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Joined: May 2001
Posts: 160
C
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Is there a service load demand factor for 6 independent heat pumps with supplemental heat strips for a 20,000sqft mega house?Or do I total all 6 units and add 65% of the total heat strips?Refer to 220.82C
Chris

2017 / 2014 NEC & Related Books and Study Guides
Joined: Jan 2004
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Chris, looks like nobody wants to make a comment on your question. The code doesn't exactly address this question when you are talking about multiple HVAC systems. At least I've not run across it. Usually a "normal" home has a single HVAC system and you can use the specs on the equipment and apply 220.82(C). Somethings that you might consider are first of all, find out if the heat pump is running when the electric backup is running. Secondly, Pick the largest heat pump at 125% and add the other five at 100%. Then do the same with the electric heat strips. The only thing not accounted for is the fact that they could all start at once (heat pumps) if the power were to be off for a while and then came back on. Possibly some staging could be implemented. I don't think there would be a problem demand wise under normal operating conditions calculating the load this way.
To answer your original question: "Is there a service load demand factor..." I don't know of any but I'm just another Inspector with an opinion. We have good Engineers who monitor this board and maybe they would add their thoughts.


George Little
Joined: May 2001
Posts: 160
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George
Thank you for the reply/opinion.Yes,the heat pump is running when the heat strips come on.I believe that your answer covered it.It is always good to hear from an inspector that is well informed.
Chris

Joined: Dec 2002
Posts: 318
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I do not know of any demand factors as conditions vary heavily. You would have to go back to the mechanical engineer who designed the setup as he would know how much things could have been oversized and could possibly figure out a demand factor to use that was acceptable to the AHJ. Otherwise the above method is correct.

It is my understanding that even if the power were out for a long time, the units would not come on at the same time due to random time delays built into typical units to avoid instant on with just a split second power return potentially damaging the the units.

Joined: Mar 2005
Posts: 1,213
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Most HVAC systems are designed for a particular outside temperature. If the outside temperature drops below that level (as it will maybe a few nights a year or a few nights a decade, depending on the design temp), the HVAC system will run continuously, unable to bring the temperature up as high as the thermostat is set.

Unless the mechanical engineer specifies otherwise, I would size the service to allow for all of the strip heaters to operate simultanously, otherwise you will be tripping breakers. Now, you *may* be able to uprate certain components based on low ambient temperature if they're in unconditioned spaces (since this will only happen when it's very very cold), but probably not. You might want to check and see what size transformer the poco has to serve this house, too, since they love to undersize those suckers and it's hard to justify a 600A service if the poco only has a 50kVA pole pig serving it.

Joined: Jan 2004
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Actually, 220.51 might also shed some light on the matter. I'm not sure if it would be considered "Fixed Electric Space Heating".


George Little
Joined: Apr 2002
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George:
I would stick with your original opinion, and concur BTW. Reading 220.51 tends to lead me into baseboard/cabinet type heating units, not strip type.

FWIW, I don't see to many heat pumps here, an occasional unit on an IG pool, none on resi/comm.



John
Joined: May 2001
Posts: 160
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Thanks to all for your responses.They were very helpful
Chris


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