SkillsUSA preliminary contest Industral Motors Controls Written test Q2

[fmtjfw]

A wall mounted box has an EMT into its left side & down from its bottom. Each contain 10 #4 wires which are spliced in the box. Minimum box size?


Also code references?

 

[cpinetree]

I went with 9 x 12 x 6

I think you only need 9 x 9 x 3 (2011 NEC 314.28(A)2)

10 - #4 THHN max conductors in an 1 1/2" EMT

1.5" x 6 = 9"

I don't want to be the one pulling it in I would go 1 size larger to 2" EMT and a larger Pull box (12 x 12 x 6) labor is way more expensive than the materials, and why fight it.

 

[fmtjfw]

My Answer

My Answer

B

(2) A wall mounted box has a conduit into its left side & down from its bottom. Each contain 10 #4 wires which are spliced in the box. Minimum box size?

To calculate the box size you need to determine the size of the conduit needed to properly hold 10 #4 conductors. Given the size of the conduit. Since we have angle splice, we multiply by 6 for both length and width.

Table C.1 [70-746] is for EMT. Under type THWN-2 the #4 row, you find that the smallest conduit that will hold 8 conductor is 1? (which holds 10).

You can also do it the hard way with Chapter 9 Table 5 type THWN-2 the #4 row which gives 0.0824 in2 per wire or a total of 0.824 for all wires. The Table contains 0.814 in the 40% column for 1? EMT. Now dividing 0.814/0.824 gives 0.988. According to table note 7 under these circumstances if the division gives 0.8 or higher, the conduit is big enough.

Either way we get 1? conduit. Multiplying that by by 6 gives 9, so the box must be at least 9x9. The depth of the box must be enough for the conduit to fit in the side so the 1 inch deep box can't be used.
The smallest box is 10?12?3 from the list

Code Ref:
Table C.1 OR Chapter 9 tables 3 & 5 and note 7
314.28(A)(2)
Index:
Conduits ? Conductors, number in
Boxes (outlet device, pull, and junction) ? Junction, pull see Junction Boxes
Junction Boxes ? Size ? Conductors 4 AWG and larger

 

[don_resqcapt19]

B
...
The smallest box is 10?12?3 from the list
...

I don't see a 10x12x3 on the list, "B" is 9x12x6.

I guess the answer really depends on the definition of smallest.

 

[fmtjfw]

I don't see a 10x12x3 on the list, "B" is 9x12x6.

I guess the answer really depends on the definition of smallest.

The sizes are random, not out of a catalog.

The sizes are chosen so that the smallest may be by L?W or volume and give the same answer.

 

[GoldDigger]

But when you are dealing with bending space requirements rather than fill requirements, a box with the same or larger volume but wrong dimensions fails.

Sent from my Droid Maxx

 

[don_resqcapt19]

But when you are dealing with bending space requirements rather than fill requirements, a box with the same or larger volume but wrong dimensions fails.

Sent from my Droid Maxx

But there is a box on the list that meets the requirements and is much smaller by volume than the one in answer B.

 

[don_resqcapt19]

The sizes are random, not out of a catalog.

The sizes are chosen so that the smallest may be by L?W or volume and give the same answer.

There is nothing to say that the box has to have its back mounted to the wall. You could mount the 8x16x3 box on the wall with the opening up and the 16" side against the wall. This box mounted that wall meets the requirements of the code and is smaller in volume than the 9x12x6.