NM-B cable is one of the most widely recognized wiring products for residential branch circuits in the North American market. It combines insulated copper conductors, a grounding conductor, and a non-metallic outer sheath in one installation-ready cable assembly.
However, buyers should not select NM-B wire only by jacket color or a familiar name such as Romex cable. Conductor size, conductor count, voltage rating, temperature rating, certification scope, installation environment, cable marking, and packaging length must all be confirmed before ordering.
This guide explains the structure, applications, ratings, limitations, size selection, and buying checks for 600V NM-B wire used in residential and building wiring projects.
Quick Answer: What Is NM-B Cable?
NM-B cable is a type of non-metallic sheathed cable primarily used for residential branch circuit wiring in normally dry locations.
A typical NM-B cable contains:
- Two, three, or sometimes four insulated copper conductors
- One bare copper equipment grounding conductor
- PVC conductor insulation
- A nylon covering over the insulated conductors
- Fillers or separator material where required
- A flame-retardant non-metallic outer jacket
Standard NM-B cable is commonly rated:
- 600 volts
- 90°C conductor insulation
- Ampacity based on the 60°C conductor rating
- For normally dry locations
- For installation according to NEC Article 334
- Listed to UL 719 when supplied as a UL-listed product
NM-B cable is widely used for switches, lighting, receptacles, and fixed household appliance circuits. It is not designed for direct burial, wet locations, or unprotected outdoor installation.
What Does NM-B Mean?
The name NM-B describes the cable category and its temperature-rated construction.
- NM means non-metallic sheathed cable.
- B identifies the modern NM cable construction using conductors with 90°C-rated insulation.
The non-metallic description refers to the outer sheath. The electrical conductors inside the cable are normally copper.
Although NM-B conductor insulation is rated for 90°C, this does not mean the cable should automatically be sized using the 90°C ampacity column. NM-B ampacity is generally limited to the 60°C conductor rating under applicable NEC requirements. The 90°C insulation rating provides thermal capability for the cable construction and certain adjustment or termination considerations, but it does not increase the normal branch-circuit ampacity by itself.
Is Romex Cable the Same as NM-B Cable?
The terms are often used interchangeably in everyday conversation, but they do not mean exactly the same thing.
NM-B cable is the generic cable type.
Romex® is a registered brand associated with Southwire’s non-metallic sheathed cable products.
A buyer may therefore purchase NM-B cable from different qualified manufacturers, but not every NM-B cable should be described commercially as Romex cable. For technical specifications and purchase orders, “Type NM-B” is the more accurate generic designation.
NM-B Cable Structure Explained
The construction of an NM-B cable affects electrical performance, stripping behavior, installation speed, and long-term reliability.
1. Copper Conductors
NM-B wire normally uses soft or annealed copper conductors.
Smaller sizes such as 14 AWG, 12 AWG, and 10 AWG commonly use solid conductors. Larger sizes may use stranded conductors to improve handling and bending.
The copper conductor must meet the required diameter, strand construction, resistance, and applicable ASTM specification. Common references include ASTM B3 for soft or annealed copper wire and ASTM B8 for concentric-lay-stranded conductors.
2. PVC Conductor Insulation
Each current-carrying conductor is covered with thermoplastic PVC insulation.
Typical conductor identification includes:
- Black for an ungrounded conductor
- Red for an additional ungrounded conductor
- White for a grounded neutral conductor
- Other identification arrangements for special multi-conductor constructions
The conductor insulation provides electrical separation and heat resistance within the cable assembly.
3. Nylon Covering
The insulated conductors may include a nylon covering over the PVC insulation.
This layer helps protect the underlying PVC during production, handling, stripping, and installation. It must not be confused with the overall outer sheath of the completed NM-B cable.
4. Equipment Grounding Conductor
Standard NM-B cable normally includes a bare copper grounding conductor.
The ground wire is not included in the conductor count shown in common cable names such as 12/2 or 12/3.
