Theory homework question turining into a disaster.

[ApexTech]

Greetings. I'm currently a student enrolled in "Apex Technical School" and I have advanced to segment 2 of electrical. I'm currently stumped by a question on a hand out my theory instructor handed out. I would appreciate the help due to the fact I have been going back and forth through "The Ugly's" for some clarity but still no luck. Well here goes the question. Determine the ampacity and the minimum raceway required to install 8-4 awg "THWN" in rigid pvc schedule 40 in an ambient temperature of 118F? (Ambient temperature).

 

[infinity]

You need to find two things, one the ampacity after derating for number of conductors in the raceway and ambient temperature and two, the fill for those 8 conductors in Sch 40 PVC. The second part is answerable by looking in the tables in Chapter 9. The first part is not answerable unless you make some assumptions about whether or not all 8 conductors are in fact considered CCC's.

 

[Dennis Alwon]

I guess you need to assume that all the conductors are current carrying conductor's but that is not stated.

Use Table 310.15(B)(3)(a), Table 310.15(B)(17), Table 310.15(B)(2)(a) and chapter 9 Tables 4 for PVC and Table 5 for the 8 THWN.

 

[MasterTheNEC]

I would also have to figure you are dealing with 8 CCC's due to the lack of additional information in the question.

In accordance with 310.15(B)(3)(a) you would have to apply a 70%(0.70) adjustment for the 8 current carrying conductors.

Next you expressed an ambient temperature of 118F, In accordance with 310.15(B)(2) and Table 310.15(B)(2)(a) which says the correction for 118F is 0.75, since the question is specifically telling you that a THWN conductor is being used and not that it's a THHN/THWN or THWN-2 conductor.

So you have a 0.70 value for the adjustment and a 0.75 value for the correction.

Now, you did not express CU or AL, and since AL was not expressed we have to assume CU was the conductor of choice.[See 110.5 if you do not understand that statement]

So assuming (best we can do) you have a 4 AWG CU @ 75 degree ratings. The ampacity of the conductor before any adjustments or corrections is 85 Amps.

Now without knowing any additional variables... we are only being asked what the ampacity of the conductors are now after the condition of use has been applied......

85A x 0.70 x 0.75 = 44.625 amps

So based on the condition of use the new ampacity of the 8- 4 AWG conductors is 44.6

Hope this was helpful....

 

[jumper]

The first mistake is trying to use an "Ugly's" book and not a NEC, the electrical code book, to find the answer.

 

[hurk27]

A little note to members, when we have a student doing homework or test work, we try to teach them how to find the answers on their own without giving them the answer.

It goes along with the saying "give a man a fish you feed him for a day, teach him how to fish you feed him for a lifetime"

Don't worry I have done the same.

 

[MasterTheNEC]

As an added note - In your question the last part about the minimum size raceway is simple and I think Dennis covered it well. The ANNEX C will tell you the number of conductors (since they are all the same size, it is an easy one) but it may be something to examine since the question specifically mentioned the minimum. While I do not like writing questions that would attempt to trick the student, I can't say all instructors are that way.

So with that said lets go ahead and do the raceway fill calculation as well for kicks and giggles and because I am extremely bored today.....:jawdrop:

OK, you have stated that Schedule 40 PVC was the raceway of choice. So first we need to build on the previous calculation in knowing that we are dealing with 8-4 AWG CU Conducotrs of the THWN insulation type. Now we need to determine the area of a 4 AWG THWN Conductor.

If crack open that dusty NEC sitting in the corner, and migrate over to Table 5, and locate THWN. You will notice that 4 AWG THWN has an approximate area (in2) of 0.0824. Now, since all of the conductors in your question are the same size (4 AWG) you can multiple 0.0824 x 8 giving you 0.6592 as your overall approximate area (in2) value.

Now since the raceway was Schedule 40 PVC, move on over to Table 4 and locate the table associated with Schedule 40 PVC. You will notice that it has (6) columns but do not worry because you are only going to be dealing with (1) column in your question, that is the 40% column for "Over 2 wires".

Now your previous math said that you have an overall 0.6592 (in2) area and since you have over 2 wires in your PVC Sch. 40 Raceway you need to look and see what the smallest raceway in that column will permit 0.6592 (in2) of wires. You will notice that the trade size 1 1/4 is too small at 0.581 so you have to move up to trade size 1 1/2 which has 0.794 (in2) area and it the minimum size you can use for your 8- 4 AWG CU THWN Conductors.

Now just for FUN.....is that what Annex C would have said?...Check it out....Did you look.....did you notice that even Annex C said the 1 1/4 was too small as it would only handle 7 wires while the 1 1/2 can handle up to 9. So it would have worked either way but now you know....How it's done. (well at least when all the conductors are the same size anyway...lol)

Hope this helps and good luck with your studies...

 

[MasterTheNEC]

A little note to members, when we have a student doing homework or test work, we try to teach them how to find the answers on their own without giving them the answer.

It goes along with the saying "give a man a fish you feed him for a day, teach him how to fish you feed him for a lifetime"

Don't worry I have done the same.

Well I prefer to show them the way......teach them the method.......then they can FISH for themselves...

Plus...I'm always one click away from dang Moderators booting me....lol...:angel:

 

[GoldDigger]

A little note to members, when we have a student doing homework or test work, we try to teach them how to find the answers on their own without giving them the answer.

It goes along with the saying "give a man a fish you feed him for a day, teach him how to fish you feed him for a lifetime"

Don't worry I have done the same.

