Grounding A Separately Derived System

DOE-HDBK-1092-2004

4.12 GROUNDING A SEPARATELY DERIVED SYSTEM

NEC 250.30 covers the rules for grounding separately derived systems. The system grounding

conductor for a separately derived system shall be grounded at only one point. That single

system grounding point is at the source of the separately derived system and ahead of any

system disconnecting means or overcurrent devices. Where the main system disconnecting

means is adjacent to the generator, converter, or transformer supplying a separately derived

system, the grounding connection to the system grounded conductor can be made at or ahead

of the system disconnecting means.

The preferred grounding electrode for a separately derived system is the nearest effectively

grounded structural metal member of the building or the nearest effectively grounded water

pipe. If neither is available, concrete-encased electrodes or made electrodes are permitted.

In a grounded, separately derived system, the equipment grounding conductors shall be bonded

to the system-grounded conductor and to the grounding electrode at or ahead of the main

system disconnecting means or overcurrent protection device. The equipment grounding

conductor should always be connected to the enclosure of the supply transformer, generator, or

converter.

The grounding electrode conductor, the main bonding jumper, the grounded conductor, and the

equipment grounding conductor are calculated, sized, and selected by the rules listed in

Sections 4.7 through 4.10. (See Figure 4-7.)

Figure 4-7. The grounded (neutral) conductor can be used to carry both normal neutral

current and abnormal ground fault current.

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