NEC Service Size vs Reality

[Ragin Cajun]

We have been doing the engineering on several fast food restaurants. Jobs have been both new and upgrades.

Haven't had one restaurant that had a utility transformer larger than 150KVA, or ~420A rated load.

Haven't had one restaurant that had one year max demand larger than 123 KVA based on utility billing records and 15 minute max demand for an all electric building. That's even after the upgrades!


Yet the NEC calc's call for at least a 1000A service. Seems kinda crazy when you have a transformer good for only 420A that very adequately handles the load!

The 2005 handbook says that the "new" calcs were based on a study of actual restaurants.

The NEC's demand numbers are WAY off!

What am I missing here?!




RC

 

[ericsherman37]

You can get pretty decent demand factors incorporated into the calc when you have a bunch of kitchen equipment... but I don't know why there is a possible discrepancy between reality and the code calc. Maybe the NEC calc is overkill to encourage people to have restaurants engineered?

 

[iwire]

What am I missing here?!

I don't think your missing anything, I think you have a very good handle on it.

Install what is required by code and make money on it.

 

[acrwc10]

220.87.(1) is pretty clear and it sounds like you fulfilled that accurately. The new calculation numbers is dealing with "standards" with a lot of unknown built into it. But you could always sell a 1000 amp upgrade based on the numbers if you were so inclined

 

[benaround]

RC,

Don't forget that all the POCOs' equipment is outdoors and in free-air most of the time.

You don't hear about services starting fires very often, not so true about POCO transformers.

Resturaunts are filled with people, the top of a POCO pole, not so many people.

 

[Ragin Cajun]

BTW, these are all restaurants by the same LARGE national chain, basically the same size building, same equipment, etc. Basically prototypes. Changes between they are minor.

We have provided the drawings with 1000A services. I just can't help wondering . . .

RC

 

[steve66]

We should expect the NEC to have somewhat of a safety margin built in.

The local climate, and the exact HVAC details of your resturaunt could have a lot of effect on the actual electric use. A resturaunt in Alaska might come a lot closer to the actual calculation than one in California. (I think its interesting that this calculation requires you to use both the heating and cooling loads which is different than the standard calculation.)

Also, for the 15 min. demand @ 420 amps, you are going to have some peaks and valleys in current over that 15 min. So it's still hard what the actual peak current might have been.

 

[chris500]

Many of the prescriptive codes have a substantial margin of error built into the requirements. IRC and IBC come to mind, and they call for approximately 200% of what is calculated to need.

 

[ronmath]

Without looking at your loads I could not tell you if you are wrong or not, but we also do a lot of national chain type restaurants and the services range from 600 to 800 amps. Rarely do we hit 1000 except for larger restaurants with electric heat etc. I am assuming you are using 220.88 New Restaurant calcs? Make sure you calculate the standard method also to see which one is lower. We also do a number of grocery stores and the services always are much larger than reality serves. IMO the NEC is a little out of touch with reality in service calcs, ex. 150 watts counted for every 2 feet of track. Any energy code will limit track lighting to much less than this. Also to count 200 watts per foot of any "show" window in the service calcs is another one. I believe (some of you guys can maybe back me up on this one) the code at one time had a provision for sizing services based on known loads for "prototype" buildings such as you are describing, would be nice to get that back in there.

 

[Ragin Cajun]

Without looking at your loads I could not tell you if you are wrong or not, but we also do a lot of national chain type restaurants and the services range from 600 to 800 amps. Rarely do we hit 1000 except for larger restaurants with electric heat etc. I am assuming you are using 220.88 New Restaurant calcs? Make sure you calculate the standard method also to see which one is lower. We also do a number of grocery stores and the services always are much larger than reality serves. IMO the NEC is a little out of touch with reality in service calcs, ex. 150 watts counted for every 2 feet of track. Any energy code will limit track lighting to much less than this. Also to count 200 watts per foot of any "show" window in the service calcs is another one. I believe (some of you guys can maybe back me up on this one) the code at one time had a provision for sizing services based on known loads for "prototype" buildings such as you are describing, would be nice to get that back in there.

AMEN (From the choir). Exactly my point.

220.88 yields slightly less, just under 1000A.

BTW, this is a big "fast food" chain restaurant.

RC