recently attended an IRC Combination Dwelling Inspector
Exam Preparation course in Virginia sponsored by Northern
Virginia ASHI. Some of the topics discussed were how to
solve typical problems presented in the exams. I would like
to share one of the typical electrical problems and share
the process of producing the correct answer. The below will
be based upon the 2003 International Residential Code, 4th
temperature 135 degrees (57°C)
the ampacity of number 12 AWG conductors in a type NMC-B
12-2 w/g cable that is bundled with six similar cables run
through an attic in Las Vegas, Nevada.
the question is not asking the maximum size overcurrent
protection device - it is asking the maximum ampacity so
table 3605.5.3, where you might go first, does not apply
to this question.
we must go to section 3605.4.4 to determine what temperature
rating column to use when determining the ampacity of type
NM cable conductors. We see in this section that for type
NMC-B cable we use the 90°C column.
we locate table 3605.1 and in the 90° column for copper
conductors we see that the allowable ampacity for number
12 is 30 amperes.
we must locate the ambient temperature correction factor
due to the attic conditions. Again in the 90° copper
conductor column we see the correction factor is .71 in
the 56-60 ambient temperature row. We take 30 times .71
and the result is 21.3 amperes.
we must modify this result with the conductor proximity
adjustment factor since there are cables bundled for distances
greater than 2 feet (section 3605.3). The problem indicated
"with six similar cables" indicating our total
cables is seven. Since these are 12-2 conductors we count
both the hot and neutral for a total of 14 conductors (both
hot and neutral are current-carrying, we would also count
both if it was 12-2 and a 240 volt circuit, both are current
carrying. The only time you do not count the neutral in
a condition like this is if the problem indicated 12/3 cable;
the neutral only carries the difference in a 120-v multiwire
circuit and the neutral is not used in a true 240-volt circuit).
Looking at table 3605.3 we see the adjustment factor is
.50 in the 10-20 conductors row. Taking our previous answer
of 21.3 amperes times .50 we see our total ampacity is now
10.65, we could round this to 11 amperes.
this helps us better understand use of the electrical tables
in the IRC.
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