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Grounding versus Bonding



250.30 Grounding and Bonding of Separately Derived AC Systems.

Author's Comment: A separately derived system is a premises wiring system with no direct electrical connection to conductors originating from another system [Article 100 definition and 250.20(D)].

All transformers, except autotransformers, are separately derived because the primary circuit conductors do not have any direct electrical connection to the secondary circuit conductors. Figure 250.67

Generators that supply a transfer switch that opens the grounded neutral conductor would be considered separately derived [250.20(D) FPN 1]. Figure 250.68
(A) Grounded Systems. Separately derived systems must be system bonded and grounded in accordance with the following:

A neutral-to-case bond must not be on the load side of the system bonding jumper, except as permitted by 250.142(B).

(1) System Bonding Jumper. Bonding the metal parts of the separately derived system to the secondary grounded neutral terminal by the installation of a system bonding jumper ensures that dangerous voltage from a secondary ground fault can be quickly removed by opening the secondary circuit's overcurrent protection device [250.2(A)(3)]. Figure 250.69

DANGER: During a ground fault, metal parts of electrical equipment, as well as metal piping and structural steel, will become and remain energized providing the potential for electric shock and fire if the system bonding jumper is not installed. Figure 250.70
The system bonding jumper must be sized in accordance with Table 250.66, based on the area of the largest ungrounded secondary conductor [250.28(D)].

Question: What size system bonding jumper is required for a 45 kVA transformer, where the secondary conductors are 3/0 AWG? Figure 250.71

(a) 4 AWG     (b) 3 AWG     (c) 2 AWG     (d) 1 AWG

Answer: (a) 4 AWG, Table 250.66

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©2005 Mike Holt Enterprises, Inc.