box volume

[tlaidman]

while teaching box fill it was brought to our attention that the volumes listed in NEC 314.16 are less than if you do the calcs yourself. does anyone know why.

 

[radiopet]

I believe that the "*" shown on the table 314.16(A) has the statement

" * Where no volume allowances are required by 314.16(B)(2) through 314.16(B)(5)"

So basically the Maximum Number of Conductors* is just that.....if using the above allowances for devices ( yokes) and so on....hickys what have you...then you have to revert to the actual volume which will then define which box you can use.

In other words...those maxium number of conductors are for junctions and pull boxes and so on that do not need allowances made for devices and such as shown in 314(B(2) through 314.16(B)(5)

Hope this helps

 

[radiopet]

another analogy.......the NEC says in Annex C how many conductors can be in a certain size raceway.......as long as they are the same size and insulation.....now throw in different sizes...and well the chart is useless......

Hows that as an analogy........see my point

 

[tlaidman]

yes but if I'm teaching a class and we take a 4x4x1.25 inch box that is 20 cubic inches and the table say 18 cubic inches then they either can't multiply or they have a reason for lowering the volume can't any one tell me why they do that.

 

[tlaidman]

where I went to school to find volume you multply length times width times depth which is 4x 4x 1.25 equals 20 cubic inches. Your math is 4 times 1.25 equal 5 and it is square inches not cubic inches.

 

[radiopet]

well that is fine and dandy but the NEC gives you the specific specs on a SQUARE BOX and how it is applied. If it does not meet the list then you defer to the markings in the box itself.

Sorry if my answers offend you......I am simply explaining how it is taught in every school I have been an instructor at....the table tells you the exact allowance it is set at......you can't alter it on your own simply because you think the math is incorrect....trust me the NEC on this is not in error.

Again I can only give you the concept of how the NEC states it.....I am sure their are many things in the NEC that can confuse...on this account the box in question is a Square Box.....4" x 1 1/4"....and since it is square it is listed on the table.....and the CU IN is 18".....

Why they DO what they do....Man I can't tell you that...I was only attempting to explain it in how we teach it.

 

[radiopet]

P.S....not MY math.......I am giving you NEC math

 

[websparky]

OK..... I think I see the problem!

The OP is asking why the table says 18cu. when the apparent size of the box is 4" x 4" x "1 1/4". The math on this is 4x4=16x1.25=20. The actual size of the interior of the nominal 4 square box is 3 13/16 x 3 13/16 x 1 1/8. The math is 3.815 x 3.815 = 14.55, x 1.125 = 18.19 or 18cu. just like the code book!

However, I still don't get

.....a normal square box say 4" x 1 1/4" you only use the 4" x 1 1/4" which would give you 18 cubic inch.....

either!

 

[iwire]

I agree with Dave if you measure the usable volume of the box it will be very close to 18 ci

 

[radiopet]

OK..... I think I see the problem!

The OP is asking why the table says 18cu. when the apparent size of the box is 4" x 4" x "1 1/4". The math on this is 4x4=16x1.25=20. The actual size of the interior of the nominal 4 square box is 3 13/16 x 3 13/16 x 1 1/8. The math is 3.815 x 3.815 = 14.55, x 1.125 = 18.19 or 18cu. just like the code book!

However, I still don't get either!

lol....No I said using the TABLE on the listed box as 4" x 1 1/4" as shown you use 18 Cu In....not that it addes up to 18 CU IN.....in other words use what the code book says use if the box matches the table....

 

[radiopet]

Ok......now you are confusing what I am saying....I was trying to say based on the BOX the OP listed...it is shown in the list of boxes as 18 Cu In...and that is the NEC's math.....my my math.....I consult on the NEC...not explain how they write it or their mathmatical intent...lol

a normal square box say 4" x 1 1/4" you only use the 4" x 1 1/4" which would give you 18 cubic inch because the table lists it as a square box.....

 

[radiopet]

Ahhh...sorry I thought you were questioning the method for using the calculation and why you have to use 18 Cu In instead of doing your own math and coming up with the volume...

I was not questioning your math.....nor the NEC's....I was simply trying to explain how to do the math...based on the NEC code for the BOX you listed and explaining when it does not meet the boxes listed what you need to do.

Sorry for any confussion on my part...was only trying to help

 

[bkludecke]

After learning this, doing this and teaching this for 30++ years I've come to the conclusion that this box fill stuff is necessary but really needs to be used as a guideline (i'm already ducking).

Here me out; the NEC give the same volume for a simple snap switch, 1000w dimmer, GFCI recept, occupancy switch, timer switch, standard recept......... Then it gives the same volumes for wire sizes regardless of insulation type, stranded v solid ......

You show me a good neat and workmanlike installation that will accomodate the device, fixture, cover etc and I will pass it.

You show me a sloppy, nasty, ugly non-proffesional installation that complies with the box fill req. and I will find a dozen reasons to fail it.

So there.

Bob on the left coast.

 

[Cavie]

Since 20 in plastic boxes are not in the table 314.16(a), do we use table 314.16(b) and figure 2.25 in for each #12 and 5 in for each yoke?

 

[celtic]

Since 20 in plastic boxes are not in the table 314.16(a), do we use table 314.16(b) and figure 2.25 in for each #12 and 5 in for each yoke?

We would follow 314.16(A)(2) and 314.16(B), paying close attention to 314.16(B)(5).

 

[radiopet]

Since 20 in plastic boxes are not in the table 314.16(a), do we use table 314.16(b) and figure 2.25 in for each #12 and 5 in for each yoke?

Actually if the box is not listed in the chart then the BOX should have the volume on it. The you use the conductor calc. as stated to determine the fill.

 

[Pierre C Belarge]

Table 314.16(A) is for metal boxes.

If you are installing plastic boxes, yes you can use Table 314.16(B) if you like, but the number of different size conductors is already stamped in the box.

 

[Cavie]

Yes, I understand the # of conductors is stamped on the box. it does not allow for a device. You must use the tables in (b) to determin the amount for each device. Therfore, it stands to reason that you must use the table to also determin the amount for the wires, Yes?? in that case, the 20 cu box will not hold 9 #12's.

 

[al hildenbrand]

Cavie,

I think of it this way.

The 20 in? box will hold 9 #12 if I put on a blank cover.

The 20 in? box, with a device on a yoke, still holds 9 #12, and 2 of those 9 are counted as the device before I count the rest of the conductors.

 

[Cavie]

See, them "Color 8 x 10 glossy pictures with the ........". works every time. thanks, but i'm still confused. 314.16 (b) calles for 2.5 cu. in. for a #12 and 5 cu. in for a yoke.

3 #12 rx = 7 x 2.5 =17.50 yoke= 2.5 x 2 = 5

17.50
+ 5.00
_______
22.50 ?? Help me out here, i never was to good at Alge'bra