S.O.S. !!! Understanding The National electrical Code Vol 1 2014 page 148

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NRG

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California
  1. I understand the 25A listed at 75 degree terminals landing on 25a on table 310.15(B)(16)
  2. considering the >100 amp rule but I am confused because the terminals are sized to 75 degrees
  3. since the 25 amp calculation falls on 25amps exactly on the 75 degree column is that the reason for sizing it to a #10 or is it the >100 amp rule or somthing else?? :slaphead: PLEASE HELP I AM PREPARING TO TAKE THE ICC E2 EXAM VERY SOON AND BUMPING MY HEAD ON THIS LOGIC




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steve66

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Illinois
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Engineer
I believe its due to the rule that #12 must be protected at 20 amps or less unless specifically permitted otherwise.

I don't remember the reference offhand.

Edit: 240.4(D) in the 2008 code.
 

charlie b

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Lockport, IL
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I agree with Steve, and his reference is the same in the 2017 edition. You need a conductor that can handle 25 amps. Table 310.15(B)(16) says that at 75C, a #12 can handle 25 amps. But at the bottom of the table there is a reference to 240.4(D). If you can only protect a #12 at 20 amps, then you can't use it to supply a load of 25 amps. It is a subtle rule, and a bit of a tricky question.
 

Dennis Alwon

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Chapel Hill, NC
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Look at T 310.15 and then look at the asterisks next to wire sizes 14-10. The asterisk is noted at the bottom of the table to see 240.4(D)
 

RD35

Senior Member
Many many moons ago, I connected up my first ever electric water heater. Was still a green horn and working to get familiar with the code book. Connected the 4500w heater with #12 wire on a 2P 20A breaker. I figured that since the heater would pull 18.75A and would never run for 3 hours without cycling off that it was considered non-continuous. Inspector never said a word so I assumed it was correct. (I now realize he just flat out missed it.) When I finally was introduced to 422.13 I went out and changed that water heater over to #10 and a 30A breaker. :ashamed1: Pretty embarrassing but had to do the right thing!
 
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