Power Factor and Apparent Power

[chriskthx]

I received some sample voltage drop calculations from a peer that I am reviewing and am a little lost on a constant they are using to determine (VA).

Their equation is VA = 1.32(Power), in this case the power is the luminaire wattage (400W). Where is the 1.32 derived from?

The system is a 240/480V single phase system. A power factor of 0.9 was used.

Thank you.

 

[topgone]

I have no idea where 1.32 comes from! But since you mentioned "power" and if power means "horsepower", 1.32 is very close to the inverse of 0.746, which is the equivalent kW to one HP (1/0.746 = 1.34). I maybe wrong, please wait for others to chime in.

 

[rcwilson]

Could the 1.32 constant inlcude a fudge factor to account for ballast loss in the fixtures?

400W x 1.32 = 528 VA. At 0.9 pf, 528VA is 475.2 W. If my guess is right, the ballast loss would be estimated at 475-400 = 75 watts per 400W fixture.

It's been a while since I checked lighting calculations so I don't recall if this number is in the ball park, it sounds high for modern ballasts.

 

[zog]

I received some sample voltage drop calculations from a peer that I am reviewing and am a little lost on a constant they are using to determine (VA).

Their equation is VA = 1.32(Power), in this case the power is the luminaire wattage (400W). Where is the 1.32 derived from?

The system is a 240/480V single phase system. A power factor of 0.9 was used.

Thank you.

Sure it is not 1.732?

 

[markstg]

Sure it is not 1.732?

But he said it was a single phase system, so why would SQ RT of 3 come into Play? Hmmmmm Civil Engineers, guess I'll go check the stuctural calcs for the light pole base.

 

[zog]

But he said it was a single phase system, so why would SQ RT of 3 come into Play? Hmmmmm Civil Engineers, guess I'll go check the stuctural calcs for the light pole base.

Yeah I know, but that was the only connection I could make to 1.32

 

[BJ Conner]

Highway lighting

Highway lighting

The equation would work if the 1.32 was the reciprocial of the Power Factor. 1/1.32 = 0.75 which is a pretty bad power factor.
Send it back and ask for more detailed information. Check the specifications for the fixtures, it should say what the minimum power factor is. A power factor of 0.9 is available for most fixtures, 0.8 is common. I would reject one with 0.75.

 

[Bob Magee]

Ballast Efficacy Factor? Could they be considering 1.32 for the ballast and harmonic? 'Just a thought with what little info is available.

 

[mivey]

Bob Wilson had it right in post #3. They are assuming about a 84% loss factor for the ballast. 84%*90% = 76%. 1 / 76% = 1.32