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Grounding versus Bonding

PART VI.

Sections

250.122 Sizing Equipment Grounding (Bonding) Conductor Continued

(B) Increased in Size. When ungrounded circuit conductors are increased in size for any reason, the equipment grounding (bonding) conductor must be proportionately increased in size.

Author’s Comment: Ungrounded conductors could be increased in size to accommodate voltage drop, because of excessive heating from harmonic currents, fault-current studies, or future capacity.

Question: If the ungrounded conductors for a 40A circuit are increased in size from 8 AWG to 6 AWG, the equipment grounding (bonding) conductor must be increased in size from 10 AWG to _____. Figure 250–170

(a) 10 AWG     (b) 8 AWG     (c) 6 AWG     (d) 4 AWG

Answer: (b) 8 AWG

The circular mil area of 6 AWG is 59 percent greater than 8 AWG (26,240 cmil/16,510 cmil) [Chapter 9, Table 8].
The equipment grounding (bonding) conductor for a 40A protection device can be 10 AWG (10,380 cmil) [Table 250.122], but it must be increased in size by a multiplier of 1.59.

Conductor Size = 10,380 cmil x 1.59
Conductor Size = 16,504 cmil
Conductor Size = 8 AWG, Chapter 9, Table 8
(C) Multiple Circuits. When multiple circuits are installed in the same raceway or cable, only one equipment grounding (bonding) conductor is required. This conductor must be sized in accordance with Table 250.122, based on the largest overcurrent device protecting the circuit conductors. Figure 250–171

(F) Parallel Runs. When circuit conductors are run in parallel [310.4], an equipment grounding (bonding) conductor must be installed with each parallel conductor set and it must be sized in accordance with (1) or (2).

(1) Based on the ampere rating of the circuit-protection device in accordance with Table 250.122. Figure 250–172

(2) Based on the ampere rating of the ground-fault protection in accordance with Table 250.122 where ground-fault protection of equipment is installed if: Figure 250–173

(1) Maintenance and supervision ensure that only qualified persons will service the installation.

(2) Ground-fault protection is set to trip at not more than the ampacity of a single ungrounded conductor.

(G) Feeder Tap Conductors. Equipment grounding (bonding) conductors for feeder taps must be sized in accordance with Table 250.122, based on the ampere rating of the circuit-protection device ahead of the feeder, but in no case is it required to be larger than the circuit conductors. Figure 250–174


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