Chloe Zhang
Grounding a plastic junction box is not a straightforward process, as plastic is an insulator and non-conductive material. In some countries, like the UK, there is no need to ground a plastic junction box. However, if you are looking to improve the grounding of a plastic junction box, there are some possible solutions. One option is to replace the plastic box with a metal one, which can provide a better grounding path. Alternatively, you can try using a small hose clamp to secure a grounding jumper to the outside of the armour on the MC (metal-clad cable) coming into the box.
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What You'll Learn
Plastic boxes can't be grounded because they are non-conductive
Plastic electrical boxes cannot be grounded in the same way as metal boxes because plastic is an insulator, and does not conduct electricity. Metal junction boxes always need to be grounded, and are self-grounding if they are properly secured to the box.
Metallic-sheathed cable, also known as BX cable, must be used with a metal box for proper grounding. This is because the cable is grounded via its sheathing, and requires contact with a metal box to achieve this. Plastic-sheathed cable, or NM cable, on the other hand, uses an internal copper wire for grounding and does not rely on the conductivity of the box.
If you are using a metal box, the best practice is to insert a green grounding screw into the threaded hole in the back of the box or enclosure. This is not possible with plastic boxes as they are non-conductive and electricity will not flow through them.
In some cases, it may not be necessary to ground a plastic box at all. For example, in the UK, it is not a requirement to ground a plastic junction box as it is made of an insulated material.
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Using a metal box provides a ground
Alternatively, when installing with conduit, the metal conduit itself can serve as the ground path. In this case, the network of metal conduit provides the path for fault current back to the panel, and no additional ground wire is required. This is because the ground pin on the receptacle is connected by screwing it into the metal box.
However, it is important to note that some sources suggest that simply screwing the receptacle into the box may not be sufficient for bonding, and that a pigtail from the ground to the metal box might be required by code. A pigtail is a short wire that connects the ground to the box, ensuring a solid bond path to ground.
Additionally, when using a mud ring, there may be issues with making a solid connection between the metal box, the mudring, and the receptacle strap due to the wall finish material. In this case, a pigtail is necessary to ensure a reliable ground path.
It is always important to follow local electrical codes and consult with a qualified electrician to ensure that any electrical work is performed safely and correctly.
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Using a squeeze/clamp metal connector to secure the MC to a box knockout
To secure the MC to a box knockout using a squeeze/clamp metal connector, you must first ensure that your metal box has the necessary knockout holes in the correct locations. Knockouts are the little metal discs covering the holes in the sides or back of a metal electrical box. You will need to remove as many knockouts as there are cables entering the box, leaving the others in place.
The next step is to remove the knockout. If the knockout has a straight slot in its centre, insert a straight-bladed screwdriver into the slot and twist back and forth until it comes off. If there is no slot, you will need to punch it out with a hammer and screwdriver. Once you have knocked it out, grab the knockout disc with pliers and twist until it is completely free.
Now you are ready to attach the connector to the box. You can either attach the connector to the MC cable and then join it to the box, or mount the connector on the box before feeding the wires through. It is generally easier to mount the connector on the box first. Feed the wires through the connector, leaving a minimum of 6-8 inches of exposed wire inside the box.
Finally, tighten the connector to secure the MC cable. You can use a locknut-type clamp, which has a threaded cylinder and locknut on one end and a saddle-type clamp with two screws on the other. Insert the threaded end into the knockout hole, then thread the locknut onto the end from inside the box and tighten with pliers. Insert the cable through the clamp saddle and tighten the screws to secure the cable.
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Using a small hose clamp to clamp the bare end of a grounding jumper
Plastic junction boxes cannot be grounded because they are non-conductive. However, you can ground the wires within the box.
If you are using a plastic junction box, you can ground the wires by using a small hose clamp to clamp the bare end of a grounding jumper to the outside of the armour on the end of each MC (metal-clad cable) coming into the box. This method is suggested by an electrician with over 20 years of experience.
Firstly, ensure you have a small hose clamp that fits the bare end of your grounding jumper. The grounding jumper should be bare, with no insulation, so that it can make effective contact with the metal armour of the MC. Place the clamp over the bare end of the jumper and tighten it securely, but be careful not to overtighten as this could damage the wire.
Next, insert the MC into the plastic junction box, ensuring that the metal armour on the end of the MC is exposed and accessible. You may need to strip back any insulation on the MC to expose the armour. Once the MC is in place, use the small hose clamp to clamp the bare end of the grounding jumper to the metal armour of the MC. Ensure that the connection is secure and that the jumper cannot move or become dislodged.
Finally, use a wire nut to connect the grounding jumper to the switch or outlet ground screw. This will ensure that the ground path is complete and will allow for easy modification if the box is upgraded to metal in the future.
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No need to ground a plastic box in the UK
If you are in the UK, there is no need to ground a plastic junction box. This is because plastic is an insulated material. If you have already run a ground wire, you can throw a wago or connector block and leave it there for future use.
In general, if there is no exposed metal, there is nothing that needs to be grounded. If there are exposed metal parts, these must be bonded to a common PE point, which must have a path back to an earthing point.
In the UK, legislation means that you must fit a residual current device (RCD) to outgoing AC connections. This is a safety device that quickly cuts off the power if an electric fault is detected, preventing serious electric shock. It is important to always follow the latest electrical safety advice and consult a qualified electrician if you are unsure.
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Frequently asked questions
A plastic box cannot be grounded because it is non-conductive. However, all the grounding conductors from the circuit are made together.
In the UK, you do not need to ground a plastic junction box because it is made of an insulated material.
One option is to use a small hose clamp to clamp the bare end of a grounding jumper to the outside of the armour on the end of each MC coming into the box. Then, use a wire nut to connect any/all of these jumpers and a jumper from the switch/outlet ground screw.
You can use a squeeze/clamp metal connector to secure the MC to a box knockout. This should bond the armour to the metal box.
