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NEC 240.21(C)(1) Transformer Secondary Conductors
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Author:  Win Engineer [ Wed Sep 17, 2025 6:42 am ]
Post subject:  NEC 240.21(C)(1) Transformer Secondary Conductors
I'm curious how long has this been in the NEC? The reason I ask is because I am currently working on a project for a major company and the facility I am in (large industrial manufacturing site) probably has close to 100 208-volt panels fed from 480V/208V wye transformers and very few have main breakers in the panels or any other form of secondary protection. The facility is old with most equipment being installed in the late 1960's to early 1970's. Was this around back in that time frame? I find it hard to believe that this client would have built a plant with their engineering expertise that was not to code, but it certainly would not be to standard today.


240.21(C)(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer
having a 2-wire (single-voltage) secondary, or a three-phase,
delta-delta connected transformer having a 3-wire (single-voltage)
secondary, shall be permitted to be protected by over‐
current protection provided on the primary (supply) side of
the transformer, provided this protection is in accordance with
450.3 and does not exceed the value determined by multiplying
the secondary conductor ampacity by the secondary-to-primary
transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than
delta-delta, 3-wire) transformer secondary conductors are not
considered to be protected by the primary overcurrent protec‐
tive device.
Author:  bbaumer [ Wed Sep 17, 2025 6:50 am ]
Post subject:  Re: NEC 240.21(C)(1) Transformer Secondary Conductors
I don't have time to look through old versions of the NEC right now but if you like, you can look it up via the NFPA free access site or Google.

https://www.nfpa.org/codes-and-standard ... lopment/70
Author:  Win Engineer [ Wed Sep 17, 2025 7:53 am ]
Post subject:  Re: NEC 240.21(C)(1) Transformer Secondary Conductors
bbaumer wrote:
I don't have time to look through old versions of the NEC right now but if you like, you can look it up via the NFPA free access site or Google.

https://www.nfpa.org/codes-and-standard ... lopment/70


Thanks for the link. It appears that between 1996 and 1999 they made some changes to the wording in that section.
Author:  mdempsey [ Fri Sep 19, 2025 2:32 am ]
Post subject:  Re: NEC 240.21(C)(1) Transformer Secondary Conductors
when a panelboard is supplied through a transformer, the OCPD is required on the secondary of the transformer per NEC 408.36(B) unless the secondary side is protected by the OCPD on the primary side per 240.21 (C) (1)
Author:  JBD [ Sun Sep 21, 2025 2:47 pm ]
Post subject:  Re: NEC 240.21(C)(1) Transformer Secondary Conductors
Win Engineer wrote:
bbaumer wrote:
I don't have time to look through old versions of the NEC right now but if you like, you can look it up via the NFPA free access site or Google.

https://www.nfpa.org/codes-and-standard ... lopment/70


Thanks for the link. It appears that between 1996 and 1999 they made some changes to the wording in that section.


Yes.
Prior to the 90s it was common for transformer secondary conductors to not have a single OCPD when feeding panelboards.
Part of this had to due with the NEC and its requirements the Lighting and Branch Circuit Panels to have a main breaker while Power Panels did not need one.
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