Pull any outlet out of a modern American house and you will almost certainly see a flat plastic-jacketed cable running into the back of the box. That cable is Romex, the residential electrician’s default wire since the 1960s. What is Romex wire at a technical level is non-metallic sheathed cable, abbreviated NM-B in electrical code language. The Romex name itself is a trademark owned by Southwire Company, but the word has become the de facto term for any NM-B product, the same way Kleenex stands in for tissue. Understanding how it is built, where it is allowed, and which gauge matches which circuit protects you from some of the most common wiring mistakes.
The Basic Construction
Romex consists of two or three insulated copper conductors plus a bare copper grounding wire, all wrapped in a non-metallic PVC jacket. The insulation on each conductor is color-coded for function, and the jacket itself is color-coded by wire gauge since 2001.
- 14 gauge (15-amp circuits): White jacket
- 12 gauge (20-amp circuits): Yellow jacket
- 10 gauge (30-amp circuits): Orange jacket
- 8 gauge (40-amp circuits): Black jacket
- 6 gauge (55-amp circuits): Black jacket
Before 2001, all NM-B cables had a white jacket regardless of gauge, which is why older houses require you to check printed text on the jacket to know what you are working with.
2-Wire vs 3-Wire Cable
Romex is sold as either 14/2, 12/2, 10/2, 12/3, etc. The first number is the gauge, the second is the count of insulated conductors (not counting the ground).
14/2 or 12/2 has a black (hot), a white (neutral), and a bare copper ground. Used for most standard circuits powering outlets and lights.
14/3 or 12/3 adds a red conductor for a second hot. Used for three-way switches, split-receptacle outlets, and 240-volt circuits like electric dryers on older installations before modern code required separate neutrals for multi-wire branch circuits.
Where Romex Can Be Used
Romex is approved for most interior residential applications within the walls, attics, and crawl spaces of dwellings. It must be protected from physical damage and enclosed in framed assemblies or conduit where exposed.
NEC allows Romex in residential buildings up to three stories tall. Commercial buildings and multi-family above three stories typically require armored cable (MC) or conduit instead, depending on local code adoption.
Where Romex Cannot Be Used
- Outdoors or exposed to weather (use UF cable instead)
- Buried underground (use UF cable or THHN in conduit)
- Inside conduit as a regular conductor (use THHN)
- Wet or damp locations like swimming pool equipment
- Exposed to temperatures above 140°F for extended periods
- As service entrance cable (needs SE or SER)
The NM in NM-B stands for non-metallic sheathed, dry locations only. Using it in wet areas violates code and creates real safety hazards because the PVC jacket breaks down under moisture and UV.
Amperage and Distance Limits
Wire gauge determines how much current the cable can safely carry. The common pairings:
- 14 gauge: 15 amps, lighting and general outlets
- 12 gauge: 20 amps, kitchen and bathroom outlets, most modern branch circuits
- 10 gauge: 30 amps, electric dryers, water heaters, window AC units
- 8 gauge: 40 to 50 amps, electric ranges, larger AC condensers
- 6 gauge: 55 to 65 amps, subpanels, large appliances
Long runs cause voltage drop. For runs over 100 feet, upsize one gauge. A 12-gauge 20-amp circuit running 150 feet should step up to 10 gauge to avoid voltage drop under load.
Installing Romex Correctly
Romex must be stapled within 12 inches of every box and every 4 1/2 feet along runs. Use insulated cable staples like Gardner Bender’s PS-175 or the Arlington NM9 series, which do not pinch the jacket. Crushing the insulation with a hammered staple is a common failure point.
When passing through wall studs, drill holes at least 1 1/4 inches from the face of the stud or install a metal nail plate. Any wire closer than that risks being hit by drywall screws or cabinet installation screws.
Secure cables to boxes with clamps or integrated cable connectors. Plastic old-work boxes have built-in tension tabs for a single cable each. Metal boxes use separate plastic cable clamps or Romex connectors. The outer jacket should extend at least 1/4 inch inside the box before the individual conductors break out.
Working with Romex Safely
Turn off the breaker for the circuit you are modifying. Test with a non-contact voltage tester like the Klein NCVT-2 before touching any wires. Use wire strippers designed for NM cable, like the Klein 1412 or Ideal T-5, which strip the outer jacket cleanly without nicking inner conductors.
Leave 6 to 8 inches of conductor inside each box for easy connections and future access. Cut the outer jacket back so 1/4 inch extends past the clamp, and use wire nuts rated for the quantity and gauge of conductors you are joining. Ideal Twister Pro or 3M Scotchlok are industry standards.
Common Mistakes to Avoid
- Running Romex through conduit (not allowed for full lengths)
- Burying it directly underground (use UF-B instead)
- Stapling too tight and crushing the insulation
- Mixing 14 and 12 gauge on the same circuit (downsizes ampacity to 14)
- Exceeding box fill capacity with too many conductors
- Leaving sheathing unstripped inside boxes (violates code)
Cost and Where to Buy
Copper prices drive Romex cost, which fluctuates quarterly. As a rough reference:
- 250 feet of 14/2: $80 to $120
- 250 feet of 12/2: $120 to $180
- 250 feet of 10/2: $200 to $280
- 50 feet of 6/3: $120 to $180
Home Depot, Lowe’s, and electrical supply houses all carry standard sizes. For large jobs, electrical supply houses typically beat home-center pricing by 10 to 20 percent and stock spools in 500-foot and 1,000-foot reels.
Understanding what is Romex wire and how to use it correctly separates DIY rewires that pass inspection from ones that fail. Always check your local code amendments since some jurisdictions restrict NM-B in specific applications, and pull permits for any new branch circuit. The $50 permit fee buys you inspector eyes on the work, which is cheaper than any future fire insurance claim denial.
