Understanding NEC Box Fill Rules
The **Box Fill Calculation** is a safety measure mandated by the National Electrical Code (NEC Article 314.16) to prevent overheating and physical damage to conductors. A box that is too full can make it difficult to terminate wires properly and can lead to dangerous conditions.
Volume Allowance Components (NEC 314.16(B))
The total required volume is the sum of the allowances for five categories. The allowance for each category is based on the **unit volume** of the largest conductor.
Conductors ($\mathbf{C}$)
Count **all** current-carrying (hot and neutral) conductors. Each wire counts as **one allowance**.
Devices ($\mathbf{D}$)
Each yoke or strap for switches or receptacles counts as **two allowances**, regardless of the number of terminals.
Grounding Wires ($\mathbf{G}$)
All equipment grounding conductors (including pigtails) combined count as a total of **one allowance**, based on the largest ground wire size.
Clamps/Fittings ($\mathbf{S}$)
All internal cable clamps or support fittings combined count as a total of **one allowance**, regardless of the number of clamps.
Important Note:
Conductors originating and terminating within the box (like pigtails) that do not leave the box are generally **not** counted as separate conductor allowances, but the largest conductor size is determined by the size of the pigtail or other wires in the box.