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Thursday, February 4, 2016
Question:
Re: CQD answer published Wednesday, January 27, 2016 -Feed Through Panelboard
You missed the boat on this one. But so did Santiago.
Typically “feed through” panels (he did say “Square D 200A main breaker feed through panel”) are referring to panels with lugs on the main bus bars. The main breaker protects these lugs and any number of additional spaces. So Santiago is most likely asking about protecting those 2/0 coppers with the new 200 amp mail breaker in this feed through panel. His concern is if the EMT and associated fittings would be a legal equipment grounding conductor. So far, so good. On the surface this whole installation appears to be okay.
But Santiago does mention that the current service conductors (to be converted to feeders) are routed through the attic. What both of you are missing is the needed derating of those 2/0 copper conductors. Now we don’t know if the 2/0’s are THW, or THHN/THWN, or THHN/THWN-2, or whatever. The old table 315(B)(6) just listed 2/0 copper as okay for 200 amp residential services and feeders. And some would argue that the older editions of the code didn’t include a requirement for derating such conductors. But the 2014 changed all that with the new wording in 310(B)(7). Now we are still allowed a lower ampacity of 83% for these dwelling unit service and feeder conductors. But derating must be taken into consideration with the updated wording.
So 2/0 with a 75 degree insulation has an ampacity of 175 amps, and the 90 degree insulation has an ampacity of 195 amps. If we need 83% of the 200 amps, that would be 164 amps.
We don’t know where this installation is located so we would have to guess what the ambient temperature of the attic could be. It is accepted that attics regularly get above 140 degrees, even 150 degrees or higher, in northern climates during the summer. Southern climates can get even warmer.
For example if THHN/THWN-2 (but probably THW’s or RHW’s) conductors were installed for feeders; we would start with the 195 amps and reduce it to 71% if the ambient was in the 141-149 degree range. Thus 195 x .71 = 138.45 amps. That’s still too low for even using these 2/0’s. Since 310(B)(7)(2) doesn’t allow for rounding up, even the 2/0 THHN/THWN-2 would be too small with a 150 amp branch breaker feeders there feeders.
Pulling in three new 3/0 THHN/THWN-2 into the existing 1.5 inch EMT would start the math with 225 amps and thus improving the derated ampacity to 159.75 amps. But once again we’re guessing where this is installed and thus what the attic temperatures might be.
Attics are so much fun. Matthew Hermanson
A
Answer:
Hey Matthew thanks for your comments, they are excellent points. There are lots of "what if's and variables in the original question but missing the boat is always possible. We discussed the "feed through panel" yesterday and if the existing 2/0 conductors are on the load side of the new 200 ampere main circuit breaker they are feeders. And if the attic is not an air conditioned space then yes an ambient temperature correction factor does apply. But then the typical ambient temperature limit for the panelboard and circuit breakers (40 degrees C) would also be exceeded. If the conductors are outside the building and only enter the attic space for a short distance (less than 10 % of the circuit length) then 310.15(A)(2) Exception could be used to "ignore" the ampacity of the conductors inside the building. If the 2/0 conductors are THW then the 175 ampere value in the 75 degree C column from Table 310.15(B)(16) can be used. Applying a correction factor of .88 from Table 310.15(B)(2)(a) based on an outdoor temperature not higher than 105 degrees F would result in a conductor ampacity of 154. Using the provisions of 240.21(B)(2) or 240.21(B)(5) if applicable could allow the installations. Yes, there are a lot of "if's".