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adding a mini-split to same circuit as heat-pump

Post in 'DIY and General non-hearth advice' started by RustyShackleford, Oct 3, 2012.

  1. RustyShackleford

    RustyShackleford Minister of Fire

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    Folks, I am guilty of thread-jacking over in the "green room" and was asked to bring the discussion here. The discussion concerned "ductless mini-split" systems, miniature room (or several room) sized heat pumps noted for astounding efficiency (25+ SEER), due to several factors, perhaps chiefly that they do not require backup heat in most US climates - the heatpump can continue to operate efficiently at very low outdoor temperatures. A poster there asserted that:

    Each ductless system should have its own circuit.

    And I posted from there:

    Why ? This is a big issue for me, because I want to add a mini-split for an addition. I'd put the compressor (outdoor unit) near the compressor for my existing heat-pump, which is on a 60-amp circuit (from an old inefficient HVAC) but has only 35-amp labeling and 16 amps running current. So the 12,000 btu/hr mini-split I'm looking at (20-amp breaker and 5.5 amp RLA) could EASILY live on the same circuit. I hate to run another circuit, because it's far from the breaker box and I'm short on spare slots. When the heat-pump dies (it's 15 years old and only 11 SEER) I'd add a second mini-split and that'd handle my whole house, hopefully. If it really MUST be on a separate circuit, I suppose I could run a sub-panel off the 60-amp circuit.

    The circuit is controlled by a 60 amp double-pole breaker in my main box. I'd run the 60-amp rated wire to two fused service disconnects, one for the old heat-pump (an existing one with 35 amp fuses) and one for the new mini-split (with 20 amp fuses, as spec'ed for the mini-split). Either daisy-chained per usual wiring techniques, or via a blind junction box in the crawlspace; whatever I do, I'd still use 6ga wire, since of course it's a no-no to have any wire on the circuit that can't handle the breaker-size current

    If that isn't kosher, I'd do as you suggest and run a sub-panel off the 60 amp breaker. It'd have 35 and 20 amp breakers for the two units. Hopefully I can position the sub-panel so as to be close enough to both compressors - I think the service disconnect only needs to be "within sight" and I can easily achieve that. I wonder if it would be a problem that 35+20 is more than 80% of 60 ?

    Also, the wire going from the 60-amp breaker in the main box to the disconnect for the existing heatpump is 6-2 (IOW, no neutral). I know subpanels are required to keep neutral and ground separate, but I'd have no neutral. Neither does the existing heat-pump have a neutral (just two hots), and it turns out the mini-split has only two hots and a ground also. So hopefully it's ok not to have separate neutrals and ground in the sub-panel if those are the only loads.

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  2. Jags

    Jags Moderate Moderator Staff Member

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    I would be looking at the sub-panel with appropriate breakers and driving a new grounding rod close to the sub-panel. Full disclosure - I am NOT a certified electrician.
  3. begreen

    begreen Mooderator Staff Member

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    Sub-panel is my thought also. I'm not sure if an additional grounding rod is required. But I like appropriately sized breakers and feeds for the individual loads.
    dtaylor likes this.
  4. Jags

    Jags Moderate Moderator Staff Member

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    The only reason I added the grounding rod is because of the lack of a common ground in the existing wires. A 60 amp sub-panel should be grounded in my opinion - but again...I am NOT an electrician, but I am subject to overkill.==c
  5. begreen

    begreen Mooderator Staff Member

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    I definitely was thinking of carrying the ground from the main panel to the sub-panel, and a neutral if required.
  6. Highbeam

