240V wiring colours are the standardized insulation color codes used to identify the specific function—hot, neutral, or ground—of conductors in a 240-volt electrical circuit. Getting these colours right changes everything from troubleshooting speed to preventing a catastrophic dead short or equipment destruction when a neutral is accidentally landed on a hot bus bar. The most common confusion occurs when DIYers mix up pure 240V circuits (which require no neutral) with 120/240V split-phase circuits (which do), or when they fail to re-identify a white wire used as a hot conductor.
The Core 240V Wiring Colours Table: US vs. International Standards
Before you strip a single wire, you must know which regional standard your installation falls under. North America relies on the National Electrical Code (NEC), which governs 120/240V split-phase and 208/240V three-phase systems. The UK, EU, and most of the world follow IEC 60446 (harmonized in the UK as BS 7671), which uses entirely different colours for 230/240V single-phase and 400V three-phase systems.
| Conductor Role | US/Canada (NEC) 120/240V Split-Phase | US/Canada (NEC) 208/240V 3-Phase | UK/EU (IEC 60446) 230/240V Single |
|---|---|---|---|
| Line 1 (Hot A) | Black | Black | Brown |
| Line 2 (Hot B) | Red | Blue (or Orange for High-Leg Delta) | N/A (Single Phase) |
| Line 3 (Hot C) | N/A | Blue (Standard Delta/Wye) | N/A |
| Neutral | White or Grey | White or Grey | Blue |
| Earth / Ground | Bare, Green, or Green/Yellow | Bare, Green, or Green/Yellow | Green/Yellow |
Pure 240V vs. 120/240V Split-Phase: A Worked Numeric Example
In North American residential wiring, the utility transformer delivers 240V across two 'hot' legs that are 180 degrees out of phase. The voltage between either hot leg and the neutral is 120V; the voltage between the two hot legs is 240V. Understanding whether your appliance needs pure 240V or split-phase 120/240V dictates your wire count and colour selection.
Let's look at a real-world numeric example: running a 40A circuit from a 200A main panel using 8 AWG THHN copper wire pulled through EMT conduit.
- Scenario A: 40A Pure 240V Baseboard Heater. This is a resistive load that only needs 240V. It requires two hot wires and one ground. You will pull Black (Hot 1), Red (Hot 2), and Green (Ground). No neutral is required. At roughly $0.85 per foot for 8 AWG THHN, 50 feet of this 3-wire setup costs about $127 in conductors.
- Scenario B: 40A 120/240V Electric Range. A modern range uses 240V for the heating elements, but requires 120V for the digital control board, clock, and oven light. It requires two hots, one neutral, and one ground. You will pull Black (Hot 1), Red (Hot 2), White (Neutral), and Green (Ground). The 4-wire setup for 50 feet costs about $170. The white neutral carries only the unbalanced 120V return current, which is typically less than 5A.
If you attempt to wire the electric range without a white neutral and instead try to use the green ground as a return path for the 120V components, you create a severe shock hazard. If the ground connection fails, the entire metal chassis of the range becomes energized at 120V. This is why the NEC mandated 4-prong (hot-hot-neutral-ground) receptacles for new dryer and range installations, retiring the old 3-prong standard.
Where You Meet This in Practice: Re-Identification and Retrofits
The most frequent code violation regarding 240v wiring colours happens when installers use standard 2-conductor NM-B (Romex) cable for pure 240V loads like water heaters or baseboard heaters.
A standard 10/2 NM-B cable contains a black wire, a white wire, and a bare ground. If you connect this to a 30A double-pole breaker for a water heater, the white wire is no longer a neutral—it is acting as a second hot conductor carrying 120V to ground (and 240V line-to-line).
The NEC 200.7(C) Re-Identification Rule:
When a white or grey wire is used as an ungrounded (hot) conductor, it must be permanently re-identified at both ends and at every accessible splice point. You do this by wrapping the wire with black or red electrical tape, or by painting it with an indelible marker. Leaving the white wire un-taped at the breaker panel or the appliance junction box is a guaranteed red flag during an electrical inspection, as the next person working on the panel will assume it is a neutral and could accidentally bond it to the neutral bus bar, creating a dead short.
For those working in the UK or EU, the rules are governed by BS 7671 (IET Wiring Regulations). In a standard 230/240V single-phase system, Brown is always Line (Hot), Blue is always Neutral, and Green/Yellow is Earth. The UK does not use split-phase 240V in standard residential properties; 400V three-phase (Brown, Black, Grey) is reserved for industrial or large domestic loads like heavy EV chargers.
Frequently Asked Questions About 240V Colours
Can I use blue and yellow wires for residential 240V?
Yes, but with caveats. In the US, blue and yellow are typically reserved for 208V/480V three-phase systems or as 'travelers' in 3-way switch loops. However, if you are pulling THHN in conduit for a 240V circuit and run out of red wire, blue is an acceptable alternative for the second hot leg, provided it is clearly documented and consistent throughout the run. Never use white, grey, or green for a hot conductor without proper re-identification.
What if my house has old cloth-sheathed wiring without colour codes?
Pre-1950s knob-and-tube or early cloth-sheathed cables often lack modern colour coding, or the colours have faded to a uniform dusty brown. In this scenario, colour is irrelevant. You must use a non-contact voltage tester and a multimeter to verify the function of every single conductor before touching it. Upgrading these circuits to modern NM-B or THHN is highly recommended for safety.
Does the colour of the wire insulation affect its ampacity?
No. The ampacity of a wire is determined by its gauge (AWG), the conductor material (copper vs. aluminum), and the temperature rating of the insulation (e.g., 60°C for NM-B, 90°C for THHN), not the colour. A 10 AWG white wire and a 10 AWG black wire from the same manufacturer have the exact same current-carrying capacity. The colour strictly denotes the conductor's role in the circuit.
Why do some 240V tools have 3-prong plugs while my dryer has a 4-prong?
Tools like table saws, welders, and air compressors are pure 240V loads. They only need two hots and a ground, hence the 3-prong plug (like a NEMA 6-15 or 6-50). Dryers and ranges require 120V for internal electronics, necessitating a neutral wire and a 4-prong plug (like a NEMA 14-30 or 14-50). Always match the receptacle and plug configuration to the appliance's specific voltage and neutral requirements.
