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# Load Calculation for Heat Pump and Air Handler

#1
04-28-08, 10:17 PM
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Load Calculation for Heat Pump and Air Handler

I'm trying to do the load calcs and have a few questions. I think I got everything else figured out but not sure how to calculate the load on the heat pump and the air handler. The HVAC guy says the air handler which has heat is 10 KW. He said the heat pump is a double pole 15. Also this is on a 3 family plus a common area so there are 4 AHs and 4 HPs, all identical.

Doing the math I get 17,200w total for the AH and HP per unit. (10 KW + (30*240)) = 10,000 + 7200. For the 4 units its 4*17200=68,800w!!!

The load calculator I got from the local inspection department says, "Where it is unlikely that two or more noncoincident loads will be used simultaneously, it shall be permissible to use only the LARGEST load(s) that will be used at one time for calculating the total load of the service."

So, do I only need to use the 10 kw for my calcs or do I have to use the full 17.2 kw per unit? I searched and saw some examples where the HP was calculated at 65%. The calculator I got says all loads for HVAC must be done at 100%.

Also, one of the electricians looked at the HW heater and said it was only 3500 w. I looked at it and it says 4500 at 240v, 3500 at 208v or something like that. When do you use 240v vs 208v for appliance calculations? I noticed the range I was looking at has the 240v/208v values as well. I don't understand the calcs on it either because it says its 11.7 kw at 240v with a 40A breaker. 11700w/240v=48.75A. how's that work?

I'm getting an electrician to do the service entry and the actual load calcs. I just want to understand it myself.

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#2
04-28-08, 11:02 PM
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ok this will be little instering with the load caluacatons.,,

for the HVAC and if the airhandler do have heat strip in there we always figure either load is largest senice you mention electric heat strip that is 10 KW which that is far more than the Heatpump unit will be running so just go that route with that way so it will be taken care of it unless your Air handler unit is designed to run both HP and electric heat but i really doubt it due they have interlock design to prevent that way.

and there is a additonal figures will be throwen in called load deveristy which it mean not everything will be running the same time. so you can get by with slightly smaller figures.

the electrician will able help you with that one and right now i don't have the NEC book with me at the moment but i know somewhere there is a table for that.

As far for the voltage for your waterheater and your electric heat strip in the airhandler it can change a bit depending on the POCO supply voltage and if it is off from 3Ø network or straght 120/240 v system

that the reason why you see the WH have dual marking due it will work either voltage the 240 v will read 4500 watts or 3800 watts at 208 volts [ if i recall it right unless stamped diffrent ]

ditto with your airhandler with heat strip you will get full 10 KW at 240 volts but it will be lower at 208 volts if the same element unless the unit is order to get full rateing on 208 volts.

hope it help you with this question you been asking for but however i am not able help you with the load demand figure due i don't have the code book with me right now unless other electicians here can chime in here as well.

Merci,Marc

#3
04-29-08, 09:19 AM
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I talked to the HVAC guy this morning. He was starting to set the heat pumps. He said the both the HP and the AH with heat could run at the same time if it was cold enough outside and the HP couldn't provide enough heat. The plate on the heat pump says it operates at 5.9A/240 V but pulls 38A/240 at startup. 5.9*240 is only 1416. Can I use that value for my calculations? He said it was a double pole 15. If it pulls 38A at startup, assuming you double the 15A to get 30A for the double pole breaker, wouldn't the 38A draw pop the breaker? He also said to run 10 wire out to the HPs. If its a double pole 15, would you size the wire based upon 15A or double it up to 30A? Also, what is the difference between a double pole 15A and a standard 30A breaker?

A lot of questions. Just trying to understand.

#4
04-29-08, 09:24 AM
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If I remember your situation correctly (3-family bldg, with 125A service to each unit, all electric) then you can use table 220.84 for computing a multi-unit demand factor. For 3-5 units in the building, you can multiply the demand of each unit by 0.45 (45%) when computing the load for the entire building. The common area loads do not get reduced by the demand factor.

When calculating the HVAC load for each unit, you pick the largest of the following:

1) 100% of the heat pump compressor plus air handler.
2) 100% of the heat pump compressor plus air handler plus 65% of the supplemental heat strips, if both the compressor and heat strips can activate simultaneously.
3) 65% of the supplemental heat strips if the heat strips and compressor cannot be active at the same time (interlocked system).
[...other options omitted which do not apply...]

Last edited by ibpooks; 04-29-08 at 09:41 AM.
#5
04-29-08, 09:34 AM
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Originally Posted by landlord373
He said the both the HP and the AH with heat could run at the same time if it was cold enough outside and the HP couldn't provide enough heat.
Okay, based on that you calculate the load of this heat pump on the service as 100% of the compressor running load (1416W) plus the air handler plus 65% of the heat strips.

He said it was a double pole 15. If it pulls 38A at startup, assuming you double the 15A to get 30A for the double pole breaker, wouldn't the 38A draw pop the breaker?
No, a 15A double pole is a 15A double pole; you do not add to 30A. The breaker has a mechanism which allows for a large burst of current when the motor first starts.

