ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
690.8(B)(2): Circuit conductors shall be sized to carry not less than the larger of 690.8(2)(B)(a) or (2)(b).
(2)(a) is the bit about multiplying by 1.25 twice.
(2)(b) is the bit about multiplying by 1.25 once and applying conditions of use.
OK so far.
(2)(c): "The conductor selected, after applications of conditions of use, shall be protected by the over current protective device, when required."
I interpret this to mean that if the derated ampaciity falls below the OCPD size below the one you have determined is the correct size, then you either have to lower the OCPD size to the first one above the derated ampacity, or you have to upsize the wire until its derated ampacity is higher than the OCPD below the one you have chosen, as long as it is less than 800A, in which case the ampacity of the wire must be equal to or greater than 95% of the rating of the OCPD. In other words, you can round up to the next OCPD size as long as the current is less than 800A, like everywhere else in the code.
William Brooks PE and James Dunlop PE do not agree. In their NABCEP Installation Professional Resource Guide, on page 67 in an example problem, they say:
"Step 5: Determine if 15A OCPD can protect the conductor under conditions of use [690.8(B)(2)(c)]: 12 AWG -> ampacity = 30A X 0.8 X 0.58 = 13.92A (fails because 14A fuse will not protect this conductor under conditions of use)."
It seems to me that if the code meant to say that the derated ampacity must be equal to or greater than the rating of the OCPD it would have said that, and that "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.
What say you? It makes a difference in system design.
(2)(a) is the bit about multiplying by 1.25 twice.
(2)(b) is the bit about multiplying by 1.25 once and applying conditions of use.
OK so far.
(2)(c): "The conductor selected, after applications of conditions of use, shall be protected by the over current protective device, when required."
I interpret this to mean that if the derated ampaciity falls below the OCPD size below the one you have determined is the correct size, then you either have to lower the OCPD size to the first one above the derated ampacity, or you have to upsize the wire until its derated ampacity is higher than the OCPD below the one you have chosen, as long as it is less than 800A, in which case the ampacity of the wire must be equal to or greater than 95% of the rating of the OCPD. In other words, you can round up to the next OCPD size as long as the current is less than 800A, like everywhere else in the code.
William Brooks PE and James Dunlop PE do not agree. In their NABCEP Installation Professional Resource Guide, on page 67 in an example problem, they say:
"Step 5: Determine if 15A OCPD can protect the conductor under conditions of use [690.8(B)(2)(c)]: 12 AWG -> ampacity = 30A X 0.8 X 0.58 = 13.92A (fails because 14A fuse will not protect this conductor under conditions of use)."
It seems to me that if the code meant to say that the derated ampacity must be equal to or greater than the rating of the OCPD it would have said that, and that "protected by the OCPD" is defined elsewhere in the code, namely that you can round up to the next size up if the current is less than 800A. It seems to me that the 14A fuse is compliant. BTW, the 15A to 14A switch is theirs, not mine.
What say you? It makes a difference in system design.
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