12 awg 25 amp breaker

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xguard

Senior Member
Location
Baton Rouge, LA
I've ran into a few examples recently of 12 awg installed on a 25 amp breaker supplying the primary of a 15 kva dry-type transformer. Is this allowed? I couldn't find an exception that would allow it but that doesn't mean there's not one.
 
What is primary voltage and number of phases?

still too small of conductor for 600 volt single phase and pushing it pretty hard for 480 three phase.
 
xguard said:
I've ran into a few examples recently of 12 awg installed on a 25 amp breaker supplying the primary of a 15 kva dry-type transformer. Is this allowed? I couldn't find an exception that would allow it but that doesn't mean there's not one.
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The install would have to comply with 240.21(B)(3).

Cheers, Wayne
 
xguard said:
480 volt primary, 3 phase
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I suppose to some degree this might be acceptable if there is a fixed load supplied. It is pushing the 12 AWG to some degree if the load were 100% of transformer rating.

What is there for secondary protection might be the next question?
 
kwired said:
I suppose to some degree this might be acceptable if there is a fixed load supplied. It is pushing the 12 AWG to some degree if the load were 100% of transformer rating.

What is there for secondary protection might be the next question?
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They land in a panel but I don't have the mcb size available unfortunately.
 
I don't see how anything that's been described here gets around this:

240.4(D) Small Conductors.
Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.
240.4(D)(5) 12 AWG Copper.
20 amperes
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xguard said:
Can you explain further? I'm having trouble seeing how that section would allow this. Thank you.
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240.4(E) says Tap Conductor shall be permitted to be protected against overcurrent in accordance with the following:
(1) ...
(2) ...
(3) 240.21(B), Location in circuit.
(4) ...
(5) ...
(6) ...

240.21(B)(3) is labeled "Taps Supplying a Transformer..."

What you have described, #12 conductors, from a 25A c/b, supplying a transformer, are Taps Supplying a Transformer.
 
david luchini said:
240.4(E) says Tap Conductor shall be permitted to be protected against overcurrent in accordance with the following:
(1) ...
(2) ...
(3) 240.21(B), Location in circuit.
(4) ...
(5) ...
(6) ...

240.21(B)(3) is labeled "Taps Supplying a Transformer..."

What you have described, #12 conductors, from a 25A c/b, supplying a transformer, are Taps Supplying a Transformer.
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I guess I'm hung up on the definition of a "tap conductor".

From 2023 NEC:
Tap Conductor: A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.

The phrase "ahead of its point of supply" has led me to believe the tap doesn't originate from a circuit breaker but would be supplied by some other means such as another conductor. Am I misunderstanding this phrase or something else in the definition?
 
david luchini said:
What you have described, #12 conductors, from a 25A c/b, supplying a transformer, are Taps Supplying a Transformer.
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No.
They are simply feeders to the transformer, because #12 may be protected by 25A breakers if it is not a general circuit.
 
xguard said:
The phrase "ahead of its point of supply" has led me to believe the tap doesn't originate from a circuit breaker but would be supplied by some other means such as another conductor. Am I misunderstanding this phrase or something else in the definition?
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Yes. If you land a #12 conductor on the load side terminals of a breaker, then those terminals are the point of supply of that conductor. If the breaker is 25A, and there is no upstream 20A or smaller breaker, then the overcurrent protection ahead of its point of supply is 25A. Which is more than the 20A limit in 240.4(D). Therefore, the conductor would be a tap conductor, and will need to comply with the rules in 240.21(B).

There is a slight circularity in the way these requirements are written, as the ampacity of the #12 AWG is 25A if it and its terminations are at least 75C rated. So in applying 240.21, we need to understand that the overcurrent device referenced is one sized to comply with 240.4 excluding 240.4(E).

Cheers, Wayne
 
jim dungar said:
because #12 may be protected by 25A breakers if it is not a general circuit.
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That is not correct. 240.4(D) requires #12 copper to be protected at 20A or less unless "specifically permitted in 240.4(E) or (G)." (E) covers tap conductors, and (G) covers "specific conductor applications." As transformers (Article 450) are not listed under (G), the only compliance path for 240.4(D) is through 240.4(E), tap conductors. Therefore the #12 would have to comply with the tap rules.

Cheers, Wayne
 
david luchini said:
A feeder to a transformer is a general circuit. There is nothing in 240.4(E) or (G) that suggests otherwise.
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My error. Mea culpa.
But isn't 240.21(B) 3 for the specific case of a transformer with the combined primary and secondary lengths is 25' total. It doesn't necessarily apply to all transformer feeders.
 
jim dungar said:
But isn't 240.21(B) 3 for the specific case of a transformer with the combined primary and secondary lengths is 25' total. It doesn't necessarily apply to all transformer feeders.
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Yes, that's correct. The original post asked about #12 conductors from a 25A c/b feeding a transformer. As Wayne pointed out in post #4, that condition could be acceptable if the installation meets the requirements of 240.21(B)(3).
 
It would still have to be a 75C conductor like THHN and a 75C termination.
A cable such as NM or UF would not be permitted, based on its ampacity in 334.80.
Or in older buildings type 'TW' wire.
 
wwhitney said:
Yes. If you land a #12 conductor on the load side terminals of a breaker, then those terminals are the point of supply of that conductor. If the breaker is 25A, and there is no upstream 20A or smaller breaker, then the overcurrent protection ahead of its point of supply is 25A. Which is more than the 20A limit in 240.4(D). Therefore, the conductor would be a tap conductor, and will need to comply with the rules in 240.21(B).

There is a slight circularity in the way these requirements are written, as the ampacity of the #12 AWG is 25A if it and its terminations are at least 75C rated. So in applying 240.21, we need to understand that the overcurrent device referenced is one sized to comply with 240.4 excluding 240.4(E).

Cheers, Wayne
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I'm struggling with the load side terminals being considered separately from the overcurrent protection. If that's true then it seems like a case with a breaker with interchangeable lugs could be viewed differently than one where the terminals that are not interchangeable. What do you think of this distinction?
 
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