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Home > Home Wiring USA Archive: NEC 1999 > Definitions and Calculations > Box Fill Calculations (NEC 1999)

Box Fill Calculations (NEC 1999)

By Warren Goodrich
Box Fill Calculations (NEC 1999)

IF YOU ARE USING CONDUCTORS SMALLER THAN 4 Awg. AND ALL THE SAME SIZE CONDUCTORS IN THE BOX, USE THE FOLLOWING

We must first find the maximum numbers of conductors allowed in the box that you are using. When you are calculating the total number of current carrying conductors in a box, in addition to counting all actual current carrying conductors as one current carrying conductor for each conductor {any color of conductor except, green or bare}, you must add the following as current carrying conductors as follows; Article 314.16.B Each device {switch or receptacle} found in the box must be counted as 2 more current carrying conductors, all grounding conductors in that box must be counted as a total, as one more current carrying conductor, and all clamps must be counted with the total number of clamps found in the box as one more current carrying conductor. Keep in mind that you are not supposed to count conductors that neither enters your box nor leaves your box. [pigtails only, these are ignored].

If you are using a steel box that is listed in the NEC chart 314.16.A, then the chart should give you the maximum current carrying conductors allowed in the steel box. A further calculation of adding up the number of current carrying conductors that you must count as being in that box, by your wiring design must be calculated. The total number of current carrying conductors that you designed to be in that box must be compared to the maximum allowed in that box. The total number of current carrying conductors that you designed to be in that box must not exceed the total number of current carrying conductors allowed in that box.

You may also use the following method of calculation if you don’t have a Code book, are using a size of steel box that is not listed in chart 314.16.A, or you are using a plastic or fiber box. If you are using a type of box that is not listed in the NEC chart 314.16.A, then you must measure the box, then multiply the height times the length times the depth of the box to find the cubic inch capacity of that box. Now you must look in the NEC chart 314.16.B to find the cubic inch required per conductor rated by the size of the conductor that you are using. The chart mentioned says that 14 Awg. = 2 Cu. In. per conductor, 12 Awg. = 2.25 Cu. In. per conductor, 10 Awg. = 2.5 Cu. In. per conductor, 8 Awg. = 3 Cu. In. per conductor, 6 Awg. = 5 Cu. In. per conductor. 314.16.B If you are using all conductors of the same size in your box, then you must count the number of current carrying conductors [all colors including white but not counting green or bare] entering your box. Also do not count conductors that neither enters your box nor leaves your box. [pigtails only, these are ignored].

Now add to your number of current carrying conductor list, by counting all of the grounding conductors as one conductor [green or bare], no matter how many grounding conductors, just add the one conductor to your total number of current carrying conductor list. All grounding conductors [green or bare] must be counted as a total of one current carrying conductor.

Clamps are also counted the same as grounding conductors, one current carrying conductor must be added for the total of all clamps found in the box. All clamps found within your box that are entering the box at least ½”, no matter how many, count as one current carrying conductor, only, for all of these clamps. Now add this one conductor count to your total number of current carrying conductor list, if any of these clamps are present. A single gang plastic or fiber box will have no clamps to consider. They are exempt from a clamp requirement.

Devices must count as 2 conductors for each device. Count the number of devices {switches or receptacles}. Multiply the total number of devices times 2. The answer from multiplying the total number of devices by the 2 is the total number of current carrying conductors you must add to your total number of current carrying conductor list.

Now this final total of your current carrying conductor list is the answer to the total number of current carrying conductors installed in your box. Now multiply the cubic inch required for the size of conductors in your box found in the 314.16.B by the answer that you found in your total number of current carrying conductor list. This is the total cubic inch required for all of the conductors, equipment, and devices in your box. Compare this total cubic inch required to the total cubic inch capacity of your box. You must not exceed the capacity of your box with the total cubic inch required by your conductors, equipment, and devices that you installed in that box.

IF YOU ARE USING CONDUCTORS SMALLER THAN 4 Awg. AND DIFFERENT SIZE CONDUCTORS IN THE SAME BOX USE THE FOLLOWING

If you are using different size conductors in the same box. You must perform the same conductor count and calculation as if using all of the same size conductors in your box as described above except the following. You must change the current carrying conductor calculation into separate calculations for each size conductor times the assigned cubic inch required in the 314.16.B. You must also change the counts calculation for your devices and clamps calculating them as if they were the largest conductor in the box, when you use the 314.16.B. Ignore any smaller conductors for the device and clamps calculations section. Only consider these equipment and devices as the largest size conductor present in your box to calculate the current carrying conductors that you must add to your list for these equipment and devices. You must also change the one conductor count for the total number of grounding conductors using the largest grounding conductor size found in the box, when you use the 314.16.B.

Then add all of the cubic inch requirements from your list, all added together, for the total cubic inch required and compare that total cubic inch required to the cubic inch capacity of your box. You must not exceed the total cubic inch capacity of the box by the total cubic inch required for the conductors, equipment, and devices present in your box, as you have calculated.

IF YOU ARE USING CONDUCTORS 4 Awg. OR LARGER THEN REFER TO 314.28 OF THE NEC

If you have boxes with conductors larger than 4 Awg. that are installed in a box, then you must calculate in a different manner. In short, a box with 4 Awg. or larger conductors with “U” or angle pulls the box fill must be calculated by adding the diameter of all of the conduits on the same side of the box and in the same row for the first total. Then add each row installed on that same side of the box. Now pick the largest one row total, of each of the rows, and use that in your calculation. Ignore the other rows on that side of the box not in that same row. 314.28.A.2 Now pick the largest conduit in that largest row and multiply that conduit diameter times 6. 314.28.A.2 Now add the total diameters of all of the conduits in that same row to that times 6 multiplication answer. 314.28.A.2 Ignore all of the other rows on that same side. This answer is the minimum distance in inches that the opposite wall of that box must be from that side that you calculated. 314.28.A.2 Now calculate each side of the same box for the answer to the distance required to the opposite wall of the box from each side that you calculate. This is the size of box required to contain the 4 Awg. or larger conductors. 314.28.A.2

If you are using nonmetallic sheathed cables with 4 Awg. or larger in them then you must measure the widest point across the nonmetallic sheathed cable and treat this measurement as the diameter of a conduit in your calculation. The intent of the nonmetallic sheathed cable widest point is to find the size of conduit required to contain than nonmetallic sheathed cable. The calculations to discover the size of a box with 4 Awg. or larger conductors gets more complicated but it should give you a general idea on the subject. 314.28.A.2

This document is based on the 1999 national electrical code and is designed to give you an option, as a self-help, that should pass minimum code requirements. While extreme care has been implemented in the preparation of this self-help document, the author and/or providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

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