📦 Box Fill Calculator
Calculate electrical box fill capacity
Standard: 15, 20, 30 cu in
How to Use This Calculator
Enter Box Size
Input box volume in cubic inches. Standard sizes: 15, 20, 30 cubic inches. Select box type (standard, octagonal, round).
Enter Wire Counts
Enter number of wires by size (14 AWG, 12 AWG, 10 AWG, 8 AWG). Each wire size has different fill requirements per NEC.
Enter Devices and Grounds
Enter number of devices (switches, outlets) and ground wires. Each device = 2 cu in, each ground = 1 cu in.
Calculate and Review
Click "Calculate Box Fill" to see total fill, fill percentage, and whether box is within NEC limits (75% max fill).
Formula
Fill (cu in) = Σ(Wires × Fill per Wire) + (Devices × 2) + (Grounds × 1)
Wire Fill: 14 AWG = 2.0, 12 AWG = 2.25, 10 AWG = 2.5, 8 AWG = 3.0 cu in
Maximum Fill = Box Volume × 0.75 (75% per NEC)
Example 1: 30 cu in box, 6×14 AWG, 2 devices, 1 ground
Step 1: Wire fill = 6 × 2.0 = 12.0 cu in
Step 2: Device fill = 2 × 2 = 4.0 cu in
Step 3: Ground fill = 1 × 1 = 1.0 cu in
Step 4: Total fill = 12.0 + 4.0 + 1.0 = 17.0 cu in
Step 5: Fill % = 17.0 / 30 × 100 = 56.7% (within 75% limit) ✓
Example 2: 20 cu in box, 8×12 AWG, 1 device, 2 grounds
Step 1: Wire fill = 8 × 2.25 = 18.0 cu in
Step 2: Device fill = 1 × 2 = 2.0 cu in
Step 3: Ground fill = 2 × 1 = 2.0 cu in
Step 4: Total fill = 18.0 + 2.0 + 2.0 = 22.0 cu in
Step 5: Fill % = 22.0 / 20 × 100 = 110% (exceeds 75% limit) ✗
About Box Fill Calculator
The Box Fill Calculator is an essential tool for electricians, electrical engineers, and contractors who need to calculate electrical box fill capacity for NEC (National Electrical Code) compliance. This calculator implements standard box fill formulas based on wire sizes, devices, and ground wires, helping you ensure proper box sizing and code compliance.
When to Use This Calculator
- Electrical Design: Calculate box fill for electrical installations
- Code Compliance: Verify NEC box fill requirements
- Box Sizing: Determine appropriate box size for wiring
- Safety Analysis: Ensure adequate space for safe connections
- Educational Use: Learn and understand box fill calculations
Why Use Our Calculator?
- ✅ NEC Compliant: Uses standard NEC box fill requirements
- ✅ Multiple Wire Sizes: Supports 14, 12, 10, and 8 AWG wires
- ✅ Complete Analysis: Shows fill breakdown, percentage, and compliance
- ✅ Easy to Use: Simple inputs for wires, devices, and grounds
- ✅ Time Savings: Instant calculations eliminate manual math
Understanding Box Fill
NEC Requirements: The National Electrical Code (NEC) limits box fill to 75% of box volume to ensure adequate space for wires, connections, and heat dissipation. Box fill is calculated based on wire sizes, devices (switches, outlets), and ground wires.
Wire Fill: Each wire size has specific fill requirements: 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in, 8 AWG = 3.0 cu in. Larger wires require more space. Fill is calculated as number of wires × fill per wire.
Device and Ground Fill: Each device (switch, outlet) = 2 cubic inches, each ground wire = 1 cubic inch. Devices require extra space for connections and heat dissipation.
Tips for Best Results
- Accurate Counts: Count all wires, devices, and grounds accurately
- Check Box Size: Verify box volume from manufacturer specifications
- Stay Within Limits: Keep fill below 75% of box volume for NEC compliance
- Account for All Wires: Include all conductors entering the box
- Verify with NEC: Check local NEC requirements for specific applications
Frequently Asked Questions
How do I calculate box fill?
Box fill = Σ(Wires × Fill per Wire) + (Devices × 2) + (Grounds × 1). For example, 6×14 AWG wires (12 cu in), 2 devices (4 cu in), 1 ground (1 cu in) = 17 cu in total fill. The calculator does this automatically.
What is the maximum box fill?
Per NEC, maximum box fill is 75% of box volume. For example, a 30 cu in box has maximum fill of 22.5 cu in. Exceeding this limit violates code and can cause overheating and connection failures.
How much space does each wire size require?
Wire fill per wire: 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in, 8 AWG = 3.0 cu in. Larger wires require more space. Fill is multiplied by number of wires of each size.
Do I count all wires in the box?
Yes, count all conductors entering the box, including hot wires, neutral wires, and ground wires. Each wire size has specific fill requirements. Devices and grounds have separate fill requirements.
What if box fill exceeds the limit?
If box fill exceeds 75% limit, use a larger box or reduce number of wires/devices. Exceeding limits violates NEC code and can cause overheating, connection failures, and fire hazards. Always stay within code limits.