The US Electrical Wire Color Code Reference
The following table outlines the standard NFPA 70 (NEC) color designations for the most common AC power systems encountered in US residential and light commercial work. Always verify voltage with a meter before touching any conductor, regardless of its insulation color.
| Function | 120V / 240V Single-Phase | 208V / 240V 3-Phase | 277V / 480V 3-Phase | Practical Notes |
|---|---|---|---|---|
| Line 1 (Hot) | Black | Black | Brown | Always assume live; feeds standard 120V receptacles and 240V appliance L1. |
| Line 2 (Hot) | Red | Red | Orange | Used for 240V split-phase (dryers/ranges) or 208V 3-phase Phase B. |
| Line 3 (Hot) | N/A | Blue | Yellow | Strictly 3-phase applications; rare in residential except for large HVAC. |
| Neutral | White | White | Gray | Current-carrying conductor. Must be insulated. Can be gray in 120V if white is unavailable, but white is standard. |
| Ground | Green / Bare | Green / Bare | Green / Bare | Equipment Grounding Conductor (EGC). Never carries current under normal operation. |
⚠️ Callout: The Switched Hot Exception
In a standard switch loop (power to the light fixture, then down to the switch), the white wire running down to the switch is actually a hot feed. The NEC requires this white wire to be re-identified with black or red tape at both ends to indicate it is a hot conductor. If you see a white wire connected to a brass switch terminal, treat it as 120V hot.
In a standard switch loop (power to the light fixture, then down to the switch), the white wire running down to the switch is actually a hot feed. The NEC requires this white wire to be re-identified with black or red tape at both ends to indicate it is a hot conductor. If you see a white wire connected to a brass switch terminal, treat it as 120V hot.
Regional Variants and the "Rows People Get Wrong"
If you are working on imported machinery, transitioning between international job sites, or wiring an ESP32/Arduino project that interfaces with mains relays, you must understand how US colors map to international standards. The IEC 60446 standard (used in the EU and modern UK) uses entirely different colors for neutral and phases.
| Function | US (NEC) | EU / Modern UK (IEC) | Old UK (Pre-2004) |
|---|---|---|---|
| Phase 1 | Black | Brown | Red |
| Phase 2 | Red | Black | Yellow |
| Phase 3 | Blue | Gray | Blue |
| Neutral | White / Gray | Blue | Black |
| Ground | Green / Bare | Green/Yellow Stripe | Green |
The Rows People Get Wrong (And Why It Matters)
When reading the US reference table above, DIYers and junior apprentices consistently misinterpret three specific rows in the field:
- The Neutral Row (White/Gray): The most dangerous assumption is that "white equals neutral." In 3-way and 4-way switch circuits, white wires are routinely used as travelers or switch legs. Furthermore, in 277V/480V commercial panels, gray is the neutral. If you land a 120V white neutral on a 480V panel's gray neutral bus, or vice versa, you risk catastrophic fault currents. Always trace the white wire or test it; do not trust the color blindly.
- The Ground Row (Bare): In modern NM-B (Romex) and THHN, bare copper is strictly ground. However, in pre-1970s SE (Service Entrance) cable used for ranges and dryers, the bare, uninsulated wire was legally used as the neutral. If you are retrofitting an old 3-prong dryer outlet to a modern 4-prong NEMA 14-30, that bare wire is your neutral, not your ground. You must pull a new 10/3 cable with an insulated ground to meet current OSHA and NEC safety guidelines.
- The Line 2 Row (Red): Hobbyists often assume red is exclusively for 240V split-phase appliances. In a 208V 3-phase wye system, red is Phase B. If you are wiring a commercial HVAC contactor and treat red as a secondary 120V hot rather than a distinct 208V phase, your motor rotation will be wrong, and your phase-to-phase voltage readings will be 208V, not 240V.
Safe Interpretation When Markings Are Faded or Missing
In homes built between the 1950s and 1980s, heat, attic temperatures, and cheap dye lots cause wire insulation to fade. Black turns dark brown or gray; white turns yellow or cream. Painters also frequently overspray outlets, masking the wire colors entirely. Here is the exact bench-and-jobsite procedure for identifying mystery wires safely.
- De-energize and Verify Dead: Turn off the breaker. Use a Non-Contact Voltage Tester (NCVT) like the Klein NCVT-2 to check for the presence of voltage. Then, use a contact multimeter (e.g., Fluke 117) set to AC Volts. Test Hot-to-Ground, Hot-to-Neutral, and Neutral-to-Ground. All must read 0.0V before you touch bare copper.
- Identify the Ground First: If the bare/green wire is painted over, scrape a small section of insulation with a wire stripper's notch or a utility knife. Ground wires will show bare copper underneath. In older BX/AC armored cable, the armor itself acts as the ground via a bonding strip; there may be no separate ground wire at all.
- Energize and Test for Hot: Cap all exposed wires with wire nuts, turn the breaker back on, and carefully remove the nuts one by one. Use your multimeter (black probe on a known ground, red probe on the mystery wire). A reading of 114V–126V identifies the hot (Line 1). A reading of ~220V–252V against ground indicates a 240V hot leg.
- Identify the Neutral: The remaining insulated wire should be the neutral. To confirm, measure Neutral-to-Ground while a load (like a space heater) is running on the circuit. You should read a very low voltage (typically 0.5V to 2.0V) due to the resistance of the neutral wire carrying return current. If you read 120V between the mystery wire and ground, you have a multi-wire branch circuit (MWBC) or a miswired switch loop, not a neutral.
- Re-Identify with Phase Tape: Once identified, wrap the ends of the wires with high-quality vinyl electrical tape. Use 3M Scotch 35 (or equivalent) in specific colors: Black for hot, White for neutral. Never use a Sharpie marker. Marker ink fades within months, bleeds into the PVC jacket, and looks identical to dirt or black insulation under dim panel lighting.
Pro-Tip for Faded NM-B Jackets:
If the outer gray/white jacket of Romex is severely faded and you can't tell if it's 14 AWG (15A) or 12 AWG (20A), look at the bare ground wire. On 14/2 NM-B, the ground is 16 AWG. On 12/2 NM-B, the ground is 14 AWG. Use your wire strippers to gauge the bare copper; it is the most reliable physical indicator of the circuit's ampacity when the printed jacket text is illegible.
If the outer gray/white jacket of Romex is severely faded and you can't tell if it's 14 AWG (15A) or 12 AWG (20A), look at the bare ground wire. On 14/2 NM-B, the ground is 16 AWG. On 12/2 NM-B, the ground is 14 AWG. Use your wire strippers to gauge the bare copper; it is the most reliable physical indicator of the circuit's ampacity when the printed jacket text is illegible.
Ultimately, wire color is a secondary verification method. Your multimeter and an understanding of circuit topology are your primary safety tools. When in doubt, pull new wire rather than guessing the function of a degraded, 60-year-old conductor.
