Calculate minimum junction box volume per NEC 314.16. Add all conductors, devices, clamps, and fittings to find the correct box size.

NEC 314.16: Each conductor counts as one unit based on its size. Devices count as 2 units, equipment grounds count as 1 unit total (largest conductor size), and internal cable clamps count as 1 unit.

Quick add – common cable types

Conductors and fittings

Item description AWG size Qty

+ Add conductor / item

Grounding conductors

Equipment grounds

Number of ground wires All grounds = 1 unit (largest size)

Please add at least one conductor.

Minimum box volume required

0 cu in

Volume breakdown

NEC Table 314.16(A) – Standard box volumes

Box type Volume Max 14 AWG

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Practical Notes

A junction box must be big enough to hold the wires and devices in it safely without overcrowding, which can cause overheating and damage the insulation. The electrical code sets the minimum volume by counting an allowance for each item. This calculator adds them up and tells you the minimum volume the box needs.

Each conductor counts as one allowance based on its gauge, for example 2.0 cubic inches for 14 gauge, 2.25 for 12 gauge, and 2.5 for 10 gauge. All the grounding conductors together count as one, a switch or receptacle counts as two, internal cable clamps count as one, and each fixture stud or similar fitting counts as one.

For instance, two 12 gauge cables with a ground, ending at one device, work out to four conductors, one ground, and two for the device, for seven allowances of 2.25, or about 15.75 cubic inches. Compare the result with the volume marked on the box, and use a larger or deeper box if it is too small. Always follow the local electrical code, and have a qualified electrician do the work where it is required.