Dwelling Unit Service Load Calculation

[Mitch22123]

Hello all,

I am a new residential plans examiner (new to the 2017 NEC, have been using the 2018 IRC) trying to learn how to calculate the load of a service panel based on a picture of the panel schedule (this is all my AHJ requires). Mainly, this has been for de-rating of the main breaker for solar additions to meet 705.12. I have been confused with how to calculate in the HVAC loads. Many of the houses here are using 2 A/C units, and I have heard a multitude of different ways I should be calculating these.

My questions are:
1. Working off of a panel schedule only, is using the MCB x .8 an approved way to calculate the load of the appliance?
2. Where do the Air handler circuits fall into this? Typically I will see 1 or 2 15A circuits for them. Are these calculated at 100%?
3. With multiple AC units, are both units calculated at 100%?


Example:

1500 SQFT Dwelling Unit, has a microwave 15A, dryer 20A, refrigerator 15A, disposal 15A, cooktop 40A, dishwasher 15A, 2 A/C units 40A & 30A condenser circuits, (x2) 15A air handler circuits.

1500 x 3VA = 4500VA
Small appliance x2 = 3000VA
Laundry = 1500VA
Microwave = 1500VA
Dryer = 5000VA
Refer= 1200VA
Disposal = 700VA
Cooktop = 7680VA
Dishwasher = 1500VA

Total = 25,180VA
Using the optional method 220.82

First 10k @ 100%= 10,000VA
15,180VA Remaining
Remainder @ 40% = 6072VA
Total = 16,072VA

AC @ 100%=
AC1 = 7680VA
AC2 = 5760VA
AH1 = 1500VA
AH2 = 1500VA

Total = 32,512VA
Total Amps = 136A



Thank you for the help in advance.

 

[wwhitney]

Hello all,

I am a new residential plans examiner (new to the 2017 NEC, have been using the 2018 IRC) trying to learn how to calculate the load of a service panel based on a picture of the panel schedule (this is all my AHJ requires).

Basically not possible to do, as 220.82(B)(3), (B)(4), and all of (C) refer to the nameplate ratings. So at a minimum the panel schedule should be annotated with the nameplate ratings of the equipment connected to each branch circuit.

Cheers, Wayne

 

[Mitch22123]

Basically not possible to do, as 220.82(B)(3), (B)(4), and all of (C) refer to the nameplate ratings. So at a minimum the panel schedule should be annotated with the nameplate ratings of the equipment connected to each branch circuit.

Cheers, Wayne

This was my thought as-well.

Thank you!

 

[suemarkp]

Cooktop seems a little off. Is it really a cooktop or an electric range? Range would be typical on a 40A breaker, whereas just a cooktop would typically be a 30A. If its a range, they are typically in the 12KW ballpark.

Refrigerator typically isn't counted as it can be included as part of the kitchen small appliance loads. But if you put it on its own circuit you count it, or if it on another 1500VA kitchen circuit then you can add the additional 1500VA.

Is there a water heater, or is it gas?

Yes, you add the HVAC loads if you have multiples of them. It isn't clear to me what nameplate value to use for HVAC when doing the optional calculation. Using the MCA value is safest, but may be larger than necessary if you have multiple units. If you were using the standard calc, it would be the sum of compressor RLA, fan FLA, and control electronics load plus 25% of the largest one motor/compressor. MCA hits you with that 25% addition on each unit.

MCB x .8 doesn't tell you anything. Residential loads generally aren't continuous, so you could have a calculated load of 199A on a 200A breaker.

You can't do an accurate load calc from a panel schedule. If you do just use breaker values instead of nameplate VA, you will get a more conservative number so if it still works you're fine. But if it is too big, you need to dive into nameplates, especially on cooking equipment and HVAC.

 

[Mitch22123]

Cooktop seems a little off. Is it really a cooktop or an electric range? Range would be typical on a 40A breaker, whereas just a cooktop would typically be a 30A. If its a range, they are typically in the 12KW ballpark.

Refrigerator typically isn't counted as it can be included as part of the kitchen small appliance loads. But if you put it on its own circuit you count it, or if it on another 1500VA kitchen circuit then you can add the additional 1500VA.

Is there a water heater, or is it gas?

Yes, you add the HVAC loads if you have multiples of them. It isn't clear to me what nameplate value to use for HVAC when doing the optional calculation. Using the MCA value is safest, but may be larger than necessary if you have multiple units. If you were using the standard calc, it would be the sum of compressor RLA, fan FLA, and control electronics load plus 25% of the largest one motor/compressor. MCA hits you with that 25% addition on each unit.

MCB x .8 doesn't tell you anything. Residential loads generally aren't continuous, so you could have a calculated load of 199A on a 200A breaker.

You can't do an accurate load calc from a panel schedule. If you do just use breaker values instead of nameplate VA, you will get a more conservative number so if it still works you're fine. But if it is too big, you need to dive into nameplates, especially on cooking equipment and HVAC.

Good point on the range thank you.

Yes typically we are seeing separate circuits for the refrigerators now.

It was a gas water heater, wired into another circuit.

That's what I am figuring out now, the city does not want to require pictures of all the appliances ( I can understand why) but also I cannot do an accurate load calc if they aren't given. I have just been requesting the nameplates if they calc was close based on breaker size.

MCB x .8 is how I have been told by superiors to calculate the loads. Not sure where they have gotten this from.

I agree with using the MCA unless pictures of the name plates are provided.

Thanks for your help!

 

[Birken Vogt]

I do these kind of calcs weekly for power gen sizing, so have some familiarity with this.

1. Working off of a panel schedule only, is using the MCB x .8 an approved way to calculate the load of the appliance?
2. Where do the Air handler circuits fall into this? Typically I will see 1 or 2 15A circuits for them. Are these calculated at 100%?
3. With multiple AC units, are both units calculated at 100%?

1. MCB x 0.8 will always give a grossly inflated number. Sometimes it will get you close on cooking equipment.

2. I assume you are talking gas furnace + AHU but clarify. The best way if nameplates are available is to do AHU motor load alone. The igniter and/or inducer will not be running during cooling season which is the higher of the 2 loads (heat vs cool). Full size heat pumps are rare here.

3. Outdoor condensing units are calculated both motors at nameplate plus 25% of the one largest motor in the house which is usually the largest compressor. Then the Article 220 calc runs it through a diversity schedule depending.

1500 SQFT Dwelling Unit, has a microwave 15A, dryer 20A, refrigerator 15A, disposal 15A, cooktop 40A, dishwasher 15A, 2 A/C units 40A & 30A condenser circuits, (x2) 15A air handler circuits.

My observations on these type of appliances. MW=1200. Dryer = 26A if electric, if gas it gets lumped in the laundry circuit 1500 VA. Refrigerator usually 300-700 watts only. Disposal variable. Cooktop variable, often a circuit with a much higher breaker is serving a downsized cooktop or has been converted entirely to gas so you have to go checking. Dishwasher = 1100. AC and AHU see above.