    Highbeam Minister of Fire

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    There are some rules about subpanels and whether or not they are in an attached building determining what to do with the neutral and whether or not ground rods are needed. The warning alarms for being in over our heads is blinking. You might think you are being conservative but in fact causing an unsafe condition. Neutrals are not common grounds, they are neutral, a deep topic.
  7. seige101

    seige101 Minister of Fire

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    Best bet is to rip it all out and go new. You can run proper sized circuits to outdoor non fused disconnects then to the new equipment. Each disconnect will need to be located within 6' of the outside unit. For good measure code requires a 120v GFCI protected receptacle within 25' of the outside AC units.
  8. begreen

    begreen Mooderator Staff Member

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    A ground rod is required if the sub-panel is in a separate building, it is not required in the same building. But a separate ground and neutral are required to be run to the sub-panel (4 wire feed). The neutral bus is independent and separate from the ground bus.
  9. RustyShackleford

    RustyShackleford Minister of Fire

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    Yes, however I'm not sure it makes sense to run (and have) separate ground and neutral in my sub-panel. The reason is because the only loads in the sub-panel will be 240v HVAC appliances. Each of these - my existing heat pump and the mini-split I am adding - does not have a neutral - just the two hot legs and a ground. So I could have a neutral in the sub-panel I suppose, but nothing would be connected to it !
  10. begreen

    begreen Mooderator Staff Member

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    As I understand it, current code as I read it requires a 4 wire feed to the sub-panel. My code book is old, so check to see if there is an equipment exception that covers your need. The ground is required regardless.
  11. Thomas Anderson

    Thomas Anderson Member

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    The rule I've always heard is a single grounding point for the entire building's electrical system. I think perhaps otherwise a voltage can develop between the two grounding points due to external factors and send current between the two grounding rods by way of your electrical system. And depending on where neutral is bonded to ground (e.g. if in multiple places), it can cause neutral to have different values across the electrical system, which can cause all sorts of problems. Neutral, and to a lesser extent, ground, are supposed to be a single common value throughout. However, if your heat pumps are not connected to the building's ground or neutral, then this rule would seem not to apply, and a grounding rod would be called for. Dangerous voltages could develop without a ground. I don't think AC power would even work with just 2-phase hots and no ground or neutral. The grounding rod will serve for both ground (which isn't supposed to normally carry a current, just under fault conditions) and neutral (which does normally carry a current) for your heat pumps. I think this combined neutral/ground is out of code as of 1996. I personally would find it questionable to have this setup though, whether to code or not, as the multiple neutrals could set up the possibility of different voltages throughout your house. Perhaps your existing heat pump has its own internal transformer and circuitry which negates this concern, but a sub-panel would not and should definitely have a neutral if not both a neutral and ground.

    FWIW, I would do either of several possible solutions (after consulting an electrician):
    1. If there aren't too many turns between your heat pumps and your panel, and the holes are big enough, or you can gain access to those places to help it along, you can tape a new 4-wire cable to your existing 2-wire in order to pull through the 4-wire to replace the 2-wire. Or if that won't work, think about a new route for a 4-wire, such as up through your eaves and across your attic and back down the wall above your panel. Then install a new 240V sub-panel for your heat pumps, each with their own breaker not to exceed a total of 60A.
    2. Remove your existing heat pump entirely, replacing it with a new 120V mini-split in addition to a new 120V mini-split for your addition (this could be a multi-zone unit with just one outdoor condenser unit or two separate outdoor units). Leave the 6/2 wire in the wall but wrap one of the leads with white tape on both sides and connect the black wire to a 120V breaker in the panel and the white tape wrapped wire to the neutral bar. On the other end, add a new 120V sub-panel with an independent ground (NOT bonded to neutral) with breakers for each heat pump. Or if you do a multi-zone unit, you might be able to leave out the sub-panel.
    3. Remove your existing heat pump entirely, replacing it with a new 240V multi-zone mini-split unit that only accepts two hots and no neutral, and connect it to your 6/2 wire just as your old heat pump was, with an independent ground. Add indoor units to replace your existing system plus the new addition. That may not meet current code though.
    4. Leave the old system as-is. Get a new, low-power (e.g. 9k-12k BTU, 26 SEER) 120V mini-split for your addition, and tap an existing 120V line somewhere else such as an outdoor receptacle box which has at least a 20A breaker (or has 12/2 wiring throughout and can upgrade to a 20A breaker). This low-power, high-efficiency mini-split probably only requires a 15A breaker. Place a breakered/fused cut-off switch on the new line between your tap and the condenser. The down side to this option is that you may trip the panel breaker if your heat pump is running and you also plug in an electric lawn mower or some other big load. But if you don't use big loads on that circuit anyway, this might be the ideal choice. And a super-high-efficiency mini-split probably only uses 5A when it's running anyway.
    All that said, I'm not an electrician, but I have wired several of my own houses to be safer than code (and passed inspection with flying colors). If this were my house, I would consult an electrician to be certain in all cases but #4 above. #4 is pretty safe. But that's your call.
  12. RustyShackleford

