Industrial Grounding/Bonding Nightmare

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Don Pedigo

New User
Location
Tennessee
I am assessing and making line drawings for an older facility in Springfield Tennessee. In my research, I've discovered three different grounding/bonding techniques on the 208/120 panels. One of which is completely wrong. The local inspector says per article 250.30 that the 208/120 system should be treated as a new service. By that he's telling me that I only carry my 3-conductors and grounded conductor from the transformer and bond the can. I see this way done on all of the old panels. The newer equipment has what would be called a four wire system. I've read forums that support the four-wire system but, I also see how 250.30 could be viewed as the inspector sees it. This plant is having serious voltage fluctuations and outages. I know grounding is the issue. Can you please clarify the correct way? Also, shouldn't EVERY panel hit the building steel? I haven't really gotten into the 480V panels so if you could include that in your answer I would greatly appreciate it.



 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The fact that you referenced 250.30, for your 208Y/120 panels we assume separately derived systems.
Your Code reference somewhat answered your question. Like most NEC info, there have been modifications over the years, so today's rules might differ sligtly from previous rules and the nomenclature has definitely changed.
Today you would be required to install phase and grounded conductors between the panel and SDS
and in additton you would need a equipment bonding jumper between the source and the first disconnecting means ('08 terminology). That bond could be accomplished by a wire conductor or
a wiring method meeting the requirements of 250.118.
You must also establish a grounding electrode system which can be connected at the first disconnect or the SDS but must connect where the system bonding jumper is installed.
These pictorials may help:(click to enlarge)
transformer.jpg
transformer bonding 2.jpg

As far as your 480v panels if they are supplied by and SDS the same rules apply. If they are utility supplied non service sub panels the only difference between those and a panel rated 250vor less to ground is the application of 250.96 and the requiremt of bonding around concentric or eccentric knockouts

I know of no NEC requirements for a non-service panel to be bonded to building steel other than doing so as one possibility of meeting the requirements of 250.30 for panels supplied by a SDS.

Bonding all panel to building steel is sometimes an engineering requirement but in those cases it is supplemental as building steel can not be used as an equipment ground
 
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ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
The inspector is correct and 250.30 is the correct reference. There is no problem with bonding at the first disconnect of a SDS. It is the way I prefer to do it and there is no reason it would cause voltage swings.

If you had an open neutral, that could cause some problems.
 
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