NCEES Question #536 - Book's answer is correct

[tobeeepe]

No. 40% is for conduit area and not for current. You take the current as it is. Table has the 40% multiplication for area done for you. Column 4 from left in Table 4 has 100% which is 2341. And 2341*0.4=937 is what you read from the 40% column.
In fact, it is even easier when you used Table C.4 in Annex C. All you do is find THHN #1 row and go right looking for 6 conductors. You go past 5 then hit 9 which is the right size. It is Metric 53 and Trade size 2 (top of the column). You didn't have to do any current calculation.

But I am not an expert in this area. So let's see if the experts approve my approach.

I shouldn't have mentioned 'current' in my repsonse. I was actually refering to area. If you use Table C.4, you don't have to calculate area.

 

[GAZOO]

I didn't mean to apply 40% to the current I ment to apply it to the total area for the 6 #1 conductors which is 604.8 mm^2. but I think once we get the total area we just apply it the over 2 wire column and the conduit size already has the 40% calculated. I would beleive your approach is correct, but it will be good to have an expert approval on both ways of solving such NEC problem.

thx

 

[tobeeepe]

I didn't mean to apply 40% to the current I ment to apply it to the total area for the 6 #1 conductors which is 604.8 mm^2. but I think once we get the total area we just apply it the over 2 wire column and the conduit size already has the 40% calculated. I would beleive your approach is correct, but it will be good to have an expert approval on both ways of solving such NEC problem.
thx

BTW, you should be able to start a new topic now since you have posted more than twice.

A question I have is: How do we know (in the PE exam) if the cable is COMACT STRANDING or not? In other words, whether to use Table C.4 or C.4A ?

 

[Dark Knight]

OK

You guys made me redo the problem. Thanks. I was rusty.

From Table 5 of Chapter 9 I get the Approximate area if #1 THHN is .1562 sq in.

We have 6 sets???( Is that what you said Gazoo????)

6 times .1562 is the area your wires will occupate in the conduit. That would be .9372 sq in.

Now you can attack the problem in different ways. If you use the right NEC Table you will be OK. Table 4, on page 70-627 of the NEC has 100%, 60%, 53% 31% and Over 2 wires (40%). I used that last column because there are more than 2 wires(6) running inside the conduit. As a comment, that last column gives you the max area that can be occupied on that conduit.

That is one way. Table C.4, the one tobeepe used, is also a right approach. I cannot say I favor one over the other. But the problem with C.4 is that you have to be careful with the insulation type of the wire. If you are aware of that is as good as Table 4 of chapter 9, where the insulation is not a big player. That table, for THHN, says that you can "legally" put up to 7 #1's in a 2" IMC. Be careful if you use this one. Remember the insulation type.

For Table 4(Chapter 9) You only have to know your needed area and pick a conduit in order to not exceed that and you will be OK. Again, that table gives you the max allowed. If you are under it you are OK.

These are my thoughts and my opinions. Of course, I am not Einstein, but I think I am OK in this one.

By the way, the size I would use is 2" IMC.

Hope that helps....

 

[Dark Knight]

BTW, you should be able to start a new topic now since you have posted more than twice.
A question I have is: How do we know (in the PE exam) if the cable is COMACT STRANDING or not? In other words, whether to use Table C.4 or C.4A ?

Assume always the C.# table(without the A) If you are required to use the A they will have to tell you.

 

[tobeeepe]

Assume always the C.# table(without the A) If you are required to use the A they will have to tell you.

Luis said "That table, for THHN, says that you can "legally" put up to 7 #1's in a 2" IMC.".

I think Luis meant to say "... up to 9 ...", because the number under 2" column for THHN in C.4 is 9.

Is it correct ? Not trying to be nitpicky but trying to make sure I understand what you said...

Thanks for your answers!

 

[Dark Knight]

Luis said "That table, for THHN, says that you can "legally" put up to 7 #1's in a 2" IMC.". I think Luis meant to say "... up to 9 ...", because the number under 2" column for THHN in C.4 is 9.

Is it correct ? Not trying to be nitpicky but trying to make sure I understand what you said...

Thanks for your answers!

OOOOOOPPPPPPSSSSSS.......Brain fart by me. You are right. Did not have the NEC open at the moment I was writing. Was writing from the top of my head after looking all the tables I mentioned.

Thanks tobeepe. On the other hand I also warned I am not Einstein. :beerchug:

Nine (9) it is.

 

[tobeeepe]

OOOOOOPPPPPPSSSSSS.......Brain fart by me. You are right. Did not have the NEC open at the moment I was writing. Was writing from the top of my head after looking all the tables I mentioned.
Thanks tobeepe. On the other hand I also warned I am not Einstein. :beerchug:

Nine (9) it is.

Thanks so much! I am new to NEC and learning a little bit every day....

 

[mudpuppy]

Sorry to bring up ancient history with this thread, but this problem (NCEES 536) has been bothering me since I first came across it when studying 7 months ago. I've been baffled by it until a couple days ago. I'd been approaching it the same way tobeeepe was in the original post at the top of this thread, which was to vary the angle of the current through the line. It finally clicked with me that what I was effectively doing was varying the power factor of the load plus the line (i.e the power factor seen at the source), while the question was clearly asking for what happens when you vary "the load power factor".

I feel kind of stupid now for not understanding this for so long, but it drives home the sage advice of BIO: "READ THE PROBLEM CAREFULLY" (and also the answers in this case).