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277/480V Systems | Special Bonding Requirements?

This forum is for all things grounding and bonding. Please include the edition of the NEC you are referring too in your post for the most accurate answers.

*All MOD responses are based on the 2023 National Electrical Code

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Greetings Sparky Hub Family,

We get loads of questions about grounding and bonding, and one of them is always about the bonding of metallic raceways or cables enclosing feeders and branch circuits at voltages such as 277V and 480V. It seems that folks want to do some "Extra" bonding but are not 100% sure of how to make it happen and be code compliant. Well, let us look at the rule that pertains to this topic and set the record straight for all those inquiring minds.
250.97 Bonding for Over 250 Volts to Ground.

For circuits of over 250 volts to ground, the electrical continuity of metal raceways and cables with metal sheaths that contain any conductor other than service conductors shall be ensured by one or more of the methods specified for services in 250.92(B), except for (B)(1).

Exception: If oversized, concentric, or eccentric knockouts are not encountered, or if a box or enclosure with concentric or eccentric knockouts is listed to provide a reliable bonding connection, the following methods shall be permitted:

(1) Threadless couplings and connectors for cables with metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate metal conduit, one inside and one outside of boxes and cabinets
(3) Fittings with shoulders that seat tightly against the box or cabinet, such as electrical metallic tubing connectors, flexible metal conduit connectors, and cable connectors, with one locknut on the inside of boxes and cabinets
(4) Listed fittings
Firstly, you have to establish if you have "Over 250 volts to ground" for 250.97 to even be applicable. Clearly, 277V is 277V to Ground, and 480V is 480V Phase to Phase but also Phase to Ground, at least in the normal sense. So, the rules in 250.97 most certainly will apply.

Now, on 120V and 240V systems, this rule obviously doesn't apply when it comes to the "enhanced" requirements found in 250.97. However, you are still required to bond your raceways and cables, and that is achieved through the listed fittings, installation practice, and their raceways or cables of course, also with any wire-type bonding jumpers where necessary to maintain that bonding reliability demanded by 250.4.

So, if you happen to be working on a system with "Over 250V to Ground", like the 277V and 480V grounded systems as depicted in our discussion then you will need to meet all the requirements of 250.97. If you are still unclear on the rules then how about watching our video on the subject below and then sharing some LOVE by telling others about this amazing discussion forum.

Best Regards,

Paul W. Abernathy, CMECP®
Fast Trax® System
Electrical Code Academy, Inc.
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