Grounding and Bonding one building with multiple laterals from one transformer

shortcircuit1

Senior Member
Hello,

I have a 4000A switchboard, 200A service for fire pump and lastly a 1200A service trough. All three laterals fed from the same transformer.

If you have one building/structure with 3 laterals from the power company transformer, do I need to design a grounding electrode system for each lateral or do you just use one common grounding electrode system for the building, have a common grounding electrode and then bond the services from the common grounding electrode? If so, how do you size the bonding wire from the common grounding electrode to the other services.

Thanks.
 

jaggedben

Senior Member
Common grounding electrode system. That's true on any building/structure anywhere all the time in all situations. The only sort of exception to that is where the code permits an 'auxiliary electrode' (or, in the one stupid case of 690.47(D) in the 2014 NEC, requires an auxiliary electrode) for a piece of equipment. An auxiliary electrode is not considered part of the grounding electrode system and is not for grounding the service.

Each service's GEC would be sized separately per 250.66. For bonding jumpers in between electrodes, you'd use whatever otherwise applicable requirement calls for the largest size. i.e. it would be based on the largest service size or the maximum requirement for the particular electrode if it is only bonding that one electrode.
 

shortcircuit1

Senior Member
Thanks for the reply.

This is how I sized my MBJ,common GEC and GEC's to different laterals. I have a common GEC copper bus from which I will tap to individual grounding electrodes.

4000A SWBD has 6 sets of 4#750MCM, so MBJ will be 300kcmil as per 250.102(C).(750x3x0.125x1000=281,250cmil)
Common GEC size(From the neutral bus in SWBD to the common copper ground bus)=750x3=2250>1100kcmil(CU) which gives me 3/0 GEC.
GEC from the common ground bus to 200A fire pump service sized based on 250.66 gives me #2CU(4/0 conductor size).
GEC from the common ground bus to 1200A trough sized based on 250.66 gives me 3/0CU (Based on 6 conductors of 350kcmil in each conduit and POCO has two sets).

Do these calculations look good?
 

shortcircuit1

Senior Member
Sorry for the delayed response. I did that based off of table 250.102 (C)(1) note#1. I counted the equivalent area for parallel supply conductors in one set which was 750x3. Did I interpret that wrong? Do I have to take 750x3x6?
 

kwired

Electron manager
Hello,

I have a 4000A switchboard, 200A service for fire pump and lastly a 1200A service trough. All three laterals fed from the same transformer.

If you have one building/structure with 3 laterals from the power company transformer, do I need to design a grounding electrode system for each lateral or do you just use one common grounding electrode system for the building, have a common grounding electrode and then bond the services from the common grounding electrode? If so, how do you size the bonding wire from the common grounding electrode to the other services.

Thanks.
You have one grounding electrode system for the building. All grounded service entrances and separately derived systems in the building must connect to it in some way.

Largest GEC in 250.66 is 3/0 copper 250 aluminum.

SSBJ's can be required to be larger, but a conductor to an electrode is not a SSBJ.

You have options on how to arrange things. Could run a separate GEC from each service to the electrode, could run a common GEC and taps to it from each service, could run a common GEC to a common point anywhere between the load end of service drop/lateral/etc and the service disconnecting means (though your setup sounds like wouldn't normally have any such point)

Location of electrode(s) vs your service disconnecting means might help persuade you to do it one way vs another in some cases.
 

shortcircuit1

Senior Member
Thank you! My scenario is second one where I am placing a common ground buss bar in the main electrical room and running GEC(Tap) from each service lateral to this buss bar. If I understand correctly the tap from buss bar to each service lateral is based on table 250.66(GEC). Am I correct in assuming this? For example if you have a 1200a service then you would run 3/0CU (Based on 6 conductors of 350kcmil in each conduit and POCO has two sets).
 

kwired

Electron manager
Thank you! My scenario is second one where I am placing a common ground buss bar in the main electrical room and running GEC(Tap) from each service lateral to this buss bar. If I understand correctly the tap from buss bar to each service lateral is based on table 250.66(GEC). Am I correct in assuming this? For example if you have a 1200a service then you would run 3/0CU (Based on 6 conductors of 350kcmil in each conduit and POCO has two sets).
Yes, and your 200 amp service likely only ends up with a 4 AWG tap to that common point.
 
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