Box Fill Calculator

Calculate NEC 314.16 box fill volume by entering conductors, looped conductors, internal cable clamps, device yokes, and equipment grounding conductors — with a box cross-section visualization, category breakdown, five preset scenarios, and a knowledge-check quiz.

AVAILABLEAUTOMATED CHECKS ONLYSTANDARDS-AWAREBEGINNERNECWHAT DO THESE STATUSES MEAN?

NEC • 314.16(A)/(B)/(C)5 FILL CATEGORIES5 PRESET SCENARIOSBOX CROSS-SECTION VIZ

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Overview

Box fill is one of the most routine NEC field calculations — and one of the most error-prone because 314.16(B) applies different counting rules to five distinct categories of items in the box. Conductors that enter or pass through count individually by AWG. Looped or coiled unbroken conductors with at least twice the required free-conductor length count as one allowance each. Internal cable clamps — regardless of how many are present — count as a single allowance based on the largest conductor in the box (small external fittings and locknuts do not count at all). Each device or equipment yoke counts as two allowances based on the largest conductor connected to that specific yoke. All equipment grounding conductors combined count as a single allowance based on the largest EGC. Getting any of these wrong produces an overfilled box that fails 314.16(A). This calculator tracks all five categories independently, applies the correct volume allowance from NEC Table 314.16(B) (1.50 in³ for 18 AWG through 5.00 in³ for 6 AWG), sums the total required volume, and compares it against the box's listed allowable fill. The Results tab shows a proportional Box Cross-Section visualization, a Fill Contribution Breakdown by category, and a Volume Comparison chart. The Trace tab walks through each category's calculation with NEC section citations. A single-size mode simplifies entry when all conductors are the same AWG. Special conditions include looped/coiled conductors, isolated grounding conductor sets (NEC 2017 treatment), and barrier-separated compartments. Five presets cover Receptacle, Switch with Clamp, Crowded, Undersized (intentional FAIL), and Isolated Ground (NEC 2017). The Learn tab covers the five counting rules, common mistakes, and a knowledge-check quiz. Supports NEC 2017, 2020, and 2026.

App preview

A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

Inside the lab

Example data — not a real field report

Box Fill Calculator — Overview with Crowded preset — Overview tab: enter conductor groups, EGCs, device yokes, clamps, fittings, and box volume. The Crowded preset loads a realistic multi-category scenario.

Box Fill Calculator — Results tab showing FAIL — Results tab: side-by-side Volume Comparison, Category Breakdown by cu in and %, and a plain-language Interpretation calling out that the 34 cu in box is 3.25 cu in short.

Box Fill Calculator — Calculation Trace — Calculation Trace: every fill category walked through step-by-step with the NEC 314.16(B) section cited for each rule so you can defend the number.

Box Fill Calculator — Undersized preset FAIL — Undersized preset: intentionally overfilled box demonstrating how a common receptacle install can fail 314.16(A) once clamps, yokes, and EGCs are counted correctly.

Box Fill Calculator — Iso Ground 2017 preset — Isolated Ground (NEC 2017) preset: shows the separate volume allowance for an isolated grounding conductor set in addition to the standard EGC allowance.

Box Fill Calculator — Code Logic tab — Code Logic tab: the five 314.16(B) counting rules and Table 314.16(B) volume allowances (1.50 in³ at 18 AWG up to 5.00 in³ at 6 AWG) in one reference view.

Box Fill Calculator — Learn Mode — Learn Mode: guided explanation of common counting mistakes plus a knowledge-check quiz to confirm you can apply each of the five categories correctly.

What you'll do

01Apply the five NEC 314.16(B) counting categories — conductors, clamps, support fittings, device yokes, and equipment grounding conductors — and state the volume allowance rule for each
02Look up the volume allowance per conductor from NEC Table 314.16(B) for conductor sizes 18 AWG through 6 AWG and apply it correctly by category
03Explain why multiple internal cable clamps count as only one allowance and why external locknuts and fittings do not count at all
04Determine the device yoke volume contribution for a box containing two different yoke types connected to different AWG conductors
05Explain why all equipment grounding conductors — regardless of count — are counted as a single allowance sized by the largest EGC
06Identify the NEC 2017 treatment of isolated grounding conductors as an additional allowance separate from the standard EGC allowance

What this lab is not

Not a design document, stamped calculation, or approval record.
Not a substitute for engineering analysis by a licensed professional.
Not a reproduction of NEC tables — values are computed and cited, and you must verify against the code edition enforced in your jurisdiction.
Not connected to live equipment and not a safety device.

Calculation basis

References NEC. Results are computed from the entered inputs and cited back to the referencing clause or table number — no code text or table is reproduced verbatim.

Simplifying assumptions are stated in the lab itself; where a model is intentionally conceptual, the lab says so rather than implying study-grade accuracy.

Standards basis: Standards-aware. Validation state: Automated checks only (last checked 2026-08-15).