240.21(C)(1) and 240.4(D)

Status
Not open for further replies.

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
That's an interesting question. Assuming a 1.5kVA or larger transformer, which would be properly protected as "primary only" by the 5A fuse, I would have to so no, its not legal.

If the #14 has an ampacity of 20, then it would appear to be properly protected per section 240.4(F), by the 5A primary OCPD with the 1:4 secondary to primary transformer ratio. That is to say the #14 secondary conductor is protected at 20A, by the primary OCPD.

I think this would violate 240.4(D), which says #14 shall be protected by an OCPD not greater than 15A, except as specifically permitted in 240.4(E) or (G). I don't see any exception permitted for 240.4(F).
 

eprice

Senior Member
Location
Utah
Is the following legal?

480V circuit, 5A fuse --> 480 - 120V XFMR --> 14 AWG --> 1500VA load

I am assuming that the 14 AWG is copper. I believe this is legal under the 2008 NEC. I don't have a copy of the 2011 NEC, but I believe the ampacity for 14 AWG copper has been changed to 15 amps in the 2011, and what you describe will not be legal under the 2011.

240.4(D) doesn't change the ampacity of the 14 AWG. It only regulates what overcurrent protection it can be used with.
 

eprice

Senior Member
Location
Utah
I don't see any mention of the transformer size in 240.21(C)(1), only its ratio which in this case is 1:4. I don't see any mention of transformers or ratios in 240.4(D). A 5 amp fuse is not larger than 15 amps :D
 
Last edited:

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I don't see any mention of the transformer size in 240.21(C)(1), only its ratio which in this case is 1:4. I don't see any mention of transformers or ratios in 240.4(D). A 5 amp fuse is not larger than 15 amps :D

While it is true that a 5A fuse is not larger than 15 amps, 240.4(D) doesn't say that small conductors shall be protected by a OCPD having a rating or setting larger than 15A for #14...it says the overcurrent protection shall not exceed 15A for #14. If the overcurrent protection for the #14 secondary was 5A, it wouldn't be able to carry the 1500VA load. The overcurrent protection for the #14 secondary is 20A, which would violate 240.4(D).
 

eprice

Senior Member
Location
Utah
The overcurrent protection for the #14 secondary is 20A, which would violate 240.4(D).

No, in the case we're discussing, the overcurrent protection for the #14 is a 5 amp fuse through a 1:4 transformer. This is a case which 240.4(D) doesn't address. 240.21(C)(1) does address this case, and the described scenario seems to comply with that section.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
No, in the case we're discussing, the overcurrent protection for the #14 is a 5 amp fuse through a 1:4 transformer. This is a case which 240.4(D) doesn't address. 240.21(C)(1) does address this case, and the described scenario seems to comply with that section.

I disagree. The overcurrent protection for the #14 is not 5 Amps. It is 20 Amps. The OCPD is rated at 5 Amps, but the protection given to the secondary conductors is 20 Amps. 240.4(D) says you cannot protect #15 at greater than 15 Amps except as specifically permitted in 240.4(E) or (G). If the code intended to allow protection as such, 240.4(D) would have said "Unless specifically permitted in 240.4(E), (F) or (G).

But it does not say that. The protection of transformer secondary conductors buy the primary OCPD as allowed in 240.4(F) is not exempt from complying with 240.4(D).
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Thanks for all the replies, guys. :cool:

Assume 75 degree terminations for this. Ed, you're right, the 60 degree column for 14 has dropped to 15A.

I saw this in a graphic from Mike Holt, and it threw me. In the 2005, 210.4(D) changed to begin including this scenario under the small conductor rule, since before that you could serve multioutlet loads that could be overloaded if you added a transformer to the mix.

I don't think the graphic is right, I don't think you get around 240.4(D) by adding a transformer, but I'm not entirely sure sitting here not looking at the NEC right this second.

Keep em comin'... :)
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Thanks for all the replies, guys. :cool:

Assume 75 degree terminations for this. Ed, you're right, the 60 degree column for 14 has dropped to 15A.

I saw this in a graphic from Mike Holt, and it threw me. In the 2005, 210.4(D) changed to begin including this scenario under the small conductor rule, since before that you could serve multioutlet loads that could be overloaded if you added a transformer to the mix.

I don't think the graphic is right, I don't think you get around 240.4(D) by adding a transformer, but I'm not entirely sure sitting here not looking at the NEC right this second.



Keep em comin'... :)

I have some of Mike's books, where is the graphic located? Maybe I have it and can look at it.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
George,
I agree. I don't see anything that would let you provide a 20 amp OCPD for the #14 transformer secondary conductors and that is what you have in that slide.
 
Status
Not open for further replies.
Top