Box Fill Calculator: How Do I Calculate Box Fill

The volume of the wiring enclosure box determines how many conductors and wiring devices can be contained in the box, as well as their sizes. Based on the International Residential Code (IRC), the volume allowed in a box includes conductors, internal clamps, support fittings, devices or equipment, and equipment grounding conductors. Understanding how to calculate box fill requires knowledge of the International Residential Code (IRC) and National Electrical Code (NEC) requirements.

How Do I Calculate Box Fill?

Box fill accounts for the number of wires in a wiring enclosure box and the total volume of conductors, devices, and fittings contained in the box. To calculate box fill based on the size and type of electrical boxes, use the tables in section E3905.12.1 of the IRC as seen below. This information can also be found in the National Electrical Code Solar Provisions NEC Table 314.16(A) and NEC Table 314.16(B).

Table E3905.12.1 [Table 314.16(A)]: Maximum Number of Conductors in Metal Boxes

*Where no volume allowances are required by 314.16(B)(2) through (B)(5)

Calculating the Right Amount of Box Fill

Determine the allowable box volume that’s marked on the box itself, and then use the following sections to add all of the electrical components present in the box to make sure you don’t exceed the allowable volume.

Box fill = Maximum number of conductors - conductor fill - clamp fill - support fitting fill - device or equipment fill - equipment grounding fill 

You’ll also need to account for the volume allowance per conductor as calculated in the following section.

How to Measure for Box Fill

How to Calculate the Volume Allowance Per Conductor

The volume allowance needed for each conductor depends on the largest conductor in the box based on its AWG (American Wire Gauge) measurement. If the largest conductor is 18 gauge, then the free space within the box required for each conductor will be 1.5 cubic inches.

Use the following table to calculate the volume allowance required per conductor in cubic inches:

Table E3905.12.2.1 [Table 314.16(B)] Volume Allowance Required Per Conductor

How to Calculate Conductor Fill

Conductors that terminate or are spliced within the box should be counted once, provided they originate outside of the box, and conductors that pass through the box without a splice or termination should also be counted once. Each loop or coil of unbroken conductor with a length equal to or greater than twice that required for free conductors (according to Section E3406.11.3) should be counted twice. Conductors that don’t leave the box shouldn’t be counted.

How to Calculate Clamp Fill

Internal cable clamps account for a single volume allowance, as specified in Table E3905.12.2.1, based on the largest conductor in the box. If the clamping mechanism is located outside of the box, an allowance isn’t required. Consult the building code for more information on clamp assembly allowances.

How to Calculate Support Fitting Fill

A single volume allowance is made for where one or more fixture studs or hickeys are in the box.

How to Calculate Device or Equipment Fill

For each yoke or strap containing one or more devices or equipment, a double volume allowance should be made based on the largest conductor connected to the device, devices, or equipment supported by that yoke or strap.

How to Calculate Equipment Grounding Conductor Fill

A single volume alliance is made where up to four equipment grounding conductors or equipment bonding jumpers enter a box. Each additional grounding conductor will require a ¼ volume allowance.

Who to Hire to Determine Box Fill

Electrical work requires an understanding of electrical systems and the associated building codes. You can hire an electrician near you to determine the appropriate box fill and help with your wiring project.