Can you tape a black wire white to make it a neutral?

Yes for conductors 4 AWG and larger. NEC 200.6(B) permits identification by a distinctive white or gray marking that encircles the conductor at its terminations. For 6 AWG and smaller, 200.6(A) does not allow general field re-identification.

What size wire can be re-identified as a neutral?

Four AWG and larger. That is the dividing line between NEC 200.6(A) for 6 AWG and smaller and 200.6(B) for 4 AWG and larger.

Can green tape be used to mark a neutral?

No. Green and green with yellow stripes are reserved for equipment grounding conductors under NEC 250.119 and may not be used for grounded or ungrounded conductors.

Can you use black wire with white tape as a neutral?

For a 4 AWG conductor, yes. NEC 2023 Section 200.6(B) states that an insulated grounded conductor 4 AWG or larger shall be identified by a continuous white or gray outer finish, by three continuous white or gray stripes along its entire length on other than green insulation, or at the time of installation by a distinctive white or gray marking at its terminations that encircles the conductor or insulation. That last option is what electricians call taping the ends, and it is code compliant for 4 AWG and larger.

The size line matters

Section 200.6(A) covers grounded conductors 6 AWG or smaller, and the options there are narrower: a continuous white or gray outer finish, three continuous white or gray stripes on other than green insulation, or white tape, painting or other white marking at terminations only for specific cases such as conductors in cables that are exposed to the elements, single conductor Type MI, or a fixture wire in a luminaire. There is no general field re-identification allowance for 6 AWG and smaller building wire. So taping a 12 AWG black conductor white and calling it a neutral is a violation, while doing the same to a 4 AWG conductor is not.

How to do the marking correctly

The marking must encircle the conductor, not sit as a flag on one side, and it must be applied at every point where the conductor is accessible, which means at both terminations and in every junction box or pull point along the way. Use white or gray only. Green is reserved for equipment grounding conductors under 250.119 and can never be repurposed. Wrap at least three full turns of tape or use a printed white marker sleeve so the identification survives handling.

The parallel rules on ungrounded conductors

The same logic runs the other way for hots. Section 200.7(C)(1) permits a white or gray conductor in a cable to be used as an ungrounded conductor where it is permanently reidentified at each location where it is visible and accessible, which is the rule that makes the taped white switch leg in a 14/2 cable legal. Section 200.7(C)(2) covers the reidentified white conductor as the supply to a switch loop. For 4 AWG and larger, 200.7(C) allows the same reidentification, which is how a white 4/0 in a feeder becomes a phase conductor.

System identification

In a building with more than one nominal voltage system, 200.6(D) requires each system to have its grounded conductor identified by a different means, and 210.5(C) requires the ungrounded conductors of each system to be identified by phase or line and system. Documenting the scheme in a legend at each panelboard is required by those sections and is what an inspector will look for.

Field takeaway

Four AWG and larger: black conductor with white or gray marking encircling it at every accessible point is permitted. Six AWG and smaller: use factory white or gray insulation, because field marking is not allowed for general building wire.