Wendell Swedberg
05-17-2008, 02:36 PM
The below is the question and answer for an ICC test question.
I understand what they did to get to the answer (A)......my question is why are they not using the 60 degree C column of Table 3605.1 because the terminals are rated for 60 degree centigrade to get the allowable ampacity?
In the 60 degree column, their isn't a rating for 15A, so you go to the next highest ampacity which is 20 amp...and that corresponds to 15 AWG?
Am I way off base on my thinking??
W
Given:
A NM cable branch circuit serves dwelling receptacles
15 amp circuit breaker protects the circuit
15 amperes is the maximum load
The branch circuit runs through the attic
The maximum attic temperature is 110 degrees F
The conductors are copper with an insulation rating of 90 degrees C
The terminals are rated for 60 degree centigrade
What is the minimum conductor size permitted in the NM cable?
A) number 14
B) number 12
C) number 10
D) number 8
The answer is (A). The answer comes from 2006 IRC Section E3605.4, E3605.4.4 and Table E3605.1 and Table E3605.2. Conductors must be sized to carry the load after the application of correction factors for ambient temperatures which are found in Table E3605.2 Dividing the load by the applicable ambient correction factor will provide an adjusted ampacity value which can be used to determine the minimum conductor size for the load served with the ambient correction included. The conductor in question is NM cable which has conductors rated for 90 degrees C. This information can be found in Section E3605.4.4 So: a 15 amp load divided by .87 [the correction factor .87 is derived from the 90 degrees C column [based upon conductor insulation] of Table E3605.2 for conductors living their lifetime in an ambient temperatures of 105-113 degrees F] = 17.24 amps. To service the load in question with the conditions and materials stated, a conductor with a minimum adjusted ampacity of 17.24 amps is necessary. Number 14 with an insulation of 90 degrees C actually has an ampacity of 25 amps, and this information is found in Table E3605.1, making choice 'A' the only correct answer.
Note: Rounding down or dropping the fraction of less than 1/2 ampere is not an option for ampacity or overcurrent calculations
I understand what they did to get to the answer (A)......my question is why are they not using the 60 degree C column of Table 3605.1 because the terminals are rated for 60 degree centigrade to get the allowable ampacity?
In the 60 degree column, their isn't a rating for 15A, so you go to the next highest ampacity which is 20 amp...and that corresponds to 15 AWG?
Am I way off base on my thinking??
W
Given:
A NM cable branch circuit serves dwelling receptacles
15 amp circuit breaker protects the circuit
15 amperes is the maximum load
The branch circuit runs through the attic
The maximum attic temperature is 110 degrees F
The conductors are copper with an insulation rating of 90 degrees C
The terminals are rated for 60 degree centigrade
What is the minimum conductor size permitted in the NM cable?
A) number 14
B) number 12
C) number 10
D) number 8
The answer is (A). The answer comes from 2006 IRC Section E3605.4, E3605.4.4 and Table E3605.1 and Table E3605.2. Conductors must be sized to carry the load after the application of correction factors for ambient temperatures which are found in Table E3605.2 Dividing the load by the applicable ambient correction factor will provide an adjusted ampacity value which can be used to determine the minimum conductor size for the load served with the ambient correction included. The conductor in question is NM cable which has conductors rated for 90 degrees C. This information can be found in Section E3605.4.4 So: a 15 amp load divided by .87 [the correction factor .87 is derived from the 90 degrees C column [based upon conductor insulation] of Table E3605.2 for conductors living their lifetime in an ambient temperatures of 105-113 degrees F] = 17.24 amps. To service the load in question with the conditions and materials stated, a conductor with a minimum adjusted ampacity of 17.24 amps is necessary. Number 14 with an insulation of 90 degrees C actually has an ampacity of 25 amps, and this information is found in Table E3605.1, making choice 'A' the only correct answer.
Note: Rounding down or dropping the fraction of less than 1/2 ampere is not an option for ampacity or overcurrent calculations