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I had a friend wire 10-3 line for a new heat pump. He connected the red and black conductor wires in the house to the 2-pole 60 amp breaker and the neutral white to the neutral bus bar. The outdoor disconnect box (pictured) has terminals for 2 conductor loads, a terminal that the neutral ends at neutral. The heat pump condensing unit only has 3 terminals (L1, L2, GRD).

My question is: why don't I have to run the neutral to the unit? Should I disconnect it from this terminal and breaker neutral bus bar? My friend said I can land the ground to it but didn't explain why. Thank you for your help in this matter.

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Neutral is not ground

That thing you are calling a "neutral bar" is bolted directly to the steel of the switch box. It cannot be a neutral, it must be a ground.

Neutral is an active, live conductor that carries current under normal conditions. It is normal for it to have "voltage drop", or more accurately "voltage rise". Ground is a safety shield, and should never flow current except during fault conditions.

In your installation, your appliance does not use neutral. Cap the wire off (Put a wire-nut on it) and wrap the nut with tape because they love to fall off single wires. The stud you have neutral on, is for ground. A neutral bar would be insulated from chassis, since we are anywhere but the main panel.

Conflating/interchanging neutral and ground defeats the purpose of running grounds. People tend to get misled by a couple of things.

  • seeing the inside of a main panel where neutrals and grounds gaggle on the same bar. That is an exception because the main panel is the one location where neutral is bonded to ground to assure the entire system's voltages doesn't float wildly and stays within 120V of earth (which puts neutral quite near earth).
  • dealing with 3-prong dryer and range connections where the neutral is attached to dryer frame. That is actually bootlegging ground, and is dangerous, except that it is legalized in those particular installations on the logic that the connections are so rarely disturbed.
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Cap off the neutral with a wirenut, connect the ground to the "neutral" bar in your disconnect

Since your heat pump does not need the neutral, you simply cap it off with an appropriately sized wirenut. The ground, then, lands on the ground bar in your disconnect (even though it gets called a neutral bar, it's really a ground bar here). That way, everything is safely grounded, and the neutral's available in case the heat pump circuit gets repurposed for something else that actually needs it.

Also, did your installer pull a no-no?

Whoever ran the circuit to this disconnect apparently pulled NM cable through the conduit, which is a no-no as NM can't be used in wet locations (such as the inside of an exterior conduit), atop being an impractical nightmare to pull through conduit. The correct thing to use instead would be a set of 10AWG THHN/THWN wires; it shouldn't be hard to fix, though, thankfully, as with the breaker off, you can yank the NM out of the conduit and run THHN in its place, although you may have to add a junction box at the other end of this conduit run to transition between the THHN-in-conduit and NM.

(The reason why I can figure out it's NM from all the way over here is because the paper separator present in NM (and AC) but not in wet-location cables such as UF is visible poking out from the fitting at the bottom right of the box, by the way.)

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  • You could use it for the convenience outlet (like you're probably supposed to have by code), but like that's ever going to happen... – Mazura Feb 2 '19 at 4:54
  • @Mazura -- some fancier A/C disconnect boxes have the convenience outlet built-in, even – ThreePhaseEel Feb 2 '19 at 14:03
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You better hope he didn't run NM-b, because that's a wet location and against NEC for a few reasons. It's can't be run in conduit or used in wet. That needs to be THHN/THWN. It sure looks like it with the paper. That's not UF-B because the sheath is grey which could be used and is solid core and not stranded like TH.

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  • NM can be run in conduit, it's just impractical to do so due to fill/pull force issues. – ThreePhaseEel Nov 8 '19 at 12:41
  • TY, could have swore I read full raceway was a heat concern with NM and was only protected runs, can't find it in newest book. Again TY. – odiebugs Nov 8 '19 at 16:11
  • The fill rules handle that concern, btw :) Also, good eye on spying it's NM, I missed that at first :) – ThreePhaseEel Nov 9 '19 at 0:38
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The other answers posted here are correct about the use of neutral and using THHN instead of NM in the conduit. But one thing that nobody else has mentioned is that you said that the other end of the wire connected to a 2 pole 60 amp breaker and said you were using 10 gauge wire. #10 wire is only good for up to 30 amps. In the event of overload, your wiring will be melting down. Also, I recognize the disconnect box you are using. It is also rated at 30 amps. So you should change out the breaker inside to a 30a breaker or smaller.

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  • No, they need to follow what the nameplate for their air conditioner says -- see NEC 440.6, 440.21, 440.35, and 440.53 – ThreePhaseEel May 11 '20 at 14:57

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