Final Exam Question #73 Low-Voltage and Power Limited Systems

[Ballew]

In going through the Low Voltage workbook based on the 2005 NEC the following question appears on the final exam:

73. The ac ohms-to-neutral impedance per 1,000 ft of 4/0 AWG aluminum condctor in steel raceway is ____.
(a) 0.06 ohms
(b) 0.10 ohms
(c) 0.22 ohms
(d) 0.11 ohms

I say the answer is (b) 0.10 ohms based on Chapter 9, Table 9. I am referencing the steel conduit column under Alternating current resistance for aluminum wires and the 4/0 AWG size row. The second number in the the column is that for ohms ber 1000 feet which is 0.10. The Answer Key says the correct answer is (c) 0.22 ohms and that the answer is found in Chapter 9, Table 9.

I am trying to figure out where I went wrong with this question.

Thanks!

 

[augie47]

I would have answered .1 also. Guess we may both learn something here,

 

[GoldDigger]

The phrase "ohms to neutral" is not well defined, and that may be part of the problem.
1. Is it the end-to-end impedance or the loop impedance including neutral for a single phase (line to neutral) load?
2. If the current as well as the impedance measurement is one way, the effect of the iron raceway on the 60 Hz. inductive reactance will be much greater than for a non-ferrous raceway or for balanced current.
I am still looking for a justifiable way to get their answer, but it is a bad question!

 

[Little Bill]

In going through the Low Voltage workbook based on the 2005 NEC the following question appears on the final exam:

73. The ac ohms-to-neutral impedance per 1,000 ft of 4/0 AWG aluminum condctor in steel raceway is ____.
(a) 0.06 ohms
(b) 0.10 ohms
(c) 0.22 ohms
(d) 0.11 ohms

I say the answer is (b) 0.10 ohms based on Chapter 9, Table 9. I am referencing the steel conduit column under Alternating current resistance for aluminum wires and the 4/0 AWG size row. The second number in the the column is that for ohms ber 1000 feet which is 0.10. The Answer Key says the correct answer is (c) 0.22 ohms and that the answer is found in Chapter 9, Table 9.

I am trying to figure out where I went wrong with this question.

Thanks!

What/Who's workbook are you using?
I know when I used Mike Holt's books they had some errors in the answer sheets. There was a list on the website that you looked at to see if the one you thought was wrong was there. Also, if it wasn't and you were pretty sure there was an error, you could report it and they would check it. If you were correct they would add that to the corrections list. I actually found a couple that wasn't on their list. I reported them and they added them after verifying.

Also, I think you were correct in your answer according to the table.

 

[Little Bill]

The phrase "ohms to neutral" is not well defined, and that may be part of the problem.
1. Is it the end-to-end impedance or the loop impedance including neutral for a single phase (line to neutral) load?
2. If the current as well as the impedance measurement is one way, the effect of the iron raceway on the 60 Hz. inductive reactance will be much greater than for a non-ferrous raceway or for balanced current.
I am still looking for a justifiable way to get their answer, but it is a bad question!

It appears they are looking for what Table 9 shows. They want you to be able to look up the answer according to the specifics of the wire type, raceway, etc. The table shows the .10 that the OP chose using Al wire, size 4/0, and steel raceway/conduit.

 

[Smart $]

...
I am still looking for a justifiable way to get their answer, but it is a bad question!

The answer could be 0.11...

...IF the effective z is at 0.85 PF...


...the author did not realize he asked for the ohm-to-neutral value without consideration for phasing, and presumed single phase total circuit impedance when making up the answer key...


:slaphead: