Wire insulation colors are legally mandated safety identifiers that dictate the voltage, phase, and function of a conductor; guessing based on faded jackets or regional assumptions is how people get shocked or burn down panels.
The Master Reference: Color Coded Wires by Region
The following table maps the exact insulation colors required for standard single-phase and three-phase AC power across the three dominant global frameworks: the US National Electrical Code (NEC), the International Electrotechnical Commission (IEC 60446), and the UK's BS 7671 (which harmonized with IEC in 2004 but still has legacy installations).
| Conductor Function | US / Canada (NEC) | EU / Int'l (IEC 60446) | UK (Post-2004) | UK (Pre-2004 Legacy) |
|---|---|---|---|---|
| Line / Hot (Single Phase) | Black (or Red) | Brown | Brown | Red |
| Neutral | White (or Grey) | Blue | Blue | Black |
| Ground / Earth | Bare, Green, or Green/Yellow | Green/Yellow | Green/Yellow | Green (or Bare) |
| Line 2 (240V Split / 3-Phase) | Red (or Blue for 3-phase) | Black (or Yellow) | Black | Yellow |
| Line 3 (3-Phase Only) | Blue | Grey | Grey | Blue |
| Switched Hot (Travelers) | Red, Black, Blue (re-identified) | Brown w/ sleeve, or other colors | Brown w/ sleeve | Red w/ sleeve |
Rows People Get Wrong: Common Color Code Traps
Even experienced DIYers and junior techs misinterpret specific rows in the table above. Here are the three most dangerous traps and how to handle them.
Trap 1: The "White Wire is Always Neutral" Fallacy
In US residential wiring, NEC Article 200.7(C)(1) allows a white wire to be used as a hot conductor in a switch loop, provided it is permanently re-identified with black tape, paint, or heat-shrink at both termination points. If you open a 3-way switch box and see a white wire connected to a dark brass screw with no black tape on it, the previous installer broke code. Treat that white wire as a 120V hot until proven otherwise.
Trap 2: Old UK Black vs. IEC Blue
If you are working in a UK home built before April 2004, the neutral wire is black. In any post-2004 UK home (or anywhere in the EU), black is a Line/Hot conductor for 3-phase or 2-way switching. Plugging a pre-2004 black neutral into a modern IEC black hot terminal will cause a dead short and trip the main breaker instantly.
Trap 3: Bare Copper vs. Green/Yellow Earth
US NM-B (Romex) cable uses bare copper for ground. European and UK cables use Green/Yellow. When wiring imported 230V machinery (like a European dust collector or CNC router) in a US shop, do not assume the machine's internal green wire is a signal ground; it is the safety earth. Always bond it to your facility's equipment grounding conductor.
Faded, Missing, or Unmarked Wires: Safe Interpretation Protocol
THHN wire left in a hot attic for a decade fades. Black turns purple or grey; red turns pink. Cloth-covered wire from the 1950s often has no color coding at all. Never rely on visual inspection alone for compromised jackets.
Follow this exact measurement protocol using a CAT III or CAT IV rated DMM (like a Fluke 117 or Klein MM400):
- Measure Line-to-Neutral (L-N): If it reads 114V–126V (in a 120V system), you have a valid hot and neutral pair.
- Measure Line-to-Ground (L-G): If it also reads 114V–126V, your ground path is intact.
- Measure Neutral-to-Ground (N-G): This should read less than 2V. If N-G reads 120V, you have an open neutral or a bootleg ground (a severe hazard where neutral and ground are bonded downstream of the panel).
- Identify the Hot: The wire that shows 120V to both the known ground and the known neutral is your Line/Hot. Re-identify it immediately with colored electrical tape.
Decision Tree: Which Standard and Wire Color Applies to You?
Use this decision path to determine exactly what wire to pull for your next branch circuit. Do not mix standards within a single junction box.
| Condition / Question | If YES | If NO |
|---|---|---|
| Is the installation in the US or Canada? | Follow NEC. Go to Cable Type. | Is it in the EU, UK, or Australia? Follow IEC/BS 7671. Go to Cable Type. |
| Are you pulling pre-sheathed cable (NM-B / Twin & Earth)? | You are locked into the manufacturer's internal colors. Buy standard 12/2 or 2.5mm² T&E. | You are pulling individual conductors in conduit (THHN / PVC). Go to Conduit Pick. |
| CONDUIT PICK (US NEC 120V 20A): | Buy: 12 AWG THHN Black (Hot), 12 AWG THHN White (Neutral), 12 AWG THHN Green (Ground). | |
| CONDUIT PICK (EU/UK 230V 20A): | Buy: 2.5mm² PVC Brown (Line), 2.5mm² PVC Blue (Neutral), 2.5mm² PVC Green/Yellow (Earth). | |
Practical Application: Matching Colors to Device Terminals
Once you have the correct color coded wires at the device box, the physical termination follows a strict mechanical standard across almost all North American and IEC-compliant receptacles, switches, and breakers.
- Brass / Gold Screws: Connect the Hot / Line wire here (Black in US, Brown in EU/UK). This is the ungrounded conductor that carries the voltage potential.
- Silver Screws: Connect the Neutral wire here (White in US, Blue in EU/UK). This is the grounded current-carrying conductor.
- Green Screws: Connect the Ground / Earth wire here (Bare/Green in US, Green/Yellow in EU/UK). This is the equipment grounding conductor; it carries zero current under normal operation and exists solely to trip the breaker during a fault.
GFCI and AFCI Line vs. Load: When wiring a GFCI receptacle, color alone does not tell you which set of screws to use. The LINE terminals (usually marked with a sticker or distinct molding) must receive the hot and neutral coming directly from the breaker panel. The LOAD terminals feed downstream devices. Reversing line and load will leave the receptacle energized but completely disable the ground-fault protection, creating a lethal shock hazard. Always use the manufacturer's included "LINE" tape marker on your incoming black and white wires before connecting them.
