Reidentifying a White Conductor in Cable

Why this matters

Everyone who opens a box after you trusts the colors. A white conductor is assumed to be a neutral, and a neutral is assumed to be safe to handle with the breaker on and to land on the silver screw of a fixture. Two-conductor cable gives you one black and one white, and plenty of jobs need two ungrounded conductors: a 240 V baseboard heater, a water heater, a switch loop. NEC 200.7(C) (NEC 2023, 2020 where the section moved, as adopted, local amendments checked) is the rule that lets that white carry voltage, and its conditions exist so the next person cannot be fooled. Miss the tape, or use the white as the wrong leg of a switch loop, and you have built the trap that gets somebody's fingers across 120 V.

Before you open the box

A reidentified white is live whenever its circuit is, and the box that holds it may hold more than one circuit. Switch loops are the classic case: the switch box can carry a loop from a light fed by a different breaker than the receptacles beside it. Turn off the breaker, lock and tag it, and verify absence of voltage on every conductor in the box, whites included, with a CAT III or better meter proven on a known live source before and after (NFPA 70E Article 120, current edition; 29 CFR 1910.333(b)(2)). A white reading 120 V to ground in a box you thought was dead means a second circuit or a reidentified conductor you have not traced: stop and find its breaker before touching anything.

What 200.7(C) requires

In substance (check the current edition for the wording):

1. Only in a cable assembly (or a flexible cord, under its own conditions). A white conductor in a cable may be used as an ungrounded conductor. In a raceway with individual conductors, the rule does not apply: pull a conductor of the right color instead. You never reidentify a white in EMT or PVC.
2. Permanently reidentified at every termination and every place it is visible and accessible. Marking tape, paint, or another effective means, and the marking encircles the conductor's insulation. Both ends of the cable count as terminations, and so does any junction box the cable passes through where the conductor is exposed.
3. A color other than white, gray or green. Black or red is normal. Green is reserved for equipment grounding, and gray, like white, already means grounded conductor.
4. In a switch loop, supply only. Where the reidentified white is used for a single-pole, three-way or four-way switch loop, it is used only as the supply to the switch, never as the return from the switch to the outlet.

The same rule covers gray and white-striped insulation, and the bare or green equipment grounding conductor is untouched by any of it: it stays bare or green and lands on the green screw.

Why supply and never return

The switch-loop condition reads like trivia until you trace where each conductor ends up. In a loop fed at the light, the cable between light and switch carries two jobs: always-hot down to the switch, and switched-hot back up to the light.

With the white as supply:

• In the light box, the reidentified white is spliced to the incoming feed's black. A white in a bundle of blacks, wearing black tape, is plainly not a neutral.
• At the switch, it lands on a switch terminal.
• The switched return is the black, and it lands on the fixture's black lead.
• The fixture's white lead, which feeds the screw shell of the lampholder, connects only to the feed's white, which is a real neutral.

With the white as return: the white leaves the switch and lands on the fixture's black lead. If the tape falls off or was never applied, the next person changing the fixture sees two whites in the box and puts them together with the fixture white. With the switch open, nothing looks wrong. With the switch closed, the switched hot is spliced straight into the neutral bundle: a bolted fault through the switch, or, in a worse arrangement, a lamp shell connected to the switched hot.

The supply-only rule keeps every white that reaches the luminaire a true neutral. That is the whole reason it exists.

The loop, drawn

A single-pole switch loop in 14/2 NM, fed at the light:

 LIGHT BOX                          SWITCH BOX
 ---------                          ----------
 feed BLACK (hot) ---+
                     +-- loop WHITE, taped --- SW
                         (supply TO switch)   term 1

 fixture BLACK ------- loop BLACK ------------ SW
                         (switched return)    term 2

 feed WHITE (neutral) --- fixture WHITE

 feed BARE + loop BARE + fixture ground -----> SW
                                          green screw

 TAPE: loop WHITE at BOTH ends, encircling it,
       black or red, plus any box in between

A single-pole switch has no polarity, so which of its two brass screws takes the white does not matter. What matters is the light box: the white is in the hot splice, the black goes to the fixture.

240 V circuits in two-wire cable

The other everyday case. A 240 V baseboard heater, a 240 V water heater with no neutral, or a condenser fed with 10/2 NM uses the white as the second ungrounded conductor.

