BoxFillCalculator.com vs Southwire

Definitions and review context

A box fill calculator is a digital tool that converts conductor counts, device counts, grounding allowances, and fittings into a cubic-inch compliance check. The table below compares resource types and access methods for Southwire and BoxFillCalculator.com.

Box fill refers to the enclosure-volume accounting method in NEC 314.16. The practical difference between calculators is rarely just the final number; it is whether the tool shows the assumptions behind that number, including how it treats internal clamps, device yokes, grounding conductors, and conductor size selection.

Author: Jack Shi is the editor of Box Fill Calculator. He researches each guide against the published text of NEC Article 314.16 and cross-checks every conductor-count and box-volume figure so the guidance stays accurate and practical for electricians, inspectors, and DIY installers.

• National Electrical Code for the governing box-fill rules.
• Junction box for the physical enclosure problem calculators are modeling.
• American wire gauge for conductor-size references that change allowance values.

Southwire box fill or Southwire conduit fill: they answer different questions

People who search for a "Southwire box fill calculator" are often looking for conduit fill, because Southwire's calculator list is built around conduit and cable rather than boxes. On the Southwire calculators page we checked in October 2026, the tools listed include a Re3 Conduit Fill Calculator (conduit fill percentage per NEC guidelines), a Re3 Cable Pulling Calculator (conduit size, fill percentage, pulling tension, sidewall pressure and jam probability) and a Re3 Voltage Drop Calculator. We did not find a box fill calculator on that page. Check the page for its current list.

Box fill and conduit fill are different calculations under different parts of the NEC. Box fill (Section 314.16) counts volume in cubic inches: every conductor, device, clamp and fitting takes an allowance, and the total must fit in the box volume. Conduit fill (NEC Chapter 9, Table 1) compares the total cross-sectional area of the conductors with a percentage of the conduit area: 53% for one conductor, 31% for two and 40% for three or more.

A box fill example with the numbers shown

The numbers below were run through the same engine as our calculator. Assume a 21.0 cubic inch box and 12 AWG conductors, which carry a 2.25 cubic inch allowance each.

• A device counts twice the largest conductor connected to it. Internal clamps count once in total, and all equipment grounding conductors count once in total.
• These are example inputs. Your box volume comes from the volume marked on the box or from the NEC table for its dimensions, and the conductor sizes and fittings come from your installation.

A conduit fill example from our tool

Three 12 AWG THHN conductors in 1/2 inch EMT fill about 13.1% of the conduit, well under the 40% limit for three or more conductors. For a mixed set, such as three hot conductors plus a smaller ground, the conduit fill calculator adds each conductor area and names the smallest listed size that fits. See the conduit size calculator on the conduit fill calculator page for the method.

Which tool to use

• Use a box fill calculator when the question is whether a box is big enough. Use a conduit fill calculator when the question is conduit size.
• Use Southwire's calculators for cable pulling, voltage drop and Southwire product selection, which are outside what this site does.
• Whichever tool you use, confirm the result against the NEC edition and any local amendments your inspector enforces. A calculator is an aid to the code, not a replacement for it.

Related tools and guides

• Box fill calculator: check a box against NEC 314.16.
• Conduit fill calculator: fill percentages and the smallest conduit for a conductor set.
• Box fill chart: box capacities and conductor allowances.
• NEC code reference: the rules behind both calculations.

Frequently asked questions