Ray holder calculation question

[Dsg319]

I’m currently studying for my WV masters. Today I was doing some calculation questions and came across this question.

Determine the MINIMUM size ungrounded (phase) 75degree ALUMINUM service entrance conductors required for a 150ampere service of a one-family dwelling with a 120/240volt ,3wire, single phase electrical system.

table 310.12 give you 2/0 aluminum.
Or 150x83%=125amps. Next size up is 2/0 rated for 135amps.

Simple right, well after the quiz going through checking my answers and this is what I came across for theirs.

150amperes x 83%124.5
*note-size 1/0 AWG aluminum conductors with an ampacity of 125amperes should be selected from table 310.12.

Am I correct in saying they are wrong, because they are applying the 83% to there service rating than using that number (124.5ampere) as there service size selection out of T310.12 to select an even smaller size conductor.

 

[tortuga]

150x83%=125amps. Next size up is 2/0 rated for 135amps.

I agree with this 2/0 AL (135A conductor) for a 150A residential dwelling service.
You must be on the '17 code? they brought back T310.12 for the 2020, and it confirms that.

 

[Dsg319]

I agree with this 2/0 AL (135A conductor) for a 150A residential dwelling service.
You must be on the '17 code? they brought back T310.12 for the 2020, and it confirms that.

I was in the 2020. I used both the 83%rule and the table 310.12 to cross check myself.

 

[tortuga]

OK sorry I missed that your on the 2020.
I just re-read 310.12(A)
The 83% rule is still in there, I just thought they switched back to the table.
So it appears you can use either the table or the 83% rule.
(EDIT I still dont see how 1/0 AL would be a correct answer)

 

[Dsg319]

OK sorry I missed that your on the 2020.
I just re-read 310.12(A)
The 83% rule is still in there, I just thought they switched back to the table.
So it appears you can use either the table or the 83% rule.
(EDIT I still dont see how 1/0 AL would be a correct answer)

Yeah they seem to be using the 83% rule and the table combined together to reduce sizes twice.

 

[tortuga]

Yeah that's odd.

For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load
associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling
unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating.
If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

I dont think you can apply the 83% to the service rating then take that value to the table 310.12, but that's what they seem to have done.