OT Watts to amps

NOWWWWW WW lets say you have 4 fixtures each with two 40 watt tubes plus a ballast,,,,, ,,,,, , lets say thats "about" 100 watts each,,,,, ,,,, then 4 of those would be a rough total of 400 watts,,,,, ,,,,,400 watts divided by 120 volts = 3.3 amps SOOOOO OOOOO OOOOO OOOOO O Yes a timer rated to carry n switch 8.3 amps ought to work fine

BUTTTTT REMEMBER this doesnt take into account the possibly lousy Power Factor of the ballast as some, especially the older, are poor as its an inductive load (its a transformer)

ANOTHER BUTTTTT TTTTT still if the timer is rated at 8.3 amps and your load is only 3.6 (but thats if youre using a pure resistive unity PF which the ballast is NOT load) THATS A SAFETY FACTOR OF DOUBLE so you ought to still get by fine. (Those would have to reallyyyyy be poorrrrr r ballasts if an 8.3 amp rated timer couldnt handle what computes to be a 3.6 amp resistive load) Just be aware if you use watts and treat the ballast as resistive (which its NOT) I wouldnt push things right up to the limit and use so many lamps the straight wattage calculations might say its 7 or 8 amps !!!!! !!!!!

Watts = Volts x Amps and thats for a pure resistive Power Factor = unity 1 load. Howeverrrrr rrrrr if the load is inductive (current lags the voltage) the apparent power DOES NOT equal the real power. It takes lagging current n energy to build and maintain the magnetic field surrounding the inductor PLUS it takes in phase (resistive) current to flow throug the straight DC resistance of the ballast. The utility hates poor power factor cuz they have to generate n supply energy for BOTH the lagging current to maintian the magnetic field of a motor/inductor PLUS the in phase resistive current SOOOOO O their generators have to work harder n theres more inefficiencies.

Hope this helps, DO NOT use so many fixtures the wattage calculations go up near the amps limit of the device TO SOLVE THIS EXACTLY we would need to know the exact loads and power factors but a safety factor of two is sureeeee eee plenty. Not knowing the power factor and just guessing I sure wouldnt go much over 5 or 6 amps using straight wattage calculations but 3 to 4.5 or should be fine regardless.

Sorry we cant give any exact answer.

John T Long Retired EE and a tad rusty on all this buttttt tt if you use conservative raitings and ESPECIALLY if you use modern high PF ballats there shouldnt be any problems

Charles

John T (rusty on this but knew it well a good while back, still interesting to me, however)

For an incandescent lamp, the load placed on the power system is very close to a pure resistive load, and the relationship of volts x amps = watts applies.

However, flourescent lamp ballasts are not resistive, and exhibit a property called reactance. This reactance causes the alternating voltage and current to become out-of-sync with each other, resulting in an offset in time between the applied voltage and the current drawn by the ballast.

This offset between voltage and current reduces the amount of real power available for a given level of voltage and current. The term "power factor" is used to describe the reduction in actual power as compared to the simple product of volts x amps. A power factor of 1.0 means that the voltage and current are in sync with each other, and the volts x amps = watts relationship applies.

For a cheap magnetic flourescent ballast, the power factor can be as low 0.5, meaning that in order to run a 40 watts flourescent tube, the fixture must draw .66 amps at 120 volts, as opposed to a current draw of .33 amps for a resistive load. This means you could only run half the wattage of flourescent fixtures on your timer as you could if you were using incandescent lights.

Better electronic ballasts have power factors on the order of 0.9, meaning that the increase in current because of the induced offset between voltage and current is only about 10%.

In your case, for 8.3 amps at 120 volts, you could run 996 watts of incandescent lighting, which we'll round to 1000 watts. If you were using flourescent lights with a high power factor electronic ballast (they'll be labeled as "high power factor" or something similar), then you could run about 900 watts of lighting for that same 8.3 amps. If you're using cheap flourescent shop lights with a magnetic ballast, you may only be able to run 500 watts of lights with 8.3 amps of current.

Loads with low power factors, like motors, welders, etc., drive the power company nuts. You only pay for the actual power you use, i.e., the meter automatically compensates for any offset between voltage and current. However, the power company has to size its distribution system to carry the maximum current, not the maximum power.

