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Thread: Yet another ampacity question

  1. #1
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    Yet another ampacity question

    According to 110.14(C), conductors are to be sized to the lowest temperature rating of any terminal, device, or conductor of the circuit, in accordance with the equipment terminal temperature rating. However, conductors with insulation temperature ratings higher than the termination's temperature rating can be used for conductor ampacity adjustment, correction, or both.

    The ampacity of each 12 THHW conductor in a dry location is 30A, based on the values listed in the 90°C column of Table 310.16. If we bundle nine current-carrying 12 THHN conductors, the ampacity for each conductor needs to be adjusted by a 70% adjustment factor.

    Adjusted conductor ampacity = 30A × 0.70 = 21A

    But under one hundred amps we have to use the 60° column which puts #12 at 20 amps

    So the correct answer is twenty amps???
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  2. #2
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    Quote Originally Posted by shputnik View Post
    According to 110.14(C), conductors are to be sized to the lowest temperature rating of any terminal, device, or conductor of the circuit, in accordance with the equipment terminal temperature rating. However, conductors with insulation temperature ratings higher than the termination's temperature rating can be used for conductor ampacity adjustment, correction, or both.

    The ampacity of each 12 THHW conductor in a dry location is 30A, based on the values listed in the 90°C column of Table 310.16. If we bundle nine current-carrying 12 THHN conductors, the ampacity for each conductor needs to be adjusted by a 70% adjustment factor.

    Adjusted conductor ampacity = 30A × 0.70 = 21A

    But under one hundred amps we have to use the 60° column which puts #12 at 20 amps

    So the correct answer is twenty amps???

    We don't have to use the 60C column if all parts of the circuit are rated 75C. For instance, if the terminals in a panel are not marked then we would have to use 60C but if we know everything is rated 75C then you can use the 75C column. In your example, it doesn't matter simply because you would have to use a 20 amp ocpd anyway.
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  3. #3
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    Quote Originally Posted by Dennis Alwon View Post
    We don't have to use the 60C column if all parts of the circuit are rated 75C. For instance, if the terminals in a panel are not marked then we would have to use 60C but if we know everything is rated 75C then you can use the 75C column. In your example, it doesn't matter simply because you would have to use a 20 amp ocpd anyway.

    So for below 100 amps and it is NOT stated then I use the 60 degree column....But if it is stated on breaker then I would use the 75 degree column.
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  4. #4
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    Quote Originally Posted by shputnik View Post
    So for below 100 amps and it is NOT stated then I use the 60 degree column....But if it is stated on breaker then I would use the 75 degree column.
    Well, not just stated on the breaker. The panelboard label must also state the panelboard itself is rated for 75°C terminations.

    Additionally, the equipment you are connecting to on the other end must also be rated for 75°C terminations.

    With that said, #12 is a poor example for this discussion because Code limits the circuit OCPD to 20A when using #12 copper [240.4(D)(5)]. So even if you could otherwise qualify the #12 for up to a 25A circuit, you cannot supply it with a 25A breaker... except for other-than-common circuits that fall under 240.4(G) Specific Conductor Applications. Motor circuit conductors are a prime example where greater than 20A is permitted.
    Last edited by Smart $; 02-11-17 at 04:07 AM.
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  5. #5
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    You have to be careful with the terminology. In your example, the adjusted ampacity of the conductor was 21 amps. The footnote at 310.15(B)(16) refers you back to 240.4 which limits the rating of the overcurrent device, hence the branch circuit, to 20 amps, however,
    240.4(D) also notes "unless permitted in 240.4(E)or (G)" so the 20 amp limitation does not apply in those case (motor applications for an example).
    Bottom line, it depends on what you are trying to determine. The amacity of a conductor and the size of the branch circuit do not correspond. This is a favorite "trick" in test taking.
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  6. #6
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    I have seen only one device with 60 Deg C terminals, a dryer plug in my house when I moved in, probably dates to when dryers where first installed.
    That rule for 100 amps and under, you can basically ignore, except for a test question.
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    It seems everyone is pushing this right there to the edge... but don't be so quick to jump.

    For example: http://www.leviton.com/OA_HTML/Produ...minisite=10251. This is a Leviton spec' grade 125V 15A NEMA 5-15R receptacle. Note the operating temperature...
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    Quote Originally Posted by Smart $ View Post
    It seems everyone is pushing this right there to the edge... but don't be so quick to jump.

    For example: http://www.leviton.com/OA_HTML/Produ...minisite=10251. This is a Leviton spec' grade 125V 15A NEMA 5-15R receptacle. Note the operating temperature...
    Does that have something to do with the terminal rating? It states that the information is on the device so couldn't it have 75 degree terminals?

    • Mechanical Specifications
    • Terminal ID: Brass-Hot, Green-Ground, White-Neutral
    • Terminal ID: Brass-Hot, Green-Ground, Silver-Neutral
    • Terminal Accom.: 14-10 AWG
    • Product ID: Ratings are permanently marked on device
    • Termination: Side
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  9. #9
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    Quote Originally Posted by infinity View Post
    Does that have something to do with the terminal rating? It states that the information is on the device so couldn't it have 75 degree terminals?

    • Operating Temperature: -40C to 60C

    As you know, a product 100A and less is only rated for 60C, unless listed and marked otherwise. This is mostly an academic rule, as a lot of modern equipment is marked otherwise, but the burden of proof still remains for product documentation and the device itself to mark it for 75C.

    As others have mentioned, #14, #12, and #10 are not the best example for the general ampacity calculation, as the copper conductors in these sizes are limited in most applications by the small conductor rule to maximum 15A, 20A, and 30A OCPD circuits respectively, regardless of if terminal equipment is marked otherwise for 75C. One counterexample is HVAC equipment that has a different algorithm entirely.
    Last edited by Carultch; 02-13-17 at 11:20 AM.

  10. #10
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    Quote Originally Posted by infinity View Post
    Does that have something to do with the terminal rating? It states that the information is on the device so couldn't it have 75 degree terminals?
    The subject of 110.14(C) is the equipment operating temperature limit (i.e. rating). You can use the terminal temperature rating as a guide, but if the equipment temperature limitation, i.e. its maximum operating temperature, you have to use the equipment temperature limitation.

    I don't have any of those receptacles on hand, but even if they are marked as having 75°C terminals, you still could only use the 60°C conductor rating. I have a sneaking suspicion the Leviton model is not the only model, or make for that matter, with 60°C maximum operating temperature rating.
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