Multiple AC unit feeder diversity

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ike5547

Senior Member
Location
Chico, CA
Occupation
Electrician
My understanding of sizing heating/cooling units for best overall efficiency and comfort means they will be sized to run quite a bit of the time . . . it was designed for the duty cycles run near or at 100%.

If you oversize a unit by too much you get less run time but comfort levels do suffer.


This is correct. An AC needs to remove the humidity as it removes the heat. An AC that is oversized will cool the space down but it will not dehumidify very well. So it has to be sized not to cycle too much.

Ü
 

Smart $

Esteemed Member
Location
Ohio
Yes you did imply that and that was incorrect.
No, it was quite likely correct for the circuit mentioned.

Now if he happened to mention A/C and furnaces, that'd be a different story.

Correct about what? That you can't predict if a group of thermostatically controlled equipment will never run together? Well congratulations on knowing what we all did. :)
Thank you, Mr. Sarcastic. ;)

We can say the same thing about demand factors for dryers and ranges.

There is no way to assure that all the ranges or dryers will not be used at the same time.
But yet Code provides us with an "out" for these loads. Hmmm. Why would they do that to us? :D
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
With A/C's you can pretty much guarantee that they will all try to start as well as run concurrently when power is restored after a failure during hot weather.
 

kwired

Electron manager
Location
NE Nebraska
With A/C's you can pretty much guarantee that they will all try to start as well as run concurrently when power is restored after a failure during hot weather.
Or they all try to start five minutes after power is restored if all of them have five minute anti-short cycle controls incorporated. Five minutes seems to be a popular setting for such controls anyway.
 

kwired

Electron manager
Location
NE Nebraska
Heating/cooling equipment will run nearly continuously at full capacity when conditions are near the design limits.

Ranges and dryers usually only have a warm up period then are followed by maintenance warming after that which can vary but often will be pretty limited compared to the initial warm up period, and the range has even less diversity because of the fact we usually don't use all the surface elements and all the oven elements every time we use the appliance, but when we do not everything is running at 100% for very long.
 

iwire

Moderator
Staff member
Location
Massachusetts
You've never been to Florida or Arizona in the summer, apparently. :huh:

No, never been.

But I do know that proper HVAC design does not result in 100% run time at the design temperature.

Do some HVAC systems run 100% of the time? Sure they do, just like many restaurant panels are running at 110% load. But neither is proper design
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
No, they will not run continuously if designed properly.
But they may run for an hour or so when first started. And the issue in question is probability of concurrence because of synchronization and/or long run times, not continuous load as defined by the NEC.
 

kwired

Electron manager
Location
NE Nebraska
No, they will not run continuously if designed properly.
Now I get to ask you if you even read what I wrote?

Nearly continuously when near design limits is not continuously, but definitely leaves little extra room to be able to justify using any kind of demand factors.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
But I do know that proper HVAC design does not result in 100% run time at the design temperature.
While common, that is an inefficient design philosophy.

Efficient HVAC design would be to select a design outdoor temperature at, say, the 99% percentile, and size the equipment to just meet the load at that design temperature. That gives you maybe 3 days a year when the system is slightly overtaxed and can't quite keep up, running at 100%. That's the tradeoff for more efficient and better performance the rest of the year. Avoiding oversizing in this way should provide more even temperature (less short cycling) and improved humidity reduction in the cooling season.

Cheers, Wayne
 

iwire

Moderator
Staff member
Location
Massachusetts
While common, that is an inefficient design philosophy.

Efficient HVAC design would be to select a design outdoor temperature at, say, the 99% percentile, and size the equipment to just meet the load at that design temperature. That gives you maybe 3 days a year when the system is slightly overtaxed and can't quite keep up, running at 100%. That's the tradeoff for more efficient and better performance the rest of the year. Avoiding oversizing in this way should provide more even temperature (less short cycling) and improved humidity reduction in the cooling season.

Cheers, Wayne

Right, 3 days a year it might run 100% but the design is not aimed at 100%.
 

Smart $

Esteemed Member
Location
Ohio
Right, 3 days a year it might run 100% but the design is not aimed at 100%.
And those 3 days a year the multiple A/C units mentioned in the OP would definitely be coincidental loads, thus disqualifying them from an application of 220.60.

:happyyes:
 

iwire

Moderator
Staff member
Location
Massachusetts
And those 3 days a year the multiple A/C units mentioned in the OP would definitely be coincidental loads, thus disqualifying them from an application of 220.60.

:happyyes:

Which is a strawman argument, I do not belive one person in this thread has said the OPs situation could use 220.60 and as for myself I clearly said otherwise.


Now, am I saying that cppoly can claim 'non-coincidental loads' in this case? No, I think it is likely the AC units would run at the same time unless there is some underlying reason they would not.




All my point has been is that you overstated the need or requiments for an interlock.
 

cppoly

Senior Member
Location
New York
Lot of informative posting, thanks.

So is the takeaway that that only way the NEC allows a diversity factor for feeders serving multiple AC units (for a commercial building) is through 220.60 non-coincidental loads? And you would have to justify this?

And even if you could use 220.60, there would still be the possibility of all units being turned on at the same time and running at peak (restarting after a power outage or at the hottest day of the year) and you would still need the ampacity for these events.
 

iwire

Moderator
Staff member
Location
Massachusetts
In my opinion the take away is that 220.60 allows some room if you can get the plan checker or inspector etc to agree on the 'likelihood' of something.

I think with a group of independent AC units that would be tough.

If we change it to say independent electric heat units vs independent AC units I think you could get a plan checker to agree it is unlikely the heat and cooling will run simultaneously. (Unless they are designed to)
 
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