THHN, NM-B and SER cable can all show up on an electrical material list, but they are not three interchangeable versions of the same product. They are constructed differently and intended for different types of installations.
Understanding those basic differences makes it easier to build an accurate material order—and helps prevent arriving at a job with the wrong wire or cable.
Quick Comparison: THHN vs. NM-B vs. SER
| Type | What It Is | Common Purchasing Context |
|---|---|---|
| THHN / THWN-2 | Individual insulated conductor | Conductors pulled through an approved raceway system |
| NM-B | Multiple insulated conductors contained in a nonmetallic cable assembly | Common residential dry-location wiring applications where permitted |
| SER | Multi-conductor service-entrance cable, Type R | Feeders and other permitted service/distribution applications |
What Is THHN Wire?
THHN refers to a type of insulated electrical conductor. Unlike a cable assembly that bundles several conductors together inside an outer jacket, THHN is typically purchased as an individual conductor.
Many products sold today carry multiple ratings, commonly including both THHN and THWN-2. The markings printed on the actual conductor are what matter, so always verify the exact product being purchased.
THHN/THWN-2 conductors are commonly associated with installations where individual conductors are pulled through an approved raceway, such as conduit.
When ordering THHN, verify:
- Wire gauge or conductor size
- Copper or aluminum construction
- Required conductor color
- Length needed
- Product ratings printed on the conductor
- Whether it is sold by the foot, spool or other quantity
Color can matter just as much as size when preparing a professional material order. Separating conductor quantities by color before ordering can save time once work begins.
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What Is Romex?
"Romex" is a name commonly used in the trade when referring to nonmetallic-sheathed cable, particularly NM-B cable.
Instead of ordering individual conductors separately, NM-B combines the required conductors into one cable assembly enclosed by an outer nonmetallic jacket.
It is commonly encountered in residential construction and similar dry-location applications where this wiring method is permitted.
When ordering NM-B cable, verify:
- Cable gauge
- Number of insulated conductors
- Required length
- Product type and markings
- Whether the application permits NM-B
A description such as "14/3" or "12/2" communicates important information, but it should not be treated as the complete installation specification. Always verify the actual product and project requirements.
What Is SER Cable?
SER stands for Service Entrance Cable, Type R. It is a multi-conductor cable assembly and is commonly encountered in service and feeder-related work when its use is permitted for the particular installation.
Contractors may encounter SER cable when supplying panels, subpanels and other distribution equipment, but the correct cable still depends on the complete installation and applicable requirements.
When ordering SER cable, verify:
- Conductor sizes
- Number and configuration of conductors
- Copper or aluminum construction
- Overall cable rating
- Length required
- Manufacturer markings
- Requirements of the specific installation
Is THHN the Same as Romex?
No.
THHN generally describes an individual insulated conductor, while NM-B is a complete multi-conductor cable assembly.
They therefore affect more than the material order. They are part of different wiring methods and may require different supporting materials.
For example, an installation using individual conductors may also require raceway, connectors, couplings, straps and other fittings. A cable-based wiring method has its own boxes, connectors, supports and installation requirements.
Is SER Cable the Same as Romex?
No. Although both are jacketed cable assemblies containing multiple conductors, they are different product types intended for different applications.
A buyer should never substitute one for another simply because the conductor sizes appear similar.
Why the Exact Product Marking Matters
Informal product names are useful when talking through an order, but the label on the actual material provides much more information.
Before buying wire or cable, look for information such as:
- Conductor or cable type
- Gauge/size
- Conductor material
- Voltage rating
- Temperature rating
- Wet/dry-location ratings where applicable
- Manufacturer
- Listings and certifications
When replacing existing material or ordering against project specifications, a photo of the existing wire markings or the exact model/specification can help eliminate ambiguity.
Don't Choose Wire Based Only on Price
The least expensive material is not a bargain if it is the wrong type for the job.
When comparing wire and cable, compare equivalent specifications first. Only then does a meaningful price comparison become possible.
This is especially important when one listing is priced by the foot while another is priced by a coil, spool or complete package.
What Information Should You Have Before Ordering?
For the fastest, most accurate material search, have these details ready:
- ☐ Wire or cable type
- ☐ Gauge/conductor size
- ☐ Number of conductors
- ☐ Copper or aluminum
- ☐ Required color, if applicable
- ☐ Quantity or footage
- ☐ Exact manufacturer/model if specified
- ☐ Any required ratings or listings
Find the Right Electrical Material Before the Job Starts
One-Sided Electrical Supply carries electrical materials for contractors and customers, including wire and cable, conduit and fittings, electrical devices, breakers and other installation supplies.
If you know the exact product you need, browse our inventory online. If you are trying to locate a specific material, model or quantity, contact us with the details of what you are looking for.
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