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How to Choose the Correct Size Electrical Box

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How to calculate the the size of an electrical box required for a specific number or wiring conductors and devices.

There are many instances when you need to add or move an electrical box during a minor renovation or repair. Whether you are working in your garage, attic, or kitchen you need to know that there is a limit to how many wires you can safely place in an electrical box. Keep in mind that not all electrical boxes are the same size.

The National Electric Code

The National Electrical Code (NEC) restricts the number of conductors that are allowed in a single electrical box. The number of conductors allowed is determined by the size of the box and the size of the conductors. A conductor that terminates or is spliced within the box is counted once, and each conductor that passes through the box without splice or termination is counted once.

The size of the box is determined by its volume which can be easily calculated by multiplying Height x Width x Length to get the total cubic inches or the volume of the box. Some boxes will have its volume listed on the box, but many will not. Most of the PVC, or nonmetallic, boxes will have its volume listed on the box.

The minimum volume required per conductor is as follows:

(G = wire gauge) Standard residential wiring is typically 12 or 14G.

18G - 1.50 cubic inches per conductor.

16G- 1.75 cubic inches per conductor.

14G- 2.00 cubic inches per conductor.

12G- 2.25 cubic inches per conductor.

10G- 2.50 cubic inches per conductor.

8G- 3.00 cubic inches per conductor.

6G- 5.00 cubic inches per conductor.

 

Electrical Box Sizes

Devices

If the electrical box is also going to house a switch or receptacle space allowance must be made for the device being used. The space allowance for a switch or receptacle is based on the size of the device. Electrician’s use the term "strap". The number of straps is based on the size of the device to account for the area that it covers.

For the do it yourselfer, each switch or receptacle is counted as one strap. A box with two standard switches or receptacles would be counted as two straps. Each strap that is mounted in the electrical box is equivalent to two of the largest conductors entering or exiting the electrical box, even if the largest conductors are not connected to the electrical device that is also mounted in that electrical box.

It is a common practice to use a box extension to increase the volume of a box. An extension will have an open back that will provide more space for the wiring to fit inside the electrical box. Sometimes extensions are used to move the front of the box out to a new finished wall without having to remount the box.

 

4-inch Octagon Box Extension

Square Box Extension

Tips

If the electrical box has an internal clamping mechanisms, as shown below, to hold the electrical wire in position, the clamping mechanisms take up space within the electrical box and must be given a space allowance. One or two internal clamps in an electrical box are the equivalent of one of the largest conductors entering or exiting the electrical box.

Electrical Box with Clamps

Pigtails that are contained within the electrical box and do not leave the electrical box are not counted.

Wiring Pigtails

Multiple ground wires are only considered to be one conductor, but they must be counted as the largest ground wire that enters the electrical box. Example: There are two 14 gauge and one 12 gauge ground wire in the electrical box; allow for one 12 gauge wire for all three ground wires.

Each wire is accounted for. A wire that enters an electrical box and is connected to another wire within the electrical box which then exits the box is considered to be two wires for space calculations.

Wire nuts or other connectors used to join wires together are not considered in the space allocation calculation.

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Posted on Feb 11, 2011
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Daniel Snyder

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