Help with a test question

[Livin'Free]

I took my JW exam yesterday(I think I did well) and there was a question I wasn't sure of?

How many 3/0 THHN conductors can you install in a 4"x4" metal wireway?

4"x4"=16 sq. in. X .2=3.2 sq. in.
3/0 THHN =0.2679 sq. in.
I came up with 11.944

The three answers to choose from were:
(11.9) 11 wires
(11.9) 12 wires
None of these

I chose A...11 wires

I guess they are looking to see if you know when to round up or down, but i couldn't find any reference in the code to rounding up or down?

Thanks for any insight...

 

[infinity]

If 20% is your maximum fill I don't see how you could exceed that. If you rounded up to 12 conductors you would have exceeded the 20%. I agree with your answer of 11 conductors.

 

[Livin'Free]

Thanks....I couldn't see how you could use 12, but some guys answered 12.

 

[peter]

Just to be ornery, I would say 12 would be OK. If my humble math is correct, it would actually be .004625% over the limit. Is this really going to matter in the real world?
The thing is, 12 would allow you 6 single phase circuits or three 4 wire circuits or four 3 wire circuits. 11 would allow only 5 single phase circuits, two 4 wire circuits or three 3 wire circuits. What is the cost difference between a 4" square wireway and a 5" wireway in terms of both money and space available?
Of course, in a test, you are not dealing with the real world so use the 11 conductor answer and score another point. On the other hand... [I'll leave out thoughts of Cousin Vinnie from New Jersey, quick trips to the recently landscaped ground 22 floors below and the 6' under very quickly bit.]
~Peter

 

[eric stromberg]

Peter,
As long as we are talking about Exam Pet Peeves, here is one of mine:

I have seen several questions such as the following:
A conduit has four conductors in it, what is the ampacity if...

the rest of the question has all the particulars of ambient temperature, &tc

My issue with questions like this is, do they expect me to use 310.15(B)(2)(a) to derate the conductors???? To me, four wires in a conduit represent a MBC that is coming off of a wye transformer. In this case, the neutral is not a CCC and so there are only 3 CCC in the conduit and 310.15(B)(2)(a) derating does not apply. But this is not the way exam questions are worded. Arrrrrgh!

And now, back to our regularly scheduled program...

 

[infinity]

If this is a test question the answer has to be based on the NEC which says fill is 20% or less. Just because the 12 conductor is slightly over the 20% doesn't mean that installing 12 conductors would be code compliant, or for that matter the correct answer.

 

[don_resqcapt19]

If this was a raceway, then the correct code answer would be 12.

Chapter 9, Table 1, Note(7) When calculating the maximum number of conductors permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal of 0.8 or larger.

I don't know of a rule that says this for wireways.
Don

 

[Livin'Free]

Yeah...I found that for conduit or tubing, but could not find a similar rule for wireways.

 

[Inspectorcliff]

sounds like good math

sounds like good math

Sometimes it is easy to forget one environment when we spend so much time in another environment. Such as packing our tool pouch and pipe benders. Versus being an inspector sitting on our duff and eating donuts waiting for the apprentice to mess up. You always know it?s the apprentice, because when you ask the JW on the job "who did this?" Oh, it was the apprentice. If it wasn't for them there wouldn't be any jobs getting done. 11 sounds like a good test answer; it probably wouldn't be identified in the field. As previously stated.:roll:

 

[infinity]

If this was a raceway, then the correct code answer would be 12.

I don't know of a rule that says this for wireways.
Don

I thought of that too but couldn't find a rounding up factor that applied to wireways either.My guess is that there isn't one therefore you cannot exceed the 20% limit.

 

[nakulak]

To me, four wires in a conduit represent a MBC that is coming off of a wye transformer. In this case, the neutral is not a CCC and so there are only 3 CCC in the conduit and 310.15(B)(2)(a) derating does not apply. QUOTE]

Doesn't your contention that the neutral is not current carrying depend on a perfectly balanced circuit ? (or were you only referring to balanced loads and not mw branch ckts ?)

 

[coulter]

...Doesn't your contention that the neutral is not current carrying depend on a perfectly balanced circuit ? ...

Nope. Summation I = 0. Or to put it another way. What ever current the neutral is carrying, one of the phase conductors isn't.

carl