Tom Henry Calculations errors?

[Ryan Ray]

Page 187 of Tom Henry's Calculations book states that if you omit AC as the smaller due to the heat being larger, you use the next largest motor as the largest motor to be increased 25%. 220.60 states that you still us the AC as the largest motor even if it's omitted due to heat being the larger load. Has anyone seen this in Tom Henry's book? Am I missing something?

 

[Tommy1776]

Greetings Ryan Ray

Are you performing a calculation using the NEC 220.60, Noncoincident Loads section?

I have one of Tom Henry’s books for study, which book are you looking at?

Tommy

 

[don_resqcapt19]

This was a code change in the 2020 and revised again in the 2023. The language is very poorly written and there are code experts that read the requirement differently. Based on the first draft report it will be revised again for the 2026 but still will not be correct.

 

[Ryan Ray]

Greetings Ryan Ray

Are you performing a calculation using the NEC 220.60, Noncoincident Loads section?

I have one of Tom Henry’s books for study, which book are you looking at?

Tommy

Yes. I'm using his 2020 Calculations For The Electrical Exam book. It says to omit the AC if the heat is larger and not to use the AC as the largest motor load if it's omitted. The NEC does not say this and the NEC states to add 125% not 25% to the largest motor load.

 

[wwhitney]

Yes. I'm using his 2020 Calculations For The Electrical Exam book. It says to omit the AC if the heat is larger and not to use the AC as the largest motor load if it's omitted. The NEC does not say this and the NEC states to add 125% not 25% to the largest motor load.

No, the second and last sentence of 2020 NEC 220.60 says "Where a motor is part of the noncoincident load and is not the largest of the noncoincident loads, 125 percent of the motor load shall be used in the calculation if it is the largest motor."

"Used" is not the same as "added". You would use 125 percent instead of 100 percent, i.e. add 25 percent.

As to what those exact words mean, it's a bit unclear to me. ("Used in the calculation" could mean "added to the final total" or it could mean "used in a comparison, not necessarily ending up in the final total.") The scenario the sentence is trying to cover is this one:

You have a feeder supplying among other things a pair of non-coincident loads, say a 60A FLA motor and a 65A non-motor load. Which load is the larger load as far as calculating the required feeder ampacity?

You can't say unless you know the makeup of the other loads on the feeder. If there are no other motor loads on the feeder, then the 60A FLA motor has the larger impact on the feeder ampacity, as when it is included you need to factor it at 125%, and 125% * 60 > 65. But if there is, say, a 100A FLA motor on the feeder, then the 65A non-motor load has the larger impact on the feeder ampacity, as the 60A FLA motor would never get a 125% factor.

Cheers, Wayne

 

[Ryan Ray]

No, the second and last sentence of 2020 NEC 220.60 says "Where a motor is part of the noncoincident load and is not the largest of the noncoincident loads, 125 percent of the motor load shall be used in the calculation if it is the largest motor."

"Used" is not the same as "added". You would use 125 percent instead of 100 percent, i.e. add 25 percent.

As to what those exact words mean, it's a bit unclear to me. ("Used in the calculation" could mean "added to the final total" or it could mean "used in a comparison, not necessarily ending up in the final total.") The scenario the sentence is trying to cover is this one:

You have a feeder supplying among other things a pair of non-coincident loads, say a 60A FLA motor and a 65A non-motor load. Which load is the larger load as far as calculating the required feeder ampacity?

You can't say unless you know the makeup of the other loads on the feeder. If there are no other motor loads on the feeder, then the 60A FLA motor has the larger impact on the feeder ampacity, as when it is included you need to factor it at 125%, and 125% * 60 > 65. But if there is, say, a 100A FLA motor on the feeder, then the 65A non-motor load has the larger impact on the feeder ampacity, as the 60A FLA motor would never get a 125% factor.

Cheers, Wayne

Yes, the NEC states to use the AC as the largest motor even if it's omitted by the heat being larger. Tom Henry's book states to use the NEXT largest motor if AC is omitted. That's the contradiction I'm seeing.

 

[wwhitney]

Yes, the NEC states to use the AC as the largest motor even if it's omitted by the heat being larger.

I don't think that's unequivocal, and it also makes no sense. As Don stated in post #3, the language in 220.60 has been changing every year as they try to get this right. The latest language in the 2026 NEC First Draft is found in 120.6 (article and section renumbered):

"For two or more noncoincident loads, it shall be permitted to use the single largest of these loads in the calculation of the total load. Determination of the largest noncoincident load shall include treatment of any motor-operated and combination loads as specified in 120.11(A)."

I certainly don't see that as saying to use the AC as the largest motor for the extra 25% even when it's omitted due to the heat being larger.

Tom Henry's book states to use the NEXT largest motor if AC is omitted. That's the contradiction I'm seeing.

That approach is the only reasonable approach, regardless of whether the NEC language in the year you are subject to has succeeded in expressing that idea.

Cheers, Wayne

 

[Ryan Ray]

I don't think that's unequivocal, and it also makes no sense. As Don stated in post #3, the language in 220.60 has been changing every year as they try to get this right. The latest language in the 2026 NEC First Draft is found in 120.6 (article and section renumbered):

"For two or more noncoincident loads, it shall be permitted to use the single largest of these loads in the calculation of the total load. Determination of the largest noncoincident load shall include treatment of any motor-operated and combination loads as specified in 120.11(A)."

I certainly don't see that as saying to use the AC as the largest motor for the extra 25% even when it's omitted due to the heat being larger.


That approach is the only reasonable approach, regardless of whether the NEC language in the year you are subject to has succeeded in expressing that idea.

Cheers, Wayne

The issue is I'm taking the 2020 admin exam and it's unclear to me as how to calculate these loads. "Where a motor is part of the noncoincident load and is not the largest of the noncoincident loads, 125% of the motor load shall be used in the calculation if it is the largest motor." 2020NEC.
That makes zero sense but I guess I have to assume it's how they want it done on the test.

 

[wwhitney]

The issue is I'm taking the 2020 admin exam and it's unclear to me as how to calculate these loads.

The only answer I can give you is that the sentence is sufficiently ambiguous that it would be a poor practice to cover this on the exam. And given the unreasonableness of the interpretation of including the extra 25% for a motor excluded from the load calculation by 220.60, it would be doubly poor to expect the test taker to use that interpretation.

Note also that it is only an extra 25%, definitely not an extra 125%.

Cheers, Wayne

 

[Psychlo]

Page 187 of Tom Henry's Calculations book states that if you omit AC as the smaller due to the heat being larger, you use the next largest motor as the largest motor to be increased 25%. 220.60 states that you still us the AC as the largest motor even if it's omitted due to heat being the larger load. Has anyone seen this in Tom Henry's book? Am I missing something?

Unfortunately, this has been a problem for almost five years now. It's a shame the NFPA hasn't fixed it yet. They've been informed by many about the issue. The 2020 addition to 220.60 was bad, and 2023 revision was no help. In fact, it arguably made things worse. It's too bad, because this kind of negligence tends to diminish respect for the NEC by those who rely on it for their profession.

It's most frustrating to those who are taking exams, since there is no way to know which way the exam preparer interpreted the second sentence of 220.60.

Here is a video that discusses this very problem with the new wording of 220.60 - for both 2020 and 2023.