Thank you for your question regarding proper grounding for portable electric generators (genset). Any generator weather it is portable of fixed-in-place must be grounded per the National Electrical Code (NEC). Specifically, NEC Article 702.10 requires that (A) Separately Derived Systems must be grounded in accordance with NEC Article 250.30, and (B) Non-Separately Derived Systems shall be bonded to the system grounding electrode. The NEC identifies that there are two methods of grounding:
1. For Separately Derived Systems the genset must have its own grounding electrode conductor.
2. For Non-Separately Derived Systems the genset must have a grounding electrode conductor from the genset to the ATS (automatic transfer switch), which is in turn connected to the main service (panelboard) grounding electrode conductor.
Method #1 is most common for commercial and industrial applications, called a Separately Derived System and is defined by NEC 702.10(A), which states that the generator must comply with the normal grounding requirements found in NEC 250.30. This is due to the fact that the transfer switch of a separately derived generator interrupts all conductors, including the grounded circuit conductor. Also, in this type of installation the neutral is derived from the generator ground, not from the main distribution ground as it is during normal power. This is why an additional pole is required in the transfer switch (4-pole ATS) for these systems.
Method #2 is most common for residential or portable generators, called a Non-Separately Derived System and is defined by NEC 702.10(B), which states that the equipment grounding (bonding) conductor must be bonded to the existing main electrical panel grounded electrode system. This is why only a 3-Pole ATS is required.
Also of note is that posted signage is required under NEC Articles 700.8(B), 701.9(B) and 702.8(B) identifying all emergency and normal sources of power connected at that location, including cases where the grounding conductor from an emergency genset is connected to a remote grounding source.
We hope you find this information useful. If you should have any further questions, please do not hesitate to call us at anytime.
The Engineering Team at E&S Grounding Solutions