Inrush current


Well-Known Member
Aug 12, 2007
New York
Inrush current is the short-time current drawn by an electrical device at the moment of energization. Two devices with high inrush current are of particular interest to the entertainment industry:

1. Tungsten lamps with cold filaments. These can have an inrush current 10-15 times the steady-state operating current.

2. Switch mode power supplies with highly capacitive inputs. These devices, used in many automated and LED luminaires, can have an inrush current 20-50 times their steady-state full-load current, albeit for a much shorter duration than tungsten lamps. Switch mode power supplies draw inrush current at energization whether or not the device they feed is turned on or off by its controller.

Inrush current must be accommodated by power devices, overcurrent protective devices, and switch and relay contacts that are correctly rated for the type of load they are feeding.

In addition, inrush current is the limiting factor when asking "How many LED fixtures can I put on a 20 amp circuit?". It is also the reason that large lighting rigs with hundreds to thousands of switch mode power supplies must be sequenced at turn-on using branch circuit switching, rather than energizing the entire rig at once with a main switch or circuit breaker.

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