In North American AC wiring, a black wire marks a hot/live conductor; other regions and DC uses vary—always verify with labeling or tests.
Quick answer and context
In household and light-commercial AC systems across the United States, a black conductor is normally an ungrounded or “hot” line. It carries voltage from the source to the load. OSHA 1910.304 and the widely adopted NFPA 70 (NEC) reserve white or gray for the grounded neutral and green or green-yellow for the equipment grounding path. Those reserved colors leave black and other non-reserved finishes for the hot role. That habit lines up with decades of field practice and with safety labeling that keeps work predictable.
Outside North America, color use shifts. Under the harmonised European scheme, brown marks the live conductor, blue marks neutral, and green-yellow marks the protective earth. Older UK circuits used red for live and black for neutral, which is one reason an unfamiliar box can mislead a DIYer. In low-voltage DC electronics and many vehicle harnesses, black often indicates the negative return. Labeling on the device and a meter test end the guesswork.
Black wire meaning by context
| Context | Typical role of a black wire | Standard or note |
|---|---|---|
| U.S. single-phase AC branch circuits | Hot (ungrounded) conductor | White/gray reserved for neutral; green/green-yellow reserved for equipment ground by OSHA/NEC |
| U.S. three-phase in small shops | One of the phase conductors | Common set: black, red, blue for A-B-C with white neutral and green ground |
| UK legacy fixed wiring (pre-harmonised) | Neutral in many circuits | Old scheme used red live, black neutral; verify before touch |
| UK/EU modern fixed wiring | Phase L2 in some three-phase sets | Harmonised set: brown live, blue neutral, green-yellow earth; black appears as a phase |
| DC electronics / automotive | Often negative/return | Device and harness diagrams rule; test first |
What the black wire indicates in home wiring
When you open a junction box or a switch loop in a U.S. dwelling, a black conductor is normally a hot line. The National Electrical Code reserves white or gray for the grounded neutral and green or green-yellow for the equipment grounding conductor. By reserving those colors, the code leaves black, red, blue, and other finishes to identify ungrounded lines. So when you see black on a cable in a branch circuit, treat it as energized until you prove otherwise with a tester.
This mapping also helps with polarity and safety devices. A breaker opens the hot leg, not the neutral. A single-pole switch sits on the hot feed so the load is de-energized when the switch is open. Marking hot legs with black or similar non-reserved colors gives a clear cue during work and inspection.
Switch loops, travelers, and re-identification
Older switch loops sometimes brought power to the light first and sent a switched feed down on the white leg. In those cases the white at the switch should be re-identified with tape or sleeve to show it is hot. Black is commonly used for the always-hot feed and for travelers in a three-way set. If you meet a white on a switch that is taped or dyed, assume it is carrying line potential. Never re-purpose green or green-yellow for anything else.
Three-phase panels at home or shop
Small shops and some homes with larger equipment may have a three-phase panel feeding machines. In many installations the phase set is black, red, and blue with white neutral and a green equipment ground. A black conductor there is one of the phase legs. If a high-leg delta is present, the high leg carries orange marking at terminations. Panel directories and labels tell the story; read them before any change.
Common device connections that use black
Standard receptacles tie the black hot to the brass-colored terminal and the white neutral to the silver one. On a light switch, the two black leads usually carry line and switched line, while the neutral bypasses the switch unless the device needs power for electronics. GFCI and AFCI devices use a “LINE” pair for incoming hot and neutral; a downstream “LOAD” pair feeds protected outlets and usually carries a black hot as well. Ceiling fans often ship with a black motor lead and a blue light kit lead; the black lead joins the switched feed you intend for the fan motor.
What black wire indicates in DC circuits
Color traditions change in low-voltage gear. In automotive and many consumer devices, black is often the negative return, with red as the positive feed. In panels built to industrial machinery standards, black may appear on AC control lines while blue marks DC control. Because these norms are not universal, you need the wiring diagram, the label, and a meter test before making a connection. Never tie a dark conductor to chassis or to a supply negative until tests show the match.
When black is not negative
Telecom racks and some LED drivers use different schemes. You may see black used on a positive return where a manufacturer has chosen white or blue for negative. Multi-conductor control cables also mix jacket colors for channel grouping without linking color to polarity at all. In short, device labeling and terminal markings outrank color conventions.
Regional differences you should know
UK fixed wiring moved to a harmonised color set years ago. Brown marks live, blue marks neutral, and green-yellow marks the protective earth as set out by the IET harmonised colours guidance. Older property can still carry the earlier colors where red was live and black was neutral. On mixed-era upgrades, junctions sometimes blend both schemes, so tagging and clear notes are part of tidy work. Across wider Europe, the IEC scheme uses brown, black, and gray across three phases, blue for neutral, and green-yellow for protective earth.