Grounding a Generator

Whether the generator is portable of fixed-in-place, and whether its for life safety or optional standby, it must be grounded per the National Electrical Code (NEC). The NEC identifies that there are two methods of grounding:

1. Connect a grounding electrode conductor from the genset to the ATS (automatic transfer switch), which is connected to the panelboard / main service grounding electrode conductor.
2. Provide the genset with its own grounding electrode conductor.

Method #1 above is referred to as a Non-Separately Derived System and is defined by NEC 702.10(B). In this type of installation the equipment grounding (bonding) conductor must be bonded to the grounded electrode system, which is typically the main distribution panel/switchboard. Also, in this type of scenario, a 3-Pole ATS is required. Refer to the diagram below for further clarification:

Method #2 is called a Separately Derived System and is defined by NEC 702.10(A). In this type of installation, the generator must comply with the normal grounding electrode requirements outlined in NEC 250.30. This is due to the fact that where the generator operates as a separately derived system and the transfer switch interrupts all conductors, including the grounded circuit conductor, the generator must comply with normal grounding electrode requirements. Also, in this type of installation when the genset is active (utility power is off), the neutral is derived from the generator ground, not from the main distribution ground as it is during normal power. Due to this, an additional pole is required in the transfer switch, thus the separately derived system requires a 4-Pole ATS. See the example below:

It should be noted that MOST genset systems in commercial and industrial applications are separately derived systems. Usually, the only time that nonseparately derived systems occur is in residential or portable generators.

One final note: A sign must be posted at the location of the remote grounding connection identifying all emergency and normal sources of power connected at that location in compliance with NEC 700.8(B), 701.9(B) and 702.8(B). This refers to cases where the grounded circuit conductor from an emergency source is connected to a grounding electrode conductor anywhere except at that source.

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