We completed an industrial plant and seem to be having issues getting the Switchgear Study completed. We have several cabinets which feed from a 480/240 transformer and they are over the 25 feet distance. We seem to be having issues with Section 240.21 of the NEC. We were told there is may be an exception in the 2023 code which allows the transfomer to be protected by the primary side breaker in an industrial facility. I figured this was the place to go and ask about this so here I am. Can anyone help us with the code interpetation with the 240.21 Code issues with the delta/delta transformer?
Transformer Overcurrent Protection at Industrial Plant
- Thread starter DDuffeInc
- Start date
Deltaforce
Senior Member
- Location
- India
- Occupation
- Electrical Engineer
If primary breaker current rating suit secondary conductors, tap rule not apply
- Location
- Illinois
- Occupation
- retired electrician
The industrial rule is in 240.92(C).
However since you have a 3 wire to 3 wire, delta/delta transformer, the transformer primary OCPD is permitted to protect secondary conductors of any length provide that the provisions of 240.21(C)(1) are complied with. This will likely require that the primary OCPD have a rating that does not exceed 125% of the rated primary current.
This is not new for the 2023 and has been in the code for many code cycles.
However since you have a 3 wire to 3 wire, delta/delta transformer, the transformer primary OCPD is permitted to protect secondary conductors of any length provide that the provisions of 240.21(C)(1) are complied with. This will likely require that the primary OCPD have a rating that does not exceed 125% of the rated primary current.
This is not new for the 2023 and has been in the code for many code cycles.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Are you bringing everything up to current codes?
A secondary side OCPD was often not required prior to the 90s.
I believe it changed about the same time that the difference between Power Panels and Branch Circuit panels disappeared.
A secondary side OCPD was often not required prior to the 90s.
I believe it changed about the same time that the difference between Power Panels and Branch Circuit panels disappeared.
- Location
- Illinois
- Occupation
- retired electrician
I don't think there was ever a code rule that permitted the transformer secondary conductors to not be protected other than the 2-wire to 2-wire and 3 wire delta delta transformers.
However it was common practice not to provide such protection in industrial installations.
Originally all of what we know as the tap rules were exceptions to 240.21. The rule in 240.21 required the conductors to have protection at their point of supply. None of the exceptions addressed secondary conductors without protection.
This is what we are looking at also. Thanks for the quick reply. I knew we would get answers from the Mike Holt forum which are very helpful.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yep.
So if it was allowed back then must it be changed now. As an engineer i would make note of theses instances, with the recommendation to install secondary main OCPD as part of Arc Flash mitigation not necessarily because it is now enforced.
- Location
- Illinois
- Occupation
- retired electrician
Going back to the 75 code, the oldest I have, I don't see where it was allowed. Back then the exceptions applied to both feeder taps and transformer secondary conductors.
I think it was ignored because the code did not directly address that issue, however it remains my opinion that lacking a specific exception in the old versions of 240.21. the parent text that requires conductors have overcurrent protection at their point of supply.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You are right. My memory must not be what it was.
Back in the 70s, the language in 384.16(d) specifically required that any required panelboard OCPD must be located on the secondary of all transformer (except 2-wire ones).
- Location
- Illinois
- Occupation
- retired electrician
I think what happened was that many code users thought the provision in 450 for primary only protection also provided protection for the secondary conductors.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Maybe, but my mentors were more of the mindset that transformer secondaries were like service entrances, so MLO Power panels were allowed if neutrals were not used for most of the loads. This lives on in areas like 240.92(C).