Use NM-B in permitted dry locations, UF-B where its listing permits wet or direct-burial use, and MC cable only where its particular construction and listing allow. Metal armor alone does not make a cable suitable for wet locations. Start with 2020 NEC Article 334 for NM, Article 340 for UF and Article 330 for MC.
Watch JCR explain where wiring methods live in the NEC
Victor maps the wiring-method families in Chapter 3. Use that map to find the article for your cable, then check its permitted uses and installation rules.
Watch JCR explain where wiring methods live in the NECWatch the existing JCR lesson on YouTubeStart with the location
| Cable | Common use | Key limit |
|---|---|---|
| NM-B | Branch-circuit wiring in permitted dry building locations | Not for wet or damp locations or direct burial |
| UF-B | Underground feeders and branch circuits, including permitted direct burial | Burial cover and physical protection still apply |
| MC | Feeders and branch circuits where its construction is permitted | Wet-location and direct-burial suitability depend on the complete cable’s listing |
Southwire’s building-wire guide is a useful check on NM-B and UF-B applications. For MC, read the exact product specification. Different MC constructions have different uses.
Can you put Romex in outdoor or underground conduit?
NM-B cannot be used in a wet location just because it is inside conduit. The inside of an underground raceway is a wet location under 300.5(B). A raceway aboveground in a wet location is covered by 300.9. Select conductors or cable permitted for that environment.
Romex is a brand name. Read the actual cable marking rather than treating the brand as a permission to use it anywhere. Southwire’s NM-B specification identifies dry-location use.
Learn to choose the right wiring method
Turn lessons like this into test-day skills. Find the rule, work the calculation and practice with JCR’s mock exam questions.
Live classes teach the 2020 NEC. Check your state’s exam requirements before choosing a course. NC prep includes NC business and law. For another state, also study any business-and-law material required for your license. Need a code book? JCR carries the 2020 NEC softbound book.
Already licensed? Earn your CE with JCR
Choose in-person classes, live Zoom or recorded courses. Review NEC navigation, grounding and bonding, and other electrical topics. Check the course’s NEC edition, state approval and credit hours before enrolling.
See CE courses and class datesCan MC cable go outside or underground?
Some MC cable can. Ordinary metal armor is not enough. Check 330.10, 330.12, the cable marking and the manufacturer’s instructions. Wet-location and direct-burial ratings are separate checks.
For example, Southwire’s PVC-jacketed MC feeder cable is a specific wet-location construction. That does not extend the same permission to every MC product. Use fittings listed for the cable and environment too.
Three more checks before selecting a cable
- Ampacity: NM-B and UF-B ampacity is limited to the 60°C conductor value under 334.80 and 340.80. For NM-B, 334.80 lets you use the 90°C rating for correction and adjustment. The final ampacity still cannot exceed the 60°C value. Section 340.80 does not include that allowance for UF-B.
- Protection and support: use the cable’s own article for fastening, physical protection and permitted building applications.
- Burial: direct-burial permission does not set the cover depth. Use Table 300.5 for the wiring method, location and circuit involved.
Continue with our ampacity adjustment guide and guide to finding wiring-method rules.
Practice question
An underground PVC raceway runs to a detached building. Can NM-B be used because the conduit keeps rain off the cable?
No. The inside of the underground raceway is a wet location. Use a wiring method permitted for that environment, then check ampacity, raceway fill and the building’s other requirements.
Common questions
Can UF-B be used indoors?
Yes. Section 340.10(4) permits multiconductor UF cable to be installed as NM cable when it follows Article 334 Parts II and III. Check the building and installation conditions.
Does thicker insulation mean higher ampacity?
No. Use the cable article, conductor rating, termination rating and conditions of use. A jacket’s appearance is not an ampacity rating.
Keep practicing
Try our most-missed NEC mock exam questions and 2020 NEC practice questions. Find the rule for each answer, work the calculation, and repeat the questions you miss.
Prepared October 25, 2026, for the 2020 NEC. Use the edition required by your licensing exam. On the job, use the locally adopted code and amendments.
