What Is 6/3 Romex? | NM-B Cable Specs & Limits

A 240-volt circuit for an EV charger, a subpanel, or a large appliance draws enough current that you need to pick the wire gauge carefully. The cable that usually comes up is 6/3 Romex, a heavy-duty nonmetallic-sheathed cable built for branch circuits that need serious power. Here’s exactly what this cable is made of, what it’s rated for, and where you’re actually allowed to run it.

What Does 6/3 Romex Actually Mean?

In US residential wiring, 6/3 Romex means a 6 AWG, 3-conductor NM-B cable with a separate bare ground wire. The “6” is the wire gauge, and the “3” counts the insulated current-carrying conductors. Southwire, the company behind the Romex brand name, labels this specific cable as “6-3 CU NM-B WITH GROUND.”

That label translates to three insulated 6 AWG copper conductors (typically black, red, and white) plus a bare 10 AWG copper grounding conductor. All four sit inside a tough PVC jacket. Because of that extra ground wire, you’ll see it described as a 4-conductor cable in some listings, but electrically it’s a 3-wire circuit with a ground.

This construction is what makes 6/3 suitable for 120/240-volt applications where you need two hot legs, a neutral, and a safety ground running to the same device or panel. The conductors are stranded with 7 wires each, and the insulation is 30 mils thick, which keeps the cable flexible enough to pull through walls and attics. You’ll also see 6/2 Romex, which skips the neutral wire, but 6/3 is the more flexible choice for new circuits.

6/3 Romex Specs and Electrical Ratings

You can confirm the physical specs on Southwire’s official product page.

Even though the cable has a 90°C insulation rating, breakers and terminals are often rated lower. That’s why a licensed electrician will verify the exact ampacity based on the 60°C or 75°C column of the NEC ampacity tables. The 55-amp figure gives you a solid ceiling for planning, but the final installation has to match the equipment instructions.

Where You Can (And Can’t) Use 6/3 Romex

6/3 NM-B is approved for dry, indoor locations only. Southwire’s documentation is explicit that this cable is not rated for direct burial and is only suitable for above-ground use in dry environments. That rules out underground runs, damp crawl spaces where it might sit in water, and exterior walls where moisture is a concern. For those situations, you’d step up to a wet-rated cable like UF-B or run THHN wires in conduit.

Running a 240-volt circuit for a workshop subpanel or a large appliance is the sweet spot for 6/3. The three insulated conductors let you wire a 120/240-volt panel with two hots and a neutral, while the bare ground handles fault currents. Just be careful with conduit sizing if you plan to sleeve it.

Wiring a new 240-volt circuit is the sweet spot for 6/3, whether it’s for a workshop subpanel or a big appliance. If you’re pricing out materials, our roundup of the best 6 AWG Romex options compares the cut lengths and specs you’ll want before you order. Southwire lists this cable in a range of reels, from 25-foot spools up to 1000-foot rolls, so you can buy closer to the exact footage you need instead of wasting a half-spool.