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Building Wire

THHN Wire vs. Romex: The Key To Successful Home Wiring

In residential and commercial wiring, Romex (NM-B) and THHN are two of the most common types of cables used. The confusion is that NM-B wire is not that far from THHN in terms of construction. Whereas THHN is a cable with a single thermoplastic high heat-resistant nylon wire, NM-B wire is a sheathed cable with multiple conductors. The conductors inside the NM-B wire are either insulated or uninsulated, and the former is actually THHN, which commonly leads to confusion between the two types of cables.

What is Romex NM-B wire?

NM-B is a non-metallic sheathed cable designed with residential applications in mind. It is available in gauges from 6 to 12 AWG, with each color-coded in different colors, and has multiple conductors that are both insulated and uninsulated. The voltage rating of the cable is 600 volts, and the temperature rating for dry conditions is 90 degrees Celsius. The cable has standard PVC insulation.

"Romex" is a brand name for non-metallic sheathed cable, specifically manufactured by the American company Southwire. While this company manufactures not all nm-b wires, Romex has become a general term that points to this non-metallic wire for digital printing machines, like a xerox.

NM-B wiring is never used outside. In the house, its primary applications are switches, lighting fixtures, and electronic appliances. It is also a common choice for interior walls. NM-B wiring can also be used in some of the rougher home applications, such as the garage, but, in these instances, they should be protected by conduit, which brings us to our next question. The cable is not practically durable and should be protected with connectors at both ends.

Can NM-B Wire Be Run In A Conduit?

Despite the existing stereotypes, nm-b wire can be run in conduit without any underlying issues. In fact, article 334 of the National Electrical Code actively encourages nm-b wiring to be run in conduit in case there is a need for protection from mechanical damage. The conduit for the NM-B wire should be of a precise size so that the cable will not be damaged.

Why is Romex banned in Illinois?

The most popular type of home wiring in most of the United States, Romex is not allowed by the local legislation in most Illinois. The reasons for NM-B wire not being used has a lot to do with bureaucracy. Check with your local county to make sure what are the exact rules surrounding Romex there.

What About THHN Wire? How is it Different?

Unlike Romex, THHN is an electrical wire with just one conductor. It is a thermoplastic high heat resistant nylon coated wire that possesses superior resistance to heatand is heat and water resistant. While NM-B wire is exclusive to residential applications, THHN is also often used in industrial and office settings, thanks to the damage-resistant nylon being one of its key components. The cable is also rated at 600 volts and has a temperature rating of 90° C in dry and 75° C in moist conditions. It is worth noting that if you take two house cables with a similar rating, THHN will likely have superior performance with heat simply because of its nylon coverage.

THHN is used to run power to the building, in lights, electrical outlets, branch circuits, control circuits water heaters, cooler systems, garbage disposers, garages, and basements. THHN is rarely used outdoors for home applications, but it can be if it has a UF-B jacket or it is double-rated as THHN/ THWN and is used in conduit.

Where to Choose One Over The Other?

  • Your regular branch wiring will likely do well with the Romex wire.
  • Running Romex inside the walls is steadier and easier.
  • On average, NM-B wire is more delicate to handle, and you need to take this into consideration when installing.
  • Suppose you need a single wire to choose THHN. If your installation calls for a multi conductor cable, go with the NM-B.
  • Always compare the prices. In some harsher home applications, like basements and garages, you may run NM-B wire in conduit, but it might be cheaper to just go with THHN instead. Check with your professional electrician if you can run NM-B wire without the conduit by simply fastening it tightly to the wall.
  • If you are in Illinois, choose THHN or another home wiring for your applications, or check with the local authorities for more information.
  • If your application is exposed to high heat, always choose THHN. If it deals with water and moisture, use THHN double-rated as THWN.
  • For outdoor home applications, use THHN with a UF-B jacket or THHN/THWN in conduit. Never use the NM-B wire outdoors.

To shop all types of residential wiring, visit We sell all types of highest-quality residential wire at the best prices in the industry, including THHN/THWN-2  and Romex NM-B wire.

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THHN Wire vs. Romex: The Key To Successful Home Wiring