Transformer Secondary Conductor Protection Review Comments

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LibertyEngineering

Senior Member
Location
Allentown, PA
Hello –



Submitted this drawing for third party review to obtain a permit. Plan reviewer sent back this comment.



#1 It appears there is no secondary protection for Transformers T12 & T13 per 240.4(f) and 240.21(c).

#2 Once the OCPD is determined for above, please be sure all conductors are in compliance with 310.15.



I assume it is because we did not show that the panels had a main circuit breaker. (the red text was added for the purpose of this post). Does anyone else see any NEC issue with this. Agree with my assumption?


Thanks for looking!
 

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xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
Hello –



Submitted this drawing for third party review to obtain a permit. Plan reviewer sent back this comment.



#1 It appears there is no secondary protection for Transformers T12 & T13 per 240.4(f) and 240.21(c).

#2 Once the OCPD is determined for above, please be sure all conductors are in compliance with 310.15.



I assume it is because we did not show that the panels had a main circuit breaker. (the red text was added for the purpose of this post). Does anyone else see any NEC issue with this. Agree with my assumption?


Thanks for looking!

You need to review 240.21. There are specific rules for not having overcurrent protection where the conductors receive their supply. Conductor length limits and how they are allowed to terminate at the equipment are specified there.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I would also want to see protection on the line side of the ATS switches, in addition to the conductor lengths mentioned in the previous post.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
You need to review 240.21. There are specific rules for not having overcurrent protection where the conductors receive their supply. Conductor length limits and how they are allowed to terminate at the equipment are specified there.
I have and it is specifically 240.21 (C) we are dealing with. I was looking to comply with (C) (2) total length is less than 10' however my concern is feeding through the ATS first. Does that violate (2) and or (3)?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
As mentioned, the ATSs require OCPD. The reviewer should have required exact details on this as ATSs are listed with very specific types and values.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
As mentioned, the ATSs require OCPD. The reviewer should have required exact details on this as ATSs are listed with very specific types and values.
don_resqcapt19 says 'I would also want to see protection'. Wanting & requires are 2 different things, would you explain why it is required by citing the NEC sections. Not questioning because I disagree but would like to know.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
I have and it is specifically 240.21 (C) we are dealing with. I was looking to comply with (C) (2) total length is less than 10' however my concern is feeding through the ATS first. Does that violate (2) and or (3)?

240.21(C)(2)(2). The secondary conductors are terminated in the ATS and then extend beyond the ATS to the panelboard.

You need an OCPD for the ATS per 110.10 and how ATS short-time withstand ratings are applied.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
don_resqcapt19 says 'I would also want to see protection'. Wanting & requires are 2 different things, would you explain why it is required by citing the NEC sections. Not questioning because I disagree but would like to know.
I would have to look at the details for the ATS. Most specify specific line side protection to get the required SCCR rating.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
240.21(C)(2)(2). The secondary conductors are terminated in the ATS and then extend beyond the ATS to the panelboard.

You need an OCPD for the ATS per 110.10 and how ATS short-time withstand ratings are applied.
That is what I was afraid of regarding 240.21(C) (2) (2)
Thanks for the reference to 110.10 makes sense. Need the OCPD to protect the equipment being fed. Like a MLO panelboard being fed by a breaker in the distribution panel. How about a panel fed from a feeder tap without OCPD at the tap because you comply with the rules. Assume the OCPD protecting the feeder conductors being tapped is protecting all.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
How about a panel fed from a feeder tap without OCPD at the tap because you comply with the rules. Assume the OCPD protecting the feeder conductors being tapped is protecting all.
Then it isn’t a feeder tap as defined by article 240 and there is no problem.
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
That would make sense, xptpcrewx also mentioned 110.10 defining the requirement for the ATS needing line side protection. This is why I love this forum. Love to learn all this

110.10 is something I cited off the top of my head as a general rule, but there is probably a more specific requirement somewhere.

What I mentioned about 240.21(C)(2)(2) is just my interpretation. Wondering if anyone else agrees though…
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
110.10 is something I cited off the top of my head as a general rule, but there is probably a more specific requirement somewhere.

What I mentioned about 240.21(C)(2)(2) is just my interpretation. Wondering if anyone else agrees though…
It is all but improbable to be able to comply with any part of 240.21(C) without The conductors terminating in an overcurrent protective device.
 
don_resqcapt19 says 'I would also want to see protection'. Wanting & requires are 2 different things, would you explain why it is required by citing the NEC sections. Not questioning because I disagree but would like to know.
110.10 is something I cited off the top of my head as a general rule, but there is probably a more specific requirement somewhere.

What I mentioned about 240.21(C)(2)(2) is just my interpretation. Wondering if anyone else agrees though…
Much of this has been mentioned, just though I would tie it all together: There is no NEC section requiring a switch be protected at its rating by an OCPD. However, most ATS manufacturer instructions will state an OCPD requirement and you are bound to follow that per 110.3(B). Also, per 110.3(B) and 110.10 you are required to observe the SCCR of the ATS. IF your available fault current is above 10k, then you likely need to provide a certain type and size of OCPD to get a higher SCCR. Sometimes you may even need to go with a larger size ATS than you need to get the SCCR (I.E. a 400A when you only need a 200). You have to consult the manufacturers literature to confirm all this.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
Much of this has been mentioned, just though I would tie it all together: There is no NEC section requiring a switch be protected at its rating by an OCPD. However, most ATS manufacturer instructions will state an OCPD requirement and you are bound to follow that per 110.3(B). Also, per 110.3(B) and 110.10 you are required to observe the SCCR of the ATS. IF your available fault current is above 10k, then you likely need to provide a certain type and size of OCPD to get a higher SCCR. Sometimes you may even need to go with a larger size ATS than you need to get the SCCR (I.E. a 400A when you only need a 200). You have to consult the manufacturers literature to confirm all this.
Thank you!
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I thought that was the point of 240.21 (C) 'without overcurrent protection at the secondary, as specified in 240.21 (C) (1) though (C) (6)
The point of 240.21(C) is to allow the overcurrent protection of the secondary conductors to be downstream of the transformer secondary instead of at the transformer secondary.
 
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