Understanding how to apply section 220.60 for noncoincident loads in single-family, multifamily, or non-dwelling applications has been a struggle for students and educators for many code cycles. In fact, when the National Electrical Code changed in the 2020 edition by adding the language regarding the 125% at the end of 220.60, it served to only cause more confusion. Well, we are here to set the record straight and close that confusing application.
Now, it is a simple process for a single-family dwelling service load calculation or feeder sizing to individual dwelling units of a multi-family dwelling. You simply compare the noncoincident loads, in most dwelling applications it will more than likely be the Air Conditioning versus the Heat. Here are the basic steps:
1) Take the A/C Loads at 100% and compare them to the Heating Loads at 100% and determine which is the larger.
2) If the lessor (lower value) of the comparison in #1 above happens to contain the largest motor in the building, in this case, the hermetic refrigerant motor compressor, you redo the comparison calculation again with the largest motor at 125% and 100% of the associated loads, such air handler and condenser fan motor, against the heat and now whichever is larger is the load used in your load calculation. Simple Right!
Now, the FUN BEGINS. We know that in "Real Life" you would never use the Standard Method for calculating a multi-family building. The optional method is always the way to go and 220.84(C) can be your friend. However, we do remember that neutral calculations are forced to be under the Standard Method and to be honest, an electrical exam could bring this question into play so we want you to be ready for anything.
GIVEN: What is the calculated load for the AC vs Heat for a (10) Story Multifamily Dwelling using the Standard Method in Part III of Article 220?
Note: The AC and Heat will not be used simultaneously.
The basic question is "how do you treat the noncoincident loads for the AC vs Heat in this (10) story multifamily dwelling".
Following the thought process we already covered earlier in this blog, let's assume that we first compare the versus the AC at 100% resulted in the Heat being the greater noncoincident load. However, we then determined the AC has the largest motor in the multifamily dwelling. As a result, we would do the math again and as a result, now the AC ends up being the larger of the noncoincident loads, based on using 125% for the hermetic refrigerant motor compressor load, if it is the largest motor load, per 220.60.
Sounds confusing so let's see an example of the above statement.
AC Compressor* 3,910 VA
AC Condensor Fan 667 VA
Air Handler 828 VA
Total AC at 100% is 5,405 VA
Heat is 5000 VA
Air Handler 828 VA
Total Heat at 100% is 5,828 VA
Heat is larger than the AC. However, the AC contains the hermetic refrigerant motor compressor, which happens to represent the largest motor in the building.
So, now we have to do the comparison yet again based on the rules in 220.60.
For Example, the AC Compressor 3,910 VA x 1.25 = 4,887.5 or 4,888 VA
AC Condensor Fan 667 VA
Air Handler 828 VA
NEW - Total AC at 100% is 6,383 VA
After our exercise above, the new calculation for the AC loads ended up being 6,383 VA and the Heat remained the same at 5,828 VA. The AC is now the larger noncoincident load per 220.60.
Now, we have to take into account the language in 220.50 as that addresses motors and hermetic refrigerant motor compressors.
220.50 states that the largest motor, as referenced to 430.24 and/or 440.6, is to be taken at 125% and the other motors at 100%. Even if we have 9 motors of the exact same size associated with the other AC systems, it is clear in 220.50 that the designer only need take one of them into consideration for the additional 25%.
Yes, the language in 220.50 is confusing and even in the proposed 2023 NEC, it makes reference to "conductors" rather than loads. However, the code-savvy user is supposed to ignore it is for conductors and just follow the same percentage rules, which is 125% of the largest motor and 100% of the remaining motors.
The use of 220.60 is only to determine the larger of the two or more noncoincident loads and only serves to also meet 220.50 compliance. In our example, units 2 through 10 would simply follow the first part of 220.60 and compare which is larger, the AC or the Heat at 100% value.
Remember, the HEAT was originally larger at 5,828 VA and the AC was only at 5,405 VA prior to the additional 25% the last sentence of 220.60, and the applicable rules of 220.50.
So let's use the numbers we have to finish this calculation- We would use 6,383 VA for the first floor AC vs Heat Load, which also covers the 220.50 for the largest motor. Now, for floors 2 through 10, we would compare the Heat vs AC at their actual 100% load values, and whichever is larger is what we use. In our example, the Heat at 5,828 VA would be used for the other 9 units as the original AC load was only 5,405 VA.
Let's do the calculation and answer the original question. The loads are (5,828 VA x 9) + 6,383 VA = 58,835 VA. So as a result, the contribution to the service calculation for the Heat vs AC is 58,835 VA.
In closing, I totally know that no right-minded individual would EVER use the standard method to calculate a multifamily dwelling. However, you never know what may appear on an electrical exam. So my motto is always to be ready!
Paul Abernathy, CMECP® | CEO & President
Electrical Code Academy, Inc. | www.FastTraxSystem.com Office: 214-945-0653
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