HVac load Calc?

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vasparky27

vasparky27

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Location
Chesapeake, Va.
scenario:

10 KW heat strips in furnace with 3 ton heat pump

Do I need to use 90 amps feeding a sub panel?
60 for the strips and 30 for the heat pump?
Why I am i wiring them on such a big load size if they are never going to be running at the same time? couldent I just feed the panel wITH 60 AMPS?
 
Re: HVac load Calc?

If the feeder will only see one load at anytime, then it only needs to be sized for the larger of the two loads.
 
Re: HVac load Calc?

I am in Va Beach and noticing that this is done widly we wire for the entire load not just the larger of the 2 any ideas as too why? and what a helper can do to to tell a 20 year sparky that hes doing it wrong and not get slugged?
 
Re: HVac load Calc?

Originally posted by vasparky27:
I am in Va Beach and noticing that this is done widly we wire for the entire load not just the larger of the 2 any ideas as too why? and what a helper can do to to tell a 20 year sparky that hes doing it wrong and not get slugged?
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he can't. it's just the way it is.

OTOH, maybe he is not wrong.
 
Re: HVac load Calc?

Doesn't a heat pump work both directions??? It cools in the summer and heats in the winter?? I thought the heat pump and the heat strips both run on the coldest days??

Steve
 
Re: HVac load Calc?

Originally posted by vasparky27:
would the heat pump and heat strips ever be on at the same time for any reason?
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In NC we are set up so that the strip heat stages in. This is to say that when the heat pump can?t do the job then the strip heat will come on to help it out. Being that I am south of you I would bet that your does also. :)
 
Re: HVac load Calc?

Originally posted by jwelectric:
In NC we are set up so that the strip heat stages in. This is to say that when the heat pump can?t do the job then the strip heat will come on to help it out. Being that I am south of you I would bet that your does also. :)
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That's pretty much how I've seen them done, too. So it's fairly likely that they will both be on at the same time.

I have seen some "low temp override" controls used to turn off the compressor if it gets too cold outside for the heat pump to be effective, so the condensing unit doesn't freeze up. I have seen some condensing unit coils literally frozen in ice -- consuming a lot of power, but producing no heat.
 
Re: HVac load Calc?

Originally posted by vasparky27:
ahh well I was hoping that I could downsize the wire can I wire it for minimum operating amps?
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If you mean can you alter the controls so that the strip heat and the compressor can't operate at the same time, then yes it could be done... but why are you wanting to do this so badly? You'll be changing the design of the equipment. :eek:

You'll lower their heating capacity if you do that and will have to modify the control setup. Depending on the design of the equipment and controls (such as: does it have an automatic defrost cycle or a low-temp shutdown on the compressor) that might be easy or pretty complicated.

You may make the HVAC contractor, home owner and builder pretty unhappy with your new control system. I see a potential voided HVAC equipment warranty in the future.

Can you read a control schematic?
 
Re: HVac load Calc?

sorry I was vague...
I meant can I wire for the listed minimum current ampacity on the nameplate or am I required too wire for minimum overcurrent amperge? and how does article 440 tie in too this? 125% of what?
 
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