Grounding Conductor Box Fill Rules - NEC 314.16(B)(5) Guide

Understanding how to count equipment grounding conductors is one of the most misunderstood aspects of box fill calculations. NEC 314.16(B)(5) provides a special rule that can significantly affect your calculations.

The Basic Rule: All Grounds Count as One

NEC 314.16(B)(5) states that all equipment grounding conductors in a box, regardless of how many there are, count as a SINGLE conductor volume allowance based on the LARGEST equipment grounding conductor entering the box.

Key Point

Four 14 AWG ground wires = ONE × 2.00 = 2.00 cu.in. (not 8.00 cu.in.)

Why This Rule Exists

Equipment grounding conductors are typically spliced together with a single wire nut or crimp connector, taking up less space than individual terminations. The code recognizes this practical reality.

When You Have Different Ground Sizes

If your box contains equipment grounding conductors of different sizes:

Count all grounds as ONE conductor
Use the volume of the LARGEST ground wire

Example

A box with:

Three 14 AWG grounds (from 15A circuits)
One 12 AWG ground (from 20A circuit)

Calculation: 1 × 12 AWG volume = 1 × 2.25 = 2.25 cu.in.

Equipment Grounding Conductors vs. Bonding Jumpers

The 314.16(B)(5) allowance applies to:

✅ Equipment grounding conductors (green or bare)
✅ Bonding jumpers connected to the box

It does NOT apply to:

❌ Grounded conductors (neutral - white/gray)
❌ Grounding electrode conductors

Common Mistakes with Ground Wire Counting

Mistake 1: Counting Each Ground Separately

Wrong: 4 grounds × 2.00 = 8.00 cu.in.
Right: All grounds = 1 × 2.00 = 2.00 cu.in.

Mistake 2: Using Wrong Size for Mixed Grounds

Wrong: Using 14 AWG volume when 12 AWG is present
Right: Always use the LARGEST ground conductor's volume

Mistake 3: Forgetting the Pigtail

Ground pigtails that originate inside the box (from the ground splice to a device) do NOT add to the count. Only grounds entering from outside the box are counted.

Calculation Examples

Example 1: Simple Receptacle Box

Component	Count	Volume
14 AWG hot conductors	2	4.00 cu.in.
14 AWG neutral conductors	2	4.00 cu.in.
14 AWG grounds (2 wires)	1	2.00 cu.in.
Internal clamps	1	2.00 cu.in.
Receptacle	2×	4.00 cu.in.
Total		16.00 cu.in.

Example 2: Junction Box with Multiple Circuits

A box with 4 cables (8 conductors + 4 grounds) all 14 AWG:

8 conductors × 2.00 = 16.00 cu.in.
4 grounds = 1 × 2.00 = 2.00 cu.in.
Clamps = 1 × 2.00 = 2.00 cu.in.
Total: 20.00 cu.in.

Calculate Correctly Every Time

Our box fill calculator automatically applies the ground conductor rule correctly—just enter the number and size of grounds.

Grounding Conductors in Box Fill: NEC 314.16(B)(5) Explained