Residential Service Load Calculations using high BTU Mini Split HVAC systems

[iustacould]

Hello and thank you for taking the time to read through and provide input on my ideas and questions!

I am currently refining my single family service load calculation for a home that will be built this summer. I was thrown a curve a few days ago dealing with the HVAC units supplying the entire project. I initially calculated my service load using packaged units (gas), but was advised that the HVAC contractor has now decided to use Mini Splits to condition the entire project. These units have NO heat strips. I am ashamed to say that I have very little experience calculating loads with Mini's and would like some feedback on my numbers. Any help or feedback is greatly appreciated as I plan to eventually have the knowledge base necessary to pay it forward. *** The HVAC contractor has not provided me with the actual nameplate ratings on any units up to this point but gave me a rundown on amps required so I derived the following information through online research***

Mini Split Spec's: (no heat strips)

Two 36K BTU Splits at 230v (requested 50 amps)
One 29K BTU Split at 230v (requested 40 amps)
Two 22K BTU Splits at 230v (requested 30 amps)
One 18K BTU Split at 230v (requested 25 amps)
Two 15K BTU Splits at 230v (requested 20 amps)
Two 9K BTU Splits at 230v (requested 15 amps)

With this information I calculated (generally) my HVAC load at 100% of the AC (Largest of 220.80(C) Optional Method.
Total HVAC Load: 68KVA

Seems fairly steep considering the number of projects i've been a part of as a Journeyman over the years.

Im trying to keep my total load under a 600 amp service (all other calculations are solid) but the HVAC load is driving me up just past this mark. (around 679).

Be gentle please as I know I've included a limited amount of information and more than a normal share of questions.

Any help understanding the basics of calculating Mini Splits vs Package units or AC vs Heat on mini splits would be invaluable.

Project is in South Carolina, under 2017 NEC guidance, Single phase 120/240 Service.

 

[suemarkp]

I see no demand tables for AC units, so I think you are stuck with the sum of the loads and you chose the correct choice. But it looks like you're using the max breaker rating and not the MCA number on the units. Since there are so many and your load is huge, I would request the make/model of the units to be installed and look up the MCA values. Might get you down to 45 to 50 KVA.

From one sample of a mini split website ,the MCAs are as follows:
36K BTU = 30A
29K BTU = 25A
22K BTU = 18A
18K BTU = 15A
15K BTU = 15A
9K BTU = 11A

This looks to be a huge residence.

 

[d0nut]

You may want to consider a 3-phase service rather than a single phase, 120/240V service. I think most utility companies would balk at a 120/240V service over 600A.

 

[StarCat]

Typical Mini Split units that heat and cool are heatpump systems, meaning they do not generally have any kind of electric resistance AUX heat.
This means your highest running load is in cooling mode.
In Heating mode the system is performing " Refrigeration " duty which is notably less than AC load. Your actual peak cooling load may approach but should not exceed compressor RLA. The Mini Splits I have worked with from Mistubishi etc. are fairly efficient. I would still run MCA for all units in question.

 

[electrofelon]

In addition to what suemarkp said, I will go a little further and say you are still over counting adding up all of the MCA numbers. This is because each MCA figure has the compressor counted at 125%. For a feeder or service you only count the largest motor at 125%. So if you take 80% of the MCA's of all but one of the largest unit, that will give you a much better number. This is not technically correct but will be close. You would take the compressor amps and knock off the extra 25% on all but the largest, if you can get the data plate.

 

[suemarkp]

You will have to dig to find the compressor RLA on a mini split and even then it may not help. Each mini split has the inside fan, outside fan, controls, and compressor all in its nameplate MCA. So you can't just take 80% of it to get the true load without the 125% factor on the compressor. 90% may be closer but even then you're guessing unless each item in the minisplit has a nameplate amp value so you can derive everything.

 

[electrofelon]

You will have to dig to find the compressor RLA on a mini split and even then it may not help. Each mini split has the inside fan, outside fan, controls, and compressor all in its nameplate MCA. So you can't just take 80% of it to get the true load without the 125% factor on the compressor. 90% may be closer but even then you're guessing unless each item in the minisplit has a nameplate amp value so you can derive everything.

