WireGaugeCalc Free calculator · NEC cited · works offline
Junction Box Size Calculator
Junction box size calculator is simple with WireGaugeCalc, since it adds the volume each conductor, device, and clamp takes up per NEC 314.16(B).
You enter the conductor sizes and counts, and the electrical junction box calculator returns the cubic inches the box must hold.
So you can confirm the box is big enough before you button it up.
Open the calculatorConductor volume per NEC 314.16(B). Add device, clamp, and ground allowances for the final count. Field aid only, verify against the current NEC and your local authority having jurisdiction.
How This Junction Box Calculator Counts Volume
Each conductor counts as a volume based on its size, so a box full of 12 AWG needs more cubic inches than the same count of 14 AWG.
Since clamps, devices, and grounds add their own allowances, the electrical junction box size calculator totals them all and compares the result to the box volume.
What This Electrical Box Sizing Calculator Uses
A few inputs set the required box volume:
- Conductor sizes and counts entering the box
- Largest conductor volume used for internal clamps
- Two-volume allowance per yoke or device
- One conductor volume for all equipment grounds together
- Conductors that pass through versus those that terminate
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
How do I count grounds in a junction box?
All equipment grounding conductors together count as a single conductor volume based on the largest ground present. The electrical junction box calculator applies that single allowance automatically.
Do pigtails count toward box fill?
A conductor that begins and ends inside the box, like a pigtail, is not counted, while each conductor that enters and terminates is. Verify your total against the current NEC 314.16(B) and your local authority having jurisdiction.
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.