ATS selection & grounding

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jkim780

Senior Member
Need some help.

1200A 480/277V 3ph 4w service that requires a 1200A GFPE main. 1000KVA 480/277V 3ph 4w generator (also with 1200A GFPE) will back up the whole building.

I would like to use the 4-pole ATS to avoid false tripping because of the GFPE.

Questions:

1) Where do you have to bond the ground and neutral to prevent them becoming parallel conductors?

My initial thought was, utility xfmr feeds the 1200A CB w/GFPE(service disc), bonded at this point, then service disconnect feeds one side of the 4-pole ATS (no bond at ATS). The generator will feed the other side of the 4-pole ATS and bonded at gen set. Will this work? why or why not?

2) If the ATS was rated for use as service equipment, can you bond at that point and still use 4-pole ATS? how?

3) Is there a service rated ATS w/GFPE main?

4) Or is this the ideal setup for the 3-pole ATS?

Thank you.
 

ReggieD

Member
Re: ATS selection & grounding

Your initial setup idea sounds good. Code is not my specialty, but I work a lot of industrial GFI setups. Since you are using a 4 pole switch you are eliminating the first error that most contractors make.
There should be no connection between ground and either neutral between the GF neutral sensor and the ATS. Also be sure to check your load neutral, after your ATS, for field grounds.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: ATS selection & grounding

Originally posted by jkim780:
Need some help.

1200A 480/277V 3ph 4w service that requires a 1200A GFPE main. 1000KVA 480/277V 3ph 4w generator (also with 1200A GFPE) will back up the whole building.

I would like to use the 4-pole ATS to avoid false tripping because of the GFPE.

Questions:

1) Where do you have to bond the ground and neutral to prevent them becoming parallel conductors?
At the service disconnect as normal for any service. 250.28

And because you do need the 4 pole switch due to the GFP you also need to bond at the generator per 250.30(A)(1)

A generator used with a 'switched' neutral is treated as a separately derived system.

A generator used with solid neutral is not a separately derived system.

[ December 28, 2005, 09:19 PM: Message edited by: iwire ]
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Re: ATS selection & grounding

jkm780 wrote: 1) Where do you have to bond the ground and neutral to prevent them becoming parallel conductors?

According to a book of mine, the most popular style GFPE, a "Zero sequence" sensor, must not see the main-bonding jumper through its sensor window. If equip. grounding bonds to neutral before the GFPE window, on the load side instead of the supply side of the service disconnect, fault current would pass through the sensor and fool it completely, since there wouldn't be any unbalanced current. *

* Richker/Hartwell. Practical Electrical Wiring 19th Ed. Minneapolis: Park publising, 2005. pg.546-7

Does Auto.Transfer Switch (ATS) location matter, as long as fault paths for all systems remain line side of GFPE-sensor window [N,Phases]?

Solid Grounding For Your Generator may depend on the type.

[ December 29, 2005, 07:58 AM: Message edited by: ramsy ]
 
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