Fastening Electrical Junction Boxes to Concrete, Brick or Block

Electrical junction boxes are metal or plastic enclosures in which electric wires meet and join together. The most common junction boxes measure 2-1/2” to 3-1/2” deep and are used according to the number of wires being joined together. Boxes that measure 2” x 3” x 2-1/2” typically have three wires, while the boxes that measure 2” x 3” x 3-1/2” usually have 5 wires. Electrical junction boxes need to be attached to a variety of base materials such as concrete, brick or block. Different types of anchors are available, the anchor chosen will depend on the specific application and user preference. Most electrical junction box applications require that the concrete fastener not protrude into the box and have a finished or smaller head to allow more space in the junction box as well as to eliminate a snag point for the electrical wires.

Tapcon® – Concrete/Cement Screws

Tapcon® concrete screws are commonly used for fastening a junction box to concrete, brick or block base material. They are the most versatile type of concrete fastener for this type of application because they work equally well regardless of the material, have consistent holding values and have a high installation success rate. The hex head anchor works best because of its positive drive when installing and the smaller head does not protrude into the junction box. Tapcons are easily removed if needed and have excellent holding values both in the 3/16” diameter and the 1/4” diameter head style. The Tapcon concrete screw requires a minimum embedment depth of 1” and a maximum embedment depth of 1-3/4”. The screw length that is used most frequently for this application is the 1-1/4” and the 1-3/4”, either in the 3/16” diameter or the 1/4” diameter. The head size on the 3/16” diameter Tapcon® is smaller than on the 1/4”. This might be a consideration when choosing the most appropriate fastener. The 3/16” diameter screw requires a 1/4” driver and the 1/4” diameter requires a 5/16” driver. When installing a concrete screw it is very important to use the correct bit size because the hole size and screw size tolerance are critical. The 3/16” diameter concrete screws require a 5/32” hole and the 1/4” diameter require a 3/16” hole diameter. The hole must also be drilled into the base material a minimum of 1/2” deeper than the screw will penetrate the base material. This provides space for materials to go during the installation process. Concrete screws are available in 410 stainless steel that can be used in environments where corrosion is a concern. Larger diameter Tapcon or LDT’s are also available in 3/8”, ½”, 5/8” and ¾” diameters for larger, heavier junction boxes.

A summary of the holding values for Tapcon® concrete screws in block is below:

(1) The Pull out values and Shear values are at one inch (1") embedment.
Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

A summary of the holding values for concrete screws in concrete is below:

Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

Wedge Anchor

The wedge anchor can only be used to fasten a junction box to concrete. The most common diameter used is the 1/4” diameter. Large size wedge anchors are available up to 1-1/4” diameter for larger, heavier junction box applications. The wedge anchor in the 1/4” diameter comes in the three lengths of 1-3/4”, 2-1/4” and 3-1/4” and the minimum embedment (see 2nd column in chart below) is 1-1/8”. The length of anchor chosen must be long enough for the anchor to be embedded into the concrete at least 1-1/8”; a 1-3/4” length wedge anchor is used for fastening material up to 5/8” thick; a 2-1/4” length wedge anchor is used for material up to 1-1/8”; a 3-1/4” length wedge anchor is used for up to 2-1/8” thick material. The longer wedge anchors can be used for thinner material. Embedding into the concrete requires more drilling, but the holding value increases as the wedge anchor is embedded more deeply into the concrete. A drawback to using the wedge anchor for fastening a junction box to concrete is that after the wedge anchor is set, the stud and nut may stick out into the box and decrease valuable space as well as become a snag point for the wiring. Keep in mind that the wedge anchor is designed for use in concrete only, which means that it is not as versatile as some of the other concrete fasteners, such as the Tapcon concrete screw or sleeve anchor.

Below is a table of the holding values for 1/4”concrete wedge anchors in concrete.

Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

Sleeve Anchor

Sleeve anchors are very versatile for fastening junction boxes to concrete, brick or block and is used by many electrical contractors for this application. The design of the sleeve anchor makes it well-suited for most base materials, especially for material that is suspect for its ability to be attached to. The most common diameter of sleeve anchor used for fastening junction boxes to concrete, brick or block is the 1/4”. The ¼” diameter comes in a number of different lengths, depending on the thickness of material that is being fastened down or on the depth of embedment. There are two available head styles for sleeve anchors: acorn nut and round head. The acorn nut is a smaller beveled nut that allows for lower profile and eliminates an edge for wires to be snagged on. The round head slotted is the most popular because the head has a very low profile and has minimal protrusion into the junction box. The acorn head comes in three lengths: 5/8”, 1-3/8” and 2-1/4”. The round head comes in two lengths: 1-1/4” and 2”.

The 1/4” Sleeve Anchor Holding values in concrete are outlined below.

Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

The 1/4” Sleeve Anchor Holding values in block are outlined below.

Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

Hammer Drive Anchor

Hammer drive anchors are well-suited for fastening junction boxes to solid concrete when a permanent attachment is required. The hammer drive anchor is designed so that when the anchor has expanded the junction box cannot be removed without risk of damage to the junction box or without cutting the head off the anchor. The head style on the hammer drive anchor is perfect for this application for several reasons: it has a low, wide profile, it takes up little space in the junction box, there is nothing for the wires to snare on, it has a larger than normal bearing surface, and it has increased pull-over values. The hammer drive anchor is available in two (2) diameters: 3/16” and 1/4”. The 3/16” diameter is not used for this application because its only length is 7/8”, which is not long enough to achieve minimum embedment. This is also true for the 3/4” long 3/4” length hammer drive. The sizes most commonly used for attaching junction boxes to concrete are the 1”, 1-1/4” and 1-1/2” length hammer drive.

Below is a chart on the values for the hammer drive anchor in concrete.

Note: All values shown are ultimate loads, safe working load is a 4:1 ratio or 25%

Fastening electrical junction boxes to concrete, brick or block is a common application. Before installing and attaching an electrical junction box to a base material, one must be certain that the anchor selected is the best one for the job. The tips offered in this article will help ensure that the installation of the junction box is properly and safely executed.