Three cable types sold into the same residential and light commercial market look almost identical on a reel and are separated by one question: where is the cable going to live. Type SE is not permitted underground. Type USE is not permitted for interior wiring. Type UF is permitted for both, and can also be installed as nonmetallic-sheathed cable, which is why it appears in switchgear rooms and in trenches on the same project.
The reason the question is decided this way is that the code did not write three separate rulebooks. It wrote one, for nonmetallic-sheathed cable, and then pointed the other two at it.
One article carries the installation rules for three cable types
Article 334 governs nonmetallic-sheathed cable, and it is not only about the type that carries that name. Two clauses reach out from other articles and pull their cable back into it.
Cable family | Its own article | What else governs it |
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Types NM, NMC, NMS | Article 334 | Nothing. This is the rulebook |
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Types SE and USE | Article 338 | Type SE used for interior wiring must comply with the installation requirements of Part II of Article 334, excluding the ampacity clause |
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Type UF | Article 340 | When installed as nonmetallic-sheathed cable, it must comply with Parts I and II of Article 334, and must be of the multiconductor type |
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So a specifier who knows Article 334 knows most of what is needed for three cable types. The exclusions are the interesting part. Type SE is deliberately cut out of the ampacity clause in 334.80, because it has its own temperature rule. Type UF is only held to Article 334 when it is being used in that role, and then only in its multiconductor form, so a single-conductor UF run in a trench is not governed by Article 334 at all.
Article 334 is the shared installation rulebook
NM, NMC and NMS differ in one dimension
The three types inside Article 334 are not three grades of the same cable. They differ in whether the cable can tolerate moisture and corrosion, and the code states that twice — once as what each type may do, and once as what the other two may not.
Type | Permitted in |
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Type NM | Exposed and concealed work in normally dry locations; fished in air voids in masonry block or tile walls |
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Type NMC | Exposed and concealed work in dry, moist, damp or corrosive locations; inside and outside walls of masonry block or tile; a shallow chase in masonry, concrete or adobe protected by a steel plate at least 1.59 mm (1/16 in.) thick and covered with plaster |
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Type NMS | Exposed and concealed work in normally dry locations; fished in air voids in masonry block or tile walls |
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The prohibition list is where the structure shows. Types NM and NMS, but not NMC, are prohibited where exposed to corrosive fumes or vapors, where embedded in masonry, concrete, adobe, fill or plaster, in a shallow chase in masonry covered with plaster, and in wet or damp locations. Those four prohibitions are, item for item, the four conditions Type NMC exists to serve.
A reader who wants to know when to specify NMC can get the answer from the prohibition against the other two, which is usually the faster route.
The third type is a different animal rather than a third moisture grade. Type NMS contains communications conductors as well as power conductors, and those communications conductors must comply with Part V of Article 800. That makes it a composite cable, and it is the reason the type exists.
The three types differ in one dimension
What decides whether the cable is permitted is the building
Article 334’s permitted-use list is unusual, and it is the clause most often misread. It does not describe the cable. It describes buildings, using the construction types defined in NFPA 220 and in the applicable building code.
Building or structure | Type NM, NMC, NMS permitted? |
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One- and two-family dwellings, their attached or detached garages, and their storage buildings | Yes |
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Multifamily dwellings of Types II, IV and V construction | Yes, except as prohibited |
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Other structures of Types III, IV and V construction | Yes, except as prohibited, and the cable must be concealed within walls, floors or ceilings providing a thermal barrier with at least a 15-minute finish rating as identified in listings of fire-rated assemblies |
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Cable trays in structures of Types III, IV or V | Yes, where the cables are identified for the use |
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Types I and II construction | Only where installed within raceways permitted in that construction type |
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Any dwelling or structure not covered above | No |
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Two consequences follow. First, the answer to "may I use this cable here" comes from a document that is not the electrical code — the building’s construction classification and occupancy, from NFPA 220 or the applicable building code, with the code’s own annex offering the determination method. Second, the permission for Types III, IV and V structures carries a concealed-work condition and a thermal-barrier finish rating, which is a fire assembly requirement rather than a cable requirement. Where a penetration in a rated assembly is involved, the sealing detail is covered in cable penetration firestopping.
A separate prohibition catches the exposed work that often follows from this. Exposed in dropped or suspended ceilings is prohibited everywhere except one- and two-family and multifamily dwellings.
SE does not go underground, and USE does not come inside
Types SE and USE are one family split by destination, and the two prohibitions that do the splitting are short.
Cable | What it must not be used for |
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Type SE | Underground, with or without a raceway; where subject to physical damage unless protected; exterior branch circuits and feeder wiring unless installed under Part I of Article 225 and supported, or used as messenger-supported wiring |
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Type USE | Interior wiring; aboveground installations except where the cable emerges from the ground and terminates in an enclosure at an outdoor location; as aerial cable unless it is a multiconductor cable identified for use aboveground and installed as messenger-supported wiring |
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So the two are complementary and the relay point is a termination. Type USE may leave the ground to reach an outdoor enclosure and it may run overhead if it is the multiconductor, identified, messenger-supported construction. It may not enter the building. Type SE may not be buried at all, with or without a raceway protecting it.
