Heat pump supply cable 8/3 or 8/2

[Greentrees]

Wiring a new heat pump with #8 wire per the manufacturers load chart. Purchased NM 8/3 plus ground. I go to install and the wiring diagram and the furnace have two line voltage lugs and (L1/L2) and a neutral lug hard to the chassis. Per NEC should this: (A) be wired like an 8/2 (L1 black, L2 red) with the bare copper into the chassis ground lug and cap the Neutral at each end of the 8/3 with wire nuts? (B) wire the white neutral leg into the ground lug (doubt it)? or (C) go buy the correct length of 8/2 and scrap the 8/3? Thanks in advance.

[timebuilder]

Wiring a new heat pump with #8 wire per the manufacturers load chart. Purchased NM 8/3 plus ground. I go to install and the wiring diagram and the furnace have two line voltage lugs and (L1/L2) and a neutral lug hard to the chassis. Per NEC should this: (A) be wired like an 8/2 (L1 black, L2 red) with the bare copper into the chassis ground lug and cap the Neutral at each end of the 8/3 with wire nuts? (B) wire the white neutral leg into the ground lug (doubt it)? or (C) go buy the correct length of 8/2 and scrap the 8/3? Thanks in advance.

1) Is this the indoor unit? If so, what are you doing for the outdoor unit? The outdoor unit is covered by article 440.

2) A neutral is NEVER tied to a cabinet ground. That is for the Equipment Grounding Conductor, the "bare wire." This leads to question 3:

3) Are there any 120 v circuits in the unit in question? If "no," then you didn't need the neutral.

Now, let's back up.

Your first correct approach to this install is to know two things from the equipment label:

1) The MCA, or Minimum Circuit Ampacity. That is a calculated value provided by the manufacturer that tells you what the ampacity load of your conductors must be. Here is where guys get tripped up: the decision making process of conductor selection.

For example, if you are using NM (non-metallic cable) you MUST use the 60°C column to size the conductors. Did you do that?

If you run some form of circular raceway, and your terminals are rated for 75°C at both ends (if you are feeding from most modern devices, you should be good at the source end) you can use the 75°C column, and pull an appropriate type of wire.

2) The next thing you need is the value of the maximum overcurrent device, or whatever word variation is used to give that data. Also, the label tells what kind of overcurrent devices are permitted.

Finally, you will need to read article 424 of the NEC, and make sure you have covered your bases there.

Now, if ANY of these questions leave you befuddled, you should call in a pro, and get a permit and inspection. In fact, if you are a homeowner, you should call your insurance Co and find out if they will void your policy if an installation that YOU perform voids your policy in the event of a loss.

Questions?

[dandyme]

party pooper

[Greentrees]

Thanks 'Timebuilder' for your response. It is a packaged unit, new construction, permits pulled. I'm the engineer, usually do furnace design, this time doing the install of unit. 8 ga is properly sized/configured per NEC. No 120v circuit in the packaged unit. The question remains 8/3 with a wirenut on the nuetral, or switch to 8/2?

[timebuilder]

What is the label MCA?

Are the unit terminations rated for 75°C?

Forgive me, but I'd like to check on this sizing.

[Greentrees]

38.5amp. NEC T310.16, 8ga 40amp, also 8ga per manufacuturer.

[timebuilder]

Good. Nothing to be gained by going to a 75°C wiring method.

I would go with the 2 conductor with ground. Remember, a disconnect is needed at the unit, and it should be placed to provide a safe working distance, as the control section often faces the building when on the ground. I would run LFNC to the unit from the disconnect. Also, the low voltage cannot be placed IN the conduit. Because the low voltage cable controls the unit attached to the conduit, it is legal to attach the low voltage to the exterior of the conduit.

Continuity of the ground/bonding must be established from the unit back to the service panel.

ALSO, 210.63: a 120v servicce outlet within 25 feet of the unit must be provided, with GFCI protection. I would run a separate pair for that.

[EEAllen]

check the wiring diagram, see if any components in there are using 120V, if so, yes u need to wire it as neutral. If not, i don't see why the wiring diagram would say it's a neutral wire, if the diagram shows there's a Ground wire, it means, that the neutral wire is for sure connected to somewhere else. Double check on the neutral. If it's actually a ground, then just ground it.