What temperature column do you use for UF cable ampacity?

The 60 C column of Table 310.16, per NEC 340.80. It does not matter that the conductors inside UF-B are marked 90 C; the ampacity of the finished cable is taken at 60 C. So 12 AWG copper UF is good for 20 amps, 10 AWG for 30, 8 AWG for 40, and 6 AWG for 55. This is the same 60 C rule that applies to Type NM cable in 334.80.

Can UF cable be run underground?

Yes. UF stands for underground feeder, and 340.10 permits it for direct burial and for interior use in wet, damp or dry locations. Direct-buried UF follows the burial depths in Table 300.5, generally 24 inches for a direct-buried cable, with reduced depths for certain residential 120-volt, 20-ampere GFCI-protected branch circuits.

Where is UF cable not permitted?

Section 340.12 lists the prohibitions: as service-entrance cable, in commercial garages and hazardous locations, embedded in poured concrete, where subject to physical damage, and exposed to direct sunlight unless listed as sunlight resistant. It also cannot be used where NM would be prohibited, such as in the ducts covered by 300.22.

Is UF cable the same as NM cable?

No. Both are nonmetallic-sheathed, but UF's jacket is molded around each conductor and rated for moisture and direct burial. NM is dry-locations only. They share the 60 C ampacity rule (334.80 for NM, 340.80 for UF) and the same securing and support intervals, but only UF goes in the ground or into a wet location.

Why is UF limited to 60 C when the wire inside is 90 C?

Because the rule is written for the cable assembly, not the bare conductor. The 90 C rating of the individual conductors still matters for the derating calculations in 310.15, where you may start from the 90 C ampacity for adjustment and correction, but the final ampacity can never exceed the 60 C value. That two-step logic is the basis of the exam's favorite UF question.

Article 340 · NEC 340.80 deep-dive

UF Cable Ampacity: The 60 C Rule the Conductor Marking Hides

Four in ten candidates miss our UF cable question, and the miss is a reasonable one: the conductors inside the cable say 90 C, so they read the 90 C column. Section 340.80 says the cable is rated at 60 C regardless. Here is the rule, why it exists, and the two-step derating question built on top of it.

Last reviewed September 2026

What UF is and where it goes

Underground feeder and branch-circuit cable, Type UF, has its jacket molded around the conductors so moisture cannot travel inside it. Section 340.10 permits it for direct burial, for interior wiring in wet, damp and dry locations, and, where listed as sunlight resistant, outdoors above grade. It is the cable that feeds the detached shed, the post light and the outdoor receptacle.

The 60 C ceiling

The cable is rated as an assembly. Type UF-B carries 90 C conductors, and that rating is still real for one purpose: under 310.15 you may start derating from the 90 C ampacity for ambient temperature and bundling. But the finished number can never exceed the 60 C value. The exam tests exactly that two-step logic: derate from 90 C, then cap at 60 C, and report the smaller.

Worked example

10 AWG copper UF-B feeds an outdoor receptacle circuit. What is its ampacity?

340.80 sends you to the 60 C column of Table 310.16: 10 AWG copper is 30 A.
The 90 C marking on the conductors does not raise that. It is 30 A.
If the run passes through a hot attic, derate from the 90 C value (40 A) by the correction factor, then compare with 30 A and use the smaller.
Breaker: 30 A maximum for 10 AWG under the small-conductor rule in 240.4(D) as well.

Burial and the rest of the article

Direct burial depth follows Table 300.5: generally 24 inches for cable, with shallower allowances for residential 120-volt, 20-ampere GFCI-protected branch circuits.
Where it emerges from the ground it is protected from damage, usually in a raceway, to the height Table 300.5 requires.
Prohibited uses in 340.12: service-entrance cable, embedded in poured concrete, hazardous locations, commercial garages, physical-damage exposure, and unlisted sunlight exposure.
Support and securing follow the same intervals as NM cable: within 12 inches of a box and every 4.5 feet.

Where candidates lose the point

Reading the 90 C column because the conductors say 90 C. 340.80 caps the cable at 60 C.
Reading the 75 C column because the terminations are 75 C. The cable rule is stricter than the termination rule; the smaller wins.
Forgetting that derating may start from 90 C. The cap is on the final number, not the starting point.
Using UF as service-entrance cable or in concrete. Both are prohibited by 340.12.
Confusing UF with NM on a wet-location question. NM is dry only.

Wiring methods are the biggest section of the exam

Cable types, raceways and their limits make up the largest block of questions. The free diagnostic shows which ones you would miss today.

Take the free diagnostic →Ampacity derating (310.15) →

UF Cable Ampacity: The 60 C Rule the Conductor Marking Hides

What UF is and where it goes

The 60 C ceiling

Worked example

Burial and the rest of the article

Where candidates lose the point

Wiring methods are the biggest section of the exam

Table 310.16 ampacity

Derating (310.15)

The small-conductor rule (240.4(D))

10 common NEC mistakes