NEC Table 314.16(B) Conductor Volume Allowances Explained

NEC Table 314.16(B) is the foundation of every box fill calculation. Understanding conductor volume allowances for each wire size is essential for accurate calculations and code compliance.

NEC Table 314.16(B) - Volume Allowances

This table provides the cubic inch volume allowance for each conductor based on its AWG size:

AWG Size	Volume (cu. in.)	Volume (cm³)	Typical Use
18 AWG	1.50	24.6	Low-voltage, controls
16 AWG	1.75	28.7	Light fixtures, controls
14 AWG	2.00	32.8	15A circuits (lighting)
12 AWG	2.25	36.9	20A circuits (receptacles)
10 AWG	2.50	41.0	30A circuits (dryers)
8 AWG	3.00	49.2	40-50A circuits (ranges)
6 AWG	5.00	82.0	55-60A circuits

How Volume Allowances Are Determined

The volume allowances in Table 314.16(B) are based on the physical dimensions of each wire size, plus additional space needed for:

Wire insulation thickness
Bending and manipulation during installation
Heat dissipation
Safe clearances between conductors

Applying Volume Allowances

Each component in the box uses conductor volume allowances differently:

Conductors [314.16(B)(1)]

Each conductor entering the box counts as one conductor volume based on its size.

Clamps [314.16(B)(2)]

All internal clamps together = ONE conductor volume (largest conductor)

Support Fittings [314.16(B)(3)]

Fixture studs/hickeys = ONE conductor volume (largest conductor)

Devices [314.16(B)(4)]

Each device = TWO × conductor volume (largest conductor to that device)

Grounds [314.16(B)(5)]

All grounds together = ONE conductor volume (largest ground)

Terminal Blocks [314.16(B)(6)]

Based on conductors connected (NEC 2023)

Practical Examples

Example 1: Standard Receptacle Box

With two 14/2 cables (4 conductors + 2 grounds):

4 × 14 AWG conductors = 4 × 2.00 = 8.00 cu.in.
2 × grounds (count as 1) = 1 × 2.00 = 2.00 cu.in.
1 × clamp = 1 × 2.00 = 2.00 cu.in.
1 × receptacle = 2 × 2.00 = 4.00 cu.in.
Total: 16.00 cu.in.

Example 2: Mixed Wire Sizes

When you have different wire sizes, use the appropriate volume for each:

2 × 12 AWG = 2 × 2.25 = 4.50 cu.in.
2 × 14 AWG = 2 × 2.00 = 4.00 cu.in.
Clamps (use largest) = 1 × 2.25 = 2.25 cu.in.
Device (use largest connected) = 2 × 2.25 = 4.50 cu.in.

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Understanding NEC Table 314.16(B): Conductor Volume Allowances