This calculator determines the minimum pull box size for low-voltage ( <= 600V for NEC prior to 2014, 1000V if 2014 NEC ) conductors. Code requirements (Art. 314) amd allowances for the physical conduit fittings are addressed. Note that code requires cable bundling if a dimension exceeds 6 ft.
'U-pulls' (where a conductor enters and exits the same side of the box): This calculator does not specifically address the Code requirement that entry and exit points of a 'U-pull' be separated by at least 6 x the raceway size: you must verify your layout complies - the calculator has no way to check this.
Several inter-related factors determine minimum box size. These factors are not easily captured on a single data-entry form, but I have tried. The first time this page is opened, it will be in 'Layout mode' (box checked at the top of the page). To use this calculator:
'Layout' the box first; for each wall, enter the quantity of conduit trade sizes that will enter the box.
Mark the walls that are removable.
Normally the page updates immediately when one of the 'sizes' fields or 'Removable' boxes is changed. In 'Layout mode', the page will not automatically update, permiting you to edit the wall information without having to wait for an update after each change. Un-check the 'Layout mode' field once you are ready for the changes to be displayed.
Once the new page is displayed, for each wall, enter the quantity of each conduit trade size entering the box.
Next, indicate the orientation of the conduits by clicking on the icons - each toggles between the applicable pair of the following:
indicates the layout will be left to right ( W dimension).
indicates the layout will be top to bottom ( H dimension).
indicates the layout will be front to back ( D dimension).
Check / un-check the 'Thru' box(es). 'Thru' means straight-thru pulls; versus corner pulls. 'Thru' pulls require larger box dimensions.
Click on the calculator icon to display the results.
To change the number of sizes in a conduit table, enter a new value in the 'sizes' field and then press the <Tab> key. You can't request more than 13 sizes in a table - there are only 13 trade sizes of conduit.
If there is more than one 'row' of conduit on a wall, enter the data for one row, observe the results, and then replace with the data for the next row. The row that produces the largest box dimension governs.
Having a removable wall on a box can significantly reduce the required dimension of the box perpendicular to that wall. If the wall is removable, the box dimension is determined not by the size and quantity of conduits entering opposite, but by the size of the conductors entering opposite. Therefore, when the 'Removable' box is checked for a wall, a field for wire size will be displayed on the wall opposite. Only the largest conductor affects box size, so only one field will appear. If a cable is entering in lieu of individual conductors, select the size of the largest conductor in the cable.
When the calculator icon appears above the 'result' field, data has changed and you need to click on that icon to recalculate the sheet.
After a recalc, you can bookmark this page and the data you have entered will be saved in the bookmark. This doesn't work if you have changed the data and not done a recalc.
Calculations are per NFPA-70 [1999, 2002, 2005, 2008, 2011, & 2014] data and requirements.