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plj 01-21-2008 09:43 AM

$ to run light @ 120 vs 240
I'm hoping you folks can educate me a little bit - I'm trying to understand the costs involved so that I can make an educated decision on which light fixtures to purchase (if any), and what voltage to have them wired them at.

I currently have 10 250 watt metal halide light fixtures at my business, they have multitap ballasts and are connected at 240 volts. I'm being advised to convert to flourescent to save $. Yes, I know flourescents consume less energy - but exactly how much less? To answer that, I need to know exactly how much my 250W MH fixtures consume, and I'm not sure how to measure that.

My limited understanding (I am not an electrician) is: Watts = volts * amps. So I measured the amps on one of the lights, it was 1.07. I also doublechecked (measured) the volts, it is 240. So my watt consumption for this fixture is 240 * 1.07 = 256.8 watts, so if I run that light for about 4 hours it costs me 1.027 kwh, for which the electric company charges me 16 cents. Right?

Now, heres the problem: I'm being told by multiple people (not electricians) that my calculations dont work, because I'm running at 240V which is a lot more efficient than 120V. They're telling me that I cant just measure the volts & amps to get the actual costs, since the 240V is a higher pressure it actaully requires less power to run the light, so my charges will be lower. Is there any truth to that? Can I simply measure the volts and amps to estimate my costs?

Btw, I am aware of demand charges etc. Also I do not need accuracy down to tenths of a penny, I just want to get close. The guys are telling me that running the exact same light fixture at 240 will reduce my energy costs by 25% compared to running it at 120V ... I'm skeptical. If its true, I want to understand how, and I want to understand how to calculate the costs.

I'd be grateful if somebody could explain this to me. Thanks.

AllanJ 01-21-2008 10:19 AM

Your 240 volt savings are mainly in the wiring.

A given wire (thickness wise) carries the same number of amperes at 240 (or twelve) volts as it does at 120 volts. More specifically the number of volts dropped (not the percentage of volts dropped) in a given wire is the same for a given number of amperes at any voltage. So you get by with one size smaller cables for so many watts if you run 240 volts. Of course you need 240 volt fixtures or multi-voltage multi-tap ballasted fixtures.

You can expect that the fixtures themselves consume the same number of watts for the same amount (lumens) of the same kind (Metal halide, etc.) light regardless of rated voltage.

Since you know the voltage, you want to know the amperage. You would need either a wired-in ammeter (the clamp on kind is not accurate enough) or do calculations based on the electric meter's spinning.

I'm told that ordinary fluorescents give more light per watt than metal halide, although I don[t know how much. Metal halide gives about the same light per watt as compact fluorescent.

plj 01-21-2008 11:58 AM

Thanks for the quick reply.


Originally Posted by AllanJ (Post 90557)
You would need either a wired-in ammeter (the clamp on kind is not accurate enough)

I use a clamp ammeter. How inaccurate are they? For example, my measurement of 1.07 amps. Is it probably off in the hundredths, or the tenths? Although a quick calc of 1.07A compared to 1.06A does make a couple of watts difference at 240V, hmmm.

elkangorito 01-21-2008 12:10 PM

The amount of power consumed by any electrical device is the same regardless of the voltage input, providing that the appliance can accept a different voltage. Apply the formula you already know: P = E x I. The power consumption of the device must remain constant, so if the voltage is doubled, the current is halved.

As AllanJ has pointed out, the only real savings you'll get by having a higher voltage supplied to your light is that of "copper losses". If you supply the light with a lower voltage, the current must increase proportionately. This will increase the power losses in the cable that supplies the light.

At the end of the day, you would be marginally better off to have your light supplied with 240v rather than 120v. Of course, this is not taking into account the actual voltage at the globe & how this voltage affects the life of the globe.

220/221 01-21-2008 01:00 PM


The guys are telling me that running the exact same light fixture at 240 will reduce my energy costs by 25% compared to running it at 120V

You should be skeptical. They are wrong.

To get an accurate comparison you need to compare the energy use ALONG WITH the light output (foot candles) on the floor.

AllanJ 01-21-2008 03:20 PM

Just checked some ad's and catalogs listing clamp ammeters and many are accurate to +=2% which should be enough for this subject.

Years ago I worked with some and the accuracy was something like +=5% which is good enough for circuit loading tests but not good enough for close energy cost comparisons.

plj 01-21-2008 06:57 PM

Excellent, it appears that my understanding is correct. Now its time for me to research the light output and quality of the new flourescents. I do already use CFL's for some applications, but still have the 2 250W metal halides in each bay of my car wash. Since I got a 10% rate hike last Feb and a 19.7% rate hike this Feb, I'll be shutting off half the lights at midnight. Researching alternative fixtures is next on the list.

I appreciate your responses very much, thank you.

frenchelectrican 01-22-2008 01:31 AM


Please correct me is the carwash bay is unheated or heated ??

and normally majorty of the carwash inside the bay area they must be a wet location luminaires not the damp location at all.

right now i try to find a manufacter do have wet location listed luminaire that can be used in carwash bay i know most used the wallpack luminaire due the MH bulbs and also keep in your mind with vandil[sp] and we have few carwash bays they used the CFL they seems work pretty good work pretty good but personally i havent see it yet but heard about it.

in heated bay like automatic car washer machinery you can used either MH or CFL enclosed wetlocation luminaire but unheated that question is how cold it get in your area .

the best way is get a electrician come out or a lighting designer [ from either electrical supply centre or lighting store] they will help you slect a proper luminaire with using less engery and yeah you can use the timer to " dim " down the luminaire during nite hours like after cetern hours or used the motion detector so it can switch the MH from low setting to high setting but they are not cheap but it can really pay off something like that so that one other option you can look at it.

Merci, Marc

plj 01-22-2008 09:41 AM

Marc, they are unheated and wet, and it does get cold here, sometimes as low as -20C.
I currently use the scottsdale MH lights manufactured by LSI industries. I have not spent any time looking at CFL's yet, I wanted to figure out what it is costing me for the MH lights first.
Thank you

AllanJ 01-22-2008 11:56 AM

"How cold" is important. Don't forget to research the air temperature (ambient temperature) recommendations for different fluorescents including CFL's. Some have noticeably less light output at cold temperatures although in enclosed fixtures they may retain enough of the heat they produce to work acceptably.

joed 01-22-2008 04:31 PM

Watts is watts. The voltage doesn't matter. Your 250 W metal halide bulbs consume 250 watts. It doesn't matter what the voltage is. If you new lamps are 250 watts they will consume the same amount of electricity.

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