Equipment sizing guidelines

[Jnox]

Recently I have gotten myself into sizing medium and low voltage switchgear and I have several questions.

Where can I find some good general sizing rules for switchgear, such as:
- Bus sizing taking into consideration MCC loading and other sources.
- Tie switch sizing in main-tie-main configuration.
- General rules when it comes to transformer sizing, such as how low is too low based on projected load.

I know much of it deals with how much HP and load is connected and how it is used.

Is there a good book, or a website that can make this a bit easier to understand, or does it mainly depend on the engineer designing the system?

 

[waross]

You will find the minimum standards in the National Electrical Code.
Not just a good idea, It's the law.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Jnox]

waross, NEC is a great sources for overcurrent protection, wire sizing etc, but as you may find it does not go into sizing based on service factors, loading etc.

 

[davidbeach]

Anticipated load * 150% is a start. Big is good. It will never be less expensive to add capacity than it is to build it in. I've seen far to many designs where somebody tried to save a buck on the initial sizing and was then cursed repeatedly, year after year, project after project. Don't make it too small. Then add a safety factor.

 

[burnt2x]

The limit you have to bear in mind is how deep the owner's pocket is. NEC will tell the minimum requirements, IMO. Make your initial estimates and present them to the owners.

 

[davidbeach]

During the 20 years or so that I did that type of design, there was only one project that I ever had any question after the fact about having over designed. That particular project was grossly over designed because the owner's reps I was dealing at the time wanted ever more; I struggled to keep the sizing down to a justifiable safety factor. I finally got them to shut up about needing more electrical capacity when I pointed out that the chiller system size that they had crammed down the throat of the mechanical engineers (only 150% of what was needed) would be unable to remove the heat that could be provided by the electrical system I thought was more than enough; never mind the people or the solar load. So, on a later project for the same owner, I really didn't appreciate comments (from people never involved with the first project) about how over designed it was.

Other than that, the only sizing complaints I ever heard or had were about things sized too small. Usually sized for the initial construction plus some minor amount. Things like a 600A MCC ('cause that's the smallest they come) fed with a 60A feeder when the code minimum would have been 50A. No spare capacity and no long term value to the owner.

 

[Jnox]

I use 80% service factor when I size equipment. Switchgear sized at 80% of projected load, then transformer rated for 80% of the switchgear. But then again, I see how much mechanical oversizes pump, fan motors and I have a general idea how much things will be loaded. Of course I do take into consideration future expansion for the customer, and always size wire/protection based on the equipment I choose.

I'm still curious if there is a good design book out there that would go into some detail on a good way to calculate this stuff.