Box fill (NEC 2026 314.16)

In 50 minutes, your students learn to count what goes in a box and pick the smallest box that holds it under 314.16.

At a glance

Level: First-year apprentices
Time: 50 minutes, with a 90-minute variant
Sections: 314.16, 314.16(A), Table 314.16(A), 314.16(B)(1), 314.16(B)(2), 314.16(B)(3), 314.16(B)(4), 314.16(B)(5), Table 314.16(B)(4)
Students need: Their own 2026 code book and a calculator
Edition: 2026 NEC
Last checked: October 2026

Class plan by minutes

0 to 5 minWarm-upDraw a device box on the board. Ask how many 12-2 cables and one receptacle fit before it is full. Take guesses and write them down.
5 to 20 minBoard workWork one box count on the board: wires, clamps, device, grounds. Have students total it before you do.
20 to 35 minSimulationPairs play Box Fill jobs 1 to 4. Walk the room and ask each pair to say what they counted and why.
35 to 45 minQuestionsAsk the three quiz questions below, one at a time. Students write an answer alone, then you give the answer and its section. They can use the code book and a calculator.
45 to 50 minWrap-upGo over the question most students missed. Return to the warm-up guesses and check them against the count.

For a 90-minute block, give the simulation 30 minutes so pairs finish jobs 1 to 6, and add 10 minutes to go back over the three questions with the code book open.

What students learn

By the end of the period, each student can count the fill in a box, look up its volume allowances, and pick the smallest standard box that holds the count under 314.16.

314.16: A box must have room for everything in it. The box volume has to be at least the fill you calculate.
314.16(A): Box volume comes from the maker's marking, the box table, or the sum of the pieces, such as a ring or extension.
Table 314.16(A): The volume of each standard metal box, in cubic inches.
314.16(B)(1): Each wire that starts outside the box and ends in it counts once. A wire that never leaves the box counts zero.
314.16(B)(2): Internal cable clamps count once in all, at the size of the largest wire.
314.16(B)(3): Luminaire studs and hickeys count once for each type, at the size of the largest wire.
314.16(B)(4): Each device yoke counts twice, at the size of the largest wire landed on it.
314.16(B)(5): Up to four grounds count once together. Each extra ground adds a quarter.
Table 314.16(B)(4): The cubic inches each wire size takes: 2.00 for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG.

Section numbers only. Read the full text in your code book.

2023 to 2026: our list has 5 changes in Article 314. See the changes

Quiz questions

1. A 6 in. pigtail is spliced to two circuit wires inside a box and never leaves the box. How many does the pigtail count?
Answer. It counts zero, because no part of it leaves the box. (314.16(B)(1))
2. One receptacle has 12 AWG wires landed on it. How many cubic inches does its yoke add?
Answer. Two allowances at the 12 AWG size, so 4.50 cubic inches. (314.16(B)(4))
3. Six 12 AWG grounding conductors enter one box. How many cubic inches do they add?
Answer. The first four share one allowance and each of the other two adds one quarter, so 1.5 allowances, or 3.375 cubic inches. (314.16(B)(5))

Teaching notes

Before class

Students need their own 2026 code book open to Article 314 and a calculator. Open this lesson on your teaching computer before class and project it. Students can play the simulation on a phone or a laptop. If they have neither, you can project the simulation and have the class call each count.

How to use the simulation in class

Run it after the worked example, so students already know how a count works.
Pairs take jobs 1 to 4 (Receptacle box, Metal box, EMT box, Two wire sizes). Each pair says the count out loud before they pick a box.
Early finishers play jobs 5 and 6 (Junction box and Ceiling light) to practice grounds past four and a luminaire stud.
When a count is wrong, ask the pair which item they miscounted and which part of 314.16 covers it.

Two discussion questions

1. A student pulls a longer pigtail so the splice is easier. Does the fill change? What if the pigtail is left long and coiled?
Look for: A pigtail that never leaves the box counts zero (314.16(B)(1)). A loop of unbroken wire that is long enough can count twice, so students should ask how long it is.
2. Your crew swaps a metal box for a plastic box that looks the same size. What do you check before you pull wire?
Look for: The volume marked on the plastic box (314.16(A)(2)), not the metal box table. Students should name the marking as the source.

Mistakes to watch for

Counting wire connectors, locknuts, or bushings. They add nothing under 314.16(B).
Counting a pigtail that never leaves the box.
Counting each cable clamp. All the internal clamps together count once.
Using the smallest wire size for the clamp or device allowance. These use the largest wire in the box.
Forgetting the device yoke counts twice.

If a student’s book is a different edition

This lesson follows the 2026 NEC. If a student has an older book, tell them to find the same topic in their book and compare the section number, and to check what their state has adopted. Hold the final answer to the code your authority having jurisdiction enforces.

Questions people ask

How do you calculate box fill?

Add the allowances for the wires, clamps, studs, devices, and grounds (314.16(B)), then compare the total with the box volume (314.16(A)). The box volume has to be at least the fill.

How many wires can go in a box?

It depends on the box volume and the wire size. Each 14 AWG wire takes 2.00 cubic inches and each 12 AWG wire takes 2.25 (Table 314.16(B)(4)). Count the clamps, devices, and grounds too, then compare with the box volume.

Do ground wires count in box fill?

Yes. Up to four grounds count once together. Each ground past four adds a quarter (314.16(B)(5)).

Do wire nuts count in box fill?

No. Small fittings such as wire connectors, locknuts, and bushings get no allowance (314.16(B)).

Ask Shorty is independent and is not affiliated with NFPA. Check every answer against your adopted code and your authority having jurisdiction. This lesson follows the 2026 NEC and was last checked October 2026. See something wrong? Tell us and we will check it against the code.