demand factor for receptacles

[malachi constant]

I am designing a double tub panel that feeds 64 15A/1P receptacles. The receptacles feed battery charging equipment for a golf cart storage area. The manufacturer says to assume a 12A load, but that it will gradually taper off to get down to ~0A after 12 hours. It's theoretically possible but highly unlikely that all 64 golf carts would be fully discharged and get plugged in at the same time.

The NEC requires you use 180VA a receptacle, to which a demand factor of 0.50 is applied after the first 10kVA. I don't think this applies here, but can't figure out what does. It requires you use a different set of demand factors for motors, electric heat, lighting, etc. What about equipment like this?
A. Could I classify it as a 180VA receptacle? (I wouldn't, even if it let me.)
B. As a continuous load with 1.25 factor applied to each receptacle? (I don't think "continuous" applies, but don't know batteries that well, maybe it does?)
C. As a non-continuous load with 1.00 factor applied to each receptacle? (This is where I'm at now.)
D. As a motor with a factor of 1.25 applied to the first receptacle and 1.00 to each thereafter?
E. Can I apply a demand factor less than 1 to reflect the fact that these are highly unlikely to all be used at full load at the same time (similar to demand factors allowed for multiple elevators, multiple kitchen equipment, etc)?

Right now I am assuming a demand factor of 1.00 for each 12A load. 12A x 120V x 64 = 92kVA. 92KVA / 208V / SQRT(3) = 256A. Is this the load you would use to size the panel, and plug it into the overall service calcs? So 400A panel?

Thoughts are appreciated. Thanks!

 

[charlie b]

You are not dealing with motors, so we can forget that set of rules. You are not dealing with continuous loads, since the maximum current is not expected to continue for 3 hours or more. The charger may take 12 hours to complete its cycle, but will not be at maximum current that whole time. Also, the NEC does not require us to use 180 VA per receptacle, but rather tells us not to use a lower value. If you wish to count each of the 64 receptacles at 1440 VA, rather than 180 VA, you are free to do so. And I would do so.

So my calculation is as follows:

• 12 x 120 x 64 = 92160 VA.
• First 10 KVA at 100% = 10,000 VA
• Remainder at 50% = 41,080 VA
• Total = 51,080 VA.
• Divide by 360, and you get 142 amps.
• I would use a 225 amp panel.

Now then, why do I think I get to use the 50% above 10KVA demand factor? Because 220.44 says I can use it, if I calculated the receptacles using 220.14(H) or (I). I used 220.14(I), and I used a value not lower than 180 VA.

 

[chris kennedy]

I don't agree, I think the OP has a 220.14(A) application.

 

[iMuse97]

The one thing I can imagine is this: they have a golf outing fundraiser at the course with a shotgun start, and 54 (or even 72) carts (do they have 27 holes? or is it 36?) return and start charging within the same 15 minute interval.

There's your scenario (quite conceivable) for a maximum load. So I would plan for that type of load. Most course managers I know would not be very happy if their system went down on a busy day.

It looks like 220.14(A) to me.

 

[Hv&Lv]

The one thing I can imagine is this: they have a golf outing fundraiser at the course with a shotgun start, and 54 (or even 72) carts (do they have 27 holes? or is it 36?) return and start charging within the same 15 minute interval.

There's your scenario (quite conceivable) for a maximum load. So I would plan for that type of load. Most course managers I know would not be very happy if their system went down on a busy day.

It looks like 220.14(A) to me.

I third that...

 

[malachi constant]

220.14(A). I knew that was the way I was supposed to do it but couldn't find it in the code. Had looked at that section, just breezed right past it without it registering. So my 256A is correct. Great, thanks!

 

[augie47]

Most panelboard manufacturers offer a 250 amp (or 300 amp) panelboard as standard.. might save you a few $$ over a 400.

 

[charlie b]

OK. I'll concede the point. 220.14(A) would govern, and the 50% above 10K demand factor cannot be used.

 

[malachi constant]

OK. I'll concede the point. 220.14(A) would govern, and the 50% above 10K demand factor cannot be used.

I applaud your effort nonetheless.

 

[dana1028]

The manufacturer says to assume a 12A load, but that it will gradually taper off to get down to ~0A after 12 hours. It's theoretically possible but highly unlikely that all 64 golf carts would be fully discharged and get plugged in at the same time.

Right now I am assuming a demand factor of 1.00 for each 12A load. 12A x 120V x 64 = 92kVA. 92KVA / 208V / SQRT(3) = 256A. Is this the load you would use to size the panel, and plug it into the overall service calcs? So 400A panel?

Thoughts are appreciated. Thanks!

I'm wondering why you are using a demand factor of 1.00 when you have a continuous load for 12 hours. I know the load will not be 12A for the entire 12 hrs....it will gradually taper off to 0.....but you do have a continuous load...and you don't know how long the 12A charge actually lasts before it starts tapering off.

 

[GeorgeB]

I know the load will not be 12A for the entire 12 hrs....it will gradually taper off to 0.....but you do have a continuous load...and you don't know how long the 12A charge actually lasts before it starts tapering off.

I'm a bit of a battery geek ... and while you are mostly correct, others may not understand.

Modern smart chargers use "full" current until some criteria are met. If these work properly, how long depends mostly on the state of charge when connected. A fully discharged pack will probably be at full for 7 or 8 of those 12 hours then taper. That depends on battery chemistry and technology. A fully charged and reconnected pack will be at full for a minute or 2. For more about batteries than you want to know, look at http://www.batteryfaq.org/. From the charger portion of that, I'm attaching one of the plots ... see the faq for details. This graph is from Deltran, one maker of chargers. While based on "12V" technology, the concepts apply to series strings.

So in conclusion ... at the end of a day, the carts won't all be plugged in and charged at the same time ... golfers finish (?) 4 at a time, with perhaps 2 carts. In my OPINION, you should be ok to allow for about 80% of them to be on at full current. All used, brought home, put on charge over perhaps a 3-4 hour block. For an hour or 4, all could be at 100% ...

 

[malachi constant]

Dana & George, excellent points.
Seeing the graph and understanding batteries more perhaps they should be considered continuous and taken with a 125% demand factor.
I issued the project already with the 100% demand factor, but put the loads on a sufficiently large 400A panel, and the overall building service was also sized about 150A larger than calculated load.

Our mechanical engineers have had similarly confounding questions related to how many air changes per hour. They need to know what the batteries are doing when charging, when fully charged but plugged in, when fully charged but not plugged in, etc. They're trying to figure out if they cut power to the charging panel for the winter, can they cut back on some of the air changes? Not a question for this forum, but related in the sense that if one could limit how many receptacles charged at the same time, then one could exclude some receptacles from the service calc, while simultaneously reducing HVAC loads.

Unfortunately (or fortunately?) this was probably one of the projects that we do it, and don't see another one cross our desk for ten years. These are the kind of problems that don't usually get fleshed out unless you repeat a project type a few times in a row. I appreciate the responses.

 

[dana1028]

Our mechanical engineers have had similarly confounding questions related to how many air changes per hour. They need to know what the batteries are doing when charging, when fully charged but plugged in, when fully charged but not plugged in, etc. They're trying to figure out if they cut power to the charging panel for the winter, can they cut back on some of the air changes? Not a question for this forum, but related in the sense that if one could limit how many receptacles charged at the same time, then one could exclude some receptacles from the service calc, while simultaneously reducing HVAC loads.

There should be meters you can hook up to a relay to turn the fans on when the toxic vapors exceed a certain level and turn off when those levels are reduced.