Electrical Boxes Volume and Fill Calculations


  • Device boxes, 
  • Pull and junction boxes,
  • Sizing of Junction and pull boxes according to NEC Section 314-28.

Today, I will explain 
Electrical Boxes Volume and Fill Calculations  as follows.



You can review the following articles in the same course for more information:

  •  Types of Electrical Conduits

  •  Conduit Fittings and Supports






Electrical Boxes Volume and Fill Calculations




  
NEC Section 314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies
This section include three parts as follows:
  • Part (A), “Box Volume Calculations,” defines the volume of a wiring enclosure or box. The calculations must take into account the volume of the box as well as the volume of any extensions such as domed covers or extension rings.
  • Part (B), “Box Fill Calculations,” describes the method for determining how much volume (fill) may be occupied by conductors, clamps, support fittings, devices (switches or receptacles) or equipment, and equipment grounding conductors.
  • Part (C), “Conduit Bodies,” covers enclosing No. 6 AWG or smaller conductors and requires that the maximum number of conductors be computed.

This section states that:
" Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C)".

Notes to section NEC 314.16:
  • The provisions of this section shall not apply to terminal housings supplied with motors or generators.
  • Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.




NEC 314.16 Part (A): Box Volume Calculations

The volume of a box is the total volume in cubic inches of the assembled sections and, where used, the space provided by plaster rings, domed covers and extension rings that are marked with their volume in cubic inches or are made from metal boxes that are included in Table 314-16(a) in the NEC.




Notes for NEC Table 314-16(a):

  • The volumes of standard boxes that are not marked with their volume shall be as given in Table 314.16(A).
  • Boxes 1650 cm3 (100 in.3) or less, other than those described in Table 314.16(A), and nonmetallic boxes shall be durably and legibly marked by the manufacturer with their volume. Boxes described in Table 314.16(A) that have a volume larger than is designated in the table shall be permitted to have their volume marked as required by this section.
  • The total volume determines the number and size of conductors and wiring devices that are permitted to be contained in the box. The cubic inch area required for each wire, clamp, support fitting, device and equipment ground is added together. The box must have a cubic-inch capacity that equals or exceeds the total of the contained items.
  • Sometimes more conductors end up in boxes than were originally intended. Where practicable, an extension ring that is the same shape as the box can be installed that will add adequate space so the original box does not have to be replaced.
  • The NEC Table 314-16(a) covers the maximum number of conductors permitted within a standard metal box. A “standard” box is one that is included in the Table. The minimum cubic inch capacity for each size is given along with the maximum number of conductors of sizes #18 through #6 that are permitted in the box.
  • This number of conductors permitted in various boxes, as shown in the Table, applies only where all conductors are the same size.  A calculation must be made of the cubic inch capacity that is required where conductors of different sizes are installed as in section 314.16(b) Box Fill Calculations.



NEC 314.16 Part (B): Box Fill Calculations

  • Section 314-16(b) includes the requirements and method for determining the minimum cubic inch area that are required in boxes that have different size conductors or any equipment installed in them.
  • The method is to add up all the allowances required for various items, and this becomes the minimum area box that is required. At that point, a standard box with a cubic area that equals or exceeds the volume required can be used.


The following two tables can be used in calculating box fill:




Conductor, Device or Type of Fitting
Counted as Conductors
Based on
Each conductor, originating outside and terminating inside the box
1
Conductor size
Each conductor passing through unbroken
1
Conductor size
Conductor that does not leave the box
0
Not Applicable
Maximum of four fixture wires smaller than #14 plus ground from fixture canopy. Must terminate in box.
0
Not Applicable
Cable clamps, one or more (internal)
1
Largest size conductor present
Support fittings, e.g. fixture studs or hickeys (per type)
1
Largest size conductor present
Device or equipment yoke, e.g. switch, receptacle, pilot light, etc.
2
Largest size connected to device
Equipment grounding conductors, all except isolated ground
1
Largest EGC present
Additional equipment grounding conductors for isolated grounding
1
Largest EGC present
Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14
2
Conductor size
Commentary Table 314.1 Summary of Items Contributing to Box Fill

Let’s take a closer look at some of the requirements that are listed on the above table as follows:
  • No allowance is required for small fittings like locknuts and bushings.
  • Where one or more fixture studs or hickeys are present in the box, a single volume allowance is required to be made for each type of fitting in the box. A fixture stud is a fitting that mounts to the top of the box, usually inserts through the knockout of a metal box and is threaded to accommodate the fixture stem. A hickey is a fitting that can be described as a coupling that has threads the same size as the fixture stem and has an oval-shaped hole on one or more sides for the fixture wires to exit inside the box. The hickey is no longer popular and has been replaced with hanger straps that are fastened to the box.
  • For each yoke or strap containing one or more devices or equipment, a double volume allowance is required for each yoke or strap. Each device or equipment is considered individually where more than one item is contained in the box. For example, if a switch has #14 wire connected to it, a volume allowance of 2 x 2.0 cubic inches or 4 cubic inches is required. If a receptacle has #12 wire connected to it, a volume allowance of 2 x 2.25 or 4.5 cubic inches must be made.
  • Where one or more equipment grounding conductors enters a box, a single volume allowance is required to be made. The allowance is based on the largest equipment grounding conductor. This applies to all equipment grounds except for an isolated equipment ground often installed on computer circuits.




Example#1:

A standard-sized box is used where all the conductors are the same size and, as shown in below figure, the box does not contain any cable clamps, support fittings, devices, or equipment grounding conductors. Determine whether the box in below figure is adequately sized or not.






Solution:


To determine the number of conductors permitted in the box, which is a standard 4 in. x 1.5 in. square box (21.0 in.3), count the conductors in the box and compare the total to the maximum number of conductors permitted by Table 314.16(A).

Each unspliced conductor running through the box is counted as one conductor, and each other conductor is counted as one conductor.

Therefore, the total conductor count for this box is nine conductors. Table 314.16(A) indicates that the maximum fill for this box is nine 12 AWG conductors, so the box is adequately sized.



Example#2:

Determine whether the box in below figure is adequately sized or not.





Solution:

The standard method for determining adequate box size first calculates the total box volume and then subtracts the total box fill to ensure compliance.

For a standard 3 in. x 2 in. x3.5 in. device box, Table 314.16(A) shows the minimum permitted box volume to be 18 in.3 and allows up to a maximum of nine 14 AWG conductors.


Total Box Fill
Items Contained Within Box
Volume Allowance
Unit Volume Based on Table 314.16(B) (in.3)
Total Box Fill (in.3)
4 conductors
4 volume allowances for 14 AWG conductors
2
8
1 clamp
1 volume allowance (based on 14 AWG conductors)
2
2
1 device
2 volume allowances (based on 14 AWG conductors)
2
4
Equipment grounding conductors (all)
1 volume allowance (based on 14 AWG conductors)
2
2
Total
16

The box fill for this situation as given in Commentary Table 314.2 is 16 in.3. Because the total box fill of 16 in.3 is less than the 18 in.3 total box volume permitted, the box is adequately sized.




Example#3:

Determine the adequacy of the device box illustrated in below figure, where two standard sized 
3 in. x 2 in. x 3.5 in. device boxes have been ganged to form a single box.







Solution:

Table 314.16(A) shows that the permitted box volume for a single box is 18 in3. Thus, the total box volume for the ganged box is 36 in.3 (2 x 18 in.3).

The total box fill, based on Table 314.16(B), is determined as given in Commentary Table 314.3. With only 26 in.3 of the 36 in.3 filled, the box is adequately sized.
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