The grounded wire color identifies the current-carrying neutral conductor that completes the circuit back to the source, which is strictly white or gray in standard US NEC wiring. If you are holding a piece of 12/2 NM-B (Romex) cable, the white wire is your grounded conductor. It is the most frequently misidentified wire on residential and commercial jobsites because the terminology trips people up. When an apprentice or DIYer hears the word 'grounded,' they instinctively look for the bare copper safety earth wire. But in electrical theory and National Electrical Code (NEC) nomenclature, the grounded wire is the neutral, and it carries the exact same return current under normal operation as the hot wire carries out.
The Grounded Wire Color Code Standard
The rules governing the grounded wire color are laid out in NEC Article 200, specifically 200.6. The code mandates that grounded conductors must be distinctly identified to prevent them from being swapped with ungrounded (hot) or equipment grounding conductors. For standard residential and light commercial systems (120V/208V and 120V/240V), the outer insulation must be continuous white or gray.
Below is the definitive color code reference for standard US AC power wiring, contrasting the grounded wire with the rest of the cable bundle.
| Wire Function | NEC Terminology | Standard US Color (NEC) | International (IEC 60446) | Typical Insulation Type |
|---|---|---|---|---|
| Current Supply | Ungrounded Conductor (Hot) | Black, Red, Blue | Brown, Black, Gray | THHN, THWN-2, XHHW |
| Current Return | Grounded Conductor (Neutral) | White, Gray | Blue | THHN, THWN-2, XHHW |
| Fault Path / Safety | Equipment Grounding Conductor | Green, Green/Yellow, Bare | Green/Yellow | Bare Copper, THHN |
| Higher Voltage Return (277V/480V) | Grounded Conductor | White with colored stripe, Gray | Blue | THHN, THWN-2 |
What the Grounded Wire Changes in a Real Circuit
Understanding the grounded wire color is not just about passing an inspection; it fundamentally changes how you size wire, calculate voltage drop, and terminate connections in a panel. Because the grounded conductor is a current-carrying wire, it is subject to the same thermal limits and voltage drop calculations as the ungrounded (hot) conductor.
A Worked Numeric Example: Voltage Drop on the Neutral
Let us look at a standard 120V, 20A branch circuit wired with 12 AWG THHN copper in a conduit, running 50 feet from the panel to a receptacle powering a 16A space heater. Many beginners calculate voltage drop only on the hot wire. This is a critical error.
The current must travel 50 feet out on the black hot wire and 50 feet back on the white grounded wire, creating a total circuit loop of 100 feet. According to NEC Chapter 9, Table 8, 12 AWG solid copper has a resistance of roughly 1.98 ohms per 1,000 feet at 75°C.
Voltage Drop: 16A * 0.198 ohms = 3.16V drop.
Resulting Voltage at Load: 120V - 3.16V = 116.84V (a 2.6% drop, well within the NEC recommended 3% maximum for branch circuits).
If you had undersized the grounded wire—say, by mistakenly using 14 AWG for the neutral and 12 AWG for the hot—the resistance of the 14 AWG wire (3.14 ohms/1000ft) would push the total voltage drop to nearly 4%, causing the heater to run hotter, less efficiently, and potentially tripping the breaker prematurely. The grounded wire must always match the ampacity and gauge of the ungrounded wire in a standard single-phase branch circuit.
Furthermore, in a Multi-Wire Branch Circuit (MWBC) sharing a neutral, the grounded wire only carries the unbalanced load. If Phase A (black) draws 15A and Phase B (red) draws 10A, the white grounded neutral only carries the 5A difference. This is why MWBCs require handle-tied or common-trip breakers; if the neutral disconnects while the phases are live, the 120V loads suddenly become a 240V series circuit, destroying appliances.
Where You Meet This in Practice
You will interact with the grounded wire color and its termination rules in three primary locations on the jobsite or in your workshop.
1. The Electrical Panel (Neutral Bar vs. Ground Bar)
In your main service disconnect panel, the grounded (neutral) bar and the equipment grounding bar are bonded together. However, in any subpanel, they must be strictly isolated. The white grounded wires must terminate on the neutral bar, which is isolated from the panel chassis. The bare/green grounding wires terminate on the ground bar, which is bonded to the chassis. Mixing these in a subpanel causes neutral return current to flow on the equipment grounding paths, creating a severe shock hazard and potential electromagnetic interference.
2. Receptacles and Devices
On a standard 15A or 20A duplex receptacle, the white grounded wire always terminates on the silver-colored screw. The black hot wire goes to the brass screw. Modern code requires these connections to be torqued to the manufacturer's specifications—typically 14 in-lbs for standard residential devices—to prevent the neutral connection from loosening under thermal expansion, which causes arcing and fires.
3. Switch Loops and Re-identification
When wiring a standard light switch, power often drops to the light fixture first, and a 2-wire cable runs down to the switch. In this 'switch loop,' the white wire in the cable is used to carry the hot feed down to the switch. Per NEC 200.7(C)(1), this white wire is no longer acting as a grounded neutral. You must permanently re-identify it by wrapping it in black electrical tape or painting it black at both termination points. If an electrician sees a white wire at a switch and assumes it is a neutral, they could create a dead short or a shock hazard.
Grounded vs. Grounding: Clearing Up the Confusion
The most common mistake in residential wiring is confusing the grounded conductor with the grounding conductor. They serve entirely different purposes, operate under different physical conditions, and have distinct color codes.
| Feature | Grounded Conductor (Neutral) | Equipment Grounding Conductor (Earth) |
|---|---|---|
| Primary Purpose | Carries normal return current back to the source. | Provides a low-impedance fault path to trip the breaker during a short circuit. |
| Current Flow | Carries current continuously during normal operation. | Carries ZERO current during normal operation; only carries current during a fault. |
| Standard US Color | White or Gray. | Green, Green with Yellow Stripe, or Bare Copper. |
| Panel Termination | Neutral bar (isolated in subpanels). | Ground bar (bonded to panel chassis). |
| Sizing Rule | Must match the ungrounded (hot) wire ampacity. | Sized per NEC Table 250.122 based on the breaker rating (often smaller than the hot wire). |
Frequently Asked Questions
Can I use a white wire as a hot wire in conduit?
Yes, but only if you permanently re-identify it. If you are pulling individual THHN wires in a conduit and run out of black wire, you can pull a white wire for your hot leg, but you must wrap it with black phase tape at every splice, termination, and junction box. You cannot use a white wire as a hot wire inside a pre-manufactured NM-B (Romex) cable unless it is a switch loop as described above.
Why is my neutral wire reading voltage with a multimeter?
If you measure voltage between the white grounded wire and the bare ground wire, you are measuring 'neutral-to-ground voltage.' A reading of 1V to 2V is normal on a loaded circuit due to the voltage drop across the neutral wire's inherent resistance. However, if you read 120V between neutral and ground, you have an open neutral (a broken or disconnected white wire somewhere upstream) or the circuit is wired with the hot and neutral reversed.
Do I need to pigtail the grounded wire if it passes through a device?
Yes. NEC 300.13(B) requires that the grounded (neutral) conductor must be continuous. You cannot use the receptacle's internal metal yoke or the silver screw terminals to daisy-chain the neutral to the next device in a multi-outlet branch circuit. If the receptacle is removed, the downstream devices would lose their neutral return path, potentially exposing them to overvoltage in a multi-wire setup. Always use a wire nut or Wago connector to pigtail the white wires, with a single tail going to the device's silver screw.
