Reference · residential conductor identification

Wire color atlas

What each color means in the wiring you'll encounter every day — line-voltage branch circuits, thermostat cable, heat-pump reversing terminals, and the exceptions that trip up techs crossing over from plumbing.

Color is a convention, not a guarantee. An old installer, a rushed retrofit, a prior homeowner, or an OEM's house rules can produce wiring where the colors say one thing and the conductor does another. This page is the starting assumption; continuity and voltage verification are the confirmation.

Line-voltage (NEC)

Residential NM (Romex) cable — color conventions per NEC 14/2 WG — single 120V circuit lighting, receptacles, most residential loads white jacket BLK WHT BARE Black · ungrounded (hot) — 120V White · grounded conductor (neutral) Bare copper · equipment ground (EGC) 14/3 WG — 3-way switches, shared-neutral travelers or multi-wire branch circuit (MWBC) yellow jacket on 12ga; white on 14ga BLK RED WHT BARE Black · ungrounded leg 1 (120V) Red · second leg OR traveler White · neutral (never a hot) Bare · ground 10/2 WG or 6/3 — 240V appliance AC condenser, water heater, dryer, range orange jacket (10ga) or black (6ga) BLK WHT* BARE Black · line 1 (120V-to-ground) White* · RE-IDENTIFY as line 2 black tape/marker at both ends Bare · equipment ground 10/3 WG — 240V with neutral electric dryer, range, split-pole loads 4 conductors total BLK RED WHT BARE Black · line 1 Red · line 2 (240V L1-L2) White · neutral (timer, light) Bare · ground ▲ The white-as-hot exception NEC allows a white conductor to serve as an ungrounded (hot) conductor ONLY when re-identified at every termination and every visible junction — typically with black electrical tape or paint. Common on 2-wire 240V loops to AC condensers and water heaters. If you see an untaped white going to an overcurrent device or load hot terminal, flag it — likely a code violation and a voltage surprise.
Typical residential cable types and conductor roles. The 14-series uses white jacket, 12-series yellow, 10-series orange, 6-series black — but the conductor colors inside follow NEC regardless of jacket.

Line-voltage conductor color code (US/NEC)

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Black Ungrounded (hot) — any phase Most common hot
Red Ungrounded (hot) — 2nd leg or switched Travelers, 240V second pole
Blue, Yellow Ungrounded (hot) in commercial / conduit Rare residential; common in raceway
White / Grey Grounded conductor (neutral) Current-carrying return
White re-identified (tape) Hot, NEC-allowed exception Must be permanently marked at both ends
Green / Green w/ yellow stripe Equipment ground (EGC) Insulated variant of bare
Bare copper Equipment ground (EGC) Most common in NM cable

Thermostat cable

Thermostat wire — 18/8 color convention 18/8 thermostat cable cross-section R W Y G C O Bn K 8 conductors · 18 AWG Standard terminal mapping verify with continuity — colors are a convention, not a spec R 24 VAC hot — from transformer's secondary red — universal W heat call — energizes heat relay or gas valve white — universal Y cooling call — energizes contactor coil yellow — universal G fan — energizes indoor blower relay green — universal C common — 24V return; powers smart stats blue usually; sometimes brown, black O reversing valve — energized in COOL mode orange — Carrier / ICP / most brands B reversing valve — energized in HEAT mode dark blue / black — Rheem, some Trane Y2/W2/Aux 2nd-stage or aux heat — varies widely brown, pink, tan — no standard ▲ Never trust the color — trust the terminal Techs have been miswiring systems from color assumption for 40 years. Photograph the old thermostat before pulling it. When in doubt, disconnect at the stat and ring-out each conductor from the equipment side with a DMM.
Typical 18/8 thermostat bundle with industry-standard terminal mapping. Red/white/yellow/green are stable; blue (C), orange (O), brown/black (B / 2nd stage) are less predictable in older installs.

Thermostat terminal color convention

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R Red — 24V hot from transformer Universal; never varies
W White — heat call Second stage = W2, often pink/orange
Y Yellow — cooling / compressor call Second stage = Y2, often blue or brown
G Green — indoor blower fan Universal
C Blue — 24V common (return) Sometimes black or brown
O Orange — reversing valve energized in COOL Carrier/ICP/most brands
B Dark blue/black — reversing valve energized in HEAT Rheem, some Trane
E / Aux / X Emergency heat, aux heat, outdoor sensor Brown/tan/grey — no standard
L Outdoor unit lockout / status Amber lamp on stat
K Combined fan+compressor signal Honeywell 'combiner' on some stats

Heat pump O vs. B — the reversing-valve trap

The most common miswire in residential HVAC: connecting a heat-pump thermostat to an O-labeled system when the stat is configured for B, or vice-versa. The symptom is always the same — the system runs but produces the opposite of what's called for (heat in cool mode, cool in heat mode). Diagnosis starts by determining which terminal the equipment actually energizes.

Reversing valve — manufacturer conventions

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Carrier, Bryant, Payne, ICP, Heil O energized in COOL Majority of US market
Trane, American Standard O energized in COOL (modern) Some older units used B
Rheem, Ruud, Weatherking B energized in HEAT The primary 'B-system' brand
Goodman, Amana, Daikin-America O energized in COOL Daikin Japan inverter = different
Lennox O energized in COOL Modern O-type
York, Coleman, Luxaire O energized in COOL (modern) Legacy B on pre-2000 units
Mitsubishi, Fujitsu, LG ductless No O/B terminal Control via proprietary bus

Exceptions and variations a career plumber should know

The conventions that will trip you up

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Dual-transformer systems Rh and Rc split Separate heat + cool transformers
Old (pre-1970s) K&T residential Black = hot, white = hot, ground = none Both conductors can be hot
Aluminum branch (1965–1972 era) Larger conductor, silver appearance Terminations require CO/ALR devices
UF direct-burial cable Grey jacket, color same inside Gasketed junctions at transitions
Armored BX / AC cable Spiral metal sheath = ground path Not EGC in older MC cable
European imports (IEC) Brown/black/grey hot, blue neutral Opposite of US convention