NEC 334.80 Meets 690.8(8)

[BarklieEstes]

Greetings,

Under the 2017 NEC, what would the minimum conductor size be for a 40a inverter output circuit using 90° TC-ER cable and a 33° ambient temperature? The inspector and I agree on the relevant codes [690.8 & 334.80] but disagree on how to apply them. For my part:

690.8(B)(1) says to multiply the maximum current by 125%, which would give: 40 * 125% = 50a.

690.8(B)(2) says to take the maximum current after adjustment, which is this case would give: 40 * 1.15 = 46a

690.8(B) says to take the larger of (1) and (2), which would give: Max(50, 46) = 50a

So now we have A) an ampacity of 40a, and B) a calculated ampacity of 50a.

334.80 says we cannot exceed the allowable ampacity using the 60° Column or the calculated ampacity using the 90° Column. Therefore:

Test 1 Allowable Ampacity: 40a @ 60° = #8

Test 2 Calculated Ampacity: 50a @ 90° = #8

The inspector's argument, I believe, is that the 50a result is not a calculated ampacity and thus that the 90° Column cannot be applied.

Thanks,
Barklie Estes

 

[BarklieEstes]

Trying to repost this due to an error in this thread, but I can't figure out how to do that or how to message an admin. Here is a revised version:

Greetings,

Under the 2017 NEC, what would the minimum conductor size be for a 40a inverter output circuit using 90° TC-ER cable? The inspector and I agree on the relevant codes [690.8 & 334.80] but disagree on how to apply them. For my part:

690.8(B)(1) says to multiply the maximum current by 125% with and not to shrink the allowable ampacity.
40 * 125% = 50a.
50a @ 90° Column = #8 conductor

690.8(B)(2) says to take the maximum current as is and to shrink the allowable ampacity. 334.80 shrinks it all the way down to the 60° Column.
40a @ 60 ° = #8 conductor

690.8(B) says to take the larger of (1) and (2), which would yield #8.

The inspector's argument, I believe, is that although 690.(B)(1) doesn't want you to shrink the allowable ampacity for some things (ambient temperatures, rooftop sunlight), it does still want you to shrink it for the purposes of 334.80. Therefore, you should increase the ampacity from 40a to 50a and shrink the allowable ampacity from the 90° to the 60° at the same time:
40 * 125% = 50a
50 @ 60° = #6 conductor

Thanks,
Barklie Estes

 

[wwhitney]

334.80 limits you to 60C except that 90C "shall be permitted to be used for ampacity adjustment and correction calculations."

690.8(B)(1) says it is "Before Application of Adjustment and Correction Factors."

As 690.(B)(1) has no ampacity adjustment and correction calculation, cables subject to 334.80 are limited to 60C for 690.8(B)(1). You could use 90C (if applicable) for 690.8(B)(2), although that doesn't do you much good.

Cheers, Wayne

 

[BarklieEstes]

Thanks Wayne!

To clarify, the requirement to use the 60° Column for a 90° cable would not be considered an "adjustment" for the purposes 690.8(B)(1)?

 

[wwhitney]

Thanks Wayne!

To clarify, the requirement to use the 60° Column for a 90° cable would not be considered an "adjustment" for the purposes 690.8(B)(1)?

No, the phrase "ampacity adjustment and correction" has a specific meaning. It refers to 310.15(B)(2) and (3).

Cheers, Wayne