NEC Box Fill Table 314.16
The two tables behind every box fill check: the volume each conductor size demands per Table 314.16(B), and the volumes of the standard metal boxes per Table 314.16(A).
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| Conductor | Volume |
|---|---|
| 18 AWG | 1.50 in³ |
| 16 AWG | 1.75 in³ |
| 14 AWG | 2.00 in³ |
| 12 AWG | 2.25 in³ |
| 10 AWG | 2.50 in³ |
| 8 AWG | 3.00 in³ |
| 6 AWG | 5.00 in³ |
NEC Table 314.16(B). Volume allowance per conductor. Each device or yoke counts as two allowances of the largest conductor connected to it · 314.16(B)(4). Up to four equipment grounding conductors count together as one allowance of the largest EGC, each EGC beyond four adds a quarter allowance, and every isolated EGC set per 250.146(D) adds one more · 314.16(B)(5).
| Box, trade size in inches | Volume |
|---|---|
| 3×2×1½ device | 7.5 in³ |
| 3×2×2 device | 10.0 in³ |
| 3×2×2¼ device | 10.5 in³ |
| 3×2×2½ device | 12.5 in³ |
| 3×2×2¾ device | 14.0 in³ |
| 3×2×3½ device | 18.0 in³ |
| 4×1¼ round/octagon | 12.5 in³ |
| 4×1½ round/octagon | 15.5 in³ |
| 4×2⅛ round/octagon | 21.5 in³ |
| 4×1¼ square | 18.0 in³ |
| 4×1½ square | 21.0 in³ |
| 4×2⅛ square | 30.3 in³ |
| 4¹¹⁄₁₆×1¼ square | 25.5 in³ |
| 4¹¹⁄₁₆×1½ square | 29.5 in³ |
| 4¹¹⁄₁₆×2⅛ square | 42.0 in³ |
Common device, round or octagon and square boxes from NEC Table 314.16(A), minimum volumes in cubic inches. For a box marked with its capacity in cubic inches, use the marked volume.
How the count works
Box fill is bookkeeping. Each conductor that originates outside the box and terminates or is spliced inside counts once at the volume for its size. A conductor passing through unbroken also counts once. Pigtails made up entirely inside the box do not count at all. Then come the hardware allowances: every device or yoke adds two allowances at the largest conductor connected to it · 314.16(B)(4). All internal cable clamps together add one allowance at the largest conductor in the box. All equipment grounding conductors together add one allowance at the largest EGC, with a quarter allowance for each EGC beyond four, and each isolated EGC set adds one more · 314.16(B)(5), 250.146(D).
Comparing against the box
Total the allowances in cubic inches and compare against the box volume. For a standard metal box that is the Table 314.16(A) volume listed above. For any box marked with its capacity in cubic inches, nonmetallic boxes included, use the marked volume. If the requirement beats the box, the fix is a deeper box, a bigger box or a mud ring that adds volume.
Do the count once, not twice
The free box fill calculator itemizes every allowance line by line from these same tables, tells you whether your box fits, and names the smallest standard box that does when it does not.
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