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Inrush Current

9 September 2011
PF Wagner, Powerbox USA

All electrical and electronic devices have a steady state operating current and an inrush current. The inrush current or input surge current, as it is sometimes called, refers to the maximum instantaneous input current drawn by the electrical device when it is first turned on. The size of the inrush current is usually much higher than the device’s steady state operating current.

An incandescent light bulb has a tungsten filament with a positive temperature coefficient. This means that the filament resistance is lower when the bulb is cold (light off) and the resistance then increases as the filament quickly heats up when AC power is applied to the bulb. Once the filament has reached its designed operating temperature, about 2,550C which is about 4,600F, the bulb resistance does not increase anymore and the bulb then draws a steady state current which keeps the filament glowing white hot, creating a little light and lots of heat.

The more efficient Compact Fluorescent (CFL) and Solid State Lighting (SSL) lights, made from LEDs, generally have a switching power supply, even if it is built into the “bulb.” For SSLs, the power supply is generally referred to as the “driver” and the entire light assembly is often referred to as a “luminaire.” Regardless, a switching power supply usually has a large capacitor on the supply’s input that must be charged up when power is first applied. This is what generates the inrush current. Once the capacitor is charged, it requires only the steady state operating current to keep it charged. It is worth noting that the capacitor is required to keep the light from having a flicker as the response time of a CFL and LED is much faster and greater than a tungsten filament to a change in voltage.

An incandescent bulb typically has an inrush current that is typically 12-15 times the amount of the steady state operating current. Thus for a 60 watt bulb that normally draws about 1/2 Amp steady state current, the inrush current is about 7.5A, when using the high end of the multiplier to be conservative.

The inrush current of a switching power supply all depends on the design of the switching power supply. Usually the inrush current is much higher than that of a 60W incandescent bulb unless special, and more costly, steps are taken to limit the inrush current to lower values. For switching power supplies, an inrush current of 30A is fairly typical for a 120VAC input. “Good” CFLs are engineered to have lower in-rush currents.

When designing a lighting system, awareness of inrush current is important and is often critical in retrofit applications.

Standard residential and commercial lights are installed on a “branch circuit.” This is basically a power limited source originating in the circuit breaker panel. Each branch circuit has a circuit breaker that is sized to the maximum amount of current the associated wiring is rated to safely carry. In the United States, these limits are defined by the National Electrical Code (NEC) along with any local building ordinances. Standard branch circuits most commonly used for lighting in commercial and residential applications are rated for 20A and 15A, respectively, where the former uses 12awg wiring and the latter uses 14awg wiring.

While some commercial lighting circuits in the USA use 240VAC, or even 277VAC, with even higher power capacity, we will keep the inrush discussion to the more common usage on 120VAC circuits. Needless to say, the same principals apply. Thus the maximum total steady state current is 20A for 12awg wired circuits and 15A for 14awg wired circuits. However, in actual practice, the allowed load is generally something less as many codes and local jurisdictions have a power limitation, such as of 80% of the maximum rating. Without getting into the details, the practical power limits for a 120VAC line come out to 1,440 watts on a 15 Amp circuit and 1,920 Watts on a 20 Amp circuit.

Measuring the inrush current on a ubiquitous 60W incandescent bulb by switching on the power during the worst case moment – when the AC is at the peak of a cycle - gives the following results. The top trace is the AC voltage applied to the bulb and the bottom trace is the AC current through the bulb.

Note that when the AC voltage is turned on at the peak across the bulb that the AC current through the bulb immediately jumps to about 7.3A peak. It then rapidly falls, as the filament heats up, to about 2A on the next negative peak, thereupon slowly falling where it fairly quickly reaches the steady state value of about 0.7A peak (0.5Arms), which is the normal operating current for a 60W incandescent light bulb. When viewing the results, keep in mind that the graph is depicting five and a half AC cycles which is less than 1/10 of a second. The initial in-rush current spike itself is only about 3/1,000 of a second long.

As previously noted, a standard 15A household circuit breaker can provide 1,440 watts of AC power. Assuming no usage derating, that is enough power for 24 sixty watt incandescent bulbs.

The worst case inrush current of 24 sixty watt incandescent bulbs would be about 175A when they are all turned on simultaneously and the AC source could provide that much peak power. If that were to happen, 175A could trip the circuit breaker and may even weld the light switch’s contacts together. In actual practice, there is generally far less than 24 bulbs on a single switch and the additional resistance and small inductance in the branch circuit wiring helps limit the inrush current to something less.