For example:
- 12/2 with ground contains two insulated 12 AWG conductors plus a ground.
- 12/3 with ground contains three insulated 12 AWG conductors plus a ground.
5. Separator or Filler
Depending on the cable design, paper, polymeric tape, filler, or separator material may be placed around the conductors.
These materials help maintain the cable shape, separate the conductors from the outer jacket, and make jacket removal easier.
6. Non-Metallic Outer Jacket
The outer jacket holds and protects the complete cable assembly.
It provides:
- Basic abrasion protection
- Cable identification
- A printable surface for ratings and traceability
- Easier handling through wood-framed construction
- Protection of the internal assembly during normal indoor installation
The outer jacket does not make standard NM-B cable suitable for wet locations, direct burial, or continuous outdoor exposure.
How to Read NM-B Cable Sizes
NM-B cable is commonly identified using the conductor size followed by the number of insulated conductors.
Example: 12/2 NM-B With Ground
This marking means:
- 12: 12 AWG conductor size
- 2: two insulated current-carrying conductors
- With ground: one additional equipment grounding conductor
A 12/2 cable normally contains:
- One black insulated conductor
- One white insulated conductor
- One bare copper grounding conductor
Example: 12/3 NM-B With Ground
A 12/3 cable normally contains:
- One black insulated conductor
- One red insulated conductor
- One white insulated conductor
- One bare copper grounding conductor
The additional insulated conductor may be needed for applications such as multi-wire branch circuits, three-way switching arrangements, or 120/240V circuits. The actual circuit design must still follow local electrical codes and equipment instructions.
NM-B Cable Voltage and Temperature Ratings
600V Voltage Rating
Standard NM-B cable is rated for a maximum of 600 volts.
The 600V marking is the cable’s insulation-system voltage classification. It does not mean that every residential NM-B circuit operates at 600V. Most household branch circuits operate at much lower system voltages.
Buyers should confirm that “600V” is clearly included in the technical datasheet and required cable marking.
90°C Insulation Rating
The insulated conductors used in modern NM-B cable are typically rated for 90°C.
This rating is important for product construction and certain code calculations, but it must not be used to justify an oversized circuit breaker.
Ampacity Limited to the 60°C Rating
For normal sizing, NM-B ampacity is limited according to the 60°C conductor rating.
This distinction is one of the most commonly misunderstood NM-B specifications.
For example:
| NM-B conductor size | Common allowable ampacity |
|---|---|
| 14 AWG copper | 15A |
| 12 AWG copper | 20A |
| 10 AWG copper | 30A |
| 8 AWG copper | 40A |
| 6 AWG copper | 55A |
These values are common reference ratings for the listed constructions shown. Final circuit sizing must consider the latest applicable electrical code, load type, ambient temperature, conductor bundling, termination ratings, voltage drop, and local authority requirements.
Common NM-B Sizes and Applications
The following table gives practical examples rather than a substitute for electrical design.
| Cable size | Common jacket identification* | Typical branch-circuit use |
|---|---|---|
| 14/2 with ground | White | 15A lighting and general residential circuits |
| 14/3 with ground | Varies by manufacturer | Three-way switching or circuits needing an additional conductor |
| 12/2 with ground | Yellow | 20A receptacle, kitchen, bathroom, garage, or dedicated circuits |
| 12/3 with ground | Varies by manufacturer | Multi-wire or circuits requiring two ungrounded conductors |
| 10/2 with ground | Orange | Certain 30A, 240V fixed appliance circuits without a neutral |
| 10/3 with ground | Varies by manufacturer | Certain 120/240V appliance circuits requiring a neutral |
| 8/3 with ground | Black | Higher-load appliances or feeder applications where permitted |
| 6/3 with ground | Black | Larger appliance, feeder, or subpanel applications where permitted |
*Jacket colors help with identification but should not be treated as the only specification. Color systems can vary by manufacturer, cable design, conductor count, and market practice. Always check the complete printed legend.