Type UF is the one that takes either assignment. It is permitted underground including direct burial in the earth, permitted for wiring in wet, dry or corrosive locations, permitted as single-conductor cable under the general rules for all conductors of a circuit, and permitted to be supported by cable trays provided it is the multiconductor type.
Type UF carries its own prohibition list, and it is almost item for item the same as Type NM’s: not as service-entrance cable, not in commercial garages, theaters, motion picture studios or storage battery rooms, not in hoistways or on elevators or escalators, not in hazardous locations, and not embedded in poured cement, concrete or aggregate. It diverges twice. It adds a prohibition where exposed to direct rays of the sun unless identified for the purpose, and it omits the building-type restriction, which belongs to Article 334.
SE above ground, USE below it, UF either
One uninsulated conductor, and two different jobs for it
Type SE and Type USE cable containing two or more conductors may have one conductor uninsulated. What that bare conductor is doing determines which rule applies.
The cable may be used where the insulated conductors carry the circuit and the uninsulated conductor is used only for equipment grounding purposes. There is also a requirement that all of the circuit conductors of the cable be of the thermoset or thermoplastic type. And there is an exception for existing installations under which the uninsulated conductor may serve as a grounded conductor — a neutral — where the installation meets the requirements for grounding separate buildings and for using the grounded circuit conductor for grounding, and where that uninsulated grounded conductor originates in service equipment.
That difference matters more than it looks. Whether the bare conductor is an equipment grounding conductor or the neutral of the circuit changes what the ampacity calculation is even about, and it is a distinction a supplier cannot infer from the reel.
The marking clause adds one more thing a buyer should ask for. Service-entrance cable must be marked as required for insulated conductors, and cable with the neutral conductor smaller than the ungrounded conductors must be marked as such. A reduced-neutral assembly is not a defect, but it has to be identifiable on the jacket.
Three types, three ampacity statements
The three families state their ampacity rule differently, and the differences are not cosmetic.
Cable | Ampacity rule |
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Types NM, NMC, NMS | Determined under the general ampacity article, but the allowable ampacity must not exceed that of a 60°C rated conductor. The 90°C rating may be used for adjustment and correction calculations provided the final derated ampacity does not exceed the 60°C value. In cable tray, determined under the tray article |
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Type SE, interior | When installed in thermal insulation, the ampacity is per the 60°C conductor temperature rating, with the maximum rating usable for adjustment and correction provided the derated result does not exceed the 60°C value |
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Type UF | That of 60°C conductors |
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The general principle behind these columns — that the highest temperature column is not automatically available — is the subject of voltage drop and conductor sizing, and the detail is not repeated here. What belongs here is that the three cable types reach the same 60°C place by three different routes, and that Type SE reaches it only conditionally.
Two bundling rules that exist because this cable is not in a conduit
Cable that is pulled through a hole in wood framing and then sealed, or laid in contact with insulation, is in a thermal condition a conduit run never sees. Article 334 has two clauses for it, and both trigger at the same threshold.
Condition | Requirement |
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More than two cables containing two or more current-carrying conductors, installed without maintained spacing, through the same opening in wood framing to be sealed with thermal insulation, caulk or sealing foam | The allowable ampacity of each conductor must be adjusted, and the exception that would normally permit a higher result does not apply |
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More than two cables containing two or more current-carrying conductors, installed in contact with thermal insulation without maintained spacing between cables | The allowable ampacity of each conductor must be adjusted |
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The threshold is more than two cables, and a two-cable hole is below it. The exception that would otherwise rescue the calculation is switched off in the first case rather than being left to the installer’s judgment, which is the clause’s real teeth.
Support, bend radius and the small numbers
Provision | Requirement |
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Support intervals | Not exceeding 1.4 m (4½ ft), and within 300 mm (12 in.) of every outlet box, junction box, cabinet or fitting |
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Hardware | Staples, cable ties, straps, hangers or similar fittings designed and installed so as not to damage the cable |
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Flat cable | Must not be stapled on edge |
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Protected sections | Cable protected from physical damage by a raceway need not be secured within the raceway |
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Unsupported cable | Permitted where fished between access points through concealed spaces in finished buildings and support is impracticable, and where not more than 1.4 m (4½ ft) from the last support to the connection to a luminaire or equipment and both are within an accessible ceiling |
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Bending radius | Not less than five times the diameter of the cable, for Types NM, NMC, NMS, SE and USE alike |
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Device without a separate box | A wiring device identified for the use with an integral cable clamp is permitted where secured at 1.4 m intervals and within 300 mm of the wall opening, with at least a 300 mm (12 in.) loop of unbroken cable or 150 mm (6 in.) of cable end available inside the finished wall for replacement |
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Conductor sizes | 14 AWG through 2 AWG copper, or 12 AWG through 2 AWG aluminum or copper-clad aluminum. Communications conductors in Type NMS must comply with Part V of Article 800 |
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Insulation | 90°C. Cables marked NM-B, NMC-B and NMS-B meet the requirement |
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Sheath | Type NM: flame retardant and moisture resistant. Type UF: flame retardant, moisture, fungus and corrosion resistant, and suitable for direct burial |
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The bending radius figure is worth separating from the more familiar conduit approach: conduit bends are selected from a table by trade size, while these cables are governed by a multiple of their own diameter. Where a pull is being planned rather than a bend chosen, the mechanics are covered in pulling tension and bend radius.
