissimilar metals and plating would be better defined? I think the NEC does not set faying surface requirements either.

Ben Jacks
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