The white wire is generally not the hot wire in a standard residential electrical system. It is designated as the grounded, or neutral, conductor. Understanding this color coding is important because the neutral wire is near ground potential, while the hot wire carries the full voltage, making a mistake a significant safety risk.
Standard Wiring Color Codes
Residential electrical wiring in North America follows standards set by the National Electrical Code (NEC) to ensure consistency and safety. In a typical 120/240-volt system, three main functions are designated by color. The white wire consistently serves as the neutral conductor, completing the circuit by returning current to the electrical panel and connecting to the earth ground at the service entrance.
The black wire is the primary hot conductor, carrying 120 volts of electricity from the circuit breaker to the load. Red wires are used as a secondary hot conductor in 240-volt circuits or as a switched hot wire. The bare copper or green wire is the equipment grounding conductor, which provides a low-resistance path to the ground in the event of a fault.
When White Is Used As a Hot Conductor
An exception to the standard color code exists, which causes confusion and potential danger. In specific wiring configurations, such as older installations or switch loops using two-conductor cable, a white wire may carry unswitched power. This typically occurs when power runs to a light fixture box, and a cable containing black and white wires runs down to a wall switch.
In this scenario, the white wire carries the constant incoming 120-volt power to the switch, making it a hot conductor. To comply with the NEC and prevent confusion, any white wire used for a purpose other than neutral must be permanently re-identified. This involves marking the insulation with black, red, or another color of electrical tape or marker at all accessible points. The marking must encircle the insulation and cannot be green or gray, as those colors are reserved for ground and neutral functions.
Essential Safety Verification
Wire color provides only an indicator of function, and relying solely on color coding is unsafe practice. Before touching any wire, verify that the circuit is dead by turning off power at the main service panel or circuit breaker. Then, test the wires themselves, treating all conductors as live until proven otherwise.
The procedure involves using a testing device, such as a non-contact voltage tester or a multimeter. While a non-contact tester provides a quick check for voltage, a digital multimeter is required for a definitive check. Professional safety standards mandate a “live-dead-live” verification process. This requires testing the meter on a known live source, testing the circuit for the absence of voltage, and finally re-testing the meter on the known live source to ensure it is functioning correctly. For low-voltage systems, the absence of voltage must be confirmed by testing between all hot conductors and between each hot conductor and ground.