SP-PH NC Electrical Prep

Calculators › Box Fill

Box Fill Calculation

Box fill answers one question: is this box big enough for everything inside it? You add up volume allowances for the conductors, devices, clamps, and grounds, then compare to the box's volume. Everything comes from NEC 314.16.

The five things you count (314.16(B))

(B)(1) ConductorsEach conductor entering/leaving = 1. A conductor that starts and ends inside the box = 0.
(B)(2) ClampsOne or more internal cable clamps = 1 (largest conductor in box).
(B)(3) Support fittingsEach fixture stud or hickey = 1 (largest conductor).
(B)(4) Device yokesEach strap (switch/receptacle) = 2 (largest conductor connected to it).
(B)(5) GroundsAll equipment grounding conductors together = 1 (largest). Add 1/4 each beyond four.

Volume per conductor (Table 314.16(B))

14 AWG2.00 in³
12 AWG2.25 in³
10 AWG2.50 in³
8 AWG3.00 in³
6 AWG5.00 in³

Example 1 — Receptacle box, two 12-2 cables

A device box has two 12-2 w/ground NM cables (internal clamps) and one duplex receptacle. All conductors are 12 AWG. What minimum box volume is required?

  1. Conductors (B)(1): 2 hots + 2 neutrals = 4 conductor allowances.
  2. Clamps (B)(2): internal clamps present = 1.
  3. Device yoke (B)(4): one receptacle = 2.
  4. Grounds (B)(5): both EGCs together = 1.
  5. Total allowances: 4 + 1 + 2 + 1 = 8.
  6. Multiply by 12 AWG volume (2.25 in³):
8 × 2.25 in³ = 18.0 in³ minimum

A 4" × 1½" square box (21.0 in³ per Table 314.16(A)) is big enough; a shallower box may not be.

Box must be at least 18.0 in³

Example 2 — Mixed wire sizes

A box has a 12-2 cable and a 10-2 cable (both with grounds, internal clamps) feeding a single 10 AWG-connected device. Count carefully — the largest conductor sets the clamp, device, and ground allowances.

  1. Conductors: 2 × 12 AWG (2.25 each) + 2 × 10 AWG (2.50 each).
  2. Clamp (B)(2): 1 × largest = 10 AWG = 2.50.
  3. Device (B)(4): 2 × largest connected (10 AWG) = 2 × 2.50 = 5.00.
  4. Grounds (B)(5): 1 × largest (10 AWG) = 2.50.
(2 × 2.25) + (2 × 2.50) + 2.50 + 5.00 + 2.50 = 4.50 + 5.00 + 2.50 + 5.00 + 2.50 = 19.5 in³ minimum
Box must be at least 19.5 in³
Exam tip: the classic trap is counting a device as 1 instead of 2, or counting each ground separately. Memorize: device = 2, all grounds = 1, all clamps = 1.
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