Results 1 to 11 of 11
  1. #1

    Heat pump supply cable 8/3 or 8/2

    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.

  2. #2
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    22,953
    Quote Originally Posted by Greentrees View Post
    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 60C column to size the conductors. Did you do that?

    If you run some form of circular raceway, and your terminals are rated for 75C at both ends (if you are feeding from most modern devices, you should be good at the source end) you can use the 75C 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?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  3. #3
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,643
    party pooper

    It`s better to be silent and thought the fool; than speak and remove all doubt.

  4. #4
    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?

  5. #5
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    22,953
    What is the label MCA?

    Are the unit terminations rated for 75C?

    Forgive me, but I'd like to check on this sizing.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  6. #6
    38.5amp. NEC T310.16, 8ga 40amp, also 8ga per manufacuturer.

  7. #7
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    22,953
    Good. Nothing to be gained by going to a 75C 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.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  8. #8
    Join Date
    May 2010
    Location
    Brooklyn, NY
    Posts
    99
    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.

  9. #9
    Quote Originally Posted by Greentrees View Post
    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.

    If , as has been said , there is no neutral load , just wire nut off the neutral , at both ends . It is not going to hurt anything .

    I look at the max fuse or HVAC rated circuit breaker rating . I size the wire , outside disconnect and inside over current protective device to that number and do not have any call backs . Except if some one diddls with the thermostat & the compressor tries to start against high head pressure . Then I advise the HVAC guy to put a time delay in the tstat circuit .

    God bless
    Wyr

  10. #10
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    22,953
    Quote Originally Posted by Lachlansmith View Post
    It`s better to be silent and thought the fool 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
    I'm sorry, but I have to ask.

    What exactly was the point of this post, quoted here?
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

    AOP Forum Rules:







  11. #11
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
    If permits were pulled then that means inspections are coming... If your an engineer then in most jurisdictions your not authorized to be doing any wiring on the unit. And if you have to ask these kind of questions you shouldn't be working on the instillation. And we shouldn't be telling you how to accomplish such a no brainer.....

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Related Forums

Plumbing Talks | Contractor MagazineThe place where Electrical professionals meet.
Comfortech 365