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How do you properly ground a portable electric generator (genset)?

Hi Dembee,

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.

Best regards,

The Engineering Team at E&S Grounding Solutions

3 Responses

  1. Phillip Stumpf

    Your original reply “Any generator weather it is portable of fixed-in-place must be grounded per the National Electrical Code (NEC)” is not accurate. According to article 250.34(a) if a portable generator meets both criteria, it “shall not be required to be connected to a grounding electrode.

  2. David Carlton

    How do you ground a 5500 watt generator? Can I use the ground in the #10/4 wire in the 4-prong twist lock or do I need to use a copper 5/8″x8′ foot rod driven into the ground?

    1. Hi David,

      Please refer to National electrical Code (2011) Article 250.34 & 205.35 for the complete text. In summary, if the generator is permanently installed and is a Separately Derived System (as is probably the case), then yes you must install an electrode system in compliance with 250.30. If it is a portable (can be carried by personnel) or vehicle mounted, it may or may not need an electrode system given certain conditions. See 250.34 for more details.

      We hope this helps,
      The Engineering Team at E&S Grounding Solutions

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