    RustyShackleford Minister of Fire

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    Thomas, thanks MUCH for the comprehensive answer. I'm out of town now, but looking forward to digesting it when I get home.
  13. RustyShackleford

    RustyShackleford Minister of Fire

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    I think there is a misunderstanding. My existing heat-pump, and the mini-split I'm thinking to add, both have connections for two hots and a ground, but no neutral. So they definitely will be connected to the building's ground regardless of what I do. The issues are, is it unacceptable to just wire a fused disconnect for each unit (both powered by the existing 60 amp circuit), and instead I need a subpanel ? If so, it is ok to only have a ground (but no neutral) in the subpanel ? Normally a subpanel must keep neutral and ground separate (but no grounding rod required if same building); but since neither piece of HVAC equipment even HAS a neutral connection, a separate (from ground) neutral bar in the sub-panel would not be connected to ANYTHING, except via a separate neutral wire back to the main panel. This seems kinda pointless, and would require me to replace the 6-2 (plus ground) wire with a 6-3 (plus ground) - hassle and expense.

    I think #1 is the way for me to go. Running a new cable would not be hard at all - just expensive (a 50ft or so length of 6 gauge 6-3 plus ground). But as I say above, I wonder if two fused disconnects would do the trick. Or a subpanel without a neutral, just a ground. I don't see a point in running a new wire just to provide a neutral that's not used.

    The problem with the other options is that I want to use a Fujitsu unit in their RLS2 line (http://www.fujitsugeneral.com/wallmountedRLS2_specs.htm), because I'm advised (locally) that they are excellent on reliability and the RLS2 line sports 20+ SEER ratings. But these are not available in 120v, and the mulit-zone units do not have as high efficiency ratings (not sure why).

    Since I listed myself as general contractor on the building permit, I was allowed to list "self" as subcontractor for HVAC, electrical, etc. A county inspector wil come tomorrow to do an "on-site consult" and we'll see what he says.
  14. Thomas Anderson

    Thomas Anderson Member

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    Oh, in that case, it makes things MUCH easier. I was totally thrown by thinking that you had no neutral wire. Your ground wire is acting as both a neutral and a ground (should be bonded inside the panel) and would probably be more correctly referred to as a neutral. You definitely do NOT want a new grounding rod at the condenser for the previously detailed reasons of not wanting multiple grounding points in your system, unless the condenser ground never contacts the neutral/ground wire to your panel (i.e. it's only a neutral and does not ground the chassis). I don't think it's up to current code to share a ground and neutral wire, but it should be safe to add a subpanel (or fused disconnect -- there's not really much difference other than that breakers can be reset) as-is without pulling a new wire if the inspector signs off on it.

    Yeah, I ran into that problem too. Installer would only do a Fujitsu. But I don't have 240V in my house because I'm on an inverter. I ended up buying a 120V Fedders 23 SEER for $800 and am installing it myself.

    I'll be curious to find out what he says.

    BTW, if you're thinking about preparedness at all, you might think about routing your heat pump drain into a tank in your basement or garage as a source of alternative fresh water.
  15. RustyShackleford

    RustyShackleford Minister of Fire

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    Building inspector came by just now for on-site consult (for this and the other wiring I'm doing in my addition).

