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NM-B is a complete, sheathed cable normally used for residential branch circuits in dry locations. UF-B is a sheathed cable selected for certain exterior loads and is marked suitable for direct burial in Southwire’s selector. THHN/THWN-2 is individual building wire, normally pulled through conduit or installed in cable trays; the THWN-2 marking is the wet-location rating. The correct choice depends on the product’s complete marking, conductor sizing, installation method, locally adopted electrical code and local amendments.
This guide explains the practical differences without treating any one label as a universal wiring permission. For a real project, verify the cable marking and installation requirements with the authority having jurisdiction (AHJ) or a qualified electrician.
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NM-B, UF-B and THHN/THWN-2 at a glance
| Type | Construction and typical use | Location or application distinction | Practical point |
|---|---|---|---|
| NM-B | Multiple insulated conductors and a grounding conductor inside an overall sheath; primarily residential branch circuits | Dry locations, exposed or concealed work | It is a complete cable assembly. Its enclosed conductors are not loose THHN conductors. |
| UF-B | Sheathed cable for specified feeders and loads such as outside post lamps, pumps, pools or hot tubs, workshops and other exterior loads | Exterior applications; Southwire’s selector marks direct burial “YES” | Confirm the exact product marking and all burial, protection and local-code conditions. |
| THHN/THWN-2 | Individual insulated building wires used for services, feeders and branch circuits in conduit or cable trays | THHN: dry locations up to 90°C. THWN-2: wet or dry locations up to 90°C, as marked. | “THHN” alone does not establish a wet-location rating; read the complete marking. |
The construction difference is the quickest way to narrow a choice: NM-B and UF-B arrive as sheathed cable, while THHN/THWN-2 is supplied as separate conductors. That difference determines how the wiring is routed and protected; it is not merely a naming preference.
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Product details and ratings should be checked against the manufacturer’s documents: Southwire’s NM-B specification, its building-wire selector and the THHN/THWN-2 specification.
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What NM-B cable is—and where it belongs
Its construction is a finished cable assembly
NM-B contains multiple insulated conductors plus a grounding conductor under one overall sheath. That format is convenient for residential branch-circuit wiring because the conductors stay together as one listed cable. The sheath is part of the cable’s identification and installation method; removing it does not turn the individual wires into a different listed product.
NM-B is intended for dry locations
Southwire describes its NM-B for exposed or concealed work in dry locations, primarily on residential branch circuits. “Dry” is an important limitation. A space that can be subject to water, condensation or other wet-location conditions requires a wiring method and product marked for that environment. Do not infer outdoor, underground or other wet-location approval from the fact that a cable has an insulated outer jacket.
The 90°C insulation statement does not set the usable ampacity by itself
The individual insulation in the cited NM-B product has a stated 90°C rating, but Southwire says the cable’s ampacity is limited to that for 60°C conductors. Ampacity is the allowable current-carrying capacity used for conductor and overcurrent-protection decisions; it cannot be selected from the insulation temperature number alone. The applicable code rules, conductor size, terminal limitations and installation conditions still control.
Why stripped NM-B conductors cannot be reused as THHN
Southwire’s specification states: “Individual conductors within Type NM-B Cable are not listed or marked as THHN conductors (or any other NEC recognized conductor type) and are not permitted to be installed apart from the complete Type NM-B Cable.” In practice, that means stripping the sheath and pulling the enclosed wires separately is not an acceptable way to obtain conduit conductors. Buy conductors that carry the required marking for the intended wiring method.
What UF-B cable is used for
A sheathed option for specified exterior loads
UF-B is also a complete sheathed cable, but it is selected for applications where the wiring is exposed to exterior conditions. Southwire’s selector lists examples including outside post lamps, pumps, pools or hot tubs, workshops and other loads. Those examples describe the product’s intended applications; they do not replace a project-specific review of the equipment, route and local requirements.
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Direct burial is a product and installation question
In Southwire’s selection table, UF-B is marked “YES” for direct burial. That answers whether the listed cable type is identified for that use, but it does not answer every installation question. Verify the jacket marking on the actual cable, the required protection at transitions and terminations, and the burial conditions required by the locally adopted code. A cable being identified for direct burial does not make every depth, location or installation detail automatically compliant.
Do not substitute UF-B solely because it is more robust
UF-B’s exterior capability does not make it the right choice for every outdoor circuit. The circuit’s conductors, equipment connections, physical protection and installation route must all be compatible. If a project calls for conductors in conduit, individual conductors with the required wet-location marking may be the more appropriate wiring method. Selection should follow the complete design and product marking rather than a perceived toughness ranking.
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Individual conductors for conduit and cable trays
THHN/THWN-2 consists of separate insulated building wires rather than a multi-conductor sheath. Southwire identifies these conductors primarily for conduit and cable trays, including services, feeders and branch circuits in commercial or industrial applications. The installer chooses the number and colors of conductors needed for the circuit and routes them through the specified raceway or tray system.
Read the whole marking, not just “THHN”
Southwire rates the cited product as THHN for dry locations up to 90°C. The same product carries a THWN-2 marking for wet or dry locations up to 90°C. Therefore, a wire marked only for THHN should not be treated as wet-location wire simply because it looks similar to a dual-rated conductor. Confirm that the actual insulation includes the THWN-2 designation when a wet location is involved.
