Load Calc for an outdoor heat pump

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liam913

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Boston
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Electrical Engineer
liam913 said:
I've seen this a bunch with FLA * 125% = MCA. But I've also seen that is not the best way to go about this.
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So what is your suggestion? FLA*125%=MCA is the standard calculation for the MCA. Would you need to add the 25% anyway for this type of load.
 
Funny they tell you the wattage of the fan motors but no compressor amps or watts.

Since the information is not there, I would used the MCA.

My reason is the compressor is the largest load but if you take 80% of the MCA you are probably two low as the MCA is usually calculated at 125% of the largest load
 
Eddie702 said:
Funny they tell you the wattage of the fan motors but no compressor amps or watts.

Since the information is not there, I would used the MCA.

My reason is the compressor is the largest load but if you take 80% of the MCA you are probably two low as the MCA is usually calculated at 125% of the largest load
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this is why I'm confused hahaha. They list the compressor model on the sheet for the outdoor unit, but don't list any ratings for it. I don't know if that is factored into the MCA they give.

Really wish HVAC companies made it more clear how for electrical.
 
liam913 said:
I have gone down a rabbit hole on how to accurately size the load in VA.
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For a NEC article 220 service load calculation I request
  • Rated load amperes (RLA) of hermetically sealed compressor motor(s),
  • Full load amperes (FLA) of the fan motor(s),
  • kW of heat strips if any,
  • Motors smaller than 1/8 horsepower or other loads 1A or more in watts.

UL standard 1995 clause 44.3(f) required equipment to be marked with all that data, "the horsepower and full load amperes (FLA) of each motor, except for hermetically sealed compressor motors, which shall be rated in locked rotor and rated load amperes (RLA)" (see UL 1995 Clause 44.10), and motors smaller than 1/8 horsepower, which may be rated in watts or amperes.
1995 clause 44.3(g) states the heat strips if any are to be marked at input amperes or watts at marked voltage. See also Clause 44.3(t) for separable heater element assemblies. It also ignored loads less than 1 amp.
So that information should be available to you. You may need to request it which is annoying.

Note UL has harmonized with international standards and 1995 is now superseded by UL 60335-2-40, I think it still requires the markings 1995 clause 44.3(f) did but have not verified that.
I think also the new product standard allows manufacturers more ways to calculate MCA including factoring in locked rotor amps (LRA) so I would say MCA cant be teased apart to determine FLA or RLA.
 
liam913 said:
this is why I'm confused hahaha. They list the compressor model on the sheet for the outdoor unit, but don't list any ratings for it. I don't know if that is factored into the MCA they give.

Really wish HVAC companies made it more clear how for electrical.
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The information is available…have you asked your ME to request it.
If you both are in the same office or not, you need to be crystal clear on the information you must have.
Put together a check list to cover all these items so the ME can share with his/her vendors…
Voltage-Phase-Fla/Rla-disconnect-how is itemed controlled-SCCR-etc….
After you design all of your circuits, you need to then calc your AFC at each item and return those values to your ME so they spec the proper SCC rating.
 
Code requires to use the MCA (conductor sizing requirements) to determine the HVAC load.

220.50(B) Air-Conditioning Equipment.
The conductor sizing requirements specified in Part IV of Article 440 shall be used to determine air-conditioning loads for hermetic refrigerant motor-compressors.
 
ron said:
Code requires to use the MCA (conductor sizing requirements) to determine the HVAC load.

220.50(B) Air-Conditioning Equipment.
The conductor sizing requirements specified in Part IV of Article 440 shall be used to determine air-conditioning loads for hermetic refrigerant motor-compressors.
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Perhaps for a general calc in part III of 220 not part IV the optional calcs.
 
I used to do a lot of Roof Top units. The MCA and MOCP are always on the name plate as well as if fuses were required that was stated.

Just for practice I used to calculate the MCA just to see if it matched the name plate and it always did.

But it is rare to see a unit not marked with amps for the compressors, fans and electric heat.

If only for the HVAC tech when he is troubleshooting
 
Eddie702 said:
I used to do a lot of Roof Top units. The MCA and MOCP are always on the name plate as well as if fuses were required that was stated.

Just for practice I used to calculate the MCA just to see if it matched the name plate and it always did.

But it is rare to see a unit not marked with amps for the compressors, fans and electric heat.

If only for the HVAC tech when he is troubleshooting
Click to expand...
That used to be the case for me, now I'd say depending on the system its a toss up, the OP is asking about what appears to be a ductless minisplit, those typically have a VFD compressor and a VFD fan and the Daikin ones I see seem to add 15% to the compressor to get MCA here is a recent one:
3MXS24WMVJU9-s.png
 
I would just use the MCA. Also, be careful, you may need a neutral to the CU portion, we have come across this requirement lately because, especially Daikin, they have gone to ECM motors in the CU and they need 120V for that fan.
 
Question for the OP: what do you mean by 'accurately size the load in VA'?

Are you trying to determine the expected typical energy use or the _maximum_ possible energy use? The energy use will vary depending upon the fan speed settings, heating or cooling mode, and operating conditions.

You can find information about the actual power consumption in different conditions on the second page of the submittal you linked, and the 'data' section of: https://www.meiersupply.com/custome...L SERIES/MUZ MSZ FXNLHZ/FX18NLHZ/FX18TECH.pdf
 
Here is how I would turn this into a test question:
A 1800 SQFT dwelling unit with a existing 100A 120/240V service is having two identical ductless heatpumps added, each has a nameplate that says the following @ 230V, MCA 12A, Compressor RLA 6.6, Fan FLA 0.76, Each indoor unit has a nameplate that says FLA 1A @ 230V. The house has the following appliance nameplates that are fastened in place, permanently connected, or located to be on a specific circuit;
Range 11500kW, Dishwasher 9A @ 120V, 50 Gallon Electric resistance water-heater 4500W, Dryer 5500W, Well pump 230V 2HP.
Using 220.83(B) what is the NEC permitted minimum calculated load for this service?
A) 103 Amps
B) 96 Amps
C) 89 Amps
D) None of the above

My answer:

Answer is (B)
ItemQtyVALine Total
General Lighting and receptacles​
1800​
3​
5400​
Small Appliance & Laundry​
3​
1500​
4500​
Range​
1​
11500​
11500​
Dishwasher 9A*120V​
1​
1080​
1080​
Water heater​
1​
4500​
4500​
Dryer​
1​
5500​
5500​
Well Pump 2HP 230V Value 12A From T430.248​
1​
2880​
2880​
​
​
​
35360​
​
​
​
​
First 8 kVA of all other loads​
​
8000​
8000​
Remainder of all other loads @ 40%​
​
27360​
10944​
​
​
​
​
100% Of New HVAC load​
​
​
​
Outdoor units MUZ-FX06NLHZ (6.6 + 0.76 = 7.36)​
2​
1766​
3532​
Indoor Units @ 1 Amp each​
2​
240​
480​
Total VA​
​
​
22956​
Total Amps (Divide by 240V per 220.5)​
​
​
96​

 
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