Something to think about... most disposals do not draw more than 7 amps, and are usually installed on a 20 amp circuit. 210.19 permits combination loads as long as the motor is not more than 50% of the branch circuit rating. Adding a few lights to the disposal circuits is actually in my thoughts a good and resourceful design, why waste anything today. As far as the lights dimming, how often is the disposal actually used? ... probably less than 5 minutes a day.
Pierre
