Non-coincident loads for standard method

[Aaron Folk]

Hello! I'm preparing for my Journeyman's test and this is my first post - sorry if this is an obvious question:

Say a dwelling unit has one a/c and one electric heater. If the heater is the larger of the two, do I still need to add the extra 25% load from the a/c (provided it's the largest of all motor like devices)? The wording on 220.60 is what's confusing me.

Thanks!

 

[don_resqcapt19]

If the AC unit is the largest motor, and it likely is, you do need to include that 25%.
This rule makes no sense, but it is what the code requires.

 

[Aaron Folk]

If the AC unit is the largest motor, and it likely is, you do need to include that 25%.
This rule makes no sense, but it is what the code requires.

Thanks don. Are there any other articles that make this rule clearer or is 220.60 it?

 

[don_resqcapt19]

Thanks don. Are there any other articles that make this rule clearer or is 220.60 it?

That is it.

 

[wwhitney]

I infer from the question that it is in reference to the 2023 NEC.

Since "used in the calculation" is pretty vague, I think it's fine to interpret the 220.60 to mean that you do one calculation with the heater and not the A/C, and one calculation with the A/C and not the heater, and take the bigger result. In the first calculation you'd use the largest motor excluding the A/C, and in the second you'd use the largest motor including the A/C. So we're still using it in the calculation, it's just an argument to only one side of a top-level "max" function.

I submitted a 2026 PI to change 220.60 to simply read "If it is unlikely that two or more noncoincident loads will be in use simultaneously, using only the single load that results in the largest total load of a feeder or service shall be permitted." Hopefully that clearly states the above procedure.

Cheers, Wayne

 

[ggunn]

FWIW, it is not unusual in the winter for me to be running the A/C in my garage music studio at the same time as the heater in the rest of the house.

 

[Aaron Folk]

I infer from the question that it is in reference to the 2023 NEC.

Since "used in the calculation" is pretty vague, I think it's fine to interpret the 220.60 to mean that you do one calculation with the heater and not the A/C, and one calculation with the A/C and not the heater, and take the bigger result. In the first calculation you'd use the largest motor excluding the A/C, and in the second you'd use the largest motor including the A/C. So we're still using it in the calculation, it's just an argument to only one side of a top-level "max" function.

I submitted a 2026 PI to change 220.60 to simply read "If it is unlikely that two or more noncoincident loads will be in use simultaneously, using only the single load that results in the largest total load of a feeder or service shall be permitted." Hopefully that clearly states the above procedure.

Cheers, Wayne

Thanks Wayne,

Yup, we use 2023 NEC in MN. I'll make sure to mention that next time.

"Used in the calculation" is what I got stuck on, as far as how to use the 125% of the motor load that it specifies. The way I interpret it is: compare 100% of the heater (not a motor) and 125% of the A/C (a motor) and drop 100% of whichever is smaller from the calculation. If the A/C is smaller, it's dropped, but since it is a motor, you must still account for its 125% demand factor by adding its extra 25% to 100% of the non-motor heater. Does that sound right?

 

[jaggedben]

FWIW, it is not unusual in the winter for me to be running the A/C in my garage music studio at the same time as the heater in the rest of the house.

Sounds like you need a system to just funnel heat from one place to the other!

 

[wwhitney]

"Used in the calculation" is what I got stuck on, as far as how to use the 125% of the motor load that it specifies. The way I interpret it is: compare 100% of the heater (not a motor) and 125% of the A/C (a motor) and drop 100% of whichever is smaller from the calculation. If the A/C is smaller, it's dropped, but since it is a motor, you must still account for its 125% demand factor by adding its extra 25% to 100% of the non-motor heater. Does that sound right?

Doing that is one way to interpret 220.60, and is conservative, so it shouldn't lead to any violation.

However, it also makes no sense to add 25% of the A/C to 100% of the heater when 125% of the A/C is smaller than 100% of the heater, as the predicate of the section is that you wouldn't have the A/C starting while the heater is on. So I'm arguing that the language is ambiguous enough that you don't have to take that step.

BTW, note that if you use are an MCA figure from an A/C, that already includes any 125% factors required.

Cheers, Wayne

 

[ggunn]

Sounds like you need a system to just funnel heat from one place to the other!

Before we replaced the single aging central unit with two smaller ones I would close off all the registers in the rest of the house other than the studio and run the A/C when I had a session going. You can imagine how well that didn't work, especially seeing as the thermostat was in the hall outside the garage and there was no direct return air path into the house.