InRush Current


InRush Current is the name given to the short duration electrical current rush which occurs when power is first applied to an Electronic Ballast or Electronic Driver (aka, electronic power supply).  LED lamps and luminaires, along with many other commercial and consumer products, have electronic power supplies that exhibit this behavior.  InRush is inherent to the way these types of power supplies are designed, and is due to the power supply momentarily having very little resistance to the flow of electricity.  During that very brief moment, significantly more current flows to the power supply.

I many cases, this InRush is not a problem.  But certain applications require accommodating this behavior so as not to overload electrical circuits or trip circuit breakers.  Lighting circuits that use dimmers and load heavy lighting circuits will need to be designed with some extra capacity.

InRush current can be 40-100 times higher than the rated steady state current of the power supply.  Calculating the total circuit load for a certain number of lamps or luminaires to be installed on a circuit is usually a simple question to answer.  But with LED lighting some extra thought is required.  For example, when using a 300W LED rated wall dimmer with a 50W LED luminaire does not necessarily mean six luminaires can be used on this dimmer.  While the luminaire may only draw 50W continuously, it will have a start-up inrush current which draws a much higher load.  These higher loads are why the total LED luminaire load rating is usually less than the maximum rating of the dimmer.  When using a dimmer for LED luminaires, you should leave at least 25% of the dimmer capacity free to accommodate the InRush condition.  To prevent problems, read the specifications of the dimmer you intend to use.  Specific system properties may require more capacity, so always consult an expert.

If you find yourself upgrading a home to LED, then make sure to replace the old dimmers with new ones rated for use with LED.  For commercial projects, electrical wall switches and remoted relays should be chosen to have extra capacity to handle the brief InRush surge, and dimmers used with LED lamps or luminaires should be rated for use with LEDs.  

Always check which type of dimmers your specific LED luminaire (or LED lamp) is designed to work with.  Many LED luminaires are offered with different drivers to accommodate different types of dimmers.  Standard LED rated dimmers use the existing 120V wiring to dim the luminaire.  0-10V dimmers and DALI dimmers completely avoid load and incompatibility issues, but do require additional low-voltage wiring.  The use of dimmers that work on the 0-10V scheme can offer greatly improved dimming performance, are simple to use, and are a top choice for residential and commercial dimming upgrades.

Remodeling a home to use LED is straight forward.  But commercial projects most often have longer runs of LED luminaires, with matching higher circuit loads.  Some extra planning will avoid nuisance circuit breaker tripping.  In regards to electrical circuit planning, one way to deal with this is to derate the circuit breaker when using it with LED luminaires.  Another solution is to use a zero voltage crossing contactor installed inline with the circuit breaker.  The zero voltage contactor delays energizing the circuit until the voltage wave is crossing zero, a thereby minimizes the in-rush current to the point that no derating of the circuit breaker is required.  For existing construction which is being updated, redistributing the loads and/or reconfiguring the circuits may also be necessary.

LED luminaires provide great benefits for new and renovation projects.  Energy savings and long life mean that these products are a win-win for property developers and owners.  Consulting an electrical engineer to help with the electrical plan is always a smart investment as well.