• Tape it at the breaker (the white lands on a two-pole breaker, not the neutral bar), at the thermostat or disconnect, and at the equipment terminals. Every termination, every accessible place.
• No supply-or-return restriction applies, because there is no switch loop: both conductors are line conductors.
• A line-voltage thermostat that breaks both legs is still two ungrounded conductors in and two out; tape the white in both cables at the thermostat.
• If the equipment needs a neutral for its controls (many water heaters do not; some appliances do), two-wire cable is the wrong cable. Pull three-wire with ground.

The neutral the switch box now wants

Old two-wire switch loops are legal as existing work. New work has a different expectation: 404.2(C) requires a grounded conductor at most switch locations that control lighting loads, so a smart switch or occupancy sensor can be installed later without a new cable. In substance, the exceptions include a raceway with room to pull a neutral later, a box that can take a new cable without removing finish, a switch that does not serve a habitable room or bathroom, and a few others; they vary by edition, so read the adopted text. The usual result in NM work is that the loop is run as 14/3 or 12/3 with ground, the white is a real neutral, and no reidentification is needed at all. Getting a neutral into an existing loop is owned by 'How to Get a Neutral to a Switch Box for a Smart Switch'.

Worked example: a fan swap that found a backward loop

The job. A tech is replacing a ceiling light with a ceiling fan in a 1990s bedroom, one wall switch, and the owner wants the fan on that switch. The breaker is off, locked and tagged, and every conductor in both boxes has been proven dead with the meter proven before and after.

What the light box holds:

• A feed cable, 14/2 NM, from the panel.
• A loop cable, 14/2 NM, to the switch.
• The old fixture's black and white leads.

What the tech finds. The feed black is spliced to the loop black. The loop white connects to the fixture's black lead. The feed white connects to the fixture's white lead. No tape on any white.

Reading it. The loop black is the supply to the switch and the loop white is the switched return to the light. That is the configuration 200.7(C) forbids: the white is the return, and it is unmarked besides. Two defects, one cable.

The correction, with no new cable:

1. In the light box, splice the feed black to the loop white, and wrap the loop white with black tape, encircling it, near the splice and up to the cable jacket.
2. Connect the loop black to the fan's switched lead (the lead the fan instructions assign to the wall switch).
3. Feed white to the fan's white. Grounds together and to the fan bracket.
4. At the switch box, the switch keeps its two conductors on its two terminals. Wrap the loop white with black tape there too.

The white is now the supply, the black is the return, and both ends are marked.

Proving it. With the fan hung and the switch box closed up, restore the breaker standing to the side of the panel. Switch on: the fan runs. Switch off: the fan stops, and a check at the fan's switched lead in the canopy, with the meter proven on a live source before and after, reads no voltage to neutral, confirming the switch opens the ungrounded conductor feeding the fan. Then close the canopy.

What would change the answer.

• The owner wants the fan and its light on separate switches. A two-wire loop cannot carry two switched legs. Pull 14/3 with ground: red and black become the two returns, the white becomes a real neutral, and 404.2(C) is satisfied in the same pull.
• The loop is in EMT, not NM. 200.7(C) does not apply to a raceway. A white found carrying the return in conduit is replaced with a colored conductor, not taped.
• The tape is on but the white still carries the return. Marking cures the visibility condition and not the supply-only condition; the swap is still required.

How to read someone else's loop

• A white spliced into a bundle of blacks in a light box, with or without tape, is almost always a loop supply. Find the other end before assuming.
• A white on a switch terminal is a loop conductor, never a neutral. If it is untaped, tape it while you are there; that is a two-minute fix that protects the next person.
• Two whites under one wire nut in a light box where one comes from the switch cable is the backward loop described above, or a neutral being carried to the switch box for a device. Trace it before you trust it.

References

• NFPA 70, National Electrical Code, 200.7(C), 404.2(C) (NEC 2023, 2020 where the section moved), as adopted by your jurisdiction.
• NFPA 70E, Article 120 (current edition); 29 CFR 1910.333(b)(2).
• See related: How to Get a Neutral to a Switch Box for a Smart Switch; Three-Way + Four-Way Switch Wiring.