Maya Ellison
Plastic junction boxes are lightweight, cheap, and easy to work with. They are also non-conductive and often come with built-in clamps for the wires, reducing overall costs. However, they tend to become misshapen under stress and can crack when hit hard. Metal junction boxes, on the other hand, are very strong and virtually impossible to bend, warp, twist, or crush. They are more challenging to work with due to the need for supplementary clamps and their sharp edges. So, when it comes to plastic junction boxes, the question of whether they need clamps depends on their design. Some have built-in clamps, while others have different mechanisms for securing wires, and some may require separate clamps.
| Characteristics | Values |
|---|---|
| Plastic junction boxes needing clamps | Depends on the style. Some have built-in clamps, others need separate clamps. |
| Plastic boxes with built-in clamps | Usually have doors that act as clamps to hold the electrical cable. |
| Plastic boxes without built-in clamps | Need separate clamps to secure the cable. |
| Metal junction boxes | Usually require clamps. |
| Plastic boxes vs. metal boxes | Plastic boxes are lightweight, cheap, and simple to work with; metal boxes are strong and provide a strong attachment to the stud. |
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What You'll Learn
Plastic boxes with built-in clamps
Plastic boxes are lightweight, cheap, and simple to work with. Many plastic boxes come with built-in clamps for the wires, which can lower the overall project cost. These built-in clamps are usually plastic tabs that you push the cable past as it goes into the box, and then they spring back and grab the cable. However, these clamps can be broken, which can cause issues with inspectors.
Some plastic boxes are designed to secure the cable without needing a clamp. Many plastic boxes have doors that act as clamps to hold the electrical cable to the box, eliminating the need for additional clamps. However, these pre-attached door clamps can stubbornly hold the cable in place, making it difficult to remove the cable.
Per Code, a single-gang plastic box for NM cable need not have a cable clamp. If you're working with the blue Carlon boxes, the most common brand in the US, the single-gang boxes have four knockouts, one in each corner. Those pop completely out and are of no use as a clamp. Some other brands of SG plastic boxes have some sort of cable clamp. For two-gang and bigger boxes, cable clamps are required.
It is recommended but not required that you use a plastic electrical box when you have Romex (or NM) cables leading in or out of the box. The electrical code does not require the use of NM cable with plastic boxes. 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.
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Plastic boxes' tendency to distort
Plastic boxes are the most commonly used electrical boxes. They are lightweight, cheap, and simple to work with. However, they have a tendency to distort when stressed and can crack if hit hard enough. This is in contrast to metal boxes, which are very strong, fireproof, and virtually impossible to bend, warp, twist, or crush.
The tendency of plastic boxes to distort under stress is due to the nature of the material. Plastic is a flexible and malleable material, which means it can be easily deformed. When subjected to high levels of stress or impact, the molecules in the plastic can move and shift, causing the box to become misshapen or cracked. This is a common issue with plastic boxes, and it is one of the main reasons why some people prefer to use metal boxes for electrical applications.
Despite this drawback, plastic boxes remain popular due to their convenience and ease of use. They are lightweight and easy to work with, making them a favourite among DIY enthusiasts. Additionally, many plastic boxes come with built-in clamps for the wires, further lowering the overall project cost. These pre-attached clamps are convenient for holding electrical cables in place, but they can sometimes make it difficult to remove the cable once it's clamped.
To address the issue of distortion in plastic boxes, some manufacturers have introduced modifications to their designs. For example, some plastic boxes feature rectangular tabs that you punch open, which then spring back to hold the wire in place. Others have round openings with a pop-in plastic valve that secures the wire. These innovative designs aim to improve the clamping mechanism and reduce the risk of distortion in plastic boxes.
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Metal boxes' strength and durability
Metal junction boxes are known for their strength and durability. They are virtually impossible to bend, warp, twist, or crush. They are often made from steel, aluminium, or stainless steel, which makes them strong and fireproof. Metal boxes are also capable of withstanding harsh environmental conditions, such as impact, moisture, or extreme temperatures. This makes them ideal for outdoor and industrial use.
Metal boxes are also more durable than plastic boxes, which tend to become misshapen when stressed and can crack when subjected to a sharp blow. Plastic boxes are also vulnerable to breakage and stripping at the nail brackets and screw holes, which can make it challenging to securely attach devices. On the other hand, metal boxes can handle heavier electrical loads and are often used to mount heavy items like ceiling fans or light fixtures.
