Box fill

[hqidesign]

Why is it if i calculate from T314.16a the box fill of any given box the table listing for cu in. is always less then the actual calculated number. for example
a 4"x1 1/2" square box is listed in the table with 21.0 cu in but if i do the calculation 4x4x1.5=24cu in. Some of the listed cu in. numbers are 2cu in. less and some are 3cu in. less like the example above. What is the logical mathematical explanation?

 

[ptonsparky]

Tape up all the holes, fill with water then measure the volume of water. Kind of like a 2X4 stud. Nominal dimensions.

my guess

 

[infinity]

Dimensions are the outside of the box which doesn't allow for the thickness of the metal. Also some boxes have rounded corners.

 

[Dennis Alwon]

I agree with Trevor. Measure a 3x2x2 and the inside dimension is less than that. That's why you need to use the table or use the cu. in marked inside the box.

 

[riverstrat]

Dimensions are the outside of the box which doesn't allow for the thickness of the metal. Also some boxes have rounded corners.

I agree with this possibility, there are occasional items in the code that tell us 'there have been no significant problems' associated with using 'this' (a particular) method.

Could be this is one of those issues.........nominal sizing.

 

[jumper]

1. Welcome to the forum.

2. You have too much time on your hands. J/K!

3. The table seems to list the minimum capacity/volume of the boxes, which as the others stated, and would be used in the if the cubic capacity/volume was not marked or not able to be read. So if I had a 4x4x1.5, I would use the chart, unless marked as larger.