If you teach a man to fish, you will find him every afternoon out on his boat drinking beer.

 

[hurk27]

Pipe fill can actully be found in the Ugly's I have the 2011 which on page 98 gives you the wire dimensions for THWN, and on page 103 will give you the fill area on sch. 40 PVC or you could use the fill tables for SCH 40 PVC found on page 88.

Adjustments can be found on page 77 and 78

 

[jumper]

Pipe fill can actully be found in the Ugly's I have the 2011 which on page 98 gives you the wire dimensions for THWN, and on page 103 will give you the fill area on sch. 40 PVC or you could use the fill tables for SCH 40 PVC found on page 88.

Adjustments can be found on page 77 and 78

Yeah, but I believe that most license exams are taken with the NEC, so I believe it should be used.

Ugly's is fine in a pinch, but using a NEC is best IMO.

 

[Dennis Alwon]

I would like to correct myself. Since all the conductors are the same you can use Annex C-- in this case Table C.10(A). You wont need to bother with Chapter 9 although it is good practice

 

[jumper]

Me no like Annex C.

Rumors say that it has mistakes in it.

 

[Dennis Alwon]

Me no like Annex C.

Rumors say that it has mistakes in it.

The 2011 had one area that was incorrect. I redid the entire annex and submitted it and the 2014 now has it all and I believe it is correct. They took my entire spreadsheet not just the correction.

 

[jumper]

The 2011 had one area that was incorrect. I redid the entire annex and submitted it and the 2014 now has it all and I believe it is correct. They took my entire spreadsheet not just the correction.

Cool. Nice job.:thumbsup:

 

[MasterTheNEC]

If you teach a man to fish, you will find him every afternoon out on his boat drinking beer.

NICE:thumbsup:

 

[kwired]

Greetings. I'm currently a student enrolled in "Apex Technical School" and I have advanced to segment 2 of electrical. I'm currently stumped by a question on a hand out my theory instructor handed out. I would appreciate the help due to the fact I have been going back and forth through "The Ugly's" for some clarity but still no luck. Well here goes the question. Determine the ampacity and the minimum raceway required to install 8-4 awg "THWN" in rigid pvc schedule 40 in an ambient temperature of 118F? (Ambient temperature).

Poorly worded question IMO. When they say determine ampacity they follow it with a conductor size - that is not how you should do things. They should give you the load, other necessary load details such as continuous or non continuous, motor, AC/refrigeration, resistance heat, etc. as all of those can have differences in how you determine size of conductors needed to supply them. Then they should tell you ambient temp, number of current carrying conductors in the raceway, etc.

All you can do with that question is derate some 4 AWG conductors to some specific conditions, but generally it is more important to come up with a conductor that can supply a particular load then it is to just derate a particular conductor. (you usually size conductors to the load, not the load to the conductor)

If anything maybe the question should have been more like "what is the adjusted ampacity of 8-4AWG current carrying conductors in a raceway with an ambient temp of 118F? A second part or even second question may involve size of raceway needed for these conductors. This is probably what the intention was when the question was written, but I still like the real life application of telling you load details and letting the student figure out conductor size , because that is more of a reality of what should happen most of the time.

Even if you had existing conductors and wanted to know if they can be used for a particular load, you should still figure out what the load actually needs and if existing are larger then they will work.

 

[MasterTheNEC]

Poorly worded question IMO. When they say determine ampacity they follow it with a conductor size - that is not how you should do things. They should give you the load, other necessary load details such as continuous or non continuous, motor, AC/refrigeration, resistance heat, etc. as all of those can have differences in how you determine size of conductors needed to supply them. Then they should tell you ambient temp, number of current carrying conductors in the raceway, etc.

All you can do with that question is derate some 4 AWG conductors to some specific conditions, but generally it is more important to come up with a conductor that can supply a particular load then it is to just derate a particular conductor. (you usually size conductors to the load, not the load to the conductor)

If anything maybe the question should have been more like "what is the adjusted ampacity of 8-4AWG current carrying conductors in a raceway with an ambient temp of 118F? A second part or even second question may involve size of raceway needed for these conductors. This is probably what the intention was when the question was written, but I still like the real life application of telling you load details and letting the student figure out conductor size , because that is more of a reality of what should happen most of the time.

Even if you had existing conductors and wanted to know if they can be used for a particular load, you should still figure out what the load actually needs and if existing are larger then they will work.

He's (she) is not the instructor so telling the student that the question is written wrong does not help them try to work through a solution. As an instructor I might give a question like this to have my student work through it and show me how they came up with their conclusions and at the end of the day that may have been the intent of the question all along.

 

[kwired]

He's (she) is not the instructor so telling the student that the question is written wrong does not help them try to work through a solution. As an instructor I might give a question like this to have my student work through it and show me how they came up with their conclusions and at the end of the day that may have been the intent of the question all along.

I know, I just dislike all the poor questions that people have mentioned here, this one is not nearly as bad as some are though.

The student also needs to realize the instructor is human and can make mistakes and if something doesn't seem right they can/should ask for clarification. Instructors should be open to the student for such questions, after all they are being paid to teach.

 

[Pharon]

I guess you need to assume that all the conductors are current carrying conductor's but that is not stated.

Yes, a poorly stated question, indeed. Especially considering that because the raceway is PVC, it cannot be used as an equipment ground path... which means that at least one of those conductors needs to be an equipment grounding conductor and therefore CANNOT be a current carrying conductor.