Alexei Volkov
When it comes to electrical boxes, there are two main types: plastic and metal. The choice between the two is primarily dictated by grounding considerations. Metal boxes, with their inherent conductivity, provide a natural pathway for grounding. In contrast, plastic boxes, being non-conductive, cannot be grounded in the same way. This fundamental distinction influences how switches and devices are grounded when installed in plastic boxes. While plastic boxes may have limitations when it comes to grounding, they are still widely used, especially among DIY enthusiasts, due to their ease of use and cost-effectiveness.
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What You'll Learn
Using a ground wire
It is important to note that a plastic box cannot be grounded due to its non-conductive nature. However, if you are using a smart switch that requires a ground to operate, you will need a dedicated ground wire. This can be achieved by connecting the green ground wire to the enclosure.
The plastic box should have a metal strip that runs up and down to where you screw in your switch. This means that as long as your switch is screwed in, it is grounded. If you need to ground NM (non-metallic) cable, you must attach the cable's internal bare copper wire to a screw located inside the metal junction box.
Alternatively, if you are using Romex or NM wiring with a metal electrical box, you can attach the bare or green grounding wire to the box with a screw to complete the grounding contact. This is important because metal-sheathed wiring depends on bonding with the metal electrical box for grounding. Using this type of wiring with a plastic box without taking other grounding measures can be highly dangerous.
If you are using metallic cable with a metal box, ensure that the cable is properly secured to the box to achieve proper grounding.
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Using ground pigtails
Ground pigtails are commonly used to ground a switch or receptacle. They are short wires that connect the device to the box in the case of a metal box or to the ground pigtail of the incoming cable in the case of a plastic box. When using ground pigtails, it is important to ensure that the pigtail wires match the wire gauge of the circuit wires.
To make a ground pigtail connection, follow these steps:
- Use a wire nut to twist together and connect the bare end of the pigtail to the circuit wire, ensuring no bare copper wire is exposed. Tug on the wires to ensure they are secure.
- When connecting a green grounding pigtail to a metal electrical box, tighten the attached green grounding screw into the back of the box.
- With plastic electrical boxes, there will only be one grounding pigtail linking the circuit wires to the device.
- Attach the free end of the pigtail to the other grounding wires using a wire connector.
- Once all wire connections are complete, tuck the wires back into the electrical box, secure the device to the box, and mount the cover plate.
It is important to note that the push-in wire connection feature on receptacles and switches should be avoided as they tend to be less secure than screw terminal connections. Additionally, the National Electric Code requires a pigtail wire to be at least six inches long.
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Plastic boxes with tabs
When using plastic boxes with tabs, it is important to ensure that the tabs are not broken. If the tabs are broken, it is recommended to replace the box. The tabs act as clamps, and if they are missing or broken, it may be a code violation.
Some people choose to cut the tabs off of plastic boxes due to their sharp edges, which can cut through the wire sheath over time. However, doing so may result in failing a rough-in electrical inspection. Cables need to be clamped when entering the box, and unnecessary or unused openings in electrical junction boxes are not allowed.
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Grounding metal-sheathed wiring
It is important to understand the grounding system in your home, as it offers backup protection against fires and shocks from short circuits. Grounding provides a safe pathway for excess electricity to flow back to the ground. This is especially important if there are any faults in the wiring system.
When it comes to metal-sheathed wiring, there are a few things to consider. Firstly, it's important to distinguish between BX cable and NM cable. BX cable, or metal-clad armoured cable, is a type of wiring that can be used to ground a metal box. It has a metal sheath that looks like a single, ribbed metal tube, but is actually a metal strand that runs in a helix-like or twisted manner around the wires. This outer metal casing can be attached to metal boxes to achieve grounding.
On the other hand, NM cable, or non-metallic cable, has a vinyl sheathing that cannot be used for grounding. Instead, it contains a bare copper wire that runs through the cable for grounding purposes. This copper wire is connected to every device and metal electrical box in your home, forming a grounding pathway.
When working with metal-sheathed wiring, it is crucial to ensure that the wiring is properly grounded. This is important for safety reasons, as an ungrounded wire can pose a serious risk. In the case of BX cable, the cable's sheathing can be connected to the ground wire, which is typically grounded at the distribution panel. This provides a pathway for electricity to flow back to the ground if there are any faults in the wiring.
It is also important to note that older wiring systems may have different grounding requirements. For example, wiring systems installed between 1940 and 1965 may be grounded through metal conduit or metal cable, rather than copper. Additionally, older BX wiring may not have been designed to have its armour grounded, so it is important to consult with an expert before making any changes to your home's wiring.
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Grounding in Canada
Grounding is an essential safety precaution in electrical systems to minimize shocks and prevent damage from lightning, short circuits, and faulty wiring. Without grounding, electricity can transmit a charge to a person or object that comes in contact with it, leading to electrical fires or electrocution.
In Canada, electrical systems must be grounded to harmlessly discharge electricity into the earth. The most common way to ground an electrical system in Canada is by attaching a grounding wire to the cold water pipe. This wire is typically made of copper and is bare since it is already grounded. When the water supply pipe is plastic, as is often the case in newer homes, the ground wire runs from the neutral bus bar to a pair of metal rods outside, driven four feet into the ground. This is known as a double ground.
In older homes, it is common to find the grounding wire connected to a metallic copper or galvanized plumbing supply pipe near the electrical panel. However, today's standards dictate that the connection should be made before the first shut-off valve leading into the house.
The Canadian Electrical Code (CEC) outlines specific requirements for grounding and bonding in residential buildings. All electrical items, devices, boxes, lights, and plugs must be grounded to the main electrical panel. Gas pipes in residential units also need to be bonded to the main electrical ground, and old metal cast sewer lines must be bonded as well.
Additionally, the CEC requires that all electrical receptacles within reach of children in residential settings be tamper-resistant (TR) and 3-slot grounded, with a maximum distance of 12 feet between receptacles. GFCI (Ground Fault Circuit Interrupter) and AFCI (Arc Fault Circuit Interrupter) breakers are also essential safety measures. GFCI breakers interrupt the circuit when there is a fault between the ground and hot wire, while AFCI breakers detect arcing in the circuit that could lead to a fire.
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Frequently asked questions
No, a plastic box cannot be grounded due to its non-conductive nature.
You can ground a switch in a plastic box by adding a pigtail with a ring terminal from your 6/32 screw to the ground wire.
A pigtail is a short wire intended to go from the device to the box in the case of a metal box or to the ground pigtail of the incoming romex.
Metal boxes are fireproof, strong, and incapable of melting, offering the greatest level of security for electrical wiring applications. Plastic boxes are easier for do-it-yourselfers to work with and are less expensive.
Plastic-sheathed cable (NM) is always used with plastic boxes, but you can use both plastic and metallic-sheathed cables with metal boxes. Metal-sheathed wiring depends on bonding with a metal box for grounding, so you must use the correct box for grounding.
