Dimmers have minimum load requirements because if the LED load isn’t drawing enough current to keep the dimmer’s switching elements closed, the dimmer will have unpredictable behaviour. This behaviour can include flickering, reduced dimming range, flashing when off, or not even working at all.
Dimmers that are designed for incandescent loads tend to have a larger minimum load requirement because incandescent fixtures generally have higher wattages than LED fixtures. This means that these dimmers will most likely require multiple LED fixtures to meet the minimum load requirement. Testing is necessary to determine the exact number of LED fixtures needed for each incandescent dimmer. Dimmers that are designed for LED loads usually have a lower minimum load requirement and typically only require one or two fixtures. The LED minimum load rating for LED load dimmers is usually given in number of lamps instead of Watts (W) or Volt Amps (VA).
Dimmers also have maximum load requirements to ensure the load does not exceed the dimmer’s design limits. Surpassing these limits can result in premature dimmer failure, flickering due to noise, as well as breaker tripping. One of the main reasons dimmers have maximum loads is because of the inrush current caused by LED drivers. Inrush current is input current of short duration during initial start-up that exceeds the steady-state current value. The graph above shows the current waveform when a device is powered up.
Because of this inrush current, an LED may be rated as 25W but the actual wattage initially going through the dimmer is much higher. This means that if an incandescent dimmer is rated at 600W, you can’t assume you can use twenty-four 25W LED fixtures. A good rule of thumb is to allow 100W for each LED fixture so in this case, the 600W dimmer can handle only six LED fixtures.