# Figuring 3-phase usage cost



## Speedy Petey (Jan 10, 2007)

Ok, this is something I have never actually had to do. 
I need to figure the actual cost to run a panel full of parking lot lights. 

The lights are 208v and are in a 3-phase panel with eight 20A circuits.
I have the load on the whole panel. Can any one help me with the calculations? 
I don't need them done for me, just the formulas.


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## RePhase277 (Feb 5, 2008)

Volts x Amps x 1.732 x PF = Watts

(watts x hours)/1000 = kilowatt-hours

kW-hrs x (cost per kw-hr) = Cost


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## dronai (Apr 11, 2011)

Also there are different charges for time of day, and demand.


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## Speedy Petey (Jan 10, 2007)

Thing is I have different amperages across the three legs.

I can figure my wattages for each 120v leg. Can I figure them separately and then just combine them? Will that achieve the same outcome?


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## captkirk (Nov 21, 2007)

Speedy Petey said:


> Thing is I have different amperages across the three legs.
> 
> I can figure my wattages for each 120v leg. Can I figure them separately and then just combine them? Will that achieve the same outcome?


just take the highest one... I will get what you need...


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## RePhase277 (Feb 5, 2008)

Speedy Petey said:


> Thing is I have different amperages across the three legs.
> 
> I can figure my wattages for each 120v leg. Can I figure them separately and then just combine them? Will that achieve the same outcome?


Yes, you can just use 120 x Amps x PF and add all three together.

Assuming that all of the loads are 120 V L-N, that is. But if you have a mix of single phase and three phase loads, you will need to do each calc separately and then add them.


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## frenchelectrican (Mar 15, 2007)

Pete.,

If you deal with the parking lot luminarie which they useally are HID type so fill in the PF number it will be either .88 for HPF { high power factor } and .60 for LPF { low power factor } ( the LPF will typically found on some MV/MH or low wattage HPS with reactor ballast sans capaitor ) 

Merci,
Marc


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## Cletis (Aug 20, 2010)

*rough*

About 21,750 per year ....


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## Cletis (Aug 20, 2010)

*?*

Oh, excuse my crudeness. But, I took average daylight in middle of new york state and no demand factor since at night. Based it on 18amp load per circuit....

What type of light BTW?


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## RIVETER (Sep 26, 2009)

Speedy Petey said:


> Ok, this is something I have never actually had to do.
> I need to figure the actual cost to run a panel full of parking lot lights.
> 
> The lights are 208v and are in a 3-phase panel with eight 20A circuits.
> ...


Is it too simple to just know what power the fixtures use with a certain lamp and then multiply times the number of fixtures...then times 1.7 because it is three phase? Then you would multiply that by whatever your utility charges.


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