Calculating Commercial Demand
- Thread starter drewski
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You need to break it down. I would make a simple table if it was just for an office. Basically, it would read:
Lighting @ 125%-------- ____________KW
HVAC @ 100%-------- ____________KW
Equipment @ 100%-------- ____________KW
Signage @ 100% 1.2 KW
Miscellaneous @ 100% _____________KW
Water Heater @ 100% ______________KW
Count your receptacles as 180VA if they are general and put them with your misc. items. Put in your HVAC load, signage is always 1.2KW. Lighting isn't really supposed to be at 125% but just do it anyway. If you read the NEC it says to size the branch circuit for 125%, but some plan reviewers dont beleive that so I just gave in pretty much.
Add these up and divide by your voltage. The total should be a little bit higher then your panel schedule (because of the extra 25% due to lighting).
There are other calculations that are optional in the NEC. rather then taking your actual lighting load you would use I believe 2.0 VA/square ft, otherwise everything else is the same.
I use this type of calculations for offices all over the country.
Lighting @ 125%-------- ____________KW
HVAC @ 100%-------- ____________KW
Equipment @ 100%-------- ____________KW
Signage @ 100% 1.2 KW
Miscellaneous @ 100% _____________KW
Water Heater @ 100% ______________KW
Count your receptacles as 180VA if they are general and put them with your misc. items. Put in your HVAC load, signage is always 1.2KW. Lighting isn't really supposed to be at 125% but just do it anyway. If you read the NEC it says to size the branch circuit for 125%, but some plan reviewers dont beleive that so I just gave in pretty much.
Add these up and divide by your voltage. The total should be a little bit higher then your panel schedule (because of the extra 25% due to lighting).
There are other calculations that are optional in the NEC. rather then taking your actual lighting load you would use I believe 2.0 VA/square ft, otherwise everything else is the same.
I use this type of calculations for offices all over the country.
mcclary's electrical
Senior Member
- Location
- VA
In your examples:
The water heater if smaller than 125 needs to be considered continuous use.
The heat needs to be considered continuous use
I wasn't aware of that. Are you saying smaller then 125 gallons? Continuous use is anything that runs for greater then 3 hours. Water heaters are either on or off dependent on the thermostat, if the elements turned on for over 3 hours I think the water would boil out of the relief valve. Which sections considers it continuous?
mcclary's electrical
Senior Member
- Location
- VA
Yes, I meant gallons. And yes that makes it continuous use 422.13
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You are right, but not the correct argument. That is only for the purpose of sizing branch circuits. That has nothing to do with load calculations. You size your breaker using 422.13 but you shouldn't artificially increase the load. For load calculation purposes the load is at 100%.
Thanks for pointing out the section.
Thanks for pointing out the section.
You are leaving out the demand factors of 220.42 and 220.44, you would also add 25% to the largest motor, see 220.18 and I think it should be mentioned that you can omit the smaller load between the AC and the Heat, see 220.60, I think you may have meant to say it, but for clarification, each sign circuit would have to calculated at 1.2 KW.
Roger
True, but there is no motor load if its an office. Also, I was assuming the person doing the load calc would only use the largest load for air conditioning, assuming they are split systems and not packaged. I do my load calcs based off of the panel schedule, I don't show a load on the condensing unit (assuming its the smaller load).
I don't use demand factors unless its necessary. If it is, say in the case of an existing service, then I can trim some fat with the demand factors if its required. Most offices I do don't ever seem to have over 10,000 VA of receptacles, and as far as 240.22 is concerned, theres no column for offices, it would fall under the "all others" category, which is calculated at 100%
I was trying to be simple with the guy that asked he seems like he doesen't do this a lot. Although your definitley right as far as the receptacles are concerned, I just never get a code rejection if I don't take a receptacle reduction after 10,000VA.
Nick
I don't use demand factors unless its necessary. If it is, say in the case of an existing service, then I can trim some fat with the demand factors if its required. Most offices I do don't ever seem to have over 10,000 VA of receptacles, and as far as 240.22 is concerned, theres no column for offices, it would fall under the "all others" category, which is calculated at 100%
I was trying to be simple with the guy that asked he seems like he doesen't do this a lot. Although your definitley right as far as the receptacles are concerned, I just never get a code rejection if I don't take a receptacle reduction after 10,000VA.
Nick
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