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	Subject - lighting questions paul.landino How would I compute the power factor of a fluorescent light fixture with two 40-watt lamps and a ballast marked 84 watts? I would like to know a formula. I also would like to know why many incandescent lamps intended for use on a 120-volt system are rated 125 or 130 volts. Thanks for your help. Mike Delaney Power(watts) = Amps * Volts Amps = Power / Volts Volts = Power / Amps Higher the voltage, lower the amperage. I know I've been told by installing the higher voltage bulbs they last longer...but I don't know how true that is. lctrc789 Well, I hope some of you guys can help me with this one it has been a while since I had to do this but here goes. Power factor is the true power to the apparent power. Divide the resistive value by the total value. V/D across the resisitive load by the total circuit voltage. or resistance didvided by the impedance or watts divided by the V/A. So what does this mean Apparent power is the volts X amps. In this case the apparent power would be 80 watts at 120 volts or 80 divided by 120 or .66 amps true power would be 84 watts or the same formula at .7 amps. The power factor would be a vector true power .7 app power would be .66 I think I am on right track here The power factor in this case would be very high but unpractical for a single use. As far as the 120 volt incandescent light bulbs go You can buy 120 ,130 and 140 volt light bulbs. the bulb is rated at the nominal voltage but in many cases we all know the voltage is 123 or 125 or greater. The higher the voltage the bulb the more life it probably have and the less wattage it does put out but usually not enough to make a difference. JimmyDee I think this is a little easier that that. 80 watts/84 watts = 95.2% I think. It takes 84 watts actual to produce 80 watts of light. That sounds about right for the new high power factor ballests. Jim side-cut1 Hey Jim, quick question for you. We are getting ready to install 26 1000 watt sports lights for a baseball diamond. Is it correct to calculate the lighting load at 125%. Just wondering. lctrc789 I would say that you must calculate the loads as per article 220-4 (b) total amperage of the ballast not the bulbs. Also read article 225-7 and it refers you to article 210-20 and that tells you that your load is at 125 % So you are right on the money to calculate this at 125 % of the ballast not the lights. side-cut1 Thanks Pat, that helps me out.