Duplex load calculations

[lead146]

I am going through the electrical exam prep book, and have come across two questions now that I have gotten wrong because I did not add in 25% of the largest motor load (A-C unit) per 220.60. My understanding was, if the A/C unit is a lessor load than the heat, it is omitted. Second, 220.60 does not state 25% of the motor load. It states 125% of the motor load. I'm confused. I'm additionally confused because throughout the course of examples on how to calculate these loads for the standard calculations, this 25% was not used. The lessor of the Heat v.s the A/C was simply omitted.

Please see Unit 7, 7.12, question 49 in the Mike holt's Electrical exam preparation book.

 

[don_resqcapt19]

This was a controversial change in the 2020 code. If the AC is eliminated from the load calculation as the lesser of the heat vs AC, and if the AC compressor is the largest motor load, you must include both the heat and 125% of the AC load.

It states 125% because you have not included that motor in the calculations. With other motor loads, you have already included them at 100%, and you add an additional 25% of the largest motor load.

 

[frofro19]

This was a controversial change in the 2020 code. If the AC is eliminated from the load calculation as the lesser of the heat vs AC, and if the AC compressor is the largest motor load, you must include both the heat and 125% of the AC load.

It states 125% because you have not included that motor in the calculations. With other motor loads, you have already included them at 100%, and you add an additional 25% of the largest motor load.

So if the AC is eliminated if it's the lesser of the two, why does it have to be included ? And why would both need to be included as they both would not be on at the same time?

 

[lead146]

I'm still not seeing where the code states 25% of the A/C or motor compressor when 220.60 says to use 125%. If the A/C is omitted as the lessor of the two noncoincident loads it would not be included in the calculation as frofro19 commented. I really just need to know whether or not I'm supposed to include 25% of the A/C load when it is not the largest noncoincident load when using the standard calculation, I would also like to see where exactly that language is in the code. And why these examples in the Mike Holt textbook are not consistent in showing that...

 

[wwhitney]

My opinion: the only rational way to handle 2 non-coincident loads is to in effect do 2 load calculations, one calculation with one load and another calculation with the other, and then use the larger. [Obviously alot of the calculation will be the same, so you can just pay attention to what the difference will be.]

220.60 has gotten a bit garbled lately, so the applicable version of it may be open to other interpretations that could result in a higher number than the above. Those interpretations do not represent what is possible in reality.

2017 NEC 220.60 was fairly simple: "Where it is unlikely that two or more noncoincident loads will be in use 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 a feeder or service." The reason for the subsequent changes is some ambiguity in the term "largest." It needs to be understood to mean "resulting in the largest final load."

Say I'm doing a standard load calc (220.60 does not apply to a 220.82 load calc), and I tell you a house has an A/C system with an MCA of 30A, where the largest motor in the A/C is the compressor with an RLA of 20A, and there is also non-coincident electric heat that draws 26A. Then I ask which is the "largest" of the non-coincident loads, the A/C or the electric heat?

The answer is "we don't know until you provide some information about the other motor loads". If the largest other motor load has a 20A FLA, then the heat is the largest load. [Heat only, get 26A+25A = 51A. A/C only, get 30A + 20A = 50A.] Whereas if the largest other motor load has a 10A FLA, then the A/C is the largest load. [Heat only, get 26A + 12.5A = 38.5A. A/C only, get 30A + 10A = 40A.]

Cheers, Wayne