The conductor size table also carries a fact that matters for sourcing. Aluminum and copper-clad aluminum are permitted in these constructions from 12 AWG upward, while copper goes down to 14 AWG, so an aluminum branch-circuit cable does not exist in the smallest size. What determines whether a conductor is actually what it claims to be is a separate question, covered in copper-clad aluminum conductor verification.
FAQ
Can Type SE cable be buried?
No. Service-entrance cable is prohibited underground with or without a raceway. Type USE is the underground member of the family, and Type UF is permitted underground including direct burial.
Can Type USE cable be used inside a building?
No. Underground service-entrance cable is prohibited for interior wiring. It is permitted aboveground only where it emerges from the ground and terminates in an enclosure at an outdoor location and is protected from physical damage.
Do the bundling rules apply to two cables through one hole?
No. Both clauses in Article 334 begin at more than two cables containing two or more current-carrying conductors. Two cables through a hole is below the threshold, though the second clause reaches cables in contact with thermal insulation at the same threshold.
Conclusion
Three cable types, one installation rulebook. Article 334 governs nonmetallic-sheathed cable and is also, by cross-reference, most of the installation requirement for Type SE used inside and for Type UF used in that role. Inside Article 334 the three designations differ in one dimension — whether the cable tolerates moisture and corrosion — and the code states it both as a permission and as a Prohibition against the other two.
The other two families are split by destination rather than by construction. Type SE does not go underground, Type USE does not come inside, and Type UF takes either assignment. The permitted-use clause that decides whether any of them may be used at all is written in terms of the building’s construction type and occupancy, which means the answer comes from the building code before it comes from the electrical code.
Residential and feeder cable from the reel
ZONG Cable supplies these constructions and prints the designation, the temperature marking and the conductor size on the jacket so that the reel can be checked against the drawing rather than against a catalog. Tell us the location, the building construction type and whether the run is interior, buried or overhead, and we will confirm which construction carries the requirement. The range sits under cable, and private-label construction for a specified project is handled under OEM support. The temperature-column question behind the ampacity clauses, the aluminum-to-copper-conductor question, the sealing of a rated crossing and the mechanics of the pull are each linked where they arise above.
Sources
- NFPA 70, National Electrical Code, 2011 edition. Read directly for this article: Article 334, Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS — 334.1 Scope; 334.2; 334.6; 334.10 Uses Permitted including items (1) through (5) with both informational notes, and paragraphs (A) Type NM, (B) Type NMC and (C) Type NMS with all their subparagraphs; 334.12 Uses Not Permitted including paragraph (A) items (1) through (10) and paragraph (B) items (1) through (4); 334.15; 334.17; 334.23; 334.24 Bending Radius; 334.30 Securing and Supporting including paragraphs (A), (B) and (C) and the informational note; 334.40 paragraph (B); 334.80 Ampacity including both bundling provisions and the cable tray reference; 334.100 Construction; 334.104 Conductors; 334.108; 334.112 Insulation and its informational note on the -B markings; 334.116 Sheath and its paragraph (A). Article 338, Service-Entrance Cable: Types SE and USE — 338.1 Scope; 338.2; 338.10 Uses Permitted including (A) and (B) with subparagraphs (1), (2) and its exception, (3), and (4) with items (a) and (b) and both informational notes; 338.12 Uses Not Permitted including (A) items (1) through (3) and (B) items (1) through (3); 338.24 Bending Radius; 338.100 Construction and its informational note; 338.120 Marking. Article 340, Underground Feeder and Branch-Circuit Cable: Type UF — 340.1 Scope; 340.2 the definition; 340.6 Listing Requirements; 340.10 Uses Permitted items (1) through (7) and the informational note; 340.12 Uses Not Permitted items (1) through (9); 340.24 Bending Radius; 340.80 Ampacity; 340.104 Conductors; 340.108; 340.112; 340.116 Sheath. Source text read from a full-text copy of the 2011 edition.
- NFPA 220, Standard on Types of Building Construction, and Informative Annex E of the code, both referenced by the code’s own informational notes for the determination of building construction types. Not read. The article states only that the code points to them.
- ZONG Cable range. https://zh-zong.com/cable/
