Pictured is the exhaust for a gas furnace, using a 3x12 double-wall insulated stovepipe. It is running up a non-load-bearing 2x4 wall sheathed in drywall. Which is apparently legal as far as thermal risk to the walls.

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The hole is 14.5" wide, and that leaves spaces at the ends and corners of the oval pipe. Someone chose this route to run some Romex, pictured. Is it legal under NEC to be that close to the exhaust?

I need to run several more circuits there, and I would be inclined to fit an EMT conduit up that same route, either alongside of or replacing the Romex. Is that legal that close to the exhaust?

3 Answers 3


The first test would be to verify the area is not hotter than the wiring can handle. Romex is rated for 90 deg C or 194F. The next check would be is the area being used as a plenum for the fresh air supply (some older homes used this airspace for environmental make up air ). If it is a plenum, romex is not plenum rated and may not pass an inspection if noticed (would need pipe) If just a hollow space romex would be ok as long as not two hot. EMT really would not help as far as temp is concerned and would limit the number of wires in the EMT or need to be derate based on the total current carying conductors.


There is nothing in the code that prevents you from running close to hot objects. You just have to derate for it.

The NEC requires you to derate because of temperatures over 86°F. Check Table 310.15(B)(2)(a) for temperature deration factors. Check the temperature of the area while the furnace is operating and derate accordingly.

NM cable is 90°C wire but can only be used at its 60°C rating regardless of temperature.

  • Two slow typing, The 60 deg ampacity rating is the max ampacity. But the 90 collum can be used for derating, the full temp rating of the 90 deg collum as long as the 60 deg ampacity is not exceed
    – Ed Beal
    Commented Jun 6, 2017 at 19:00
  • Yes, I agree Ed Beal. I should have said it can only be used at the maximum of its 60°C rating after derating for heat or current carrying conductors. Thanks for clarifying that. 😊
    – ArchonOSX
    Commented Jun 7, 2017 at 8:45

I see one romex cable already. Heat and electricity is never good but if you could maintain a 3" - 4" separation from the exhaust pipe you should be fine. I am assuming it is a double walled exhaust. Or if you could you might install a fireproof partition if you can't maintain separation.

  • With a 12" duct in a 14.5" space you can't get 3-4" of separation.
    – ArchonOSX
    Commented Jun 6, 2017 at 1:06

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