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Category Engineering Systems

Box Type

Custom Box Volume (in³)

Current-Carrying Conductors

18 AWG

16 AWG

14 AWG

12 AWG

10 AWG

8 AWG

6 AWG

Equipment Grounding Conductors (EGC)

14 AWG EGC

12 AWG EGC

10 AWG EGC

8 AWG EGC

6 AWG EGC

Internal Cable Clamps

Device Yokes (Switches/Receptacles)

Support Fittings (Fixture Studs/Hickeys)

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About

Overfilled electrical boxes cause insulation damage, short circuits, and fire. NEC Article 314.16 mandates maximum fill volumes for every box type and conductor size. The calculation is not a simple count. Each current-carrying conductor adds 2× the volume allowance from Table 314.16(B). Equipment grounding conductors, regardless of count, add only 1× the volume of the largest grounding conductor. Each device yoke adds 2× the volume of the largest conductor connected to it. Internal cable clamps, regardless of count, add 1× the volume of the largest conductor in the box. Missing any of these rules leads to inspection failures and rework costs.

This calculator implements the full NEC 314.16 methodology. It cross-references standard metal and nonmetallic box volumes against your computed fill. The result is a pass/fail determination with percentage utilization. Note: the tool assumes standard conditions and does not account for equipment bonding jumpers or unusually large splices that may require additional volume allowances per NEC 314.16(B)(5).

Formulas

Total required volume per NEC 314.16(B):

V_total = V_conductors + V_clamps + V_support + V_devices + V_grounds

Where each component is calculated as:

V_conductors = ∑ (n_i × 2 × a_i)

V_clamps = 1 × max(a_i) if any internal clamps present

V_devices = n_dev × 2 × max(a_connected)

V_grounds = 1 × max(a_egc) regardless of EGC count

Where n_i is the count of conductors at gauge i, a_i is the volume allowance from NEC Table 314.16(B) in in³, n_dev is the number of device yokes, and max(a) selects the largest conductor's allowance among the relevant group. The result V_total must be ≤ the rated box volume.

Reference Data

AWG Size	Volume Allowance	Metric Equivalent	Typical Use
18 AWG	1.50 in³	24.6 cm³	Low-voltage control wiring
16 AWG	1.75 in³	28.7 cm³	Low-voltage, fixture wire
14 AWG	2.00 in³	32.8 cm³	15A branch circuits
12 AWG	2.25 in³	36.9 cm³	20A branch circuits
10 AWG	2.50 in³	41.0 cm³	30A circuits, dryers
8 AWG	3.00 in³	49.2 cm³	40-50A ranges, subpanels
6 AWG	5.00 in³	82.0 cm³	60A feeders, large appliances

Frequently Asked Questions

NEC 314.16(B)(5) treats all equipment grounding conductors as a single volume entry. Whether the box contains one or ten EGCs, you add only 1× the volume allowance of the largest EGC. The rationale is that grounding conductors can be bundled tightly and do not require the same working space as current-carrying conductors. For example, four 14 AWG EGCs still count as only 2.00 in³ total, not 8.00 in³.

External clamps (mounted outside the box) require zero volume allowance. Internal clamps (inside the box body) require a single volume allowance based on the largest conductor entering the box, regardless of how many internal clamps are present. If your box has built-in cable clamps, you must add this allowance. Switching to a box with external clamps can recover critical cubic inches in tight fills.

A standard duplex receptacle on a single yoke counts as one device - contributing 2× the volume allowance of the largest conductor connected to it. However, if a device requires two gangs (like certain GFCI/combination units), it counts as two devices. Always count yokes, not individual outlets or switches.

Each conductor is counted at its own gauge's volume allowance from Table 314.16(B). For the clamp fill, device fill, and grounding fill, you use the largest conductor's allowance within that category. For example, a box with 14 AWG and 12 AWG conductors where the largest connected to a device is 12 AWG: each device adds 2 × 2.25 in³ = 4.50 in³.

No. Conduit bodies are governed by NEC 314.16(C), which uses a different methodology based on conduit body cross-sectional area and the number of conductors (typically limited to the conduit fill tables in Chapter 9). This calculator strictly implements NEC 314.16(A) and (B) for outlet and junction boxes.

Spliced conductors that originate and terminate within the box (pigtails) are not counted for box fill per NEC 314.16(B)(1) - only conductors that enter the box through a knockout are counted. However, each pigtail connected to a device does affect the device fill calculation since the device yoke still counts. Wire nuts and push-in connectors themselves have no specific volume deduction, but the conductors they join are already counted.