Wire color dictates safety, function, and code compliance in alternating current (AC) circuits. Misidentifying a single conductor can result in a dead short, equipment destruction, or fatal electrocution. Before stripping insulation or terminating a lug, you must identify your region's governing standard using the master chart below.

The Master AC Colour Code Reference Table

The following table maps the primary AC wiring functions to their mandated insulation colors across the three most common global standards. Keep this chart accessible on your bench or in your panel schedule.

Function NEC (US / Canada) IEC (EU / Modern UK / AU / NZ) Old UK (Pre-2004)
Line 1 (Hot / Phase) Black Brown Red
Line 2 (Hot / Phase) Red Black Yellow
Line 3 (Hot / Phase) Blue Grey Blue
Neutral (Grounded Conductor) White or Grey Blue Black
Earth / Ground (Protective Earth) Bare, Green, or Green-Yellow Green-Yellow Green-Yellow (or Bare)
CRITICAL SAFETY NOTE: Never assume a wire's function based solely on its color, especially in older homes. Previous owners or unlicensed handymen frequently misuse wire colors. Always verify dead with a tested multimeter before touching any conductor.

Regional Standard Decision Path

Use this if-then decision tree to determine which standard applies to your project and exactly what materials to procure. This path terminates in a concrete purchasing decision.

  • IF you are wiring a new residential branch circuit in the US or Canada THEN the NEC (NFPA 70) applies. DEFAULT PICK: Buy Southwire 12/2 NM-B (Romex) containing Black (Hot), White (Neutral), and Bare (Ground) for 20A 120V circuits.
  • IF you are wiring a new ring main or radial circuit in the UK, EU, or Australia THEN the IEC 60446 / BS 7671 standard applies. DEFAULT PICK: Buy Prysmian 6242Y 2.5mm² Twin & Earth cable containing Brown (Line), Blue (Neutral), and Green-Yellow (Earth) for 20A circuits.
  • IF you are extending or repairing a pre-2004 UK circuit THEN you are dealing with a hybrid environment. Treat existing Red as Line and Black as Neutral. DEFAULT PICK: Buy modern IEC cable (Brown/Blue), but you must sleeve the old Black neutral wire with blue heat-shrink tubing at every junction point to prevent fatal confusion with modern IEC Black (which is a 400V Line 2 conductor).

Rows People Get Wrong (And How to Fix Them)

When troubleshooting or extending existing circuits, these specific color code misinterpretations cause the most blown breakers, damaged equipment, and shocks.

1. The 'White is Always Neutral' Trap (NEC)

In US wiring, a white wire is the grounded neutral conductor. However, in 3-way switch loops or 240V appliance circuits (like a dryer or range), white wires are frequently used as ungrounded 'hot' travelers or line conductors. The Fix: NEC Article 200.7(C)(1) mandates that any white wire used as a hot conductor must be permanently re-identified with black tape, paint, or heat-shrink at every termination point. If you open a switch box and see a white wire connected to a brass (hot) screw without black tape, flag it and correct it immediately.

2. The Old UK Black vs. IEC Black Death Trap

This is the most dangerous color code conflict in the world. In pre-2004 UK wiring, Black is Neutral. In modern IEC wiring, Black is Line 2 (carrying 230V to ground, or 400V phase-to-phase). If an electrician extends an old UK circuit using modern IEC cable and connects the new Black (Line 2) to the old Black (Neutral), they create a dead short across the mains, resulting in an arc flash or fire. The Fix: Never mix old and new cable colors in the same junction box without explicit, permanent sleeving and warning labels on the consumer unit.

3. Using Green/Bare as a Current-Carrying Neutral

Some DIYers run out of white/blue wire and decide to use the bare copper or green ground wire as a neutral to complete a circuit. This is a severe code violation. It energizes the entire grounding system of the house, meaning every metal appliance chassis, water pipe, and outlet cover plate becomes a shock hazard. The Fix: Ground wires must only carry current during a fault condition to trip the breaker. If you need a neutral, pull a new cable with a dedicated neutral conductor.

Safe Interpretation When Markings Are Faded or Missing

In older homes, NM-B jacket colors fade, white wires get painted over, or original installers used whatever scrap wire was in their truck. When visual identification fails, rely on electrical measurement. Follow this exact verification sequence using a CAT III or CAT IV rated multimeter (like a Fluke T5-600 or Klein MM700).

Pro-Tip: Always test your meter on a known live source (like a standard outlet) before testing the unknown circuit to verify the meter's battery and fuses are intact.
  1. De-energize and Verify Dead: Turn off the breaker. Use a Non-Contact Voltage Tester (NCVT) like the Klein NCVT-3 to confirm the circuit is dead before removing wire nuts.
  2. Identify the Hot (Line): Turn the breaker back on. Carefully measure between each suspect wire and a known good ground (like a metal junction box or the bare ground wire). The wire that reads nominal voltage (114V–126V in the US; 216V–253V in the UK/EU) is your Hot/Line.
  3. Differentiate Neutral from Ground: With the circuit under load (plug in a space heater or hair dryer to draw at least 10A), measure the voltage between the Hot wire and Suspect Wire A, then Hot and Suspect Wire B. Both should read nominal voltage. Next, measure the voltage between Suspect Wire A and Suspect Wire B.
    • If the reading is < 2.0V (ideally < 0.5V), you have found your Neutral and Ground. The Neutral will usually show a slight voltage drop (0.5V - 1.5V) relative to the ground under heavy load due to the resistance of the neutral wire carrying return current.
    • If the reading is nominal line voltage, one of those wires is not a neutral/ground; it is another hot leg (e.g., a 240V circuit).
  4. Dead Testing (Resistance): Turn the breaker OFF. Set your multimeter to continuity/ohms. Measure between the suspected Neutral and the bare Ground wire at the furthest outlet on the run. You should read < 1 ohm (they are bonded at the main panel). Measure between the suspected Hot and Ground; it should read OL (Open Loop) or infinite resistance.

Concrete Default Recommendations for Your Next Run

Stop guessing at the hardware store. When planning a new AC circuit, default to these exact, code-compliant specifications based on your region. Do not substitute aluminum for copper in standard residential branch circuits unless you are explicitly trained in aluminum termination torque and anti-oxidant paste application.

For US / Canada Installations (NEC)

  • 15A Lighting/Receptacle Circuits: 14 AWG NM-B (Black, White, Bare). Max run length before 3% voltage drop: ~50 feet.
  • 20A Kitchen/Bathroom/General Circuits: 12 AWG NM-B (Black, White, Bare). Max run length before 3% voltage drop: ~65 feet.
  • 30A Dryer/HVAC Circuits (240V): 10 AWG NM-B (Black, Red, White, Bare). The white wire is mandatory for the 120V control circuits in modern dryers.

For UK / EU Installations (IEC / BS 7671)

  • Lighting Circuits (6A/10A MCB): 1.0mm² or 1.5mm² 3-core and earth (Brown, Blue, Green/Yellow).
  • Ring Main Socket Circuits (32A MCB): 2.5mm² Twin & Earth (Brown, Blue, Green/Yellow). Must be wired in a continuous ring topology.
  • Cooker/Shower Radials (32A-45A MCB): 6.0mm² or 10.0mm² Twin & Earth (Brown, Blue, Green/Yellow).

The Final Rule: When in doubt, rip out the ambiguous, unverified wiring and pull a fresh, code-compliant run using the exact part numbers and color standards listed above. Never guess on AC wire colors; the cost of new copper is always cheaper than the cost of a house fire.