What size Romex wire do I need for a 15 amp circuit?

For a standard 15 amp circuit in a residential setting, you need 14 gauge Romex, specifically designated as 14/2 with ground for most 120V applications, which contains one black hot, one white neutral, and one bare copper ground. According to the National Electrical Code Table 310.16, 14 AWG copper NM-B is rated for 15 amps at 60 degrees Celsius, which matches exactly the maximum protection allowed by a 15 amp breaker. You must never protect 14 gauge wire with a 20 amp breaker because the wire would overheat before the breaker trips, creating a fire hazard. For a 15 amp circuit, 14/2 is the minimum and standard size, used for general lighting, bedroom receptacles, and living room circuits where the expected load is not continuous and heavy. If you have a long run, over 100 feet, you should consider upsizing to 12 gauge to compensate for voltage drop, even though the breaker remains 15 amps, because voltage drop can cause lights to dim and motors to overheat. If your circuit requires two hots, such as a 15 amp multiwire branch circuit or a switched receptacle with a separate switch leg, you would use 14/3 with ground, which includes black, red, white, and ground. Always ensure the jacket is white, which is the industry color code for 14 gauge, and staple it properly within 12 inches of boxes and every 4.5 feet.