Calculating LED amperage usage

I'm calculating basic amperage (W/V=A) I have a track light with 33 LED lamps that is experiencing an unusually high failure rate. When calculating amperage would I use the 120v that the entire lamp is using or would I use a low voltage amount such as 12v or 24v since the lamps have an integrated driver?

• Schematic ... ? – Majenko Jul 15 '17 at 18:00
• are you using resistor(s) to limit the current? – karthik Jay Jul 15 '17 at 18:07
• Actually this sounds like some commercial product that you are troubleshooting. Not a design of your own. Ergo maybe DIY would be a better place for you? – Majenko Jul 15 '17 at 18:12

If these are standard $120\:\textrm{V}$ LED lamps, such as these, they will have stamped on them a "wattage." If you look at that page, you will see that these are rated with a "power consumption" of $6.5\:\textrm{W}$. Ignore the "wattage equivalent" ratings -- those are about how the lighting supposedly "appears" to a human when comparing it with a traditional incandescent bulb and not about the actual power consumption of the device.

So, let's say you had 33 of the above lamps in your track lighting fixture. This would mean a total of $33\cdot 6.5\:\textrm{W}=214.5\:\textrm{W}$. That would be about $1.8\:\textrm{A}$ when all the lights are on. (This is RMS, which means the peak, momentary current can be $\sqrt{2}$ more than that, or about $2.5\:\textrm{A}$.)

Now, all this assumes that the power factor of the lamps is 1. In reality, it could be half that much (in one you should NOT buy.) But they vary and the actual power factor for an LED lamp is hard to find when you are buying one. I tend to assume a power factor of about 0.9, or so, if I don't have any information. (But I honestly have no good justification for that figure. So I'm open to better advice on this point.) But assuming 0.9, then the circulating current would have an RMS value of $2\:\textrm{A}$ and a peak value of slightly over $2.8\:\textrm{A}$.

I'd want to plan $5\:\textrm{A}$ for your track lighting if you were intending to use those particular bulbs in it.

Assuming you apply the correct rated voltage and you always add the total ACTUAL Watts for breaker ratings.

• where 120W on 120V is 1Aac. regardless what the LED sees.

1) DIY lighting design with lamps and luminaires can cause excess heat rise can cause low MTBF (excessive failure rates or FITs)

2) Poor "AC Power Quality" with frequent lightning transients in tornado alley or Florida can also caused low MTBF rates.

3) Cheap LED's can have low MTBF from poor heatsink designs.

MR16 GU5.3 may be 120Vac rated or 12VAC (not DC rated) and may be protected with better cooling so that the free space above the lamp does not restrict the flow of convection cooling by a better design track socket or extension and NOT a cup back that restricts airflow or right at the ceiling with no ceiling vent.

MR16 GU10 come in Voltages: AC/DC12V, AC85-265V/ Which ones do you have?

Which problem? poor convection cooling? poor line quality? poor quality LEDs?

If you are calculating the amperage of the leds themselves, it would be the voltage and current through the individual leds. If you are calculating the amperage that the entire bulb uses, it would be the line voltage. This would include the power used by the internal led driver as well. The driver is likely to be 80% or higher efficiency.

Whenever you are dealing with LED lamps with integral drivers, you must supply the voltage they are rated for. If at that point they fail, then the product is defective or you have an application issue.**

It's also possible there's a quality problem with your supply power.

Often wnen line power has noise or spikes, the culprit is actually your own gear. So it is possible the LEDs themselves have dirty power supplies which are putting a lot of noise on the line. They may be too dirty for 33 of them to coexist on the same circuit. This would be caused by cheapness and should be considered a product defect. If the manufacturer's initials are LoA, FE or UT, that's your problem.

It also causes trouble when you dim LEDs that are not made for dimming.

Watts and VA

That dirty power draw can be a factor, a power-factor to be precise. Imagine a power supply draws from the AC sinewave unevenly - it doesn't use the whole sinewave. But the generator still has to generate the whole sinewave, and the wires must carry it. The part it uses (times amps, averaged out) is its watts. The entire sinewave is called VA. The ratio is called the power factor.

With very poor power factor, it's possible to hit the VA limits of a circuit (i.e. 1800 watts on 15A - without getting near it in wattage.

This will cause line noise and poor performance.

** an example of an "application issue" would be trying to use an LED as an oven light.