box volume

[George Stolz]

According to Table 314.16(B), #12 is to count as 2.25 cubic inches, not 2.5.

The easiest way I've found to figure the box fill in the field is to:

• Count the conductors with your fingers.
• Count the devices, raising two fingers for each device.
• If there are grounding conductors present (yes/no), raise another finger.
• If the clamps are internal to the box (yes/no), raise another finger.
• Studs/hickeys, raise another finger.

Multiply this number by the multiplier in the table, and compare it with the available box space.

So, suppose I'm working with a 32.0 cubic inch, two gang box. I have:
(2) 14-2 cables
(2) 14-3 cables
(2) switches
The cables contain an EGC.

Counting:
(II II) (III III) (II II) (I)
I have 15 conductors in the box, converting devices/clamps/studs/hickeys into conductors.

15 x 2.0 = 30 cubic inches.

Converting everything into a common unit, in this case conductors, is the easiest format to solve, IMO. Forget that a switch is a switch and treat it as a conductor, and the problem is far easier to solve.

(I don't think I ever had success figuring out the cheat sheet they often stamp into the boxes. I always go by the cubic inch rating stamped on the box, for less brain-strain.)

 

[George Stolz]

Here me out; the NEC give the same volume for a simple snap switch, 1000w dimmer, GFCI recept, occupancy switch, timer switch, standard recept....

Bob, I think a GFCI receptacle is roughly 4 cubic inches in volume. IMO, they are using the worst case scenario for devices with the double-conductor size requirement.

Taking into account that the code is a minimum, then it makes sense. There are situations where it's difficult to fill a box to it maximum physically (with #12's and a dimmer, for example.) There are situations where it's fairly easy to get everything into an overloaded box (with #14's and a single-pole snap switch.)

So, they picked a spot in the middle and ran with it. Perhaps?

 

[Cavie]

Made an appointment to see the eye doctor. 2.25 2.5 close. Thanks for pionting out the error of my ways. I am in your debt.

 

[George Stolz]

I am in your debt.

Not one bit.

 

[infinity]

Bob, I think a GFCI receptacle is roughly 4 cubic inches in volume. IMO, they are using the worst case scenario for devices with the double-conductor size requirement.

I just measured one I had laying around in my office. It measured 1.5" X 1.625" X 2.75" = 6.70 cu in.

 

[bkludecke]

With all of the above dialog................
I rest my case.


Bob on the left coast.

 

[hillbilly]

Here's one way to figure the box volume.
Tape over all of the KO's to keep the box from leaking.
Completely fill the box with pure water that's been cooled to 39 degrees F.
Weigh (the combination) on a precision scale.
Deduct the weight of the box (tape included) from the total.
If the scale weight is in ounces, divide the weight of the water (remainder) by .578
This will give you the box volume in cubic inches.
If you're using the Metric system, the water temp. needs to be 4 degrees C, and you divide the water weight in grams by 16.388.
This will also give you the box volume in cubic inches.
I can assemble and sell you a "Box volume Calculator Kit" including a conversion chart (different water temperatures...etc) for a nominal fee.
Everyone should have one.
steve

 

[Minuteman]

Hillbilly - Do you have a similar formula for calculating conduit fill?

 

[celtic]

What about surface tension?

 

[bkludecke]

My case is still resting. are you guys bored or high or ...............? Personally I prefer High.

Bob on the left coast.

 

[hillbilly]

My case is still resting. are you guys bored or high or ...............? Personally I prefer High.

Bob on the left coast.

How about "Highly Bored".
I forget to mention. The calculations need to be performed at sea level, so plan your vacation around them and the trip will be tax deductable.
If you're concerned about the sharp edge of the box combined with the surface tension of the water interfering with the accuracy of the calculation, I offer a adder to my kit that includes a special surfactant that you add to the water before performing the test.
The adder contains a second conversion chart for the primary conversion chart to account for the small decrease in water volume if using the surfactant.
The adder is only recommended if your AHJ is a real PITA.
steve

 

[Bob Anchorite]

More on the box...

More on the box...

So the Nec is saying that a 4x4x1.25 box is called a 4x1.25 box and that size box is assigned a volume of 18 cu inches.... is that right?

Bob

 

[George Stolz]

Yes.

Notice, the box is written into the table as "square."

 

[richard pando]

The NEC states; a) "minimum" volume, b) yes, we should use internal dimensions and c) it is only a nominal value. The skin or steel the box is made of takes quite a bit of that volume you're missing. Do a bit more math with your students-many of them can use and enjoy the practice. It hones their skills.
I hope this helps,

Richard

 

[TAZMAN]

box fill

box fill

I do not know the specifics but. I was told that they fill the metal boxes with sand , and then measure the cubic inches of sand within the metal boxes. This is probably a average as some of the boxes have curved corners and a raised area for the bonding wire.

 

[celtic]

...I was told that they fill the metal boxes with sand ....

You really believe that?

It's amazing we were able to send a man to the moon.

 

[tallgirl]

Not one bit.

George, never underestimate the value of your posts. Your methods are often simple and ellegant and more than adequate.

Seeing as I have more than 18 fingers and toes, I think I'll never be at a loss for calculating single gang box fill ever again

 

[tallgirl]

You really believe that?

It's amazing we were able to send a man to the moon.

Of course they don't use sand -- they use water, as this page explains

(Scan down to the "Big Rocks" section)

 

[TAZMAN]

box volume

Yoy guys are way to smart. I just called Raco. There engineer said that u.l. has a specified test that is to be used. They fill the boxes with water. They then convert the mili liters to cubic inches. Wonder how they keep the water from running out with those holes in the box. Dah!

 

[celtic]

Wonder how they keep the water from running out with those holes in the box. Dah!

They freeze it