How to Calculate Electrical Box Fill: Step-by-Step Guide

Learning how to calculate electrical box fill is essential for every electrician. This step-by-step guide will walk you through the process of determining if your electrical box meets NEC 314.16 requirements.

Before You Start

Gather the following information about your installation:

• Box type and volume (or dimensions)
• Number and size of conductors entering the box
• Number and type of devices (switches, receptacles)
• Whether the box has internal clamps
• Number of equipment grounding conductors

Step 1: Determine Box Volume

Find the box volume in cubic inches. This is typically stamped on the box or can be found in NEC Table 314.16(A) for standard box types. Browse our complete box reference for volumes. Common volumes include:

• Single gang device box: 14-22 cu.in.
• 4" square × 1-1/2": 21 cu.in.
• 4" square × 2-1/8": 30.3 cu.in.
• 4-11/16" square × 2-1/8": 42 cu.in.

Step 2: Count Your Conductors

Count each conductor that enters the box from outside and terminates or splices inside. Use Table 314.16(B) to find the volume for each conductor size:

• 14 AWG: 2.00 cu.in. each
• 12 AWG: 2.25 cu.in. each
• 10 AWG: 2.50 cu.in. each

Step 3: Add Clamp Fill

If your box has one or more internal cable clamps, add one conductor volume based on the largest conductor in the box. External clamps don't count.

Step 4: Add Device Fill

For each device yoke (switch or receptacle), add TWO conductor volumes based on the largest conductor connected to that device. See our device fill guide.

Step 5: Add Ground Fill

All equipment grounding conductors together count as ONE conductor volume, based on the largest ground wire entering the box. Learn more in our grounding conductor article.

Step 6: Compare to Box Volume

Add up all the fill volumes. If the total is less than or equal to the box volume, your installation is compliant.

Related Resources

• NEC 314.16 Code Reference
• Common Box Fill Mistakes to Avoid
• Junction Box Sizing Guide

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