Oil exposure and other conditions still matter
The THHN/THWN-2 specification includes a stated oil-exposure limit. That qualification is part of the product’s application information, not a general permission to use the wire around any oil or chemical environment. Check the complete manufacturer marking and the project’s environmental conditions before choosing a conductor.
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What is the difference between NM-B and UF-B wire?
Both are sheathed cable assemblies, but their intended environments differ. NM-B is primarily a dry-location residential branch-circuit cable. UF-B is selected for specified exterior loads, and the cited selector identifies it for direct burial. The outer sheath and the product listing are therefore central to the distinction.
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- Identify the environment. If the route is a dry interior location, NM-B may fit the described application. If the route is outside or underground, evaluate UF-B or another wiring method specifically marked for those conditions.
- Identify the installation method. A sheathed cable and individual wires in conduit are different methods. Do not remove NM-B’s sheath to imitate individual building wire.
- Check the complete product marking. Confirm conductor type, temperature ratings and location approvals on the cable you will actually install.
- Confirm the local rules. The AHJ determines how the adopted electrical code applies to the project, including local amendments.
Can NM-B cable be used outdoors?
NM-B is described for dry locations, so an outdoor route should not be assumed acceptable. Outdoor spaces can involve rain, condensation, soil moisture or other conditions that are not dry locations. A project requiring exterior wiring should be evaluated for a product and wiring method specifically identified for that environment, such as UF-B where its marking and the installation conditions qualify, or individual conductors with the required wet-location rating in a suitable raceway.
The practical test is not whether the sheath feels weather-resistant. It is whether the complete listed cable, the route and the installation details satisfy the applicable requirements. If the route changes from indoors to outdoors, treat the transition and the outdoor segment as part of the design rather than extending an interior cable by default.
Is UF-B suitable for direct burial?
The Southwire building-wire selector marks UF-B direct burial “YES,” and its application examples include exterior loads. That is a useful starting point, not a blanket installation instruction. Before using it underground, match the cable’s printed marking to the intended use and verify the locally required burial, protection, entry and termination conditions. Those requirements can vary with the adopted code and local amendments.
Where the cable emerges from the ground or enters equipment, the transition may require additional mechanical protection or a different wiring method. The selector’s “YES” does not establish a universal depth or cover every site condition, so those details must come from the governing requirements for the project.
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Can THHN wire be used in wet locations?
Only when the conductor carries an appropriate wet-location marking. In the cited Southwire specification, THHN is rated for dry locations up to 90°C, while THWN-2 is rated for wet or dry locations up to 90°C. A dual-marked THHN/THWN-2 conductor can therefore be selected for a wet location when the rest of the wiring method and code requirements are satisfied. A conductor marked THHN without THWN-2 should not be assumed to have that approval.
Conduit does not automatically make an interior-only conductor suitable for moisture. Raceway systems can contain water or condensation, which is why the conductor’s location rating and the raceway installation both matter.
How electrical-code adoption affects the choice
The National Electrical Code (NEC), NFPA 70, is a U.S. electrical-installation standard. The Electrical Safety Foundation International explains that NEC provisions are commonly mandated through state or local law and that the AHJ inspects installations. Adoption is not uniform: ESFI’s map effective June 2026 shows jurisdictions using different NEC editions and amendments. See the ESFI NEC overview and the June 2026 adoption map.
Consequently, a cable label can establish the manufacturer’s rating without answering every code question. The edition in force where the work occurs, local amendments, the equipment listing and the AHJ’s interpretation can affect the permitted wiring method. A national rule stated without that geographic qualification can mislead.
A practical selection checklist
- Environment: dry interior, exterior, underground or another condition that may be wet.
- Cable form: complete sheathed cable (NM-B or UF-B) or individual conductors in conduit or cable tray (THHN/THWN-2).
- Marking: read every relevant designation, including THWN-2 where wet-location use is expected.
- Application: branch circuit, feeder, service or a named exterior load; confirm that the product is listed for that use.
- Temperature and ampacity: do not select current capacity from a 90°C insulation statement alone; apply the governing ampacity and terminal rules.
- Physical route: account for transitions, mechanical protection, terminations and any underground segment.
- Jurisdiction: check the locally adopted NEC edition and amendments with the AHJ.
- Professional review: use a qualified electrician when the project conditions, equipment or code requirements are uncertain.
Common selection mistakes and how to avoid them
Using NM-B because it has a sheath
A sheath does not make a cable suitable for wet or outdoor locations. Start with the location rating, not the appearance of the jacket.
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Calling stripped NM-B “THHN”
The enclosed conductors are not marked as THHN and are not permitted to be installed apart from the complete NM-B cable according to Southwire’s specification. Purchase listed individual building wire when conduit wiring is required.
Reading THHN as a wet rating
Look for THWN-2 on the conductor. The dual marking is the relevant indication for the wet-location application described by the manufacturer.
Treating a selector entry as a complete design
A “YES” for direct burial identifies the cable’s listed application in the selector; it does not provide every site-specific burial, protection or termination detail. Those conditions still need to be checked.
Assuming one NEC edition applies everywhere
State and local adoption varies, and amendments can change how a wiring method is accepted. Confirm the rule in the project’s jurisdiction before work begins.
What “and more” means in this comparison
Building-wire catalogs include other cable and conductor families, but the distinctions documented here are limited to NM-B, UF-B and THHN/THWN-2. Other labels should not be assigned a location rating or installation method by analogy. If a project specification names another cable type, use that product’s own listing and the locally adopted code rather than inferring its properties from the three types above.
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