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The house has a gas water heater with no electrical connection. I'd like to replace it with an electric hybrid (heat pump) water heater that requires a 240v 30A circuit. Some of the newer hybrid heaters only need a 120v circuit by dispensing with the backup resistance heating element, but that doesn't really change my fundamental question of running wire to add a circuit upstairs.

Is it allowable to run a 240v 30A line beside a cold air return duct up to the 2nd floor attic? Can it touch the duct, or is space required between them? I should be able to drill a hole for the wire in the corner of the connecting space (think round flexible duct in a square space). It'll be trickier to fish 10AWG between floors that way.

A somewhat related question: Is there a minimum required distance between power wires and natural gas pipes?

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  • Where are you located? Some locations require conduit, others allow NB cable.
    – JACK
    Feb 11, 2022 at 21:38
  • TX. All the existing power wires are NM without conduit running directly in the walls and in the space between floors.
    – pmont
    Feb 12, 2022 at 19:28
  • Then what isherwood answered.
    – JACK
    Feb 12, 2022 at 19:59
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    8 AWG is larger than you need unless the distance is well over 200', or the 8 AWG is aluminum. (properly terminated and torqued, nothing wrong with that). These days are not time to overspend on copper, but a great time to trade what you have for what you need on Craigslist or Facebook marketplace. Feb 12, 2022 at 20:23
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    @pmont At 120V, that is about my threshold for even bothering to crunch the numbers to see if there's a problem. At 240V my threshold is twice that (twice the voltage half the voltage drop %). In your case punch in 240V, (distance), 5% voltage drop (override 3%), and amps off the water heater nameplate. Don't use the 125% derated number, because voltage drop is caused by actual amps not derate. Definitely don't use breaker trip (biggest novice mistake). A typical resistive water heater is 18.75A @ 240V. Feb 13, 2022 at 2:38

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To my knowledge there are no rules that say an insulated cable can't touch a duct, though you're required to protect the cable from potential damage. Sharp edges on corners and joints could present such potential.

If you can clearly see that there's no risk of damage, and if you secure the cable as you're able to in accessible areas per code, you should be fine with this plan as you've described it.

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  • Thanks @isherwood. Is stapling the wire to joists recommended? I found that a nuisance when done in-wall because it makes it much harder to pull/replace old wiring. Then I read someone say that's required by code to stop fire from traveling along the insulation.
    – pmont
    Feb 13, 2022 at 2:01
  • Recommended and required where accessible. Future-proofing is done with conduit, not loose wiring.
    – isherwood
    Feb 13, 2022 at 16:48

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