Question 1

How do I calculate box fill under NEC Section 314.16?

Accepted Answer

NEC 314.16(B) box fill is calculated by counting volume allowances: each current-carrying conductor in the box counts once; conductors passing through without splice or termination count once; a single volume allowance for all grounding conductors combined (based on the largest grounding conductor); one allowance per yoke for devices; and one allowance per internal cable clamp assembly. Multiply the conductor count by the volume per conductor from Table 314.16(B)(1) — for example, #12 AWG = 2.25 in³ per conductor — and compare the total to the box volume from Table 314.16(A).

Question 2

Do pigtails count toward NEC box fill calculations?

Accepted Answer

Per NEC 314.16(B)(1), conductors that originate and terminate within the box — such as pigtails — are not counted in the box fill calculation. Only conductors entering the box from outside are counted, along with device and clamp allowances.

Question 3

What is the volume allowance for a device like a receptacle or switch under NEC 314.16?

Accepted Answer

NEC 314.16(B)(4) requires a double-volume allowance (based on the largest conductor connected to the device) for each yoke or strap containing one or more devices. For #12 AWG conductors, a single receptacle requires 2 × 2.25 = 4.5 in³. A duplex receptacle on one yoke still counts as one device volume allowance.

Box Description	Volume (in³)
4×1-1/4 square	18.0
4×1-1/2 square	21.0
4×2-1/8 square	30.3
4×4×1-1/2 square(4-11/16 in. square)	42.0
3×2×2 device	10.5
3×2×2-1/4 device	12.5
3×2×2-1/2 device	14.0
3×2×2-3/4 device	15.5
3×2×3-1/2 device	18.0

Conductor Size	Volume per Conductor (in³)
#18 AWG	1.50
#16 AWG	1.75
#14 AWG	2.00
#12 AWG	2.25
#10 AWG	2.50
#8 AWG	3.00
#6 AWG	5.00

Table 314.16(A) — Standard Metal Box Volumes

Table 314.16(B)(1) — Volume per Conductor