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  1. #14
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    Quote Originally Posted by timjimbob View Post
    Wry,
    HP can be configured to be run either both on or "one or the other" with regard to heat strips. Defrost may run 10 minutes every 60 or 90 minutes. I meant non coincidental, not "non-consequential" loads. As cited in the code book.
    I kind of thought they might be able to do either or ? I am correct , both run in defrost ?

    Another issue , as far as stressing the service is heat . I have seen services fail much more often when temps approach 100 degrees or greater .

    Obviously , in the winter , the stress on the service from ambient temps , would not be a factor .

    Thanks ,
    Wyr
    God bless

  2. #15
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    Heating is a continuous load. Size the strip heat for 125%.

    Since the strip heat is a separate feed from the outdoor unit, the outdoor feed is sized according to 440.

    Questions?
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  3. #16
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    My original post regarded installing a HP to replace a straight cool AC. Now required by energy code. Inspector never asks about panel upgrades.

  4. #17
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    Quote Originally Posted by timjimbob View Post
    My original post regarded installing a HP to replace a straight cool AC. Now required by energy code. Inspector never asks about panel upgrades.

    Most inspectors use the International Residential Code which gets its electrical sections from the National Electrical Code. However, most inspectors are not familiar with the National Electrical Code and have no idea what its requirements are.

    The onus is on you, the installer, to make yourself aware of the requirements of the Code.
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    2 Tim 3:16-17

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  5. #18
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    Quote Originally Posted by timebuilder View Post
    Heating is a continuous load. Size the strip heat for 125%.

    Since the strip heat is a separate feed from the outdoor unit, the outdoor feed is sized according to 440.

    Questions?
    I size all my circuits for continuous load . Voltage drop , ambient temp , number of current carrying conductors , etc. .

    Actually , that is only partially true . Much / most of what I bid , the engineer does all or part of that .

    I have wired roof top package units / heat pump units that had one large circuit for the whole unit . They had circuit breakers or fuses to divvy out the load .

    Thanks ,
    Wyr
    God bless

  6. #19
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    The key words there are "package units."

    A package unit has a single number for MCA and MOD. It is a simple matter to follow those values, which include in those vales the factor of continuous load.

    It's nice to have a set of stamped prints where all of the requirements have been met and approved by a plan review. Most HVAC contractors never see such a nicety.
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  7. #20
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    Quote Originally Posted by timjimbob View Post
    My original post regarded installing a HP to replace a straight cool AC. Now required by energy code. Inspector never asks about panel upgrades.
    What size service is in these homes? Do you do a load calc to know if they really need as much aux heat as the straight A/C had in strip heaters?
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  8. #21
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    Strip heaters are on the same time as HP defrosts. Regardless of load calculation

  9. #22
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    Quote Originally Posted by timebuilder View Post
    The key words there are "package units."

    A package unit has a single number for MCA and MOD. It is a simple matter to follow those values, which include in those vales the factor of continuous load.

    It's nice to have a set of stamped prints where all of the requirements have been met and approved by a plan review. Most HVAC contractors never see such a nicety.
    For a split system , I look at the max fuse / circuit breaker size for the out door unit & figer a circuit for it . Then I try to find out about the indoor unit and figure an appropiate circuit ot circuits for it .

    More and more often , having stamped prints is not the end all & be all . I am seeing more and more prints that almost seem to be a case of an engineer just throwing some ink at the paper . :-(

    In those cases , the safest path is to generate Requests For Information , to try to get something in black and white , to get accurite / correct imformation to bid by . Before bid date .

    If you never get a substantial reply , you have some ground to stand on . If you get the job and need to ask for change orders .

    God bless
    Wyr

  10. #23
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    I have not read the entire thread, however:

    Can someone post (or was it posted), the NEC code for necessary elec capacity with a HP system? Where I am going with this is to learn the minimums I need to look for when replacing a HP.

    We do not do much in the way of HP's in the Atlanta area, however if I get my way and retire in the mountains years forward, I would like to know the rules, THX!
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  11. #24
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    I think "second opinion" gave a educated response. It's not about opinions, but what is legal. Kinda like Zimmerman/Martin case.

  12. #25
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    Quote Originally Posted by ga-hvac-tech View Post
    I have not read the entire thread, however:

    Can someone post (or was it posted), the NEC code for necessary elec capacity with a HP system? Where I am going with this is to learn the minimums I need to look for when replacing a HP.

    We do not do much in the way of HP's in the Atlanta area, however if I get my way and retire in the mountains years forward, I would like to know the rules, THX!
    Are you asking about service calculations or the branch circuits to the actual HVAC equipment ?

    Are you talking about a split system heat pump ?

    If so , the outside unit , size the wire and fuse / circuit breaker to the name plate Max fuse / circuit breaker ampacity .

    The indoor unit / air handler / fan coil unit will have a small fan moter and probably the control transformer . These do not pull much current . Then , will it have electric resistance heat strips ? If so , find out the kw of each heat strip . As has been said , they come in several kw rateings .

    I typically see 5 kw & 10 kw . I run a 30 amp circuit to 5 kw and a 60 amp circuit to a 10 kw . Best I remember , 60 amp is the max , unles the unit itself has fuses or circuit breakers to sub divide the power . For instance , a 90 amp circuit to feed a FCU with a 5 & 10 kw heat strip . With internal fuses or CB's to feed the individual heat strips .

    The fan and control transformer is typically wired with the first stage of resistance heat ( and are 240 VAC ) , since their load is small . The second stage of heat pretty much only has the heat strip connected to it .

    As far as how much this impacts the service load calculations , it depends , as has been said , how the heat pump compressor and the heat strips , are staged .

    Worse case senerio , the above example would be 90 amps plus what ever the name plate minimun circuit ampacity of the out dooe unit is . All these numbers have built in 125% rateing for continious duity .

    In a straight A/C unit ( no heat pump ) , for service load calculations , you use the largest of the resistance heat load or the A/C load , but not both , since you should not be running A/C & heat at the same time .

    Since you live in Atlanta , I am guessing you would not need as much resistance heat , as some one in , say , Maine ?

    All this does not even considerable the possiability of a dual fule heat pump . I suspose that would be another compination ?

    I , myself have a 2 stage 92% gas furnace with a 4 ton A/C . 99% of the time , the furnace never goes to the high flame heat mode .

    God bless
    Wyr

  13. #26
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    If I remember My NEC AC and heat SERVICE load calculation called it a" non-coinsidental load" One or the other. It assumes your not running both. Things have changed since the 70s. Hardly any straight electric resistance heat now, except when defrost or second stage.

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