Box Fill Chart
Use this quick lookup chart to compare common box volumes, conductor-only capacity, and practical box choices before you run the full NEC 314.16 calculation.
What This Box Fill Chart Is Good For
This page is a fast planning reference for electricians, estimators, and contractors who want to compare common box sizes before they open the full calculator. It shows conductor-only capacity by box volume so you can rule out boxes that are obviously too small.
It is not a substitute for a full NEC 314.16 calculation. Devices, internal clamps, support fittings, grounding conductors, and terminal blocks can all reduce the real capacity of a box, which is why this chart works best as a quick screening tool.
Quick Capacity Chart
The counts below are conductor-only reference values. Real installations usually need a larger box once devices, grounds, clamps, or splices are included.
| Box Type | Volume | 14 AWG | 12 AWG | 10 AWG | Best Use |
|---|---|---|---|---|---|
| Handy Box 1-7/8" Deep 4" x 2-1/8" x 1-7/8" | 13.0 cu.in. | 6 | 5 | 5 | Surface mount devices |
| Single Gang Device Box 18 cu.in. 3" x 2" x 3-1/2" | 18.0 cu.in. | 9 | 8 | 7 | Light switches |
| 2-Gang Device Box 2-3/4" Deep 4" x 3" x 2-3/4" | 28.0 cu.in. | 14 | 12 | 11 | Dual switches |
| 4" Square x 1-1/4" Deep 4" x 4" x 1-1/4" | 18.0 cu.in. | 9 | 8 | 7 | Junction boxes |
| 4" Square x 1-1/2" Deep 4" x 4" x 1-1/2" | 21.0 cu.in. | 10 | 9 | 8 | Junction boxes |
| 4" Square x 2-1/8" Deep 4" x 4" x 2-1/8" | 30.3 cu.in. | 15 | 13 | 12 | High-capacity junction |
| 4-11/16" Square x 1-1/2" Deep 4-11/16" x 4-11/16" x 1-1/2" | 29.5 cu.in. | 14 | 13 | 11 | Commercial installations |
| 4-11/16" Square x 2-1/8" Deep 4-11/16" x 4-11/16" x 2-1/8" | 42.0 cu.in. | 21 | 18 | 16 | High-capacity commercial |
Volume Comparison
These bars make it easier to compare how fast box volume grows when you move from a single-gang device box to square and large-format junction boxes.
Handy Box 1-7/8" Deep
4" x 2-1/8" x 1-7/8"
14 AWG
6 max
12 AWG
5 max
10 AWG
5 max
Single Gang Device Box 18 cu.in.
3" x 2" x 3-1/2"
14 AWG
9 max
12 AWG
8 max
10 AWG
7 max
2-Gang Device Box 2-3/4" Deep
4" x 3" x 2-3/4"
14 AWG
14 max
12 AWG
12 max
10 AWG
11 max
4" Square x 1-1/4" Deep
4" x 4" x 1-1/4"
14 AWG
9 max
12 AWG
8 max
10 AWG
7 max
4" Square x 1-1/2" Deep
4" x 4" x 1-1/2"
14 AWG
10 max
12 AWG
9 max
10 AWG
8 max
4" Square x 2-1/8" Deep
4" x 4" x 2-1/8"
14 AWG
15 max
12 AWG
13 max
10 AWG
12 max
4-11/16" Square x 1-1/2" Deep
4-11/16" x 4-11/16" x 1-1/2"
14 AWG
14 max
12 AWG
13 max
10 AWG
11 max
4-11/16" Square x 2-1/8" Deep
4-11/16" x 4-11/16" x 2-1/8"
14 AWG
21 max
12 AWG
18 max
10 AWG
16 max
Fast Field Picks
Single switch or receptacle box
Typical quick-check target: around 14.0 to 18.0 cu.in. depending on conductor size and device fill.
Practical choice: 18 cu.in. single-gang device box
It gives more room than a shallow box when the location has feed-through conductors, a GFCI, or a smart device body.
Feed-through GFCI or smart receptacle
Typical quick-check target: around 15.75 to 18.0 cu.in. on 12 AWG layouts.
Practical choice: 18 cu.in. single-gang box or deeper
A GFCI yoke, grounding bundle, and onward cable quickly consume the reserve that smaller one-gang boxes appear to offer.
Standard junction box with multiple splices
Typical quick-check target: around 18.0 to 21.0 cu.in. for common 12 AWG splice layouts.
Practical choice: 4-inch square x 1-1/2-inch deep box
The 21.0 cu.in. square box is a reliable baseline when a box is doing real splice work instead of only supporting one device.
Large junction, feeder splice, or crowded transition box
Typical quick-check target: 30.3 cu.in. and up once 10 AWG, 8 AWG, or multiple circuits are involved.
Practical choice: 4-inch square x 2-1/8-inch deep or 4-11/16-inch square box
Larger boxes leave reserve for bending space, connector bulk, and future re-termination instead of forcing an exact-limit install.
Important Notes Before You Use the Chart
- This chart is based on conductor volume allowances and box volume only. Device yokes, internal clamps, support fittings, grounds, and terminal blocks can change the final answer.
- The 14 AWG and 12 AWG counts shown in the table are conductor-only capacities from the marked box volume. They are not permission to fill a working box with that many conductors plus devices.
- 10 AWG counts on this page are quick reference values calculated from box volume at 2.50 cu.in. per conductor. Always verify the exact conductor mix and fittings in the real box.
- Use this chart to narrow your box choice fast, then use the main box fill calculator to confirm compliance on the actual layout.
Box Fill Chart FAQ
Is this box fill chart enough to prove NEC compliance?
No. It is a quick planning chart, not a final compliance tool. NEC 314.16 requires you to count the real conductors, grounds, clamps, devices, and other fittings in the actual box.
Why does a box that looks large still fail box fill?
Because legal volume is consumed by more than insulated conductors. Device yokes, grounding conductors, internal clamps, support fittings, and larger conductor sizes can erase the apparent reserve quickly.
Can I use the conductor-only maximum shown in the chart for a receptacle box?
Not usually. A receptacle or switch yoke adds two conductor allowances by itself, and feed-through conductors, grounds, and clamps also consume box volume. The chart is only a first-pass screen.
What should I open after I use this chart?
Use the main Box Fill Calculator for the exact count, then cross-check conductor allowances in the Wire Gauge Chart and code rules in the NEC Code Reference.
Use the Chart for Speed, Then Run the Exact Count
The chart helps you reject obviously undersized boxes in seconds. Use the full calculator before rough-in is finalized so conductor size, yokes, grounds, clamps, and other allowances are all counted correctly.
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