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Question about wiring minisplits — electrician’s mistake

Clay Whitenack | Posted in Green Products and Materials on

I consulted with the electrician before he ran the wiring for our new home. I knew that mini splits weren’t something they would see everyday, so I wanted to make sure everyone was on the same page. We noted that the unit needed three wires plus the ground, and everyone knew what that meant because I had to run to Lowes to pick up some extra 12/3 wire since they were running low.

Fast forward a couple weeks, and the drywall is up and the HVAC guys are starting to hook things up. Turns out, the electrician only ran 12/2 to the units. However, the HVAC guy ran some thermostat wire from the outdoor unit to the indoor unit.

Can the thermostat wire serve as the ground connection between the two units? What is the purpose of the extra ground wire?

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  1. Jimmy Black | | #1

    Physically if the copper in the tstat wire can carry the current of 12ga then it would be possible, although I can't imagine under any circumstance would this be to code anywhere, and a fire hazard. I'm not sure if code actually calls for the ground to be in the same sheathing as the other two wires, I would think it would, you're only solution would be to run a whole new 12/3 wire. Regardless, do you really want to risk burning down your house, voiding your warranty, over a few hundred to run a new wire?

    If you speced 12/3 and he ran 12/2, he should run 12/3 for free.

  2. Chris B | | #2

    I'm pretty sure you can run a separate ground as long as it's the same gauge as the other conductors and meets all the wire protection requirements. A 'reduced ground' - ground wire smaller than the conductors - is definitely not allowed.

    Heck, for very large capacities I've often seen them run individual conductors simply because it's easier to pull than 8ga bundled.

    Most likely, you need to make the electrician come back and pull a full 12ga ground.

  3. John Ranson | | #3

    Running the equipment grounding conductor separately from the other wiring is generally disallowed with some exceptions. IRC 2015 E3801.3 and NEC 2017 300.3(B)


  4. Ken Cohenour | | #4

    In most mini splits, the "extra" wire is for communications, but needs to be rated as the other wires. In romex, the ground is only sheathed in cardboard not insulation. Are there special problems like cathedral ceilings etc to prevent replacing the 12/2?

  5. John Ranson | | #5

    My $0.02: If the extra wire is a control wire, you probably still want it in the same cable. The potential for external noise / stray voltage pickup (and generation) is directly related to the size of the loop formed by the control wire and whichever other wire carries its return current. All wires in one cable makes for a small loop and less interference potential. Loose wires make for a large loop and more interference potential.


  6. Clay Whitenack | | #6

    Thanks for the response, folks. That's what I needed to hear.

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