I have usually seen compressor RLA on the data plate for mini splits. Maybe your experience is different. Obviously it's not correct to just take 80% of the MCA, but that will be very close as the compressor is by far the largest load on a mini split. If you did that with a package unit, it may have multiple compressors and the fan motors are larger so you can be significantly off. I am looking at a Fujitsu mini right now and it shows comp as 20.5, fan 1.5, other loads 1.8. Taking 80% of the MCA (28.9) only under counts it by .7 amps. It may not be worth worrying about these deductions from the MCA in some situations, but others it can be very beneficial. Right now I am working on plans for a job that needs plan review, and it has 11 RTU's. Adding up the MCA gives me 83KVA, and there is already a 200 amp panel up there. Taking the deductions I come in just under 200 amp, by the skin of my teeth, but under.

 

[iustacould]

I appreciate all the insight and information! I should have nameplate by the end of the week. I'll post some pictures later on as the project gets underway. Again thanks for all the help!

 

[kwired]

Hello and thank you for taking the time to read through and provide input on my ideas and questions!

I am currently refining my single family service load calculation for a home that will be built this summer. I was thrown a curve a few days ago dealing with the HVAC units supplying the entire project. I initially calculated my service load using packaged units (gas), but was advised that the HVAC contractor has now decided to use Mini Splits to condition the entire project. These units have NO heat strips. I am ashamed to say that I have very little experience calculating loads with Mini's and would like some feedback on my numbers. Any help or feedback is greatly appreciated as I plan to eventually have the knowledge base necessary to pay it forward. *** The HVAC contractor has not provided me with the actual nameplate ratings on any units up to this point but gave me a rundown on amps required so I derived the following information through online research***

Mini Split Spec's: (no heat strips)

Two 36K BTU Splits at 230v (requested 50 amps)
One 29K BTU Split at 230v (requested 40 amps)
Two 22K BTU Splits at 230v (requested 30 amps)
One 18K BTU Split at 230v (requested 25 amps)
Two 15K BTU Splits at 230v (requested 20 amps)
Two 9K BTU Splits at 230v (requested 15 amps)

With this information I calculated (generally) my HVAC load at 100% of the AC (Largest of 220.80(C) Optional Method.
Total HVAC Load: 68KVA

Seems fairly steep considering the number of projects i've been a part of as a Journeyman over the years.

Im trying to keep my total load under a 600 amp service (all other calculations are solid) but the HVAC load is driving me up just past this mark. (around 679).

Be gentle please as I know I've included a limited amount of information and more than a normal share of questions.

Any help understanding the basics of calculating Mini Splits vs Package units or AC vs Heat on mini splits would be invaluable.

Project is in South Carolina, under 2017 NEC guidance, Single phase 120/240 Service.

My guess is "requested amps" is the recommended overcurrent device and not actual load. actual compressor rated amps might only be around half those values.

 

[electrofelon]

My guess is "requested amps" is the recommended overcurrent device and not actual load. actual compressor rated amps might only be around half those values.

Yeah many HVAC people probably are not real knowledgeable about the MCA Vs MOCP thing and would "request" a circuit of the MOCP size. In my experience, actual running load of HVAC units is always very close to 66% of MCA.

Those "requested" values do seem real high. Attached is the data plate of the mini I mentioned earlier in this thread. Dont quote me on it, but I am assuming from the part number it is 45,000 btu/hr, and as you can see it is only 29 A MCA.

 

[iustacould]

Solid copy Electrofelon! Preciate it!

 

[kwired]

Yeah many HVAC people probably are not real knowledgeable about the MCA Vs MOCP thing and would "request" a circuit of the MOCP size. In my experience, actual running load of HVAC units is always very close to 66% of MCA.

Those "requested" values do seem real high. Attached is the data plate of the mini I mentioned earlier in this thread. Dont quote me on it, but I am assuming from the part number it is 45,000 btu/hr, and as you can see it is only 29 A MCA.

Majority of time that is what I get from the HVAC people - "it needs to be a 30 amp breaker". I bet majority of those that need to be a 30 amp breaker can use 12 AWG conductor, but unless you actually know the MCA you still risk having the rare case where it needs 10 AWG, then use of NM cable VS other conductors that can be utilized at 75C can be a factor as well.