WireGaugeCalc Free calculator · NEC cited · works offline
Box Fill Calculations
box fill calculations are fast with WireGaugeCalc, since the app totals every conductor, device, and clamp volume from NEC 314.16(B) for you.
You enter the conductor count and size, the number of devices and internal clamps, then the app adds the cubic-inch allowances and compares the total to your box.
You get a clear pass or fail in seconds, so you know the box is legal before any wire goes in.
Open the calculatorFill against NEC Chapter 9, Table 1 (40% for 3+ conductors) with Table 4 and Table 5 areas. Field aid only, verify against the current NEC and your local authority having jurisdiction.
How the box fill calculator counts every conductor
Each current-carrying conductor counts as one volume allowance based on its gauge, since the larger the wire the more space it needs.
All grounding conductors together count as a single allowance, and a yoke or strap counts as two of the largest conductor connected to it.
WireGaugeCalc applies these rules automatically, so the cubic-inch total stays correct as you change the wire size.
What goes into accurate box fill calcs
The app asks for each item that takes up space inside the box:
- Hot and neutral conductors by AWG size
- Equipment grounding conductors, counted once as a group
- Internal cable clamps, counted as one allowance
- Devices such as switches and receptacles on a yoke
- Support fittings or fixture studs in the box
- The marked cubic-inch volume of the box itself
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
Do wire nuts and tape count in box fill calculations?
No, splices, wire connectors, and tape are not counted in the box fill total. Only conductors, devices, clamps, support fittings, and grounds get a volume allowance.
How are equipment grounding conductors counted?
All equipment grounds together count as a single conductor allowance based on the largest grounding wire present. Always confirm the count against the current NEC and your local authority having jurisdiction before sizing the box.
Does a receptacle count as one or two conductors?
A receptacle or switch on a yoke counts as two volume allowances of the largest conductor connected to that device, since the strap and the conductors share the space.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.