Romex cable connectors are used to secure non-metallic sheathed cable NM-B where it enters a box or panel. They help protect the cable jacket and provide strain relief. If the Romex connector is metal, it provides a metal-to-metal bond between the cable and the enclosure for grounding.

In this guide, we’ll go over Romex connector types, sizes, when to use plastic vs. metal connectors, and how to select the right one for your application.

Because the NM-B cable has a soft oval-shaped sheath, it’s prone to damage at knockout edges, which is why the connector is needed.

Why You Need a Romex Connector

You cannot pass Romex cable into a box without a connector because it is easily damaged at the knockout edge.

A Romex-to-box connector protects the cable from sharp edges and prevents it from moving. The metal Romex connector also maintains the grounding path. Using simple plastic bushings or exposed knockouts is not code-compliant. Bushings only protect the edge of the knockout and do not provide strain relief or secure the cable.

Types of Romex Cable Connectors

Plastic Romex Connectors

Plastic Romex connectors are used with non-metallic boxes. They’re corrosion-resistant, fast to install, and commonly used in residential wiring. They are also cheap.

Plastic NM Cable Connector

Standard plastic connector with an external clamp.

Typically, a 3/8-inch connector for ½-inch knockouts is accepted by code, as long as it's listed for that use. A 1/2-inch plastic Romex connector is very often used for 14/2 NM-B Cable and 12/12 NM-B cable.

Standard plastic NM-B connectors are used with plastic boxes to clamp the NM-B jacket and prevent movement.

Plastic NM Connector with Internal Clamp (Insider)

A connector with an internal clamp is used when there's no drywall access to the outside of the box.

It installs from the outside, but the clamp is tightened from inside the box.

Overall, this connector is ideal for remodel work and tight spaces. Other names for this specific connector include push-in Romex connector and quick Romex connector.

Plastic NM Snap-In Connector

A plastic snap-in connector gets its name from its mechanism. It snaps into the knockout from the outside.

The Romex cable is passed through, and a built-in gate (set with wire strippers) locks it in place for strain relief.

This is the best option for fast, tool-less installation for plastic boxes in open framing.

Plastic NM Duplex Connector

This connector allows two NM-B cables to enter through one knockout.

It is only allowed if the connector is explicitly listed for two cables of the sizes being used.

Common in junction boxes where multiple branch circuits are joined.

Metal Romex Connectors

Metal Romex connectors are required with metal boxes or panels. They’re stronger and provide a grounding path. You will often find metal Romex connectors in utility rooms, garages, and exposed installations. Metal connectors are used when securing NM-B cables to the panel because they provide the required strain relief.

Metal Clamp NM Connector (Best Option)

This is generally the most secure NM-B connector. Most electricians and inspectors alike prefer it for the ease of use. This connector:

• Uses a solid metal body with an internal clamp

• Firmly grips the outer jacket, not the conductors

• Provides strong strain relief

• Ensures proper bonding to metal boxes or panels

• It is used in basements, garages, utility spaces, and panels

This is generally the most robust option for long-term safety, especially in metal enclosures and high-use circuits. A metal clamp connector is also used as a Romex connector for a garbage disposal.

Metal Snap-Lock NM Connector

Often, a 3/8-inch metal fitting is used in ½-inch knockouts.

Similar to its plastic counterpart, this connector snaps into the metal box, is tightened with a locknut, and is then twisted with pliers or a wire stripper to lock the clamp in place.

It is a quick option for metal boxes and panels that provides solid grounding. It can be marketed as a 3/8 Romex connector or a Romex connector 3/4 by the stores.

Metal Split NM Connector (Two-Piece)

This connector is more common in older homes and retrofits where cable ends can't be pulled, and you cannot access the cable's end.

It is installed first, then the NM-B cable is fed through and clamped down.

It is most useful when working with pre-installed or shortened cables.

Metal NM Duplex Connector

Allows two NM-B cables through a single knockout in a metal box.

Like its plastic counterpart, only usable when specifically listed for that configuration.

You will most often find this connector in high-density electrical panels.

Some additional types of Romex connectors are:

• Romex-to-EMT Connector is specifically marketed for transitioning NM-B cable into conduit.

• Weatherproof Romex connectors that protect from wet conditions. NM-B cable is generally not allowed outdoors, but it can get wet in garbage disposals.

What Is Not An NM-B Connector

The following items are not allowed as NM-B connectors:

• Plastic bushings

• Insulating throat bushings

• Open knockouts

• EMT connectors

• MC/AC connectors

• Conduit hubs

• Strain reliefs not listed for NM-B

None of these options clamps the NM-B jacket or prevents cable movement, leaving the cable exposed to damage.

Romex Connector Sizes

When selecting a connector, sizing is based on the NM-B cable's outer diameter, not just its gauge.

• ½ inch Romex connector

• Common for 14/2, 12/2 NM-B

• ¾ inch Romex connector

• For 12/3, 10/2 NM-B or tight-fitting boxes if the cable needs to move freely without any deformation.

• 1-inch Romex connector and above are used for larger sizes, like   8/3 NM-B, or where multiple conductors pass through the knockout.

Even if a connector is labelled for your wire size, always test fit. Romex jackets can vary by brand and thickness.

Choosing the Right Connector

Use this checklist:

• Use plastic for plastic boxes, and metal for metal boxes.

• Match the connector listing to the jacket diameter.

• For panels, always use clamp connectors. For disposal or wet areas, use weatherproof Romex connectors.

• Use snap-in or insider clamp connectors for remodels, and when the ease of access is a factor

• Use metal Romex connectors to maintain the bonding path in metallic boxes.

Nassau National Cable sells affordable NM-B Romex cables along with corresponding connectors.