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Box Fill Practice for the California Electrician Exam

Box fill adds up the cubic inches every conductor, device, clamp, and ground takes up, then checks it against the box. On the California exam, calculations are part of the Determination of electrical system requirements domain. Here is the pattern, one worked example, and a question to try.

Last reviewed June 2026

One worked example

A 4 by 4 by 1.5 inch square box has four 12 AWG conductors, one device, the grounds, and an internal clamp. Does it have room?

  1. 1
    Name the problem

    This is a box fill. Every conductor, device, clamp, and ground takes up cubic inches. We add them all up and compare to what the box holds.

  2. 2
    Count the conductors

    Each wire that enters and leaves counts once, sized by its own AWG (Table 314.16(B)).

    4 × 12 AWG = 9.00 cu in

    Subtotal: 9.00 cu in

  3. 3
    Add devices, grounds, and clamps

    All the grounds together count as one, sized by the largest. Each device yoke counts as two of the largest conductor. An internal clamp counts as one.

    Yokes: 1 × 2 × 2.25 = 4.50 cu in

    Grounds: 1 × 2.25 (12 AWG) = 2.25 cu in

    Clamp: 1 × 2.25 = 2.25 cu in

  4. 4
    Add it up

    That total is the volume the box has to give you.

    Required = 9.00 + 4.50 + 2.25 + 2.25

    Required = 18.00 cu in

  5. 5
    Compare to the box

    The 4 in. square × 1-1/2 in. holds 21.00 cu in. You need 18.00. The box has room. You are clear.

    Box 21.00 cu in vs needed 18.00 cu in

Now try one

Your turn. A 4-11/16 box (29.5 cu in) has four 12 AWG, two 10 AWG, one device, the grounds, and a clamp.

Add it up. What volume does the box need, in cubic inches?

  1. 1
    Add it up

    That total is the volume the box has to give you.

    Required = 14.00 + 5.00 + 2.50 + 2.50

    Required = 24.00 cu in

  2. 2
    Compare to the box

    The 4-11/16 in. square × 1-1/2 in. holds 29.50 cu in. You need 24.00. The box has room. You are clear.

    Box 29.50 cu in vs needed 24.00 cu in

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