However, suppose there were a more realistic ten 60W bulbs on a switch. Even that is high for most residential applications but could actually be low in a commercial application. In any case, the inrush current is now a more moderate 73A, something the switch and circuit breaker are designed to handle.

The issue for lighting comes in when the 60W bulbs are replaced by an electronic light such as a CFLs or SSLs. In this case, both the CFL and SSL replacements have a switching power supply powering the end light generator.

Here’s where the problem comes in. Like an incandescent bulb, very few CFL or SSL products state their inrush current. As long as the inrush current is less or roughly equivalent to the 7.3A inrush current of a 60W bulb there are no problems. Unfortunately that is rarely the case. While “good” CFLs often have fairly equivalent or an even lower in-rush current, many don’t. Many if not most of the “lower cost” SSL drivers have a 30A inrush, with some as high as 45 or even 60 amps! In comparison, even the inrush current of a 100W incandescent bulb is only about 12A.

You can often actually notice the increase in the inrush current when one changes out a few incandescent bulbs for CFLs or SSLs by listening as the light switch is turned on. A louder “snap” emanating from the wall switch is a higher inrush current surging across the switch’s contacts. A larger current surge will wear out and ultimately destroy the switch’s contacts faster than a smaller surge current will. How much faster will depend on the amount of the surge current, the quality of the switch, and how often the switch is turned on and off. However, given that most wall switches are now made in China….

As long as only a few incandescent lights are directly replaced by CFLs bulbs or SSL luminaries with larger inrush currents there are generally no problems. However, once the number starts to get too big, the new “high efficiency” lights can actually trip the circuit breaker when the energy saving, environmentally conscious end customer turns on the lights. To make matters worse, when the total inrush current is border line, this phenomenon can also be related to the ambient temperature that all the various lighting components are operating in. And it gets even worse. With the growing requirements and applications of Ground Fault Circuit Interrupter (GFCI) circuit breakers, the occurrence of false nuisance tripping due to switching surges has been growing. And that is without even changing out the incandescent bulbs!

It would be a gross understatement to say that nobody would appreciate tripping a circuit breaker when they turn on the lights. Additionally, those are trouble calls no lighting provider wants to receive, especially if they also have to tell the customer that they need to replace a circuit breaker to “:hopefully” fix the problem. (Some circuit breakers are not as sensitive to tripping on high inrush currents. Others are. Murphy’s Law says the customer with the problem will have the sensitive one.)

The only solution for this potential problem is to get CFLs or SSLs that have an equivalent or low inrush current. Since very few manufacturers state the inrush current number, good luck in finding one. At this point it is also worth noting that the problem is actually exasperated when the wattage of the incandescent bulbs goes down but the quantity on the same switch goes up. When multiple CFL or SSL replace incandescent bulbs, the inrush current on the branch circuit is the sum of all the individual inrush currents from each light source. The more light sources with individual inrush currents the greater the total inrush current.

When CFLs, and especially SSLs, replace incandescent bulbs, the inrush could be 4-5 times greater than it was with the incandescent bulbs. The power factor could also be much worse too. If so, that reduces the apparent energy “savings,” places an even greater stress on some of the branch circuit components, and could actually INCREASE the load on the energy provider. For more information on power factor, see Powerbox application note, Watts Up with Power Factor?

To assist designers and installers, Powerbox publishes the inrush current of all its SSL drivers. Refer to Powerbox’s SSL Driver Specifications page. It is worth noting that some of Powerbox’s drivers have inrush currents equivalent to and even lower than the standard 60 watt incandescent light bulb.

An alternative solution to having SSLs with integrated power supplies or associated individual drivers with large inrush currents is to use a single larger power supply to drive a number of SSLs. In this case there is only one inrush current for the one large supply.

For this solution, there are a number of different configurations. The best one depends on a number of factors, including whether the new solid state lighting is for a retro-fit or new build. For additional information on various SSL architectures refer to Powerbox’s SSL Architectures application note.

If you are interested in the remarkable characteristics of the ubiquitous 60W incandescent light bulb that has lighted up the world for the past 100 years, see:

Basics Physics of an Incandescent Light Bulb

For more information on LEDs and SSLs, see the Powerbox SSL Application Notes.

Download the LED / SSL brochure and learn more about power solutions for LED / SSL applications.
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