Southwire, for example, identifies common two-conductor constructions using white for 14 AWG, yellow for 12 AWG, orange for 10 AWG, and black for larger sizes. Some newer three-conductor constructions use different jacket colors.
Where Is NM-B Cable Used?
NM-B is primarily a residential branch circuit cable.
Common applications include:
- Residential lighting circuits
- General-purpose receptacles
- Switch wiring
- Kitchen and bathroom circuits
- Bedroom and living-room circuits
- Fixed household appliances
- HVAC or water-heating circuits when correctly sized
- Residential feeders where the selected construction and local code permit
- Concealed wiring behind drywall
- Routing through wood studs, floor joists, and ceiling joists
- Air voids in masonry block or tile walls that are not wet or damp
NM-B may be installed in exposed or concealed work in normally dry locations where permitted by the applicable electrical code. Physical protection may still be required where the cable could be damaged.
Buyers sourcing North American building wire can review KingForYou Cable’s NM-B cable product information when preparing a project specification.
Where Should NM-B Cable Not Be Used?
Standard NM-B cable should not be selected for:
- Direct burial
- Underground feeder runs
- Wet locations
- Outdoor exposed wiring
- Conduit systems classified as wet locations
- Locations subject to continuous moisture
- Cable tray applications unless specifically permitted and listed
- Corrosive or industrial environments requiring a different cable type
- Areas with severe impact or crushing risk
- Applications requiring metal armour
- Service entrance use unless the product and installation are specifically approved for that purpose
An underground conduit is normally considered capable of collecting moisture. Installing NM-B inside it does not convert the cable into a wet-location cable.
For underground or wet applications, a correctly rated UF-B, THWN-2, XHHW-2, or other approved cable should be considered according to the installation design.
Can NM-B Cable Be Installed in Conduit?
NM-B cable may sometimes be placed in a conduit or sleeve in a dry location for physical protection, subject to local code rules, conduit fill, bend radius, and installation conditions.
However, several limitations are important:
- The complete NM-B cable assembly must be treated as one cable.
- The outer jacket should not be removed to use the internal conductors as separate building wires.
- The individual conductors inside NM-B are not automatically listed and marked as THHN or THWN-2.
- Installing NM-B in outdoor or underground conduit does not make it suitable for a wet location.
- The flat cable shape can make long conduit pulls difficult.
For full conduit wiring systems, separately listed THHN or THWN-2 wire is usually more appropriate. Southwire specifically states that individual conductors removed from an NM-B assembly are not permitted to be installed separately as recognized conductor types.
NM-B vs UF-B vs THHN/THWN-2 vs MC Cable
| Cable type | Structure | Typical environment | Main advantage | Main limitation |
|---|---|---|---|---|
| NM-B | Insulated conductors and bare ground inside non-metallic sheath | Normally dry indoor locations | Fast residential installation | Not for wet locations or direct burial |
| UF-B | Conductors embedded in a moisture-resistant overall construction | Wet locations and direct burial where permitted | Suitable for underground and wet use | Stiffer and harder to strip |
| THHN/THWN-2 | Individual PVC/nylon insulated conductors | Conduit and raceway systems | Flexible circuit design and wet-location options when dual-rated | Requires an approved raceway |
| MC cable | Insulated conductors inside metallic armour | Commercial, industrial, or exposed installations where approved | Better mechanical protection | Higher cost and more complex termination |
UF-B is specifically designed for underground feeder and branch-circuit applications and can be suitable for wet, dry, or corrosive locations when listed and installed correctly.
THWN-2 provides a more appropriate solution for wet conduit installations, while MC cable should be considered where metal armour and additional mechanical protection are required.
NM-B vs NMD90 Cable
NM-B and NMD90 may look similar, but buyers should not assume they are automatically interchangeable.
- NM-B is commonly associated with the United States market and UL/NEC requirements.
- NMD90 is commonly associated with Canadian residential wiring and CSA/Canadian Electrical Code requirements.
