Motor contribution central air conditioners and series rating in residential buildings NEC 240.86(C)(2)

Tainted

Tainted

Senior Member
Location
New York
Occupation
Engineer (PE)
See this article for reference as it will be relevant to my qeustion (starting on the 3rd page): https://www.eaton.com/content/dam/e...s-ele-series-rating-technical-information.pdf

In a 208V 3-phase building, I'm proposing (4)-600 amp 3-pole circuit breakers to feed 3-phase (4)-600 amp meter banks.

Each meter bank has 15 apartments. The meter banks have internal 2-pole breakers which feed apartments.
These 2-pole breakers will series rate with the 600 amp 3-pole breakers.

Central air conditioners have motors and therefore there will be motor contribution. If everyone in the apartment will have central AC, would this screw up the series rating because of NEC 240.86(C)(2)?

I know that the motor contribution is minimal from a short circuit standpoint, but what about from a series rating system standpoint?
 
Last edited:
If the central air unit compressors have some kind of variable speed drive there is no motor contribution since the motor can't backfeed to the line.
 
petersonra said:
If the central air unit compressors have some kind of variable speed drive there is no motor contribution since the motor can't backfeed to the line.
Click to expand...
But in my opinion, as far as code goes, code doesn’t care if there is a VFD. You still have to apply the 1% rule per 240.86(C)(2)
 
electrofelon said:
I would say they both need to be met, thus there is no motor contribution (at least that would mess up the series rating).
Click to expand...
So lets say each motor FLA in the apartment is 40 amps, this means (15) 2-pole apartments per meter bank will have a total of 600 FLA for the motors combined.

Does this mean that each 2-pole breaker inside the meter bank need to be 65kAIC?

I believe the panels inside the apartment units can stay at 10kAIC?
 
wwhitney said:
Since "situations" is plural, reads like an "or" to me.

Cheers, Wayne
Click to expand...
Where are you seeing the word "situations"?

There is an "and" between (1) and (2) in the 2017. Has the wording changed since?

C) Motor Contribution. Series ratings shall not be used where:
(1) Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.
 
electrofelon said:
Where are you seeing the word "situations"?
Click to expand...
2023 NEC.

Significant change between the 2023 NEC and the 2020 NEC. 2020 NEC says "and." 2023 deleted and and now it reads as "or". Oddly the change appears in the 2023 First Draft with no associated PI or substantiation, something I've not seen before.

Cheers, Wayne
 
wwhitney said:
2023 NEC.

Significant change between the 2023 NEC and the 2020 NEC. 2020 NEC says "and." 2023 deleted and and now it reads as "or". Oddly the change appears in the 2023 First Draft with no associated PI or substantiation, something I've not seen before.

Cheers, Wayne
Click to expand...
Interesting ok. But from an electrical theory standpoint, doesn't it have to be an "and"? Again I don't have the recent wording in front of me, but With an "or" it seems you would now have a blanket statement about the sum of motor currents regardless of where they are located which makes no sense.
 
electrofelon said:
Interesting ok. But from an electrical theory standpoint, doesn't it have to be an "and"? Again I don't have the recent wording in front of me, but With an "or" it seems you would now have a blanket statement about the sum of motor currents regardless of where they are located which makes no sense.
Click to expand...
To me it still reads as an "and" type condition.
 
david luchini said:
To me it still reads as an "and" type condition.
Click to expand...
Really, are we reading the same text? It total reads as an "or" to me now. Certainly it was clearer in the 2020 version.

Edit: OK, I see one signal now that it should still be and, the use of the word "these" in (2), indicating that (2) is not a stand-alone provision. But definitely worse in 2023 than 2020. In fact, this shouldn't even be a list, I would think.

2023 NEC 240.86(C) said:
(C) Motor Contribution.
Series ratings shall not be used in the following situations:
(1) Where motor circuits are connected between the higher-rated overcurrent device of a series-rated combination and on the lower-rated circuit breaker
(2) Where the sum of these motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker
Click to expand...

Cheers, Wayne
 
Last edited:
wwhitney said:
Really, are we reading the same text? It total reads as an "or" to me now. Certainly it was clearer in the 2020 version.



Cheers, Wayne
Click to expand...
I would say it is a typo, or horrible wording. I agree making "situation" plural makes it an "or". However in (2) it says "where the sum of these.....", which references the motors in (1), which makes it "and". I'm going with that clearly both have to apply. If either had to apply, then any motor at all in that location would disallow a series rating.
 
wwhitney said:
Really, are we reading the same text? It total reads as an "or" to me now. Certainly it was clearer in the 2020 version.

Edit: OK, I see one signal now that it should still be and, the use of the word "these" in (2), indicating that (2) is not a stand-alone provision. But definitely worse in 2023 than 2020. In fact, this shouldn't even be a list, I would think.



Cheers, Wayne
Click to expand...
Yes, that's the way I read it. I don't know why they change things that don't need changing. I'm still annoyed by the 215.2 revisions all those years ago. It made sense the way it was...and it's nothing but confusion the way it's written now.
 
petersonra said:
There is no motor connected to the branch circuit. How could there be a motor contribution.
Click to expand...
each apartment has a branch circuit with central AC. Central AC is connected to branch circuit and therefore is part of the motor contribution
 
You must log in or register to reply here.
Top