THHN vs Romex compares an individual insulated building wire with a complete sheathed cable. THHN conductors normally run in conduit or another permitted wiring system. Romex® Brand NM-B contains several conductors inside one nonmetallic jacket and is commonly used in dry residential walls where local code permits. Choose the wiring method and installation environment first, then compare the complete circuit cost.
This comparison uses the U.S. National Electrical Code (NEC) framework. The locally adopted edition, amendments, product listing, building conditions, and authority having jurisdiction (AHJ) determine what a project may use. A product designation alone does not approve an installation.
What is the difference between THHN and Romex?
THHN identifies a thermoplastic-insulated, high-heat-resistant, nylon-covered conductor type. A typical copper THHN wire has a copper conductor, PVC insulation, and a thin nylon covering. Each wire must be specified by size, color, construction, and applicable ratings.
Romex® is Southwire’s brand name. Contractors sometimes use “Romex” informally to mean NM-B cable, but NM-B is the generic product designation. Cable from another manufacturer should be specified as Type NM-B rather than presented as genuine Romex Brand cable.
For purchasing, write the required cable type and state separately whether a named brand is mandatory or an approved equivalent is acceptable.
The practical THHN wire vs NM-B distinction is how the circuit arrives: separate conductors that need a permitted wiring system, or a factory-assembled cable with an overall jacket.
For full NM-B construction, size markings, and product documentation, use our NM-B cable guide.
THHN vs Romex decision table
The table assumes copper building wire and standard NM-B cable. Every application remains subject to local code and project approval.
| Selection factor | THHN or appropriately dual-rated conductors | Romex Brand NM-B or other listed NM-B |
|---|---|---|
| Construction | Individual insulated conductor with nylon covering | Several conductors within a common nonmetallic jacket |
| Conductor quantity | Specify each required conductor separately | Select a factory cable configuration, such as 12/2 with ground |
| Dry wall cavities | Normally installed within a permitted raceway system | Common choice where the building and local code permit NM-B |
| Complete conduit runs | Usually the more practical arrangement | Verify permission, cable dimensions, fill, and pulling conditions |
| Exposed routes | Raceway selection provides the required protection | Only where permitted, with protection against physical damage as required |
| Wet locations | Verify an actual wet-location rating, such as THWN-2 | Standard NM-B is unsuitable |
| Underground conduit | Use conductors approved for the wet environment | Putting NM-B in conduit does not make it suitable |
| Installation work | Raceway installation, conductor pulling, identification, and termination | Cable routing, securing, protection, and termination |
| Future circuit changes | Repulling may be possible after checking capacity and route access | A different conductor count generally requires another or replacement cable |
| Packaging | Separate spools or reels by size and color; other arrangements by agreement | Coils or reels of a specified complete cable |
| Cost comparison | Include every conductor plus raceway, fittings, and labor | Include the complete cable plus supports, connectors, protection, and labor |
This leads to two common starting points: evaluate NM-B for permitted dry residential cable routes, and evaluate separately listed conductors for a designed conduit system. Neither starting point replaces a code review.
Conductor count: one THHN wire is not one NM-B cable
A 12/2 NM-B cable with ground contains two insulated 12 AWG conductors plus an equipment grounding conductor. A 12/3 with ground contains three insulated conductors plus a grounding conductor.
A spool marked 12 AWG THHN contains one conductor. The circuit design determines how many separate lengths are needed.
For example, a circuit requiring one hot, one neutral, and a separate wire equipment ground needs three conductor runs. A circuit with different voltage or control requirements may need a different arrangement.
Specify grounding independently. Whether an approved metal raceway may serve as the equipment grounding path depends on the complete installation and applicable rules; it should not be assumed when comparing quotations.
Use our cable jacket markings guide to check AWG, conductor count, type designation, and traceability before accepting a substitute.
Can THHN be used without conduit?
For ordinary building branch-circuit wiring, loose THHN conductors should not be routed or stapled through wall cavities as a replacement for NM-B. They normally need conduit or another wiring method specifically permitted for the application.
“Conduit” is not the only possible approved system. Certain cable-tray installations and equipment applications have their own requirements, including conductor size, marking, and permitted use. A tray allowance for a larger conductor does not authorize loose small-gauge THHN in a house wall.
Boundary 1: a wall cavity does not replace the wiring system
Concealing THHN behind drywall does not provide the permissions associated with a listed sheathed cable. Confirm the complete wiring method before substituting individual wires for NM-B.
For specification and listing checks, see our UL 83 THHN/THWN-2 wire guide.
Can Romex be installed in conduit?
A complete NM-B cable may be permitted in a dry-location conduit or protective sleeve, subject to the installation rules. “Romex can never go in conduit” is too broad.
The project reviewer should distinguish a short sleeve used for physical protection from a complete raceway run. Check the applicable fill treatment, cable dimensions, bends, ampacity adjustments, and protection at the entries. Do not calculate a flat cable’s space requirement as though it were several loose THHN wires.
For a long conduit route, separately listed conductors often offer more practical pulling and conductor selection. This is a planning judgment, not a universal prohibition on NM-B in conduit.
Boundary 2: conduit does not make NM-B suitable for wet locations
Standard NM-B remains a dry-location cable. Underground raceways and raceways in wet locations require the appropriate wet-location wiring selection under the adopted code.
A sealed-looking pipe is not evidence that its interior qualifies as dry.
For THHN, THWN, and THWN-2 temperature and moisture designations, read our THHN vs THWN-2 comparison. The purchasing check here is to verify the actual marking; “THHN” alone does not establish a wet-location rating.
