277V & 120V switch legs in the same 3-gang switch box?

[tx2step]

There will be 2 ea. 277V switchlegs + 1 ea. 120V switchleg in the same 3-gang switchbox. If the 277V switchlegs are on different phases, 404.8(B) requires a barrier (divider) between the 277V switches.

But what about the 120V switch? Does it need a barrier/divider as well?

The commentary following 404.8(B) in the NEC handbook says (in part): "...This requirement also applies to switches ganged together with any wiring device where the voltage between adjacent conductors exceeds 300-volts. Where switches or devices are on different systems and are in the same box, such as a switch on a 277-volt system and a receptacle on a 120-volt system, a careful analysis of the voltage between adjacent device terminals is required to ensure that the 300-volt limitation is not exceeded."

So with 277V and 120V switchlegs in the same switch box, what are the conditions that would create 300V or more between wires/terminals?

I usually don't mix the 2 voltages in device boxes for fear of accidentally imposing 277V onto 120V wiring...which I would think would be bad for any connected devices. So I haven't run into an occasion before where the 2 voltages will be present in the same switchbox.

Are "...identified, securely installed barriers..." required between the 277V and 120V wiring/devices? by "identified" I assume they must be "listed" for that use?

 

[iwire]

Put the barrier in between the 120 and 480 as depending on which 120 and which 277 phases you measure between you could have more than 300 volts

 

[don_resqcapt19]

There are a lot of different possible voltages based on the phase, rotation and number of transformers used to supply the two voltages ...the voltages between one conductor of the 208Y/120 volt system and the 480Y/277 volt system can range from 157 volts to 397 volts, so as Bob said, put in the barrier because you don't know what combination you actually have and over 1/2 of the combinations result in over 300 volts.