Load Calcs?

[Mike01]

Correct me if I am wrong but the code states to use rated horsepower when performing load calculations for commercial building, however looking at many different projects mechanical unit / electrical data the unit most of the time is powered by a ASD and the break HP lets say 62 so they install a 75HP motor. I have seen some firms that apply a demand factor to packaged roof tops and other mechanical equipment based on this type of information. I guess my question is it incorrect from a code standpoint to apply a demand factor to mechanical equipment? Or is this an acceptable practice? Like I stated above I have seen different firms utilize the rated HP and others apply demand factors of varying values. Just curious on others opinions.

 

[kingpb]

I believe you are talking about instances where you are looking at connected load versus running load. IMO, you need to size the branch circuit conductors along with motor and conductor protection based on the full rated Hp of the motor.

You would use the maximum operating Hp to determine the load on your panel/switchboard/MCC and for sizing the service requirements. I know some will disagree on the second point.:smile:

 

[Mike01]

Full Rated Vs Running Load

Full Rated Vs Running Load

I guess that is the part I do not understand, I would size my overcurrent protection and conductors based on the full rated HP but when doing my load clacs. Why not take the running load or apply a little demand to the overall load. I know it is a conservative approach to design to rated HP and there is nothing wrong with that but the code makes allowances for everything else, for lighting (in some instances), receptacles, kitchens, and even under article 517 for health care facilities for x-ray and other medical equipment it allows demand factors to be applied, why not with mechanical equipment? It?s funny the will not let you do it with mechanical equipment but I have seen more people get burned with the kitchen demand factors, because in my experience what is the first thing the chef does when he gets into his kitchen turns everything on, so there goes your demand factor out the window. I have always seen kitchen distributions designed for the connected load but on the load summary taking the allowed demand. The previous two firms I worked at applied 80% to all mechanical loads, where I work now 100%. IMO the code should allow for some judgment calls here, the do for almost every other type of installation. I am just curious if my previous employers used 80% no one ever raised an issue so their must be some kind of understanding amongst the reviewers. Just my 2 cents.

 

[necnotevenclose]

I'm not sure why your last company figured all of the loads at 80%. Article 430 allows you to size motors based on the FLA for the continuous, non-continuous and intermittent. 430.24 addressess calculating multiple motors. As a summation from this article and from my experiance all motor loads are calculated at 100% with the largest load calculated at 25%. There are some cases in the code where it requires you to use the RLA to size the OCPD. Take a look at this article:

http://bg.ecmweb.com/ar/electric_motor_calculations_part/index.htm

As for demand factors engineers may provide only the code required demand factors and leave the optional demand factors (residential, schools, elevators, intermittent motors) out just as a safeguard to ensure the feeders and service are sized appropriately.

 

[hbendillo]

I've yet to find a demand factor that I could use legitimately for HVAC equipment or motors in general beyond the 100% + 25% of largest rule. It would seem to me that we would have enough historical data to apply demand factors for HVAC loads but I guess the NEC is afraid of the many variables involved. I think some engineers apply some of their own demand factors for various loads. Not many jurisdictions check actual load calculations to assure strict adherence to the the NEC. The engineers probably never get in trouble either because of the usual tremendous difference between NEC demand and actual demand. As for me, I stick to the NEC.