How Romex NM-B Cable Is Built and Where You Can Run It

Why is a cable stamped 600 volts illegal to run through a damp basement? Because the rating that controls the run is the one on the insulation, and NM-B's insulation is listed for dry locations only, at the 60°C ampacity column. The cable is a stack of four layers, and a run is only as legal as the weakest layer in that stack. That is why matching the cable to the environment matters more than matching the wire gauge to the load: ampacity tables tell you how much current a conductor can carry, but they say nothing about whether the jacket will survive the room it sits in.

The four layers inside a Romex NM-B cable

Southwire's own listing for Romex Brand SIMpull nonmetallic-sheathed cable reads like a parts list: 600 volt, copper conductor, PVC insulation with a nylon sheath, PVC jacket with SIMpull technology for easier pulling. Taken in order, those are the four layers. The copper is ordinary annealed building wire, 14, 12, or 10 AWG in the sizes that fill a house, and it does the conducting. The PVC insulation is the dielectric that holds circuit voltage off the sheath and the surrounding wood, and its chemistry is what fixes the cable's temperature class. The nylon sheath is only a few thousandths of an inch thick and sits directly on the insulation; it exists to stop the insulation from being scuffed through when the cable is dragged across a stud or pulled through a bored hole, and it resists oils and chemicals that would attack PVC. The outer PVC jacket is what you actually see: it holds the conductors at fixed spacing, keeps them together as one flat cable, and takes the staples and the sharp edges of a framing cavity.

The reason the 600-volt figure belongs to the insulation and not the copper is simple physics. Copper has no voltage rating at all; voltage is a property of a dielectric, a measure of how much electric stress a material takes before it breaks down, and the metal is only the path the current travels on. The PVC insulation is what was tested to 600 V, which means the cable's voltage ceiling moves if you change the insulation, not if you change the conductor.

Why 600-volt Romex still can't run in wet spots

The same spec sheet is specific about where the cable may go: Romex Brand SIMpull nonmetallic-sheathed cable may be used for both exposed and concealed work in dry locations, with ampacity limited to that for 60°C conductors. Exposed means run along the face of a stud or joist where you can see it. Concealed means buried inside a wall or ceiling cavity, but only a dry one.

Wet service is where the polymer stack gives up. PVC takes on water over time and, in a persistently damp environment, the plasticizers that keep it flexible leach out; the material turns brittle, its dielectric strength and insulation resistance drop, and the end state is a cable that trips a GFCI or, worse, breaks down to ground. That failure is a function of jacket and insulation chemistry, which means a half-flooded basement, an unconditioned crawlspace, or a conduit that stays wet is off limits even when your load calculation was perfect. The substitute is a different cable type: UF for direct burial and wet runs, or individual THWN conductors inside conduit. Keep the wet-location list literal: buried direct in soil, inside a wet or damp conduit, under a slab, on the exterior of the building, and any spot subject to saturation.

Why NM-B ampacity stops at the 60°C column

NM-B is capped at the 60°C ampacity column, and the limiting factor is heat rather than current. Every foot of conductor makes I²R heat while it carries load, and that heat has to leave the copper, cross the insulation wall, cross the jacket bundle, and reach the air in the cavity. The temperature column a cable is listed under is set by the least heat-tolerant material in that path.

It comes down to the least heat-tolerant material in the path. In NM-B the copper inside is often the same thermoplastic-insulated conductor that would carry a 90°C rating on its own, but bundled in a jacket and stapled inside an insulated wall it has nowhere to dump heat, so the assembly is assigned the 60°C column. The 60°C column lands on familiar numbers, roughly 14 AWG at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A, and those are the values to size to unless the current code edition and the manufacturer's ampacity table give you a specific allowance.

Here is where people misread the number. Going up a conductor size does buy ampacity: swap 14 for 12 and 15 A becomes 20 A. What size never buys is a temperature class or a location. A 10 AWG NM-B run is still a 60°C dry-location cable, so pulling a 75°C or 90°C column out of the code book to squeeze more current through the same jacket is a defect no inspector will sign off on, because the rating scheme describes what the insulation survives, not what the copper could survive if it were bare.