Metal boxes also provide a stronger attachment to the stud. They allow for the use of strong screws that ensure a maximum amount of holding power. Metal boxes are also suitable for interior exposed applications, such as unfinished basements, garages, or mudrooms, where there is no complete wall system to enclose the box.
However, metal boxes can be more difficult to work with due to their sharp edges, which can injure hands. They are also heavier than plastic boxes, making them more cumbersome to install or move. Additionally, they tend to be more expensive.
In terms of clamps, some metal boxes have pre-attached clamps for gripping the electrical cable, while others require separate clamps, increasing the total cost. Plastic boxes, on the other hand, often have built-in clamps, and some are designed to secure the cable without needing a clamp at all.
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Metal boxes' difficulty to work with
Metal electrical boxes are known to be very strong, fireproof, and virtually impossible to bend, warp, twist, or crush. They are also suitable for heavy-duty applications. However, they can be more difficult to work with, especially for DIY enthusiasts.
Metal boxes have rear and side knockouts that require additional clamps, and these knockouts can be challenging to remove. Their sharp edges can also cause injuries, so it is advisable to wear gloves when working with them.
When installing metal boxes, one must decide whether to place the box face flush with the studs or to "countersink" it by the thickness of the steel. This decision can be influenced by the type of box used, as some boxes have mounting tabs that recess the box, while others do not.
Metal boxes are often used with metal-sheathed cable (armored BX cable) or metal conduit, as the metal structure provides necessary grounding. They are also essential for maintaining safety in unfinished basements and mudrooms with exposed conduit.
Overall, while metal boxes offer superior strength and durability, they can present challenges during installation, especially for those without professional electrical experience.
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Grounding concerns with metal-sheathed wiring
Grounding is an essential safety feature in electrical systems, providing a safe pathway for excess electricity to flow back to the ground in the event of a fault. It is a backup pathway designed to protect people and property from electrical hazards, such as short circuits, overcurrent, or equipment malfunction. In the case of metal-sheathed wiring, proper grounding is crucial to ensure safety and prevent potential hazards.
Metal-sheathed wiring, commonly referred to as "BX" cable, typically consists of two conductors without a ground wire or bonding strip. Older designs of BX cable may not have been intended to have the armour grounded, as they predate the NEC grounding requirement. However, it is essential to verify this information with local electrical codes and consult with professionals for accurate guidance.
When working with metal-sheathed wiring, it is crucial to address the concern of corrosion. The armour of the cable may be made of aluminum, which is highly anodic and susceptible to corrosion when in contact with other metals, such as copper, iron, or steel. This corrosion can lead to continuity issues with the armour, compromising its primary function of providing protection. Therefore, it is recommended to use an insulated ground wire to prevent corrosion and ensure a reliable installation.
To ensure proper grounding with metal-sheathed wiring, it is essential to follow specific guidelines. Firstly, a ground wire, typically made of bare copper, should be connected to the BX cable's sheathing. This ground wire should then be properly grounded at the distribution panel. Additionally, bonding between metal components, such as metal electrical boxes and conduits, is crucial to ensure all metal parts are at the same electrical potential, further reducing the risk of electric shock.
It is important to note that local building codes and regulations may vary, and it is the responsibility of contractors and electricians to consult these codes and seek official confirmation before proceeding with any electrical work. By following the necessary guidelines and taking the proper precautions, grounding concerns with metal-sheathed wiring can be effectively addressed, ensuring the safety and functionality of the electrical system.
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Frequently asked questions
It depends on the style of the box. Some plastic junction boxes have doors that act as clamps to hold the electrical cable to the box, eliminating the need for additional clamps. However, some plastic boxes have rectangular tabs that you punch open, and they spring back to hold the wire.
Plastic boxes are lightweight, cheap, and simple to work with. Holes are easy to punch out in the back or sides. Many plastic boxes come with built-in clamps for the wires, further lowering the overall project cost.
Plastic boxes tend to become misshapen when stressed. When plastic boxes receive a sharp blow, they can crack. The pre-attached door clamps, while convenient, often stubbornly hold the cable in place even when you want to remove the cable.
No, a single-gang plastic box for NM cable need not have a cable clamp. However, if there is no clamp, the code calls for securing NM cable within 8" of where it enters the box.
