Neutral ampacity calculation

[JasonMT]

Im taking a practice test for the jorneyman exam. This is a question I missed and can figure out. A branch circuit supplies a household range with a demand of 8000 watts. The ungrounded conductors have an ampacity of 40 amperes. The neutral ampacity must be at least? The answers 28 amps. Could someone please explain. Thanks.

 

[kwired]

Read 210.19(A)(3) and the exceptions that follow and then let us know if your problem has been resolved.

 

[jimdavis]

A demand factor of 70% is permitted to be applied to the neutral conductor of the range. 40A x.70=28A

 

[JasonMT]

Got it 70% but not smaller than a 10 AWG. Thanks

 

[kwired]

A demand factor of 70% is permitted to be applied to the neutral conductor of the range. 40A x.70=28A

Since you are fairly new here I will let this one go, but with people that are in school or taking exams we generally only try to point them in the right direction instead of just giving them an answer, they generally learn better if they find the solution themselves then to just be told an answer.

 

[jimdavis]

Gotcha!

 

[JasonMT]

Looks like that 70% also goes for service neutral ampacity minimums. I plugged it into another question I missed. A 120/240 volt, 200 amp service requires a neutral with a minimum ampacity of? 140 amps. Where to find that in the Nec I dont know. Ill keep looking.

 

[kwired]

Looks like that 70% also goes for service neutral ampacity minimums. I plugged it into another question I missed. A 120/240 volt, 200 amp service requires a neutral with a minimum ampacity of? 140 amps. Where to find that in the Nec I dont know. Ill keep looking.

I have to disagree with 140 amps being the correct answer, or will say more information needs to be provided before 140 can be a valid answer.

Start by reading 230.23(C) and tell us what you end up with after further evaluation of what is said there.

My answer to your question as stated is a minimum of 4AWG copper conductor or 2 AWG aluminum, add more information and I may change that answer.

 

[JasonMT]

Everything I have read leads me to 250.66. Which Leads me to 4 AWG copper or 2 AWG aluminum. This is the kind of stupid mistakes that will kill me on the test next friday. I need to stick to what the NEC says. I dont know why 70% calculated the answer the book wanted. I have found other questions and answers that have conflicted with what the NEC says.

 

[DavJ09]

Everything I have read leads me to 250.66. Which Leads me to 4 AWG copper or 2 AWG aluminum. This is the kind of stupid mistakes that will kill me on the test next friday. I need to stick to what the NEC says. I dont know why 70% calculated the answer the book wanted. I have found other questions and answers that have conflicted with what the NEC says.

Jason, make sure you jot down on the suggestion/corrections paper any mistakes you find. I suggest jotting notes down and after the test completing a detailed report, to use all your time for the exam on the exam for the test.



-- J. Davis. Sent using Tapatalk for Windows Phones

 

[kwired]

Everything I have read leads me to 250.66. Which Leads me to 4 AWG copper or 2 AWG aluminum. This is the kind of stupid mistakes that will kill me on the test next friday. I need to stick to what the NEC says. I dont know why 70% calculated the answer the book wanted. I have found other questions and answers that have conflicted with what the NEC says.

Test writers do make mistakes. If nobody challenges a question it will not get corrected either.

That said many people taking these tests are not all that certain if they have a question with no correct answer or if they are just missing something. But if you feel pretty strongly that you have an answer that is not one of the choices I would suggest making notation of it on our exam and the source of your answer.


The only way 140 is the correct answer to the question you gave is if the question contained more detail that indicated the load could be loaded to 140.

May also want to make sure there is no local amendment that would result in this answer, but based on NEC alone I don't think the question had a valid answer to go with it.

 

[texie]

Everything I have read leads me to 250.66. Which Leads me to 4 AWG copper or 2 AWG aluminum. This is the kind of stupid mistakes that will kill me on the test next friday. I need to stick to what the NEC says. I dont know why 70% calculated the answer the book wanted. I have found other questions and answers that have conflicted with what the NEC says.

Just always remember that for a service, the minimum neutral size must always be the calculated load or per 250.66 (NEC 2011) whichever is larger. In the 2014 NEC this has been changed to Table 250.101(C)(1) (this from memory-need to verify the number) but the end result is the same.
As an example, what if you had a service with 4/0 CU conductors with a calculated load of 30 amp on the neutral. At first glance it would seem that a #10 would work but that does not meet the minimum of 250.66.
The inverse of this would be if you had 100 amps of calculated load the you need to have a neutral conductor with an ampacity of 100 amps which is larger than what Table 250.66 minimum would be.