I've got 4-conductor cable now, but am looking to put in a smart thermostat (ecobee3). It requires a C wire, or the use of their adapter kit, so I'm thinking of just pulling new cable.

7-conductor is obviously cheaper than 8, but I also realize the cost difference is neglible in the long run. I've got single stage heat and cool, and a fan, so 4 wires have served well until now. I could maybe forsee adding a (de)humidifier in the future, but don't think I would ever install anything else (furnace and AC were replaced 2 years ago), rendering the extra wires redundant.

Will I be severely hampering upgrades in the future by only pulling 7 wires? The run looks to be maybe 25 feet, all in the attic.

  • In my orange colored big box store the 8 cond x50' is actually priced lower than 7 cond x 50'
    – Tyson
    Feb 18, 2017 at 17:32
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    @Tyson mine doesn't seem to have 18/8 in 50ft - only 250ft, and I definitely don't need that much
    – mmathis
    Feb 18, 2017 at 19:36
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    Sounds like temporarily out of stock, try blue or green, Or a real HVAC supply instead of diddling with these unhelpful and not-actually-cheaper bigbox stores. Feb 18, 2017 at 19:43
  • That probably just made your mind up 👍I just happened to be there waiting on paint mixing when I read this question, so I looked. Heat pumps use up more condutors, and it sounds like you've got forced air gas.
    – Tyson
    Feb 18, 2017 at 19:46

1 Answer 1


I always pull 8 conductor for several reasons. First I only keep 1 spool on the shelf and the cost is within a few cents per job. Second if a conductor gets damaged there is a spare. Third upgrades at a later date can use the same wire. Little things like the last one can help on selling a spec home in my experience.

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