What Is Romex (NM Cable)?

NM cable is what most of your house is wired with. "Romex" is a trade name used generically, the way people say thermos — the code and inspectors call it NM cable, for non-metallic sheathed cable.

NM-B (Romex) at a glance

The 60°C rule is the one that catches people. NM-B conductors carry 90°C insulation, but NEC 334.80 requires the ampacity to be taken from the 60°C column — so 8 AWG NM-B is 40 A, not the 50 A the 75°C column would allow. That is why a 50 A circuit in cable needs 6 AWG.

What is inside Romex cable?

A flat plastic jacket containing:

• Insulated current-carrying conductors — a black hot and a white neutral in the common two-conductor version
• A bare equipment grounding conductor
• Paper or nylon filler separating them

Three-conductor versions add a red insulated conductor, used for three-way switching and multi-wire branch circuits.

How do you read NM-B cable markings?

The printing on the jacket tells you what you have. Learn to read it and you can assess a house from the basement.

"12-2 WITH GROUND" means:

• 12 — conductor gauge (12 AWG)
• 2 — two insulated current-carrying conductors
• WITH GROUND — plus a bare equipment grounding conductor

"14-3 WITH GROUND" means 14 AWG, three insulated conductors (black, white, red), plus a bare ground.

Also printed: the voltage rating, the type designation, and the manufacturer.

Remember that smaller gauge numbers mean larger wire — 12 AWG is bigger than 14 AWG. See wire gauge explained.

What do Romex jacket colors mean?

Modern NM cable jackets are color-coded by conductor size. It's a manufacturing convention, not a code requirement — helpful for spotting a mismatch at a glance, but the printed markings are authoritative.

Two cautions:

• Older cable predates the convention and may be white regardless of size
• Color alone should never be the basis for deciding what a circuit can carry

Always read the printing.

Why is NM-B limited to the 60°C column?

Modern NM cable is designated NM-B, indicating conductors with a 90°C insulation rating.

The practical catch: even though the insulation is rated to 90°C, the cable's ampacity is generally applied at the 60°C column of the NEC tables. So the higher insulation rating doesn't let you load the cable more heavily — it provides margin for derating.

This trips up people who read a 90°C ampacity value and assume it applies. Your electrician applies the correct column.

Where can NM-B cable be used?

Yes:

• Concealed in walls, floors and ceilings
• Dry interior locations
• Most one- and two-family dwellings

No:

• Outdoors — use UF cable or conductors in a suitable raceway
• Wet or damp locations — including underground, in contact with concrete or masonry in some circumstances, and unconditioned crawlspaces depending on conditions
• Buried
• Exposed to physical damage without protection
• Embedded in concrete

Some jurisdictions and building types restrict NM cable further — for example in certain multi-family and commercial construction. Local rules govern.

See types of electrical wire for the alternatives.

Support and protection rules

The NEC has specific requirements that inspectors check:

Support intervals. NM cable must be secured at intervals and within a set distance of every box. Loose, drooping cable is a defect.

Protection from nails and screws. Cable running through studs must be set back from the face, or protected with a steel nail plate where it isn't. This is what stops a picture hook from piercing a conductor.

Protection where exposed. In areas where cable could be physically damaged — garage walls, basement stairwells, storage areas — protection is generally required. Running along the sides of joists in a basement is normal; running across the face of a garage wall at shoulder height is not.

Bushings and connectors. Cable entering a box needs a proper connector and the sheath must extend into the box.

Bend radius. Sharp bends damage conductors and insulation.

What are common Romex installation mistakes?

Things worth looking for in a house:

Cable used outdoors or in wet locations. A clear defect — the jacket degrades in sunlight and moisture.

Unprotected cable in exposed areas. Especially garages and unfinished basements where things get stored and moved.

Missing nail plates where cable passes through studs near the face.

Sheath stripped too far back, leaving conductors unprotected outside the box.

No connector at the box entry.

Splices outside a box. Every splice must be in an accessible enclosure — see junction boxes explained.

Overfilled boxes. Box fill limits exist because crowded conductors can't shed heat and are damaged when the device is pushed in.

Stapled too tightly, crushing the jacket and conductors.

If you find these during a walkthrough, they're worth an electrician's assessment — see home electrical inspection before buying.

Working with it as a homeowner

Whether you may do this work yourself depends on your jurisdiction — see electrical work homeowners can legally do. Even where it's permitted:

• Permits still apply to new circuits in most places
• Box fill, support and protection rules are where DIY work most often fails inspection
• Terminations and torque matter more than they look — loose connections are a leading cause of overheating

The bottom line

NM cable is the standard interior branch-circuit wiring method, and the printing on the jacket tells you the gauge, the conductor count and whether a ground is present — read it rather than trusting jacket color. It belongs in dry, concealed interior locations only, properly supported, with nail plates where it passes near stud faces and protection anywhere it could be damaged. Cable outdoors, unprotected in a garage, or spliced outside a box are the defects worth acting on.

Check circuit capacity with the home electrical load calculator, look up sizes in the wire and breaker size reference, or read wire gauge explained.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC 334.80 — NM cable ampacity taken from the 60°C column
• NEC 334.12 — uses not permitted
• NEC 334.10 — uses permitted

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.