Can you remove the Romex jacket and use the wires as THHN?
No. Do not strip standard NM-B and pull its internal conductors through a raceway as separately listed THHN.
Similar-looking insulation does not establish an independent conductor listing. Southwire explicitly states that the individual conductors in its NM-B cable are not listed or marked as THHN and cannot be installed apart from the complete cable.
Removing the jacket within a box or panel for normal termination is different: those conductors remain part of the NM-B cable installation.
Boundary 3: internal construction does not establish interchangeability
Specify separately listed building wire when the design calls for individual conductors. Do not accept dismantled NM-B as a procurement substitute.
THHN vs Romex for walls, garages, and commercial projects
Dry residential walls
Where NM-B is permitted, its assembled construction can simplify material handling. The installer routes one cable containing the selected conductor set.
Before ordering, confirm the required conductor count and the route’s support, protection, and box requirements. “Inside a house” does not establish permission for every location.
Exposed garage or workshop routes
Compare the actual route and likelihood of physical damage. A surface raceway system with appropriately rated individual conductors may fit the design, while an NM-B route may require additional protection.
A garage label alone does not answer whether an exposed cable installation is acceptable. Have the proposed route checked against local requirements.
Commercial interiors
Avoid blanket statements that NM-B is banned in all commercial buildings or allowed throughout them. Building construction, occupancy, concealed spaces, local amendments, and project specifications affect the decision.
Confirm the approved wiring method before procurement. A lower cable price has little value if the submitted product is rejected.
Routes with indoor and wet sections
Review the entire route before choosing material. If the design changes from NM-B to individually rated conductors, it needs a compliant transition arrangement, appropriate enclosures and fittings, and maintained grounding continuity. A qualified electrical professional should specify that transition.
Does THHN carry more current than Romex?
A higher insulation temperature rating does not automatically permit a larger breaker.
NM-B ampacity is limited to the applicable 60°C conductor value under the NEC framework, even though its conductor insulation is rated 90°C. Individual THHN/THWN-2 sizing also depends on terminal ratings, conductor grouping, ambient temperature, and other applicable restrictions.
Small-conductor overcurrent rules can limit a circuit below a higher-temperature ampacity-table value. Changing the wire type alone does not authorize an increase in circuit protection.
A substitution review should therefore check the load, conductor size, terminals, installation conditions, and protective device together. This article intentionally avoids a universal “same gauge, more amps” table.
THHN vs Romex cost: compare the installed circuit
A price per foot of one THHN conductor and a price per foot of NM-B cable measure different quantities.
For a meaningful romex vs thhn comparison, request both estimates against the same circuit schedule and route. Compare:
| Cost item | Individual-conductor system | NM-B cable system |
|---|---|---|
| Conductors | All required sizes, colors, and lengths | Complete cable with the required conductor count |
| Grounding | Separate conductor where specified, plus bonding provisions | Included grounding conductor, plus termination provisions |
| Route materials | Raceway, couplings, connectors, supports, and boxes | Cable supports, connectors, boxes, and required protection |
| Labor | Raceway preparation, pulling, identification, termination | Cable routing, securing, stripping, termination |
| Packaging losses | Remaining stock on each spool or reel | Remaining cable on each coil or reel |
| Delivery costs | Freight, handling, cut charges, and minimum quantities | Freight, handling, cut charges, and minimum quantities |
A quantity example without invented prices
Assume a 100 ft route and a design requiring one hot, one neutral, and one separate equipment grounding conductor.
Before adding termination allowances or waste:
- The individual-conductor option requires 100 ft of each conductor, totaling 300 conductor-ft.
- The NM-B option requires 100 cable-ft of the approved cable configuration.
- A new conduit system also requires its raceway materials and installation labor.
This is a quantity example, not a circuit-sizing recommendation. Conductor sizes and the grounding arrangement must come from the approved design.
For estimating, consider two scenarios. On a new dry wall route where NM-B is permitted, the assembled cable may reduce installation work. Where a suitable raceway already exists, individual conductors may be more economical. These are project-dependent expectations; current quotations and labor estimates determine the result.
Do not publish a fixed percentage saving without a dated bill of materials, matched scope, and local installation costs.
What to include in a THHN or NM-B quotation request
Give suppliers enough information to quote the same approved requirement.
| RFQ field | Information to provide |
|---|---|
| Destination and project | Country, jurisdiction, building use, and project specification |
| Wiring method | Complete raceway, concealed cable route, protective sleeve, or other approved method |
| Environment | Dry, damp, or wet sections; relevant exposure conditions |
| Product designation | THHN with required additional ratings, or Type NM-B |
| Brand requirement | Named brand only, or approved equivalent permitted |
| Conductors | Copper, AWG sizes, solid or stranded, colors, and conductor count |
| Grounding | Required grounding conductor and approved arrangement |
| Listing | Required certification and evidence covering the offered product |
| Quantity | Length by color for singles; length by cable configuration for NM-B |
| Packaging | Length per coil or reel, payout arrangement, labels, and pallet requirements |
| Documentation | Datasheet, marking sample, certification reference, and required test records |
| Commercial terms | Delivery destination, lead time, quotation validity, and freight basis |
Ask the supplier to identify deviations explicitly. A quotation for another conductor type, a different cable count, or an unverified listing should not be treated as an equivalent offer.
For cable assemblies, review our NM-B product information. To request a project quotation, send KingForYou Cable your specification and quantity schedule, including the installation method and destination-market requirements.
