National Electrical Installation Standards

Standards as High as Your Own

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Monday, October 9, 2017

Question:

What is the recommended practice of grounding and bonding a 480/277V generator that serves (2) 4 pole automatic transfer switches and 1 three pole automatic transfer switch? The three pole ATS is fed from a 480V to 208/120V transformer. In this case would it be permissible to locate the system bonding jumper at the generator? Do you foresee an issue with objectionable ground currents on the system? Thank you for your time.

Nick Castillo

A

Answer:

Hey Nick thanks for your question.  The NEC does not provide recommendations and sometimes there are choices. Alternating current systems must be grounded as stated in 250.20(B). The choice of transfer switches can determine whether the system is separately derived or not as that term is defined in Article 100.  Some prefer to switch the grounded conductor if one is used and others prefer to directly connect the grounded conductor from different systems together.

If the system (generator in your case) is a separately derived system a system bonding jumper must be installed at a single point as stated in 250.30(A)(1) and locating it at the generator is allowed. The system grounded conductor is not allowed to be "re-grounded" on the load side of the system bonding jumper as stated in 250.30(A). Systems installed this way reduce or eliminate objectionable current because they are only connected to ground once thereby not providing parallel paths for current.

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