Wire colours on a plug are a standardized insulation coding system that identifies the specific electrical function—line (hot), neutral, and earth (ground)—of each conductor inside a flexible cable to ensure safe termination at the plug's terminals. Getting these colours right dictates the polarity and safety grounding of the appliance; reversing line and neutral might still power the device, but it leaves internal single-pole switches and chassis components energized, creating a severe shock hazard during maintenance. DIYers most commonly confuse the physical ribbing on a flat US cord with the IEC colour code, or assume the earth wire is always solid green (it is green-and-yellow in modern IEC/UK standards).
The Global Standard: Wire Colours on a Plug Mapped
Because appliances travel across borders and DIYers frequently replace molded plugs on imported equipment, you must know which standard applies to your region. The table below maps the exact insulation colours and terminal destinations for the three most common plug wiring standards you will encounter on a workbench.
| Region / Standard | Line (Hot / Active) | Neutral | Earth (Ground) | Plug Terminal Target |
|---|---|---|---|---|
| UK / EU / IEC (IEC 60446 / BS 7671) |
Brown | Blue | Green & Yellow | L (Brass), N (Nickel), E (Green) |
| US / Canada (NEC Art. 200/250) |
Black (or Smooth) | White (or Ribbed) | Green or Bare | Brass screw, Silver screw, Green screw |
| Australia / NZ (AS/NZS 3000) |
Brown (Active) | Blue | Green & Yellow | Active, Neutral, Earth pins |
| Old UK (Pre-1970 / Pre-2004 fixed) |
Red | Black | Green | L, N, E (Requires sleeve update) |
In the US and Canada, the National Electrical Code (NEC) governs these assignments. Notice that for North American NEMA 5-15 plugs, the physical terminal screw colour is just as important as the wire insulation. The hot wire always terminates on the brass screw, the neutral on the silver screw, and the ground on the green screw. In the UK and EU, the BS1363 and Schuko standards rely strictly on the IEC brown/blue/green-yellow triad, terminating into clearly marked L, N, and E brass or nickel posts.
Where You Meet This in Practice (and the Math)
You will most frequently need to reference wire colours on a plug when replacing a damaged molded plug on a power tool, wiring an IEC C13/C14 "kettle lead" for a custom PC build, or adapting a US appliance for a UK/EU travel plug. Simply matching the colours to the terminals is only half the job; you must also verify the flex rating and, in the UK, calculate the correct fuse size.
A Worked Numeric Example: Sizing a BS1363 Plug Fuse
Unlike US plugs, UK BS1363 plugs contain an internal cartridge fuse to protect the flexible cable, not just the appliance. Replacement plugs typically ship with a 13A fuse installed. Leaving this 13A fuse in place for a low-draw appliance is a major fire hazard.
The Math:
Current (I) = Power (P) / Voltage (V)
I = 400W / 230V = 1.74 Amps
The Decision: Standard BS1363 fuses come in 3A, 5A, and 13A. Since 1.74A is well below 3A, you must swap the default 13A fuse for a 3A fuse. If a fault occurs and the cable draws 10A, a 0.5mm² flex will overheat and melt before a 13A fuse blows. A 3A fuse clears the fault instantly, preventing a fire. For appliances with high inrush currents (like a 700W blender drawing 3.04A nominal), step up to a 5A fuse to prevent nuisance tripping on startup.
For US NEMA 5-15 plugs, there is no internal fuse. The branch circuit breaker (typically 15A or 20A) protects the wiring. Therefore, you must ensure the flexible cord gauge (e.g., 14 AWG for 15A, 12 AWG for 20A) matches the plug's ampacity rating and the receptacle's breaker size.
Common Confusions and Dangerous Mistakes
Even experienced makers trip up on edge cases when wiring plugs. Here are the most common mistakes and how to avoid them.
1. The "Ribbed vs. Smooth" Flat Cord Trap (US/NEC)
Many US lamps and light-duty appliances use SPT-2 flat parallel cord. This cord has no coloured insulation inside; both conductors are bare copper. Instead of colours, the NEC relies on physical texture. The conductor with the ribbed or grooved outer jacket is the Neutral (connects to the Silver screw). The conductor with the smooth outer jacket is the Hot (connects to the Brass screw). Reversing these leaves the lamp's metal socket shell energized even when the switch is off, creating a shock hazard when changing bulbs.
2. Mixing Up Old and New UK Colours
If you are wiring a plug onto an older piece of equipment or extending a circuit in an older UK home, you may encounter the pre-2004 (and pre-1970 for fixed wiring) colour code: Red (Line), Black (Neutral), and Green (Earth). If you connect a modern brown/blue flex to an old red/black system, you must use crimp connectors and clearly mark the old black wire with a blue sleeve to indicate it is neutral, preventing future confusion. Electrical Safety First strongly advises against mixing old and new colour codes without permanent, physical sleeving markers.
3. The "Green is Always Ground" Assumption
In modern IEC and UK standards, a solid green wire is technically obsolete and non-compliant; the earth wire must be green-and-yellow striped. If you open a modern European appliance and see a solid green wire, it is likely a DC logic ground or a thermistor lead on a control board, not the mains earth. Never terminate a solid green wire to the mains earth terminal without verifying continuity to the appliance chassis with a multimeter.
Frequently Asked Questions
Can I use a US 2-prong cord on a UK appliance?
No. UK appliances designed for a 3-pin BS1363 plug often rely on the earth pin for safety grounding (Class I appliances) or to operate internal safety interlocks. Furthermore, a US 2-prong cord lacks the 0.5mm² minimum flex rating required for UK plug fusing and will not physically fit the earth aperture of a UK socket. Always use a cord that matches the appliance's Class rating and regional plug standard.
What if my replacement cable only has two wires (Brown and Blue)?
A two-wire cable (Brown/Line, Blue/Neutral) is designed exclusively for Class II (double-insulated) appliances, which do not require an earth connection. These appliances will have a "square within a square" symbol on their rating plate. You can safely wire this to a 2-pin Europlug or a 3-pin UK plug (leaving the Earth terminal empty), provided the appliance is strictly Class II. Never use a 2-wire cord on a Class I appliance with a metal chassis.
Why is the earth wire sometimes longer than the line and neutral wires?
This is a deliberate safety design known as "first-make, last-break." When wiring a plug, you should leave the green/yellow earth wire slightly longer than the brown and blue wires. If the cable is violently yanked from the plug and the cord grip fails, the line and neutral wires will pull out of their terminals first, cutting power to the appliance while the earth connection remains intact, keeping the chassis grounded until the very last millisecond.
