too many grounds

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tracker

Member
Location
Vermont
we have an 800 amp main disconnect feeding 3 200 amp and 1 300 amp subpanels they are fed with 4/0 copper with #2 copper gec Main is grounded with #4 copper to 3 ground rods Subpanel is boned to water line with #4 cooper The engineer who designed this setup wants each sub to have #6 copper to a ground rod This does not seem right Any comments?
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: too many grounds

You can add supplemental ground rods to sub-panels as long as you are not bonding the neutral again downstream and feeder EGC's are provided. You will still have a planned fault path back to source.

The idea behind it is to lower the impedance to earth (not the source) at the sub-panels.
 

s.d. cronk

Member
Location
Ohio
Re: too many grounds

I assume that when you say "main disconnect" you are referring to a service disconnect and the additional panels are feeders off of the service disconnect. If so, the water lines are not permitted to be bonded from the sub-panels per 250.104, the farthest point that the bonding could take place would be the service disconect enclosure. Secondly, you are permitted to install supplementary grounding electrodes with the GEC terminating to the respective equipment grounds, as long as you do not rely on earth as the equipment grounding path per 250.54. If I assumed incorrectly on the set-up let me know, it may change my answer.
 
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