Utility Transformers: Load Assumptions 1

[PowerNomad]

I'm trying to develop some rules of thumb for small commercial generator sizing. I'm assuming that there are already some guidelines that utilities follow when placing transformers in new commercially zoned areas. I'm mainly interested in estimates for small commercial such as restaurants, grocery stores, gas stations, light industrial, etc. This might be very generalized, such as kW per square ft. Any ideas would be greatly appreciated!

 

[rbulsara]

Are you looking for generator sizing or transfomer?

Is that for one business facility or a group (zone) of facilities? How many?

Normally generators are sized for emergency loads only, unless applicaiton you are looking for is 100% standby power. Even many standby requirements are much less than normal loads required for a 'tranfromer' sizing.

 

[tinfoil]

For a permanently installed nonutility-owned generator, you may have Code issues to comply with.

 

[skiier]

General rules of thumb? Huh?
I think the best approach is a Load calculation. Don't you?
If you want info on stuff look in your local code book. CEC and NEC basically hold your hand and show you all the calcs you need. Ever seen a code book?

 

[PowerNomad]

I fully agree that load calculation would undoubtedly be, in the end, the only way to go. What I am trying to put together is a quick reference for people looking to get an initial rough estimate of what their type of business usually needs. I admit, approaching this problem from a utility perspective is problematic... first, most people will not back up all of their loads. Second, the utility can afford to be imprecise in its assumptions.

 

[tinfoil]

The best 'rule of thumb' is to do a load calculation.

Failing that, find a similar facility near where the proposed one, and find out their loading.

BTW, are you looking to backup ALL of their load, or just some emergency load? That will have a dramatic impact on your generator requirements

 

[jghrist]

Chapter 2 of IEEE Std 241, IEEE Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book), gives guidelines for load estimation and some typical values.

 

[busbar]

While there may appear to be a ready correlation, it may pay to be exceedingly cautious of comparing “loadability,” and particularly rate-of-change-of-load characteristics for oil-insulated transformers versus generators. Comparing C57.91 (transformer) to NEMA MG1§32-33 (generator) capabilities may be a daunting task.

N.B., there are also varying types of generator duties, such as for “prime” compared to “standby” units.