WireGaugeCalc Free calculator · NEC cited · works offline
Calculating Box Fill Plastic Box
Calculating box fill plastic box is simple with WireGaugeCalc.
Enter the conductor sizes, devices, and clamps, and it adds up the cubic inches using the volume allowances in NEC 314.16(B).
You compare the total to the volume marked on the plastic box and confirm it fits.
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 Plastic Box Volume Adds Up
Each conductor takes a set volume, so the count and size of wires decide how much space the box needs.
A plastic box carries its usable volume molded into the wall, since nonmetallic boxes are not listed in the metal box table.
The tool sums every allowance so you can read the required cubic inches before you pull wire.
What Sets The Minimum Conductors Box Fill
The calculator counts each item that uses volume in the box:
- Each current-carrying conductor by its 314.16(B) volume
- One allowance for all internal clamps
- Two conductor volumes per yoke or strap
- One conductor volume for all grounds together
- The largest conductor size used for device counts
- Total compared to the molded plastic box volume
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 find the volume of a plastic box?
Nonmetallic boxes are marked with their usable volume in cubic inches, usually inside the box. Use that marked number rather than the metal box table when checking your fill.
What counts toward the minimum conductors box fill?
Count every conductor entering the box, plus two conductor volumes for each device strap and one for all clamps and one for all grounds. Compare the total to the marked volume and confirm against the current NEC and your local authority having jurisdiction.
Do equipment grounds use volume in a plastic box?
All equipment grounding conductors together count as a single conductor volume based on the largest ground present. Extra grounds beyond the first do not add more volume.
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.