OCPD required before 1,600A ATS?

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SafeTfirst

Member
Location
NW AR
Good Day. Looking at a project where it has been requested to install a ATS on an existing 1600A 120/208 3phase system. 4 sets of 4-500KCMl CU feeding existing Nema 3 MDP 10' from POCO Transformer. Is there a problem with routing feeder wiring through 1,600A ATS,( Nippled, side of MDP to back of ATS) then back into MDP? Making sure I don't need to install a 1,600A OCPD before my ATS.
On another note; Is it permissible to reduce neutral size from the Generator (55' wire ft.away) feeding Auxiliary side of ATS? I do not know what the unbalanced portion would be as I have not taken any amperage readings yet...Many Thanks!
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Good Day. Looking at a project where it has been requested to install a ATS on an existing 1600A 120/208 3phase system. 4 sets of 4-500KCMl CU feeding existing Nema 3 MDP 10' from POCO Transformer. Is there a problem with routing feeder wiring through 1,600A ATS,( Nippled, side of MDP to back of ATS) then back into MDP? Making sure I don't need to install a 1,600A OCPD before my ATS.
On another note; Is it permissible to reduce neutral size from the Generator (55' wire ft.away) feeding Auxiliary side of ATS? I do not know what the unbalanced portion would be as I have not taken any amperage readings yet...Many Thanks!

Yes, you must have OCPD ahead of an ATS. In addition, OCPD for an ATS have to be very specific as to type, size and even brand depending on the situation. The ATS will have a label detailing this info.
It sounds like in your case you are referring to a service, so your OCPD will also now be the service disconnecting means. Neutral bonding will have to be adjusted accordingly. You could also consider a SUSE rated ATS.
 

SafeTfirst

Member
Location
NW AR
Thanks, Is there a good code reference for that? Wanting to have that to back it up to the client if they refuse and do not think it necessary... or am I understanding correctly, if I have a service entrance rated ATS… that takes the place of the required OCPD you were referring to?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Yes, and is also where the EGC and GEC systems are bonded to the neutral. Everything downstream is wired a la​ sub-panels
 

Dexie123

Senior Member
Are the 4 sets of 500 enough? 380 x 4 is 1520. Being over 800 amps you can’t round up. Unless the calculated load is less? I’m a novice so just trying to understand not question.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks, Is there a good code reference for that? Wanting to have that to back it up to the client if they refuse and do not think it necessary... or am I understanding correctly, if I have a service entrance rated ATS… that takes the place of the required OCPD you were referring to?

This article may be of help to you: https://www.csemag.com/single-article/automatic-transfer-switch-protection.html
And yes, a SUSE rated ATS could negate the separate OCPD. In either case you need to consider AIC ratings carefully.
 

SafeTfirst

Member
Location
NW AR
Are the 4 sets of 500 enough? 380 x 4 is 1520. Being over 800 amps you can’t round up. Unless the calculated load is less? I’m a novice so just trying to understand not question.


The existing gear and service was installed in 1984 and was wired with 4 sets of 500's. Was thinking just match existing installation that has been powering this building complex for 34 years. 500 is rated for 430 if using THHN, but of course need to look at termination rating, which my switchgear spec sheet does not tell me:roll:...
 

Dexie123

Senior Member
The existing gear and service was installed in 1984 and was wired with 4 sets of 500's. Was thinking just match existing installation that has been powering this building complex for 34 years. 500 is rated for 430 if using THHN, but of course need to look at termination rating, which my switchgear spec sheet does not tell me:roll:...

I guess it’s up to the inspector. Let us know what he says. I would think you’d need another set. Unless there’s an exception for this scenario
 
The existing gear and service was installed in 1984 and was wired with 4 sets of 500's. Was thinking just match existing installation that has been powering this building complex for 34 years. 500 is rated for 430 if using THHN, but of course need to look at termination rating, which my switchgear spec sheet does not tell me:roll:...

Terminations will never be 90. It seems to be very common on older installations to still use next size up rule above 800A. I actually see it that way more than full rating conductors.
 

pv_n00b

Senior Member
Location
CA, USA
Good Day. Looking at a project where it has been requested to install a ATS on an existing 1600A 120/208 3phase system. 4 sets of 4-500KCMl CU feeding existing Nema 3 MDP 10' from POCO Transformer. Is there a problem with routing feeder wiring through 1,600A ATS,( Nippled, side of MDP to back of ATS) then back into MDP? Making sure I don't need to install a 1,600A OCPD before my ATS.
On another note; Is it permissible to reduce neutral size from the Generator (55' wire ft.away) feeding Auxiliary side of ATS? I do not know what the unbalanced portion would be as I have not taken any amperage readings yet...Many Thanks!

What you want to keep in mind is that you will be moving the service entrance disconnecting means when you do this. So the ATS or something ahead of it needs to be the service disconnecting means and meet the NEC requirements.

On the neutral size, that's tricky. You need to apply NEC 220.61 to see if a reduced size neutral will work, or just use a full size neutral. Is it less expensive to use the full size neutral or spend the time to maybe find out you can use a reduced size neutral? That's the question.
 
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