Box and conduit fill

Why it matters

Stuff too many wires into a box and you get crushed insulation, loose splices and heat with nowhere to go. Article 314 limits how many conductors, devices and fittings a box may hold, and Chapter 9 limits how full a conduit may be. Inspectors check both at rough-in, and both are favourite exam questions because they are pure arithmetic once you know what counts.

Awareness level only. This lesson is paper and calculator practice. Installing boxes and pulling wire is done under a licensed electrician's supervision with your employer's training.

Box fill: the idea

Every box has a volume in cubic inches, either marked on the box or listed in a table of standard boxes in Article 314. Each item inside the box "uses up" a set volume, based on the size of the conductors. The total used must not exceed the box's volume.

required volume = allowance units × volume per conductor (for the size used)
box OK if: required volume ≤ box volume

The code's table gives the volume allowance per conductor for each AWG size. Look it up in Table 314.16(B) of your edition.

What counts (the counting rules)

Section 314.16(B) tells you how many "allowance units" each item uses. In plain English:

• Each conductor that starts, ends or is spliced in the box: 1 unit. A conductor that passes through without a splice: 1 unit.
• Pigtails and conductors that never leave the box: 0.
• All equipment grounding conductors together: 1 unit (based on the largest). Some editions add a quarter unit for each additional EGC beyond the first four; check your edition.
• Each device yoke (a receptacle or switch strap): 2 units, based on the largest conductor connected to it.
• All internal cable clamps together: 1 unit.
• Each support fitting (fixture stud or hickey): 1 unit.
• Small fixture wires (from a luminaire canopy) may not count; there are conditions.

When conductors of different sizes are in the box, each item uses the allowance for its own size (devices and grounds use the largest connected or present).

Worked example (illustrative values)

The volumes below are illustrative practice values; take the real ones from your code book.

A switch box marked 18.0 in³ holds:

• two 12 AWG cables, each with a black, a white and a bare ground,
• one duplex receptacle,
• internal cable clamps.

Count:

With an illustrative allowance of 2.25 in³ per 12 AWG conductor: 8 × 2.25 = 18.0 in³. It fits exactly. Add one more 12 AWG conductor and the requirement becomes 9 × 2.25 = 20.25 in³: too much, so you would need a deeper box or a box extension.

Conduit fill: the idea

Conduit fill compares the cross-sectional area of the conductors with the internal area of the raceway:

percent fill = total conductor area / conduit internal area × 100

Chapter 9 sets the maximum fill by the number of conductors. The best-known limit is 40% for more than two conductors; one conductor and two conductors have their own limits (two conductors is the lowest, because two round wires jam easily). Chapter 9 lists the area of each conductor size and insulation type, and of each trade size and type of raceway. Informative Annex C does the arithmetic for you when all conductors are the same size and type, but you must be able to do it by hand when they are mixed.

Worked example (illustrative values)

Three conductors, each with an illustrative area of 0.0133 in², in a conduit with an illustrative internal area of 0.304 in²:

total area = 3 × 0.0133 = 0.0399 in²
fill       = 0.0399 / 0.304 × 100 ≈ 13.1 %

Well under 40%, so it passes. Remember that equipment grounding conductors in the conduit do count for conduit fill, even though they do not count as current-carrying for ampacity adjustment.

Common mistakes

• Counting each ground as one unit (all of them together count as one).
• Forgetting the device yoke counts as two.
• Counting pigtails.
• Using the 40% limit for two conductors.
• Leaving equipment grounding conductors out of conduit fill.
• Using the wrong raceway type's area (EMT, PVC and rigid conduit of the same trade size have different internal areas).

Quick recap

Box fill: count allowance units (conductors 1 each, all grounds 1, each device yoke 2, all clamps 1, each support fitting 1), multiply by the per-conductor volume for the size, and compare with the box volume. Conduit fill: total conductor area divided by raceway area, compared with the Chapter 9 limit (40% for more than two conductors). Use your edition's tables (NFPA 70; checked on 2026-10-23).

Try it

Open the Do tab. The counting rules and the conduit-fill check are written; you add the box-fill volume formula.