Hope this throws a little light on the subject. Some of these concepts aren't intuitively very obvious.

Keith

Thanks for an explination that made some sense of that term - never really understood it be4, now at least I have a general grasp of it.

--->Paul

John T

The big limits are the ballast loss and the ballast power factor. And if one looks at the ballast, its label should provide a claim of high or normal power factor, but if made in China for the lowest price ($8 two lamp fixture) it will assuredly contain the lowest power factor ballast that can be made. And that ballast will have the least efficiency from use of insufficient volume of materials and lousy materials. One can't say its a bad design, its designed to be CHEAP to make, not cheap to operate. Cheap to operate costs $ and materials to make.

Gerald J., Electrical Engineer in Iowa.

The question is, "why didn't they just say 1000 watts"? Well, it turns out that with alternating current, 1000 volt-amps equals 1000 watts only if you have a purely resistive load. An inductive load (like a motor) or a capacitive load (rare) has a power factor of less than one. Power (watts) equals volt-amps times power factor. So if you plugged a motor that requires 1000 watts into your timer, you would probably fry out the switch because it would exceed its rated amperage, even though it would handle a 1000 watt resistive load just fine.

BTW, your fluorescent lights are probably slightly inductive, but not enough to worry about. Incandescent lights are resistive loads. You should be OK powering 800 watts with your timer.

You might want to check the power factor of a few flourescent lights. Cheap magnetic ballasts can have a power factor as low as 50%, which would require double the current than the same wattage load with a unity power factor. Newer electronic ballasts can be as good as 0.9, which means the difference is only about 10%.

Keith

Next Power in "Watts" unity power factor = V x I as you are already aware. The Volt Amp rating, however, is dependant upon the Power Factor and ballasts, being inductive, are NOT unity power factor, say maybe 85% to 90% or more depending upon type n design etc.

Next if youre gonna run a 20 amp branch circuit to serve a "continuous" load you cant size light fixtures out for 20 amps continuous draw, but only 16 amps total max continuous.

FINALLY if you wanna use say two 40 watt fixtures and the total fixture load (lamps plus ballast) were 100 watts, figure 16 amps as 120 volts = 1920 Watts and at 100 watts per fixture that would mean 19 max fixtures per 20 amp branch circuit HOWEVERRRRR RRRRR R when figuring in a lousy power factor plusssss sss general conservative desing principles wayyyyy back years ago when I was designing and specifying lighting circuits I seldom placed much more then 10 or 12 fixtures per circuit.

You also have to figure in switching and its ampacity ratings dont forget.

BOTTOM LINE add up the lamps plus the ballast to get total watts and then divide by 120 to get the amps, then make your computations (how many fixtures) so the toal amp draw doesnt go over 80% of the branch circuit (thats 16 amps on a 20 amp circuit) and then I still downsize maybe an additional 10% to 20%

Sorry this is so complex but its all in the NEC and is for good safe conservative design.

Ol John T (Conservative and long retired EE) i.e. my figures may be somewhat outdated but this gives you the idea.

Fluorescent tubes are wattage rated assuming that they are driven by a "reference" ballast. The actual ballast in the fixture may drive the tube harder or weaker than the amount needed for 40 watts nominal. (This is called "ballast factor" and affects both the amount of light you get and the amount of power drawn from the circuit). In addition, there are ballast losses to consider, which add to the draw of the tubes when computing the total load on the timer/switch.

Ballast factors are usually below 1.0, and ballast efficiency is usually fairly high, so these usually roughly cancel out, but there is no guarantee. The best guide for actual draw is the ballast nameplate, not solely the nominal tube wattage.

Finally, a fluorescent fixture represents a "ballast" type load. Making or breaking a circuit to a fluorescent fixture can be harder on a switch than controlling a simple resistive heater, though it should be easier than a (tungsten) filament incandescent lamp, with the latter's high starting current. Lighting contactors, especially, are rated differently for resitive, ballast, tungsten, and HID type loads.

Your switch may be rated at 8 amps only for simple resistive loads, and thus too small for 8 amps worth of lighting fixtures, which are almost always a more stressful load.

I'd be more inclined to use a lighting control relay to switch a whole bunch of fixtures, and then control the relay with the timer, than I would be to push an 8 amp timer close to its maximum rating.