Load calculation

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augie47

augie47

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State Electrical Inspector (Retired)
Been there before but have a sudden brain freeze:

Using the optional method of calculation on a residence.

Residence has a conventional heating and cooling system (air handler with 5 kw heat, 21 amp
compressor).
Use the larger of the two loads at full value ?

(as opposed to 65% of heat rating for heat pump supplemental)
 
I read it as 100% of the heat pump and 65% of the electric heat unless they are not designed to come on together, then it would be 65% of the heat load. One would think it would be the larger of the 2 loads
 
I find the wording a bit confusing.
(1) refers to 100% of the air conditioning AND cooling which leads me to wonder if "air conditioning" includes heat.
(2) is all about heat pumps w/o supplemental heat
(3) addresses heat pumps and the supplemental heat system but mentions "central electric heat" which is what I have (with no heat pump involved)
(4)& (5) are electric space heating.

The examples in "D" don't seem to address the old style (non-heat pump) central air.
 
augie47 said:
I find the wording a bit confusing.
(1) refers to 100% of the air conditioning AND cooling which leads me to wonder if "air conditioning" includes heat.
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I don't see how -- I guess there are other forms of cooling then A/C. Heat would not fit in there IMO

(2) is all about heat pumps w/o supplemental heat
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Agreed

(
3) addresses heat pumps and the supplemental heat system but mentions "central electric heat" which is what I have (with no heat pump involved)
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This is the part that gets me because it seems to say we can take 65% of the heat load for the calculation but 100% of the a/c


(4)& (5) are electric space heating.
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Baseboards styles, I believe

The examples in "D" don't seem to address the old style (non-heat pump) central air.[/QUOTE]
 
Heat pumps generally produce a low temperature differential and so run for a long period of time and for a large percentage of the time.
Supplemental electric heat could be expected to cycle on and off under thermostatic control.
Just a guess.
 
augie47 said:
Been there before but have a sudden brain freeze:

Using the optional method of calculation on a residence.

Residence has a conventional heating and cooling system (air handler with 5 kw heat, 21 amp
compressor).
Use the larger of the two loads at full value ?

(as opposed to 65% of heat rating for heat pump supplemental)
Click to expand...

If I'm following your question correctly, for this application you would use the larger of 100% of the A/C OR 65% of the heat.
 
texie said:
If I'm following your question correctly, for this application you would use the larger of 100% of the A/C OR 65% of the heat.
Click to expand...
Agreed. The air handler w/5kW heat is considered 220.82(C)(4) electric space heating. If elements are 5kW, need to add blower kVA. Shouldn't be much, but still required.

Note that 21A compressor, assuming it is A/C only, at 240V calcualtes out to 5,040kVA and will also nee blower kVA added... so I'd say it is the larger.

If the compressor serves as a heat pump, throw the preceding out the window and calculate 220.82(C)(3).
 
Smart $ said:
Agreed. The air handler w/5kW heat is considered 220.82(C)(4) electric space heating. If elements are 5kW, need to add blower kVA. Shouldn't be much, but still required.

Note that 21A compressor, assuming it is A/C only, at 240V calcualtes out to 5,040kVA and will also nee blower kVA added... so I'd say it is the larger.

If the compressor serves as a heat pump, throw the preceding out the window and calculate 220.82(C)(3).
Click to expand...


IMO, the air handler with resistant heat is considered central electric space heating and not electric space heating as you stated
 
Smart $ said:
Being "central" does not make it not electric space heating. See 424.1.
Click to expand...

I see your point but the only time it may be an issue is if there are more than 4 separately controlled systems in a house. I am still not convinced that that is the intent.
 
Dennis Alwon said:
I see your point but the only time it may be an issue is if there are more than 4 separately controlled systems in a house. I am still not convinced that that is the intent.
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So under what item would you put a forced-air electric furnace. From a basics perspective, it is just an air handler with heating elements. ;)
 
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