Transformer Conductor Protection

LibertyEngineering

Senior Member
Location
Allentown, PA
45kVA 480V Delta/ 120/208V Wye transfomer

Using a 70A-3P circuit breaker to feed the primary side

Feeding a 120/208Vpanel with 175A-3P main circuit breaker.
Is the 175A-3P main breaker in the panel the OCP for the conductors required per 240.21? Assuming I would not comply with 240.21 (C)(1) thru (C) (6) which would eliminate the need all together.
NEC says protection is required at the point where the conductors receive their supply. To me that means for OCP of the conductors the OCP device should be located right at the transfomer first than feed the panel.
 
45kVA 480V Delta/ 120/208V Wye transfomer

Using a 70A-3P circuit breaker to feed the primary side

Feeding a 120/208Vpanel with 175A-3P main circuit breaker.
Is the 175A-3P main breaker in the panel the OCP for the conductors required per 240.21? Assuming I would not comply with 240.21 (C)(1) thru (C) (6) which would eliminate the need all together.
NEC says protection is required at the point where the conductors receive their supply. To me that means for OCP of the conductors the OCP device should be located right at the transfomer first than feed the panel.
You must comply with 240.21(C). If the MB in the panel doesnt meet one of them, then you must add another device that complies.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
You must protect your panel, either right at the panel or someplace upstream.
You must protect the conductors on the secondary side of the transformer, this protection needs to be located per 240.21(C).
There is no reason that a single device cannot protect both items.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
45kVA 480V Delta/ 120/208V Wye transfomer

Using a 70A-3P circuit breaker to feed the primary side

Feeding a 120/208Vpanel with 175A-3P main circuit breaker.
Is the 175A-3P main breaker in the panel the OCP for the conductors required per 240.21? Assuming I would not comply with 240.21 (C)(1) thru (C) (6) which would eliminate the need all together.
NEC says protection is required at the point where the conductors receive their supply. To me that means for OCP of the conductors the OCP device should be located right at the transfomer first than feed the panel.
That is the very reason for the transformer secondary conductor rules in 240.21(C)...to give you a bit of length to work with before you have to install the OCPD for the transformer secondary conductors.
 

Carultch

Senior Member
Location
Massachusetts
The first part of 240.21(C) gives you a couple of exceptions for qualifying topologies of transformers to not require protection of the secondary conductors. My term for this is "protect by proxy". The primary OCPD would protect the secondary conductors by proxy, such that a 70A breaker on the primary would act as if it were effectively a 161A breaker on the secondary, because 70A at 480V is the same kVA as 161A at 208V. This requires a topology combination, where the fault currents are guaranteed to line up across the winding pairs, such as single phase 2-wire to 2-wire, and 3-phase delta-to-delta 3-wire. Any time you have a center-tap or a wye system, the transformer doesn't qualify, since overcurrents can be redistributed, and end up in the blind spot of the primary OCPD.

The rest of the subsections of 240.21(C), are analogous rules to the 240.21(B) tap conductor rules, for situations where you do need a secondary OCPD. There are length limitations as a compromise to the practicality of wiring to the secondary OCPD equipment, and the goal to mitigate the risk of faults on the not-yet-protected conductors.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
My confusion lies in the way it is written, 'located at the point where their receive their supply'. The transfomer is what is supplying them so the location where they receive their supply is at the secondary transfomer lugs
 
My confusion lies in the way it is written, 'located at the point where their receive their supply'. The transfomer is what is supplying them so the location where they receive their supply is at the secondary transfomer lugs
I don't have the code book in front of me, but I think the point of supply wording is earlier in article 240, and 240.21c modifies that to give you some workable lengths
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
My confusion lies in the way it is written, 'located at the point where their receive their supply'. The transfomer is what is supplying them so the location where they receive their supply is at the secondary transfomer lugs
As others have mentioned, the transformer lugs are rarely capable of having a protective device attach directly to them which is why the rules in 240.21(C) exist.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I don't have the code book in front of me, but I think the point of supply wording is earlier in article 240, and 240.21c modifies that to give you some workable lengths
For reference, it's the 1st paragraph of 240.21.. "except 240.21 (A) thru (H) "
 
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