Which household circuits NM-B is built to feed

The cable was designed for one job: branch circuits. NM-B cable is primarily used in residential wiring as branch circuits for outlets, switches, and other loads. In practice that is a 15 A or 20 A circuit feeding bedroom and living-room receptacles, a 15 A circuit for lighting, a 20 A bathroom circuit on GFCI protection, a 20 A kitchen small-appliance circuit, and the laundry circuit. Stock sizes map to those jobs: 14/2 for 15 A lighting and general receptacle runs, 12/2 for 20 A circuits, 12/3 when you need a multi-wire branch circuit or a traveler pair for three-way switches, and 10/2 for a 30 A circuit such as a water heater.

Because a residential branch circuit is low voltage, dry, and protected inside framing, it lines up exactly with what NM-B offers: 600 V, dry locations, concealed or exposed work. That alignment is why the cable is the default in wood-frame houses rather than a premium line. It is inexpensive because nothing about it is built for harsher conditions, and the moment a run leaves those three conditions the default stops being the right answer.

Running a Romex branch circuit, start to finish

Pulling a branch circuit is a sequence, and each step has one thing you cannot get wrong. Set the circuit and conductor size first: pick the breaker amperage from the load, then read the conductor size off the 60°C column, since that is the number that applies to the cable in your hand.

Then drill the framing. Bore through the center of the stud, keep the hole at least 1.25 in from the edge that will face a nail or screw, and make it a little wider than the flat of the cable. Centering the hole protects the conductors from compression, and holding it back from the nailing face is what stops a drywall screw from finding the cable a year later.

Pull the cable smooth and steady. Kinks are the enemy, because a sharp fold pinches the insulation and leaves a spot that can overheat under load. If it drags, ream the hole or fit a bushing instead of yanking harder.

Secure the run with staples or straps. The first one goes within 12 in of the box, then roughly every 4.5 ft along the run. Drive them just snug; a staple that crushes the jacket squashes the insulation beneath it and is the single most common installation defect I find on rework.

Leave 6 in of free conductor at each box so you have room to terminate, then strip. Use an NM-specific stripper that scores the outer jacket without cutting into the conductor insulation; a knife that reaches the copper is a nick waiting to fail. The low-friction SIMpull jacket exists for the same reason: the less force you need to pull, the less chance you tear the layers that keep the circuit safe. Finish by landing the grounds, making the terminations, and closing the box.

How to prove the run is right before drywall

Before the drywall goes up is the only cheap moment to catch a bad run. Work a checklist. Confirm the breaker amperage matches the conductor on the 60°C column. Walk the entire path and confirm it stays in dry, indoor, accessible space. Look at every box: conductors with no nicks, jacket not crushed under a staple, 6 in of free conductor, and box fill within the count for that enclosure size.

Then test. A continuity and insulation-resistance check between conductors and from each conductor to ground catches a nick you cannot see. On receptacle circuits, confirm the GFCI and AFCI protection the room requires is installed and resets cleanly, and write the circuit number on the panel schedule so the next person can trace it.

Which means the diagnostic order is set for you. When a check fails, whether it is a tripped GFCI, a scorched staple point, or an insulation-resistance reading that came back low, the problem almost always lives in the sheath or the insulation, the two polymer layers, and not in the copper. Copper either conducts or it is broken; it rarely fails partially. Troubleshoot from the outside in: jacket, sheath, insulation, then conductor.

Choose NM-B and you get the cheapest legal way to wire a dry indoor branch circuit: easy to pull on the SIMpull jacket, easy to staple, easy to strip, and cheap to buy. What you give up is everything above the 60°C column and every location that is not dry, including wet basements, buried runs, damp conduit, and hot attics. Only a different cable type buys those back. Inside dry walls it is the least expensive path, and the moment the environment changes it is the wrong cable, with nothing in between.