    He said it was fine to run two fused disconnects off the 60 amp circuit - the existing one for my old heat-pump and a second one for the new mini-split. But he said it's quite difficult to join 6 gauge wires and I probably need some special connectors. However he said that I need to look at the specs on the old and new HVAC units carefully and if it says "max breaker size" (and not "max fuse size") , then I've gotta have breakers instead of fuses. I checked and this will not be an issue for me, but I thought I should note it.

    Instead (and not much more expensive because of the special connectors), he said I could run a sub-panel. Since it's in the same building as my main panel, it does not need a main breaker (the 60 amp breaker in the main panel serves this function). As I had hoped, since I'm only running 240v equipment off this subpanel, and that equipment doesn't have separate neutral and ground connections, it is ok for the sub-panel not to have separate neutral and ground; it would just have a ground bus.

    As I'd hoped, with either option I do not need to run a new cable from the main panel over to the HVAC area; the existing 6-2 (plus ground) wire will suffice. Also, the sums of the "minimum ampacity" for the two units cannot exceed 60 amps (I thought it would have been the sums of the "maximum fuse size" but it's not); and apparently the "running current" spec is insignificant for purposes of code and breaker sizes.

    Thanks to all who helped with this.
    Thomas Anderson likes this.
  16. begreen

    begreen Mooderator Staff Member

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    Thanks for the update Rusty. In our area the building inspector is not the electrical inspector. If he is the final authority then there you have it.
  17. RustyShackleford

    RustyShackleford Minister of Fire

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    Yes, he is one of the building inspectors whose specialty is electrical - an IS the final authority (other than the laws of electricity and physics :)
  18. jharkin

    jharkin Minister of Fire

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    To add extra clarity to info in this thread.

    Ground points: Only one per building, but if you are dealing with outbuildings, every building gets its own ground rod even if they are fed from the same service.

    Ground - neutral bonds: ONLY ONE in the entire system. All the sub panels have isolated neutral.

    The key point is that you never, ever, ever! bond neutral to ground anywhere other than the service entrance. If you have multiple bonding points and have a neutral fault, current will jump to the ground to find a path back to the service entrance. If you combine that fault with a high resistance ground or a ground break close to the service entrance you have the potential to energize the ground to line voltage. Very dangerous.

    When it comes to electricity I go right to the code book and a local pro/inspector. Its one area Im not willing to risk my life based on internet advice (in other words don't take my word for it either go ask your inspector)


    To the OP -
    I'm glad to hear you got the verdict from local code inspector , that's the only thing that matters.
  19. RustyShackleford

    RustyShackleford Minister of Fire

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    Thanks jharkin. I guess clarity is needed only because in threads like this people tend to jump to all sorts of suppositions. At no point did I ever suggest bonding neutral to ground (except where it's already bonded in my main panel). My only concern would be the complete absence of separate neutral and ground in my possible subpanel - since neither the existing cable coming from the main panel, nor the connections from the existing and proposed HVAC units, have separate neutral and ground - and that concern would only be because what I have heard of code requiring separate neutral and ground in subpanels.

    I think I am just going to go with two fused disconnects - assuming joining 6 gauge wires is not too big of an issue - the inspector seemed to think it might be and I might find it simpler just to do a subpanel.

    To be contrarian, I would take issue with the statement that the code inspector's words are all that matters. These guys are quite fallible. For years, including when my house was built, they were gving out erroneous information about the ventilation of crawlspaces, only fairly recently corrected thanks to some university research.
  20. jharkin

    jharkin Minister of Fire

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    No prob - my comments were intended for people who might run into this thread later via search, and might not realized the discussion of multiple bonding points was just referring to what might preexist in an old system, not whats considered safe or code compliant today.

    You make a good point about inspectors... I guess a better way to put it is if you want to be code compliant you have to know what codes your local jurisdiction has adopted.
    RustyShackleford likes this.

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