In the US (NEC), standard AC power wiring uses black or red for hot, white or gray for neutral, and bare or green for ground. In Europe and the modern UK (IEC 60446), brown, black, and gray are hot phases, blue is neutral, and green-and-yellow is earth. Because legacy installations frequently violate modern codes, always verify conductor function with a multimeter before making connections.
Complete Reference Table: NEC vs. IEC vs. Old UK
The table below maps the standard color codes for wiring across the most common global frameworks. This applies to standard THHN in conduit and the outer jacket colors of NM-B (Romex) and MC cables. For authoritative reference, consult the National Fire Protection Association (NFPA) for US standards and the Institution of Engineering and Technology (IET) for UK BS 7671 standards.
| Function | US (NEC) 120/240V | US (NEC) 277/480V | Europe / Modern UK (IEC) | Old UK (Pre-2004) |
|---|---|---|---|---|
| Line 1 (Hot/Phase) | Black | Brown | Brown | Red |
| Line 2 (Hot/Phase) | Red | Orange | Black | Yellow |
| Line 3 (Hot/Phase) | Blue (rare in residential) | Yellow | Gray | Blue |
| Neutral (Grounded) | White or Gray | White or Gray | Blue | Black |
| Ground (Earth) | Bare, Green, or Green/Yellow | Bare, Green, or Green/Yellow | Green/Yellow | Green/Yellow |
The Rows People Get Wrong (And Costly Mistakes)
Even experienced DIYers misinterpret specific rows in the color codes for wiring when transitioning between regions or working on older homes. Here are the most dangerous points of confusion:
- White as a Hot Conductor (Switch Loops): In older US homes, a 2-wire NM-B cable was often run down to a switch. The white wire was used as the hot feed to the switch, and the black wire returned as the switched hot. NEC 200.6(A) now requires this white wire to be permanently re-identified (typically with black electrical tape or heat shrink) at both ends. If you see a white wire connected to a brass screw on a switch or a black wire nut in a ceiling box, assume it is hot until proven otherwise.
- The 277/480V High-Leg Delta Trap: In commercial US panels, a 240V high-leg delta system uses an Orange wire for the high leg (which reads 208V to neutral, not 120V). Connecting a standard 120V appliance to the orange phase will instantly destroy the appliance and create a fire hazard. NEC 110.15 mandates the high leg be identified by orange outer finish.
- Old UK Red/Black vs. US Black/White: If you are working on a UK home wired before April 2004, Red is Live and Black is Neutral. US expats or DIYers used to American codes often assume Black is hot and Red is a secondary phase, leading to dead shorts when replacing old UK fixtures with modern IEC-colored ones.
- Gray in Residential vs. Commercial: In US 120/240V residential wiring, gray is an allowed neutral color (though rarely used in NM-B). However, in 277/480V commercial systems, gray is a hot phase (Line 3). Never assume gray is neutral without checking the panel voltage.
Safe Interpretation When Colors Fade or Go Missing
Insulation degrades. In a 60-year-old panel, white THHN can turn yellowish-brown from heat, and black wires can look dark gray in low light. Furthermore, previous owners may have used whatever scrap wire was left in their truck, completely ignoring standard color codes for wiring. Here is the exact protocol for safe interpretation:
Never trust wire color alone in a legacy panel. Turn off the main breaker, lock it out if possible, and use a Category III or IV non-contact voltage tester, followed by a multimeter, to verify the bus bars are dead before touching any conductors.
Step 1: The Voltage Drop Test (Live Circuit)
With the circuit energized and a safe load connected, measure the voltage between the suspected hot and the bare ground. A true hot will read 114V–126V (in a 120V system). Measure between the suspected neutral and ground; it should read less than 2V. If neutral-to-ground reads 120V, you have an open neutral—a highly dangerous condition where the neutral is carrying full line voltage.
Step 2: Continuity Testing (Dead Circuit)
Shut off the breaker. Disconnect the wires at both ends. Use your multimeter's continuity setting (the diode/beep symbol) to test the unknown wire against a known ground at the panel. If it beeps, it is a ground. If it shows infinite resistance (OL), it is either hot or neutral.
Step 3: Mandatory Re-Identification
Once you have scientifically identified the conductors, you must re-identify them to meet code. NEC 200.6 and 250.119 require that neutral wires be white or gray, and ground wires be green, green/yellow, or bare. Use high-quality 3M Super 33+ vinyl electrical tape or adhesive-lined heat shrink tubing to wrap the ends of the wires. Do not rely on a Sharpie marker; it fades and is not recognized by inspectors.
Frequently Asked Questions
What are the color codes for wiring a 3-way switch?
In a standard US 3-way switch setup using 14/3 or 12/3 NM-B cable, the black wire is typically the "common" (either the always-hot feed or the switched-hot return to the light). The red and white wires act as the "travelers" carrying the switched state between the two switches. The bare wire is ground. Note: Under NEC 404.2(C), if you are installing smart switches that require a neutral, the white wire in the traveler cable must be re-purposed and capped as a neutral at the switch box, requiring a 4-wire setup or a different wiring topology.
Can I use a green wire for anything other than ground?
No. NEC 250.119 strictly prohibits the use of green, green with yellow stripes, or bare conductors for anything other than the Equipment Grounding Conductor (EGC). Using a green wire as a hot or neutral is a severe code violation that creates an immediate electrocution hazard for the next person who works on the circuit, as they will instinctively touch the green wire assuming it is safe.
Why is my neutral wire reading 120V to ground?
If your white neutral wire reads 120V against the bare ground, you have an "open neutral" or "floating neutral." This means the neutral path back to the panel is broken somewhere upstream. Because the circuit is incomplete, the voltage from the hot wire is passing through the connected load (like a lightbulb or appliance) and energizing the neutral wire all the way back to the break. This is extremely dangerous; the neutral wire is now effectively a hot wire. Turn off the breaker immediately and trace the break in the neutral path.
Do DC solar and battery systems use the same color codes for wiring?
No, DC systems use entirely different standards, which causes massive confusion when integrating solar inverters into AC panels. Per NEC Article 690 and ABYC marine standards, DC positive is typically Red and DC negative is Black or White. This directly conflicts with AC IEC standards (where black is a hot phase) and US AC standards (where white is neutral). Always label DC conductors with "DC+" and "DC-" heat shrink, and never run DC and AC conductors in the same conduit without a physical divider.
