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 any large or load heavy lighting circuit, 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. Electrical wall switches and remoted relays should be chosen to have extra capacity to handle the brief Inrush surge. For example, 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. Six 50W LED luminaires is a total load of 300W. Yet while these luminaires may only draw 300W continuously, they will have a start-up inrush current which draws a much higher load. Common Circuit Breakers designed for lighting loads can handle some Inrush, yet when designing a wiring circuit for LED luminaires, you should leave at least 25% of the circuit capacity open to accommodate the Inrush condition (but specific system properties may require more capacity, so always consult an expert).

Dimmers used with LED lamps or luminaires should be de-rated at least 75% for use with LEDs (please read the instructions for your dimmer to find exact working load for LED applications). If you find yourself upgrading a home or business to LED, then make sure to replace all the old switches with quality heavy duty switches, and replace old dimmers with new ones rated for use with LED. (See our article on dimmers.)

In regards to electrical circuit planning for commercial properties with high-load LED lighting circuits, one way to deal with Inrush is to derate the circuit breaker. Also consider load balancing and reconfiguring circuits as needed. 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, which minimizes the inrush current to the point that no derating of the circuit breaker is required.

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 electrician to help with the electrical plan is always a smart investment as well.

Copyright 2019; QuantaLight Inc.