He also said to run 10 wire out to the HPs.
The larger wire size is to reduce voltage drop when the motor starts. It will lengthen the life of the motors and reduce the lights-dimming effect when the compressor kicks on.

If its a double pole 15, would you size the wire based upon 15A or double it up to 30A?
Circuits for motors are a special case for wire and breaker size. It's not as straightforward as general purpose circuits where #14 = 15A, #12 = 20A and so forth.

Also, what is the difference between a double pole 15A and a standard 30A breaker?
Well, a 15A double-pole is a standard size. I assume you mean the difference between single pole and double pole. The most fundamental difference is that single pole breakers power 120V circuits; double pole breakers power 240V circuits. DP breakers take up two slots in the panel.

#6
04-29-08, 10:15 AM
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Thanks Ben. Now I understand things a little better. Your breaker explanation and wire sizing makes total sense. And yes, this is the 3 family I was asking about before. Because the quotes I got from the electricians differed in sub panel size I am trying to figure it out myself. I'm thinking 125A should do it but want to be sure. The only piece of the puzzle I'm missing is the Air Handler draw. I'll have to look at the name plate tonight. Here is my load calc so far...

1030 sq ft per unit
3 Sm App circuits + 1 Laundry
1030*3 + 1500*3 + 1500 = 9090
3000 + (35% * 6090) = 5132

General Lighting Load 5132
Range 11700
HW Heater 4500
Air Handler ?
Heat Strips 6500 (10000 * 65%)
Heat Pump 1416

Total Load 29248
Min Service 122A

I don't have the Air Handler in there yet. Gotta figure that out. Other than that, does it look right? This is just for one unit. As Ben says, because this is a 3 family the total load on the building has different demand calcs. I'll get into that later Using the above it comes to about 345A for the building.

Also, code requires that I count one laundry circuit per unit. The units themselves don't have laundry. I have common laundry area in the basement for all three units. Can I drop that laundry circuit for the apartment load?

#7
04-29-08, 10:34 AM
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Originally Posted by landlord373
Range 11700
You can actually reduce this a bit. The first 8kW counts at 100%, the remainder counts at 40%.

Also, code requires that I count one laundry circuit per unit. The units themselves don't have laundry.
To the strict letter of the code, each unit needs the laundry load; however I think it would be an easy pitch to the inspector that all of your laundry loads will be counted in the common building service and not in the units.

Don't know how big your laundry room will be, but if you have 1-4 dryers in the laundry room, they must be counted at 100%. With 5, 85%; 6, 75%.

#8
04-29-08, 11:14 AM
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I have room for 2 dryers in the laundry area so I'll just count 2 @ 100% on the common service.

Concerning the range, the worksheet from the local inspection department says that if the range is under 12kw then you can just use 8kw, no additional calcs required. It also states that for 3 units with all ranges under 12kw that I can use 14kw for the total building service calculations. I used the actual for the unit just to be safe but the 14kw for the building service.

What is the definition of a small appliance circuit? I assumed this was the 20A circuits. The apartment unit has 3 20A circuits. Two for the kitchen and one for the bathroom. Is that the correct assumption?

#9
04-29-08, 11:18 AM
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If anyone is interested, here is the link to the load calculator I have been using. Its from my locality, Cincinnati OH but I'm sure it would work for other areas.

http://www.inspectionbureau.com/RCO%...mated_Rev1.xls

#10
04-29-08, 12:32 PM
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Originally Posted by landlord373
What is the definition of a small appliance circuit? I assumed this was the 20A circuits. The apartment unit has 3 20A circuits. Two for the kitchen and one for the bathroom. Is that the correct assumption?
The SABC specifically just means the two serving the kitchen countertop area. The bathroom circuit is not itemized in the load calc.

#11
04-29-08, 01:39 PM
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I just got done with the figures now [ finally got my NEC code book* and brought it home ] and yes Ibpooks [ ben ] pretty much hit on the nail with the figures it result the same as he give to you.

Merci,Marc

* normally i leave them in my office or in my service truck

#12
04-29-08, 08:20 PM
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Here are the plates from the AH and HP. On the AH the HVAC guys said its the 3rd heat package down which is the 9.6kw. I see a 15A 240V circuit there. Is that the AH motor circuit? WOuld that be another double pole 15 for the AH?

Sorry for the big image size. Had to enlarge it so it was readable.

#13
04-30-08, 12:54 AM
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Landlord373.,,

i took look at the photo what you provide here and speaking of the circuit hook up it should be one circuit for both electric heat and AH as long the listing show as one circuit [ few AH manufacter may listed diffrent ]

for the outdoor unit it only need 15 amp breaker without any question.

Merci,Marc

#14
04-30-08, 09:16 AM
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Looks like the air handler runs off the heat strip circuit. The motor is only 1/6HP, so not really worth considering in calculations.

The heat strip line says 240V, 40A heater, min ckt ampacity 51A, max OCPD 50A.

This means the circuit which supplies the air handler and heat strips needs to be installed with #6/2g NM-B cable and a 50A double-pole breaker.

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