I am running one 12 gauge NM cable from my panel to a 5 gang switchbox with dimmers. The dimmers collectively control 1360 watts of recessed lighting including 1 100 watt circuit of magnetic low voltage lights. The light are all connected with 12 gauge NM cable. Since 12 gauge NM cable has the capacity to carry 20 amps of current, does this mean that I have to use a 15 amp circuit breaker? The reason I ask is because it doesn't make sense to me to use a 20 amp circuit breaker that will trip at the maximum capacity of the wire. Could the 20 amp rated wire already be melting or on fire by the time the 20 amp circuit breaker trips?

See if I remeber correctly. I would go with the 15 amp breaker. 15 amps X 120 = 1800 watts, your total load is 1360 watts + 100 watts =1460 watts, well within the limits of the 1800 watts. Some electrician say thaT the circuit most be derated by 85%, but that is usually for commerical applications, but 85% of 1800 watts = 1630 watts well within a 15 amp breaker. HTH

bink is right. But you shouldn't worry about 12 guage wire melting or burning from 20 amps of current. 4500-watt water heater elements run on 12 guage wire.

12 Gauge wire will actually handle 25 amps, However the NEC limits it to 20 amps. If you use a 20 amp circuit breaker you should use #12 wire. It is perfectly acceptable to use a 15 amp breaker with #12 wire. You must use at least #14 wire with a 15 amp breaker but any size wire larger than that is permitted. If you were running a 30 amp circuit for something, then you would need # 10 wire.

Wattage is not what you use to determine wire size, You need to base the wire on the amps. In the example that rpxlpx used for water heater wattage you must consider that the heater runs off of 240 volts not 120. When voltage goes up amperage goes down. Power(watts) = amperage X voltage. So if you had 120v x 37.5amps=4500watts, 240v X 18.75amps also = 4500 watts.

Never load a breaker with more than 80% of its capacity and you will always be safe. 16 Amps is permissible for a 20 amp breaker.

Make sure you use a magnetic dimmer for the low voltage lighting if the lights indicate they require it. They typically require them and also require a neutral to the dimmer. They are also quite expensive. Good Luck, Bill.

Wattage is not what you use to determine wire size, You need to base the wire on the amps. In the example that rpxlpx used for water heater wattage you must consider that the heater runs off of 240 volts not 120. When voltage goes up amperage goes down. Power(watts) = amperage X voltage. So if you had 120v x 37.5amps=4500watts, 240v X 18.75amps also = 4500 watts.

Never load a breaker with more than 80% of its capacity and you will always be safe. 16 Amps is permissible for a 20 amp breaker.

Make sure you use a magnetic dimmer for the low voltage lighting if the lights indicate they require it. They typically require them and also require a neutral to the dimmer. They are also quite expensive. Good Luck, Bill.