- Product markings, certification files, conductor colors, voltage classifications, and market approvals can differ.
When supplying Canadian projects, review the required product designation and approval rather than substituting US-market NM-B by appearance alone. KingForYou Cable’s NMD90 vs T90 cable guide provides additional information for Canadian building-wire buyers.
How to Select the Correct NM-B Cable
1. Confirm the Target Market
Start by identifying where the cable will be sold or installed.
Confirm:
- Country
- State, province, or local jurisdiction
- Applicable electrical code
- Required product listing
- Inspection or project approval requirements
A cable described as “NM-B style” is not the same as a product carrying the required listing for the destination market.
2. Confirm the Circuit Voltage
NM-B is rated 600V, but the actual circuit may be:
- 120V
- 120/240V
- 240V
- Another permitted branch-circuit voltage
The number of insulated conductors must match the circuit arrangement.
3. Confirm the Load and Overcurrent Protection
Do not select cable size only from the appliance name.
Review:
- Continuous and non-continuous load
- Equipment nameplate current
- Breaker or fuse rating
- Motor or HVAC requirements
- Starting current
- Required neutral
- Voltage drop
- Installation conditions
- Local code requirements
4. Select Two or Three Insulated Conductors
Choose 2-conductor cable when the circuit requires one ungrounded conductor and one neutral, or two ungrounded conductors without a neutral.
Choose 3-conductor cable when the circuit requires:
- Two ungrounded conductors and a neutral
- An additional switched conductor
- A multi-wire branch circuit
- A 120/240V appliance connection
- Another code-compliant circuit arrangement requiring three insulated conductors
5. Check the Installation Environment
Confirm whether the route is:
- Normally dry
- Concealed or exposed
- Protected from physical damage
- Indoors or outdoors
- Above ground or underground
- Inside thermal insulation
- Bundled with multiple cables
- Passing through masonry voids
- Near heat-producing equipment
NM-B should only be selected when its listing and code-permitted installation conditions match the real route.
Ten Buying Checks for Bulk NM-B Cable Orders
1. Verify the Product Listing
Do not accept “UL standard,” “UL quality,” or a UL logo by itself.
Ask for:
- UL file number
- Certificate holder name
- Product category
- Covered cable sizes
- Applicable standard
- Confirmation that the factory and product are included in the listing
UL identifies UL 719 as the applicable standard category for non-metallic sheathed cable.
2. Review the Cable Print Legend
The jacket marking should be legible, permanent, and consistent throughout the production length.
Depending on the required listing and specification, confirm items such as:
- Manufacturer or trade name
- Cable type: NM-B
- AWG size
- Conductor count
- Grounding conductor indication
- Voltage rating
- Temperature information
- Certification or listing mark
- Production or traceability information
Approve the complete print legend before mass production.
3. Confirm Copper Conductor Construction
Check:
- Bare annealed copper
- Solid or stranded construction
- Conductor diameter
- Number of strands
- Strand diameter
- Grounding conductor size
- Applicable ASTM construction
Do not compare suppliers only by nominal AWG description.
4. Measure Conductor Resistance
Conductor resistance is one of the most useful checks for detecting undersized copper conductors.
Request resistance results at the specified reference temperature and compare them with the applicable standard or approved datasheet.
Abnormally high resistance can indicate:
- Reduced conductor diameter
- Incorrect strand size
- Poor copper quality
- Incorrect conductor length
- Measurement or production inconsistency
5. Check Insulation and Jacket Dimensions
Inspect:
- Insulation thickness
- Minimum insulation thickness
- Nylon covering consistency
- Overall jacket thickness
- Cable width and thickness
- Concentricity
- Surface defects
- Separation between internal conductors
A cable can have the correct outside color but still fail dimensional requirements.
6. Review Electrical Test Records
Depending on the applicable standard and purchase specification, request records for:
- Conductor continuity
- Conductor resistance
- Insulation resistance
- Dielectric voltage withstand
- Spark testing during production
- Grounding conductor continuity
The report should identify the actual cable size and production batch.
