If you run a 20 amp circuit (12/2) with shared lights AND receptacles, can you use a 15amp switches? Its my understanding that the switch is rated for the load its carrying (lights), but what about the load of the whole circuit? Lets say you have a room on one 20 amp circuit supplying outlets(15amp rec) and lights(15amp switch)you plug in a vacuum cleaner, have mini fridge running, and a fan plus lights, you pull more than 15 amps on the circuit. The breaker won't flip, but will the light switches rated for 15amp be ok?
Using 15A light switches to switch <15A on a 20A branch circuit is OK, unless you're switching receptacles!
As it turns out, NEC 404.14(A) permits the use of a snap switch (general light switch) to control any hardwired, non-motor load up to its rating, or motor loads under specific conditions (informational notes omitted):
404.14 Rating and Use of Switches. Switches shall be used within their ratings and as indicated in 404.14(A) through (F).
(A) Alternating-Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following:
(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage applied
(2) Tungsten-filament lamp loads not exceeding the ampere rating of the switch at 120 volts
(3) Motor loads not exceeding 80 percent of the ampere rating of the switch at its rated voltage
Note, though, this doesn't hold true when you're switching receptacles, as someone could plug say, a space heater into the receptacle(s) and exceed the rating of the switch as a result (informational note omitted again):
(F) Cord- and Plug-Connected Loads. Where a snap switch or control device is used to control cord- and plug-connected equipment on a general-purpose branch circuit, each snap switch or control device controlling receptacle outlets or cord connectors that are supplied by permanently connected cord pendants shall be rated at not less than the rating of the maximum permitted ampere rating or setting of the overcurrent device protecting the receptacles or cord connectors, as provided in 210.21(B).
Exception: Where a snap switch or control device is used to control not more than one receptacle on a branch circuit, the switch or control device shall be permitted to be rated at not less than the rating of the receptacle.
The reason why is in your hypothetical case, the wall switches are not in the path of the current the vacuum draws, basically. (Keep in mind that current flows in loops and that loads connected are in parallel.)