The calculation, without hidden assumptions
Size the checked pull-box dimension from raceway geometry instead of guessing from a stock enclosure. This calculator separates straight pulls from angle/U/splice geometry, shows the substituted arithmetic, checks same-conductor entry separation when supplied, and can compare the result with a proposed dimension.
How to use this calculator
Confirm the box contains conductors 4 AWG or larger and that NEC 314.28 is the applicable dimensional sizing rule for the installation.
Choose straight, angle, or U-pull/splice for the wall and row being checked.
Enter the largest raceway trade size. For angle/U/splice checks, add the trade-size sum of the other raceways in that same wall and row.
If the same conductors enter and leave through another raceway, enter that paired raceway size to check the minimum edge-to-edge entry separation.
Optionally enter the proposed box dimension. Repeat the calculation for every wall/row and axis; the enclosure must satisfy every governing requirement.
Where people use it
- •Straight feeder pull-box sizing.
- •90-degree angle pulls with multiple conduits in one row.
- •U-pulls returning through the same wall.
- •Junction-box planning where large conductors are spliced.
- •Checking a proposed enclosure dimension before procurement.
Example: 3-inch angle pull with two 2-inch raceways
For a row containing one 3 in raceway and two additional 2 in raceways, the wall requirement is 6×3 + 2 + 2 = 22 in. If the same conductors leave through a 3 in raceway, entry separation is 6×3 = 18 in, so the 22 in wall requirement governs the checked dimension.
What the result does not assume
- •This owner is a planning calculator for NEC 314.28 geometry. The adopted NEC edition, local amendments and authority having jurisdiction control the installation.
- •The 314.28 dimensional method is distinct from the 314.16 volume-based box-fill calculation used for smaller conductors; do not substitute one calculation for the other.
- •Calculate every applicable wall, row and axis. A single result does not prove the complete enclosure layout complies.
- •Raceway trade size drives these multipliers; do not substitute conductor diameter or outside raceway diameter.
- •The same-conductor separation check is meaningful only when the paired entry/exit raceways actually enclose the same conductors.
- •This calculation does not select enclosure depth, environmental/NEMA rating, accessibility, grounding/bonding, conductor ampacity, bending space under other applicable rules, or a commercially available box size.
Frequently asked questions
What is the 8-times rule for a pull box?+
For a straight pull under NEC 314.28(A)(1), the box dimension in the direction of the pull is at least eight times the trade size of the largest raceway.
How do I size an angle pull box?+
For the wall and row being checked, use six times the largest raceway trade size plus the sum of the other raceway trade sizes in that same row. Check each applicable wall and keep the governing dimension.
Does this calculator handle U-pulls?+
Yes. U-pulls use the angle/U dimensional method, and you should also check the required separation between raceway entries carrying the same conductors.
Can I use this instead of a box-fill calculator?+
No. NEC 314.16 volume fill and NEC 314.28 pull/junction-box dimensions solve different sizing problems. MAXScanner keeps them as separate canonical owners.
Why is the paired-raceway input separate?+
It lets the result check the same-conductor entry-to-entry separation independently from the wall-to-opposite-wall requirement, then show which value governs the checked dimension.
Does a passing proposed dimension prove the whole box complies?+
No. It proves only the entered geometry check. Repeat for all walls, rows and axes and verify the adopted code, enclosure, accessibility, bending space and installation details.
Semantic next steps
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