7. Check Jacket Stripping and Installation Behavior
During sample evaluation, verify that:
- The outer jacket strips cleanly
- The internal conductor insulation is not damaged
- The cable does not split excessively during bending
- The jacket is not brittle
- The conductors remain properly positioned
- The cable can be stapled or routed without abnormal deformation
- Printed markings remain readable
These checks are especially important for contractors and private-label distributors.
8. Confirm Coil or Reel Length
Common retail and project packaging may include lengths such as:
- 125 ft
- 250 ft
- 500 ft
- 1,000 ft
- 2,500 ft
- Custom metric or imperial lengths
Confirm:
- Nominal length
- Permitted length tolerance
- Continuous length
- Joint policy
- Coil weight
- Reel dimensions
- Pallet quantity
- Length verification method
9. Approve Labels and Packaging
The label should match the cable itself.
Check:
- NM-B type
- AWG size
- Conductor count
- Grounding conductor
- Length
- Jacket color
- Voltage rating
- Certification mark
- Lot number
- Barcode
- Country-of-origin information where required
- Private-brand artwork
10. Require Batch Traceability
Each shipment should be traceable to:
- Raw material batches
- Production date
- Production line
- Test records
- Packing date
- Reel or coil number
- Inspection status
For more general supplier verification steps, refer to this guide on choosing a cable manufacturer in China.
How to Evaluate an NM-B Cable Sample
Before approving bulk production, evaluate the sample in four stages.
Visual Inspection
Check:
- Jacket color consistency
- Smooth cable surface
- Clear print legend
- No bubbles, cracks, cuts, or contamination
- Correct conductor colors
- Proper ground conductor position
Dimensional Inspection
Measure:
- Copper conductor diameter
- Individual strand diameter where applicable
- Insulation thickness
- Nylon covering
- Outer jacket thickness
- Overall cable dimensions
Electrical Inspection
Test or review:
- DC conductor resistance
- Continuity
- Insulation resistance
- Voltage withstand
- Grounding conductor continuity
Practical Handling Test
Try:
- Bending the cable
- Pulling it through a representative dry installation route
- Removing the jacket
- Stripping conductor insulation
- Securing it with approved cable fasteners
- Terminating it in representative boxes or equipment
The approved sample should be retained as a reference for production and pre-shipment inspection.
Example NM-B Cable RFQ Specification
A clear request for quotation can be written as follows:
Type NM-B cable, 12/2 with ground, solid annealed copper conductors, 600V, 90°C conductor insulation with ampacity applied according to the required 60°C limitation, UL 719 listing required, yellow outer jacket, 250 ft coils, private-label carton, continuous length without joints, approved print legend, batch test report, certificate copy, and pre-production sample required.
The RFQ should also state:
- Order quantity
- Delivery destination
- Required Incoterm
- Coil or reel format
- Certification requirement
- Label language
- Barcode requirement
- Inspection method
- Sample approval process
- Third-party inspection requirement
- Delivery schedule
Common NM-B Buying Mistakes
Mistake 1: Treating 90°C as the Normal Ampacity Column
The insulation is rated 90°C, but NM-B ampacity is normally limited to the 60°C rating. Ignoring this distinction can result in incorrect cable and breaker selection.
Mistake 2: Selecting Only by Jacket Color
Jacket colors are useful identification tools, but the printed size and construction must always be checked.
Mistake 3: Ordering 10/2 When the Appliance Requires a Neutral
A 120/240V appliance may require 10/3 rather than 10/2. Confirm the equipment wiring diagram and local code.
Mistake 4: Using NM-B in Underground Conduit
Underground conduit can contain moisture. Standard NM-B is not a wet-location cable.
Mistake 5: Assuming the Internal Conductors Are THHN
The conductors are part of the complete NM-B listing and should not be removed and installed separately unless they carry their own appropriate markings and listing.
