UF-B is Underground Feeder cable — the grey, solid-jacketed cousin of Romex, listed for direct burial and for wet locations. Its conductors are moulded into a solid PVC block rather than wrapped in a loose sheath, which is what makes it watertight and what makes it so difficult to strip. Like NM-B, its ampacity comes from the 60°C column under NEC 340.80.
| Property | Value |
|---|---|
| Voltage rating | 600 V |
| Temperature rating | 90°C conductors |
| Ampacity column | 60°C per NEC 340.80 |
| Jacket | Solid moulded PVC around all conductors |
| Conductor | Copper, solid through 10 AWG |
| Sizes | 14 AWG through 4/0, commonly 14 AWG to 6 AWG |
| NEC article | Article 340 |
| Direct burial | Yes |
| Sunlight resistance | Yes, listed sunlight resistant |
| Common configurations | 14-2, 12-2, 12-3, 10-2, 10-3, 8-3, 6-3 |
| Size | Ampacity |
|---|---|
| 14 AWG | 15 A |
| 12 AWG | 20 A |
| 10 AWG | 30 A |
| 8 AWG | 40 A |
| 6 AWG | 55 A |
| 4 AWG | 70 A |
| 3 AWG | 85 A |
| 2 AWG | 95 A |
| 1 AWG | 110 A |
| 1/0 AWG | 125 A |
| 2/0 AWG | 145 A |
| 3/0 AWG | 165 A |
| 4/0 AWG | 195 A |
UF-B is direct-burial cable, so it takes the first column of NEC Table 300.5(A): 24 inches of cover in the general case.
Two reductions are worth knowing. A residential branch circuit of 120 V or less, rated 20 A or less, and GFCI protected, may be buried at 12 inches. And irrigation or landscape lighting circuits of 30 V or less in UF cable may go at 6 inches. Under a driveway serving a one- or two-family dwelling, the general requirement is 18 inches.
Where UF-B emerges from the ground it must be protected in a raceway from below grade to at least 8 feet above finished grade, per NEC 300.5(D).
They look similar on the shelf and are constantly substituted for each other. They are not interchangeable in either direction.
UF-B is grey and solid. The conductors are embedded in a single moulded PVC body with no paper filler and no air gaps. That is why it survives burial and why removing the jacket takes a utility knife and patience. NM-B is white, yellow, orange or black, with a loose sheath over paper-wrapped conductors that peels off easily.
UF-B may be used indoors wherever NM-B is permitted, so substituting UF-B for NM-B is legal, just expensive and annoying to work with. The reverse — NM-B outdoors or underground — is a code violation under NEC 334.12(B)(4) and will fail inspection.
Both are limited to the 60°C ampacity column, so the wire size for a given breaker is the same for either cable.
Circuits to detached garages, sheds and workshops. Post lights, landscape lighting and yard receptacles. Well pumps and irrigation controllers. Outdoor GFCI receptacles fed from the house. Anywhere a branch circuit has to leave the building envelope and you do not want to trench conduit.
For anything larger than a branch circuit — a sub-panel feeder, a service lateral — use USE-2 or individual THWN-2 conductors in PVC conduit instead. The 60°C limit makes UF-B expensive and bulky at larger sizes.
24 inches of cover in the general case per NEC Table 300.5(A). A GFCI-protected residential branch circuit of 120 V or less and 20 A or less may be buried at 12 inches, and landscape lighting circuits of 30 V or less at 6 inches.
20 amps. NEC 340.80 requires UF cable ampacity to be taken from the 60°C column of Table 310.16, the same restriction that applies to NM-B.
Yes. UF-B is permitted anywhere NM-B is permitted, plus wet and underground locations. It is simply more expensive and much harder to strip, so NM-B is used indoors by default.
UF-B has a solid moulded PVC jacket and is rated for direct burial, wet locations, and sunlight exposure. NM-B (Romex) has a loose sheath and is rated for dry interior locations only. Both take ampacity from the 60°C column, so the wire size for a given breaker is the same.
No — direct burial is what it is listed for. It does need conduit or another approved raceway for physical protection where it emerges from the ground, from below grade up to at least 8 feet above finished grade.