US Wiring Colour Code Reference: NEC Standards vs IEC & UK Variants

The standard US wiring colour code for 120V/240V split-phase AC branch circuits mandates black (Line 1 hot), red (Line 2 hot), white or gray (neutral), and bare or green (ground). These rules are enforced by the National Electrical Code (NEC) under Articles 200, 210, and 250. While the US system is highly standardized for residential and light commercial work, interpreting older panels, imported machinery, or 3-phase commercial systems requires a deeper understanding of regional and voltage-specific variations.

The Complete US Wiring Colour Code Reference Table

The table below covers the standard NEC (NFPA 70) requirements for the most common AC and DC systems found in North American homes, workshops, and light commercial buildings.

Regional Variants: NEC vs IEC vs Old UK

If you are wiring a subpanel for imported European machinery, working on a boat with IEC-standard marine wiring, or troubleshooting an older facility, you will encounter non-US color codes. The most critical shift occurred in 2004 when the UK harmonized its colors with the IEC 60446 standard to match the rest of Europe. Mixing these up can result in a dead short or a lethal shock.

Rows People Get Wrong (And How to Fix Faded Wires)

Even with a perfect reference table, real-world jobsite conditions and NEC exceptions cause mistakes. Here are the most common pitfalls and how to handle them safely.

1. The "White Hot" Switch Loop Exception

Under NEC 200.7(C)(1), a white wire can be used as an ungrounded (hot) conductor in a cable assembly (like NM-B Romex) for a switch loop, provided it is permanently re-identified at both ends. In older homes, you will frequently open a switch box and find a white wire connected to the brass (hot) screw. If the previous electrician failed to wrap it in black electrical tape or mark it with a Sharpie, you might assume it is neutral and get shocked. Rule of thumb: In a standard single-pole switch box, the white wire is almost always the switched hot returning to the light fixture, not a neutral.

2. 240V Pure Loads and the Missing Neutral

When wiring a 240V baseboard heater or a pure resistive load, you will use a black and a red wire (or two black wires with red tape). There is no white neutral wire in this circuit. Beginners often look for a white wire to terminate on the appliance, not realizing that both the black and red legs are carrying 120V to ground, creating a 240V potential across the load.

3. Safe Interpretation When Markings are Faded or Missing

Drywall mud, attic heat, and decades of UV exposure can turn black THHN wire gray, or white wire completely translucent. OSHA wiring design standards and basic electrical safety dictate that you never trust the jacket color on an unverified circuit.

• Step 1: Turn off the main breaker or use a non-contact voltage tester (NCVT) to identify the specific breaker for the circuit.
• Step 2: De-energize the circuit and lock out the panel if possible.
• Step 3: If you must test live, use a CAT III or CAT IV rated multimeter (like a Fluke 117 or T+PRO). Set it to AC Volts.
• Step 4: Measure the mystery wire against a known, verified bare copper ground. If it reads ~120V, it is a hot leg. If it reads 0V, measure it against a known hot leg. If it now reads ~120V, it is your neutral.

Frequently Asked Questions

What is the US wiring colour code for 277V/480V 3-phase systems?

For commercial and industrial 277V/480V 3-phase systems, the NEC mandates a different color scheme to prevent mixing up panelboards. The hot legs are Brown (Phase A), Orange (Phase B), and Yellow (Phase C). Electricians use the mnemonic "BOY" (Brown, Orange, Yellow) to remember this. The neutral is Gray, and the ground remains Green or Bare. Never use black, red, or blue in a 480V panel, as those colors are strictly reserved for 120/208V 3-phase or 120/240V split-phase systems to prevent catastrophic miswiring.

Does the US wiring colour code allow white wires to be hot in a 3-way switch?

Yes, but with strict re-identification rules. In a 3-way or 4-way switch setup, the white wire inside a 3-conductor cable (like 14/3 or 12/3 Romex) is frequently used as a "traveler" wire carrying switched hot voltage. According to the NEC, any white or gray wire used as a hot or traveler must be permanently re-identified with black or red tape, paint, or heat-shrink tubing at every point where the wire is accessible. If you are troubleshooting a 3-way switch and find a white wire on a brass traveler screw without black tape, note it as a code violation and wrap it before closing the box.

How do I identify a wire if the US wiring colour code insulation is faded or painted over?

If a wire is heavily painted or faded, visual identification is invalid and dangerous. You must perform a live voltage test using a high-impedance digital multimeter rated for your system's overvoltage category (CAT III for branch circuits). First, identify a known, clean bare copper ground wire in the box. With the circuit energized, touch one probe to the bare ground and the other to the mystery wire. A reading of 114V–126V confirms it is a hot (line) conductor. A reading near 0V indicates it is either a neutral or a disconnected wire. To confirm it is a neutral, measure between the mystery wire and a known hot wire; a reading of ~120V confirms it is the neutral return path. Once identified, wrap the wire in appropriately colored Phase Tape (e.g., black tape for hot, white tape for neutral) to restore the visual code for the next electrician.