Mistake 6: Accepting a Generic Certificate
The certificate must cover the actual manufacturer, product category, construction, and cable sizes being purchased.
Mistake 7: Comparing Quotes With Different Copper Content
Two 12/2 cables can have different conductor resistance, dimensions, length tolerances, packaging, and certification coverage. Compare technical details before comparing price.
Why Source NM-B Cable From KingForYou Cable?
KingForYou Cable supplies building wire and electrical cable products for distributors, contractors, importers, OEM customers, and project buyers.
For NM-B cable inquiries, buyers can request support with:
- Cable size selection
- Two- and three-conductor constructions
- Copper conductor specifications
- Jacket color and printing
- Coil and reel packaging
- OEM labels and cartons
- Technical datasheets
- Factory test reports
- Production samples
- Export documentation
- Batch inspection
- Project-based quotations
The most reliable purchase begins with a complete technical specification. Buyers preparing an NM-B order can submit the required AWG size, conductor count, coil length, certification, packaging, and quantity through the KingForYou Cable contact page.
Frequently Asked Questions About NM-B Cable
What is NM-B cable used for?
NM-B cable is mainly used for residential branch circuits supplying lighting, switches, receptacles, and fixed appliances in normally dry indoor locations.
Is NM-B cable rated for 600V?
Yes. Standard listed NM-B cable is commonly rated for 600 volts.
Is NM-B cable rated for 90°C?
The individual conductor insulation is typically rated for 90°C, but the allowable ampacity of NM-B cable is normally limited to the 60°C conductor rating.
What does 12/2 NM-B mean?
It means the cable contains two insulated 12 AWG conductors. A standard “12/2 with ground” construction also includes a bare equipment grounding conductor.
What does 12/3 NM-B mean?
It contains three insulated 12 AWG conductors plus a grounding conductor when marked “with ground.”
Is Romex the same as NM-B?
Romex is a brand name, while NM-B is the generic cable type. People frequently use Romex as an informal name for non-metallic residential cable.
Can NM-B cable be installed outdoors?
Standard NM-B should not be used for exposed outdoor or wet-location installation. Use an appropriately listed outdoor or wet-location cable.
Can NM-B cable be buried underground?
No. Standard NM-B is not intended for direct burial. UF-B or another properly listed underground cable should be considered.
Can NM-B cable be used in conduit?
It may be installed as a complete cable assembly in certain dry-location conduit or sleeve applications where permitted. It should not be used in wet underground conduit, and its internal conductors should not be separated and treated as THHN.
Is NM-B suitable for commercial buildings?
NM-B may be permitted in certain commercial or light-commercial applications, depending on the building type, construction, occupancy, local code, and authority having jurisdiction. It should not be assumed to be acceptable for every commercial project.
What is the difference between NM-B and UF-B?
NM-B is intended for normally dry indoor locations. UF-B is designed for underground feeder and branch-circuit applications and can be suitable for wet locations and direct burial when installed according to its listing.
What should buyers check before ordering NM-B cable?
Confirm the UL listing, cable size, conductor count, grounding conductor, voltage rating, conductor resistance, insulation dimensions, print legend, coil length, packaging, batch report, and destination-market requirements.
Conclusion
NM-B cable is a practical, installation-efficient solution for residential branch circuit wiring, but correct selection requires more than choosing a familiar jacket color.
Buyers should confirm:
- Type NM-B construction
- 600V rating
- 90°C conductor insulation
- Ampacity based on the applicable 60°C limitation
- Correct AWG size
- Two- or three-conductor configuration
- Grounding conductor
- Normally dry installation environment
- UL 719 listing where required
- Cable marking and traceability
- Electrical and dimensional test results
- Coil length and packaging
When these details are clearly defined before quotation and sample approval, importers, distributors, and contractors can reduce compliance risk, avoid incorrect cable substitutions, and maintain more consistent quality from the first sample to the final bulk shipment.
