NM-B vs. UF-B: Why Your Outdoor Project Needs the Gray Cable

Cut both cables open and you'll find the same thing inside: copper conductors with 90°C-rated insulation, plus a bare ground. Everything that makes them different is the jacket around them.

NM-B ("Romex") wears a PVC outer sheath over paper-wrapped conductors. The sheath is there to hold the cable together and protect it inside walls — dry, dark, temperature-controlled walls. Paper and plastic do fine in there. They do not do fine in water.

UF-B (underground feeder) wears a solid gray jacket extruded directly around the conductors — no paper, no air gaps, the insulation molded solid around each wire like the cable was cast in place. Water has nowhere to go. The gray jacket is also sunlight resistant: UV-stable so it doesn't break down where the cable emerges from the ground into daylight.

That's the whole thesis of this article: the conductors are the same; the jacket decides where the cable is allowed to live.

NM-B: dry locations only. The Code says it outright — NM cable is not permitted in wet or damp locations (NEC 334.12(B)(4)). Inside finished walls and ceilings, attics, dry basements: fine. Anywhere water can reach it: not fine, and not a judgment call.

UF-B: wet locations, direct burial, and sunlight. It's rated for direct burial in the earth, for wet locations generally, and its jacket is sunlight resistant for the stretches above ground. Outdoors, underground, through a damp crawlspace — that's what it was made for.

The key insight most people miss: "outdoors" is always a wet location in the Code's eyes, even under a roof overhang, even inside conduit (more on that below). If the cable leaves the dry envelope of the building, NM-B is done and UF-B takes over.

Here's the mistake I see more than any other: someone runs Romex outside, but inside PVC conduit, reasoning that the conduit protects it from water. It feels logical. It's wrong.

Conduit installed outdoors or underground is a wet location inside (NEC 300.9) — raceways in wet locations above grade are considered wet inside, because condensation and seepage are inevitable. The conduit keeps out shovels and sunlight; it does not keep out moisture. NM-B inside outdoor conduit sits in a damp tube, the paper wrap wicks moisture, the sheath degrades, and you have a failing cable inside a perfectly good conduit.

The right answers for outdoor conduit: individual THWN-2 conductors pulled through the conduit (wet-rated, and the standard pro choice), or UF-B run through conduit as a cable. What goes in the ground and how deep: the burial depth guide has the full table.

UF-B costs more than NM-B — typically 30 to 50 percent more for the same gauge and conductor count. You're paying for the solid extruded jacket and the ratings that come with it. It's also stiffer and harder to work with: the solid jacket doesn't strip as cleanly as NM-B's sheath, and pulling UF-B through conduit is a fight (its flat profile binds — upsize the conduit or pull individual THWN-2 instead).

None of that is a reason to substitute NM-B outdoors to save money. The cost difference on a 100-foot run is tens of dollars; the cost of digging up and replacing a failed underground cable is a weekend and a sore back — assuming the inspector doesn't make you do it twice.

Quick chooser:

• Indoor, dry, inside walls → NM-B. Cheaper, easier, made for it.
• Outdoor, underground, damp, or sun-exposed → UF-B. The gray cable, every time.
• Outdoor in conduit → THWN-2 individual conductors (or UF-B through the conduit). Never NM-B.
• Chicago-area note: local code leans heavily on EMT conduit for residential work, so you'll handle less NM-B here than in most of the country — but the NM-B vs. UF-B rule doesn't change. Outdoors is outdoors.

Ended the project with most of the UF-B roll untouched? If it's still good and on the roll, we buy usable leftover wire — anywhere in the country. Fast, free quote.

← Back to the full Wire Guide

• How Deep Do You Bury Electrical Wire? — once you've picked UF-B, here's how deep it goes.
• How to Read the Printing on Electrical Wire — confirm the jacket says UF-B and SUNLIGHT RESISTANT.
• Stranded vs. Solid Wire — UF-B is solid through 10 AWG; know how it terminates.