How the box fill calculator works
Group entering, passing, loop and entirely internal conductors by size. Then add clamps, each support type, device mounting allowances, entering grounds and terminal-block assemblies. Internal pigtails contribute to the largest physically present wire used for clamps/supports but do not add a conductor volume.
Required in³ = entering/passing + loop + clamp + support + device + grounding + terminal-block allowances. A device uses 2 × mounting gangs × largest connected conductor allowance per yoke. Entering grounds use one allowance for up to four, plus 0.25 for each additional ground.
Worked example
Four entering #12 conductors use 9.00 in³. One internal clamp adds 2.25 in³; one single-gang receptacle yoke adds 4.50 in³; two entering #12 grounds add 2.25 in³. Total = 18.00 in³. A verified 4 × 1-1/4 in square metal box provides 18.00 in³, so it passes exactly. The suggested preset can differ because suggestions sort all supported shapes by volume and identifier.
More original examples
Three entering #12 conductors without devices or internal hardware require 3 × 2.25 = 6.75 in³. Six entering #12 grounds require [1 + 2 × 0.25] × 2.25 = 3.375 in³; with four entering #12 circuit conductors, the combined allowance is 12.375 in³ before devices or hardware.
NEC references
NEC 2023: 314.16(A); 314.16(B)(1); 314.16(B)(2); 314.16(B)(3); 314.16(B)(4); 314.16(B)(5); 314.16(B)(6); 300.14. These are reference identifiers, not reproduced code text. Read our methodology and verification limits.
For product-specific evidence, consult Southwire manufacturer resources and Hubbell RACO box documentation. We do not publish a full reference table.
Limitations
Only ordinary, unpartitioned boxes and AWG 18 through 6. No 4 AWG or larger, conduit bodies, fixture-canopy exceptions, special integrated assemblies or unconfirmed extension-ring volume. Verify marked volume, physical depth, conductor bends and device listing; a volume comparison alone does not prove installation suitability.
For estimation only. Results compare loads under the selected NEC method; they do not guarantee permit approval or code compliance. Verify with your local adopted code and a licensed electrician.
FAQ
Do internal pigtails count?
Entirely internal pigtails have zero conductor allowance here, but their size affects the largest physical conductor for internal clamp/support allowances.
How do six grounding conductors count?
The first four share one allowance; two more add a half allowance. Use the largest entering ground size.
Is a duplex receptacle two devices?
A duplex receptacle on one strap is one yoke. Enter actual mounting gangs for unusually wide devices.
What about an unbroken loop?
In this supported six-inch free-length model, a loop at least 12 inches long receives two conductor allowances. Shorter loops receive one.
Can I add extension-ring volume?
Only enter independently verified usable marked volume. Unknown additional volume is not accepted.
Why is a different-shaped box suggested?
Suggestions choose the first supported preset by volume and identifier, not the smallest product on the market or a physically equivalent shape.
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