Junction Box Sizing Calculator
Determine minimum junction box length and height from conduit sizes for straight and angle wire pulls per NEC Article 314.28.
What Is a Junction Box Sizing Calculator?
A junction box sizing calculator determines minimum pull box or junction box dimensions based on conduit trade sizes and wire pull direction. It applies NEC Article 314.28 rules for straight pulls and angle pulls so conductors can be pulled without sharp bends.
How Junction Box Dimensions Are Calculated
For straight pulls, the box dimension in the pull direction must be at least eight times the largest conduit trade size:
$$ L_{min} \geq 8 \times t_{max} $$
For angle pulls, the dimension parallel to the wire path must satisfy:
$$ L_{min} \geq 6 \times t_{max} + t_1 + t_2 + \ldots + t_n $$
Calculate horizontal minimum from left/right straight and angle pulls, and vertical minimum from top/bottom pulls. Use the largest result for each box dimension.
For wire sizing and voltage drop, see the Voltage Drop Calculator.
Frequently Asked Questions
What is a straight pull in a junction box?
A straight pull runs wires through the box in one direction, such as from a left conduit to a right conduit without turning inside the box. NEC requires at least 8 times the largest conduit size for that direction.
What is an angle pull?
An angle pull turns wires inside the box, for example from the left side to the bottom. NEC requires 6 times the largest conduit on that side plus the sum of other conduits in the same row.
Does this calculator replace a licensed electrician?
No. Results are for planning and estimation. Always follow the latest NEC edition, local amendments, and professional guidance for installed electrical work.
Which NEC article covers pull box sizing?
NFPA 70 National Electrical Code Article 314.28 covers pull and junction boxes and conduit bodies. This tool follows the straight and angle pull formulas from that article.
What is the smallest standard junction box?
A common minimum is 4 by 4 inches when 1/2 inch conduits are pulled straight in both directions, because 0.5 inch times 8 equals 4 inches per NEC straight-pull rules.
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