Power Supply Standards 1

[noel0589]

Hi all,

I have a question and hope that someone can help. What or who determines the power supply of a building? I work for an elevator company and I see 480V about 70% of the time but then I see that sometimes there are 208V and more rarely I see 240V. For motors using wye-delta starting its better to have 480V, so why is it that there can also be 240 or 208. I know power from the utility company is downsized in a transformer before supplying a building, but does somebody know what the standards are? Thanks for any responses!!

 

[DRWeig]

Hi Noel0589,

Ideally, the choice of service is based on the relative costs, which in turn are based on the ratio of 120V load to load that could be applied at higher voltage.

Example: If my un-airconditioned warehouse building has all incandescent lighting, tons of 120V receptacle type load, and the only thing that could be selected at 480V is its elevator, it wouldnt' make sense to get a 480V service from the utility and then have to buy another transformer to step down 95% of my load to serve all the 120V equipment.

On the other hand, if I have all flourescent lighting (277V) and lots of three-phase motor loads, the money saved by having the higher voltage available offsets the cost of transformers for the less-significant 120V loads.

In theory, the electrical engineer designing the facility should look at total costs both ways and pick the one that turns out to be most economical. In practice, there are limitations on how big a 208V service can be.

I hope I've made some sense here. Please feel free to point out if I've confused...

Best to ya,

Old Dave

 

[busbar]

 
One reference for North American practices is chapter 3 of the IEEE Red book {Standard 141.}

 

[buzzp]

DRweig pointed it out, its a matter of economics. Higher voltages (up to some load level) means higher cost, generally.

 

[cuky2000]

QUESTION A: What or who determines the power supply of a building? The type of service is determine by the local electrical utility which will be provided at a particular location is ordinarily determined by one or more of the following conditions: Type of service available at that location,type of load,size of load to be served and other criterias.
NOTE: Not only there is no uniform electric service in the power industry but also this could vary within regions of the same utility company. This could be different from network-interconnected systems such as NY City and downtown areas in large cities. In other cases the service varies depending on rating capacities base on type of installation such as pole or pad mounted transformers either in overhead or underground configuration. For specific service voltage, consult the local utility installation requirement often available in the internet. See sample enclose

http://www.ceco.com/comed/library/pdfs/Rule_Book_Complete.pdf

QUESTION B: For motors using wye-delta starting its better to have 480V, so why is it that there can also be 240 or 208. Wye-Delta starter can be used in a wide range of voltage levels, however the starter will be larger as lower is the operating voltage. The Wye-Delta starter is one of the simplest and low cost available for voltage reduction method accepted by many utilities to keep the inrush current within the allowable limits. (For additional info, check faq237-613)
Beware that conventional electromechanical Wye-Delta starter could create severe transient conditions. The enclose fig & site provide the shape of the different starting curves.

http://www.lmphotonics.com/star_delta_curve.gif

QUESTION C ……does somebody knows what the standards are? Most utilities in the US have adopted the ANSI Std C84.1 -Electric Power Systems and Equipment, Voltage Ratings (60-Hertz). This standard establishes nominal rating for utilities to regulate the service delivery and it established operating tolerance at the point of power delivery to the user.