How do you calculate box fill under NEC 314.16?

Count the conductors and allowances in the box, multiply each allowance by the cubic-inch value from Table 314.16(B), then compare the required volume to the box volume. The common counts are insulated conductors, device yokes, equipment grounds, internal clamps, and support fittings.

How many times do equipment grounds count for box fill?

All equipment grounding conductors together count as one allowance, based on the largest equipment grounding conductor in the box. Do not count every ground wire separately.

How does a device yoke count in box fill?

Each device yoke counts as two conductor allowances, based on the largest conductor connected to that yoke. A duplex receptacle on one yoke still counts as two allowances, not one.

Do pigtails count in box fill?

A conductor that starts and ends inside the same box does not count as a box-fill conductor. A conductor that enters from outside and is spliced, terminated, or passes through does count.

Should I use a box fill calculator or learn the table?

Use the calculator to check your work, but learn the count. The exam tests whether you know which items count and which Table 314.16(B) value to use.

NEC 314.16 | box fill

NEC Box Fill Calculations

Box fill is not hard because the arithmetic is hard. It is hard because the exam hides the count. Get the count wrong and the calculator gives you a clean wrong answer.

Last reviewed July 2026

The rule family

Box fill lives in NEC 314.16. The setup is simple once you stop treating it like a random math problem. You are not solving for a mystery formula. You are asking one question: how many conductor allowances are inside this box?

If you want the number checked after you count it, use the free box fill calculator. If you are sitting for the exam, learn the count first. That is where the points are lost.

The count that actually matters

Each conductor that enters the box and is spliced, terminated, or passes through counts as one allowance.
A pigtail that starts and ends inside the box does not count.
Each device yoke counts as two allowances based on the largest conductor connected to it.
All equipment grounding conductors together count as one allowance.
An internal cable clamp counts as one allowance. An external clamp does not take box volume the same way.
A support fitting inside the box counts as one allowance.

Common Table 314.16(B) values

These are the values most exam questions use. Confirm the exact row in your own NEC on test day.

14 AWG: 2.0 cubic inches
12 AWG: 2.25 cubic inches
10 AWG: 2.5 cubic inches
8 AWG: 3.0 cubic inches
6 AWG: 5.0 cubic inches

Worked example: 4 in. square box with one receptacle

A 4 in. square box has 21.0 cubic inches marked. Inside the box are four 12 AWG insulated conductors, one duplex receptacle on one yoke, two 12 AWG equipment grounding conductors, and one internal clamp. Is the box large enough?

Insulated conductors: 4 allowances x 2.25 = 9.00 cubic inches.
Device yoke: 2 allowances x 2.25 = 4.50 cubic inches.
Equipment grounds: 1 allowance x 2.25 = 2.25 cubic inches.
Internal clamp: 1 allowance x 2.25 = 2.25 cubic inches.
Total required volume: 18.00 cubic inches.
Box volume: 21.00 cubic inches. The box is large enough.

Wrong answers the exam wants you to pick

Counting every ground

Five grounds still count as one allowance. The count is based on the largest equipment grounding conductor in the box.

Forgetting the yoke

A yoke counts as two allowances. This is the detail that makes a box pass or fail in many device-box questions.

Counting pigtails

A pigtail starts and ends inside the same box. It does not enter from outside the box, so it does not add a box-fill allowance.

How to drill this for the exam

Run the count without a calculator first. Say the categories out loud: conductors, yokes, grounds, clamps. Then use the calculator to check the cubic inches. If your miss keeps coming from the count, more arithmetic will not fix it. You need more box-fill reps.

Find out if box fill is actually your weak spot

The free diagnostic shows whether box fill, conduit fill, motors, grounding, or another NEC area is costing points.

Take the free diagnostic Open the box fill calculator