Chloe Zhang
Plastic electrical boxes are a popular choice for do-it-yourself home remodelers as they are easy to work with and inexpensive. They are also safe to use with Romex or NM wiring, as long as the bare or green grounding wire is attached to the box with a screw. When connecting Romex to a plastic box, some kind of wire gland or clamp is usually needed to secure the wire to the box. This could be a two-screw clamp, a bonding bushing, or a plastic cable gland.
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What You'll Learn
Using a wire clamp or gland
When connecting Romex to a plastic box, it is recommended to use a plastic electrical box. While not required, it is safer than using a metal box. This is because metal-sheathed wiring depends on bonding with a metal electrical box for grounding. Using a plastic box with metal wiring severs the ground, which is highly dangerous.
If you are using a plastic box, you can use a cable gland, which is a cable grip with a rubber grommet that tightens around the wire. Cable glands are hard to find for Romex and UF cables on major retailer sites, but they are available at electrical supply houses. You can also use an LB, which is a conduit fitting with a cover that can be used to go around a 90-degree bend to enter a surface.
If you are using a metal box, you must remove a knockout in the box. Knockouts are the little metal discs covering the holes in the sides or back of a metal electrical box. You remove as many knockouts as you need for the number of cables that will enter the box, leaving the others in place. A box should not have open knockout holes that are empty, as this compromises the protection offered by the box enclosure.
There are several different designs for clamps, but they are all plastic bushings that snap into the knockout hole in the metal box. The cable is inserted through the connector and secured with a spring tab or other devices. One version includes a little gate piece that you slide down into a slot, then clamp down on the gate with pliers to secure the cable.
About 1/2 inch of outer cable sheathing should extend into the box past the cable clamp. This ensures that the cable will be held securely and that the clamp will not compress and damage the insulation around the individual wire conductors. It is a code violation for the cable clamp to directly contact the wires themselves rather than the outer cable sheathing.
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Plastic vs. metal boxes
Electrical boxes are vital components of a building's electrical system, housing wiring connections for switches, outlets, fixtures, and transition wires. The two main types of electrical boxes are plastic and metal.
Plastic Electrical Boxes
Plastic electrical boxes are typically made from PVC or fiberglass. They are lightweight, easy to install, and inexpensive. Plastic boxes do not conduct electricity when touched by a live wire and do not require grounding. They are also easy for DIYers to work with and come with pre-attached nails for nailing into studs. However, plastic boxes can distort or crack under stress, and the nail brackets can break during installation or replacement. PVC boxes can also melt due to overheating.
Metal Electrical Boxes
Metal electrical boxes are usually made from steel, aluminum, or cast iron. They are strong, fireproof, and durable, offering excellent security for electrical wiring applications. Metal boxes provide a strong attachment to the stud, allowing for maximum holding power. They can also be used with Romex or NM wiring if proper grounding is ensured. Metal boxes are less likely to warp, bend, twist, or crush compared to plastic boxes. Additionally, some metal boxes have pre-attached clamps for gripping electrical cables. However, metal boxes are more expensive and may be more difficult for DIYers to work with due to sharp edges and the need for supplementary clamps.
Connecting Romex to a Plastic Box
When connecting Romex wiring to a plastic box, it is recommended to use a connector. Two-screw clamps or plastic cable glands can be used, although these can be pricier. Alternatively, a bonding bushing can be used to ensure proper grounding.
In summary, the choice between plastic and metal electrical boxes depends on various factors, including ease of installation, budget, durability, and personal preference. Grounding is also a critical consideration, especially when using metal-sheathed wiring, which requires bonding with a metal electrical box for proper grounding.
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Grounding considerations
When connecting Romex to a plastic box, there are several grounding considerations to keep in mind. Firstly, it is recommended to use a plastic electrical box when you have Romex cables leading in or out of the box. This is because Romex or NM wiring is typically used with plastic boxes, whereas metal-sheathed wiring is used with metal boxes for grounding purposes. Using metal-sheathed wiring with plastic boxes without proper grounding measures can be highly dangerous.
Secondly, while plastic boxes are convenient for do-it-yourself home remodelers, they cannot be grounded in the same way as metal boxes. Metal boxes provide a strong attachment to the stud and are less likely to warp or melt. If you are using a metal box, ensure that you have proper grounding by attaching the bare or green grounding wire to the box with a screw.
Thirdly, when using a plastic box, it is still necessary to bring the equipment-grounding conductor into the enclosure to ground devices such as switches and receptacles. Connect the bare or green wire directly to the green screw on the device. If another cable exits the box to feed downstream devices, use a pigtail to connect the equipment-grounding conductors in both cables to the grounding screw.
Finally, if you are using a metal box with Romex or NM wiring, special precautions must be taken to ensure proper grounding. This can be achieved by attaching the bare or green grounding wire to the metal box with a screw. Additionally, some metal boxes have pre-attached clamps for gripping the electrical cable, providing a secure connection.
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Using a two-screw connector
To begin, you must orient the clamp so that the saddle screws are facing you. This makes it easier to tighten the screws when installing the cable. You can also secure the clamp onto the cable first and then install the cable and clamp into the box.
When using a two-screw connector, it is important to ensure that the cable is securely clamped. The clamps should be tightened enough so that the cables cannot be pulled out by hand, but be careful not to over-tighten as this may crush the cable sheathing or damage the insulation on individual wire conductors.
In some cases, a giant hole behind the box may be required to accommodate the connector. This is a common issue when using a two-screw connector with a 2-gang outdoor box. To address this, you may need to ream out the threaded hub with a unibit and then use a button connector, or consider using a threaded nipple and a threaded coupling.
Additionally, when using a two-screw connector, it is important to follow standard practices to ensure your wiring installations are safe and secure. For example, about 1/2 inch of outer cable sheathing should extend into the box past the cable clamp.
It is also recommended to consult with a licensed electrician or refer to specific electrical codes and standards for your region to ensure that your connection is safe and compliant with local regulations.
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Plastic box installation
Plastic electrical boxes are easy for do-it-yourself home remodelers to work with. They are also the least expensive option and often come with pre-attached nails for nailing into studs. However, they are not as strong as metal boxes and can bend, warp, twist, or crush.
When installing a plastic box, it is recommended but not required that you use one when you have Romex (or NM) cables leading in or out of the box. The overriding concern is grounding. Metal-sheathed wiring depends on bonding with the metal electrical box for grounding. Using metal-sheathed wiring with plastic electrical boxes, without taking other grounding measures, severs that ground and is highly dangerous.
If you are installing a plastic box, you can use a plastic "push-in" connector or a two-screw connector. You can also use a bonding bushing. If you are using a plastic box for a weatherproof receptacle, you will need to remove the knockout and install a standard thread wire clamp. Run the wire through, clamp, and install onto the siding surface. You will also have to drill a bigger hole through your siding and sheathing to make room for the wire clamp.
To install a plastic box, you can use plastic NM Romex standoff brackets nailed to the joists. Alternatively, you can use blue PVC boxes with push-in plastic tabs or traditional 4" x 4" metal boxes with a metal cable connector installed at the conduit opening.
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Frequently asked questions
You can use a plastic cable gland or a two-screw connector to connect Romex to a plastic box.
Metal boxes are the safest type of electrical box as they are strong, fireproof, and incapable of melting.
Plastic electrical boxes are easy for non-experts to work with and are the least expensive option.
It depends on the situation. If you are using a plastic box, you can staple the NM12-2 to the stud. However, if you are using a metal box, you will need to attach the bare or green grounding wire to the box with a screw.
