Conductor Ampacity - InspectionNews

Wendell Swedberg · #1 (**permalink**) 04-23-2008, 01:12 PM

ICC test question:

Number 12 AWG THHN , copper conductors that are located in the attic and are part of the NM cable. The ambient temperature around the conductors will be 110F. The terminal devices are not rated for more than 60degrees C......What is the conductor ampacity for this condition?

A) 20
B) 26
C) 30
D) 35

I get 30 amps from table 3605.1.....the answer is B, 26 amps...what gives?

Thanks as always for your help.

Wen

Bob White · #2 (**permalink**) 04-23-2008, 01:50 PM

Quote:

Originally Posted by Wendell Swedberg

ICC test question:

Number 12 AWG THHN , copper conductors that are located in the attic and are part of the NM cable. The ambient temperature around the conductors will be 110F. The terminal devices are not rated for more than 60degrees C......What is the conductor ampacity for this condition?

A) 20
B) 26
C) 30
D) 35

I get 30 amps from table 3605.1.....the answer is B, 26 amps...what gives?

Thanks as always for your help.

Wen

Section E3605.2 sez that for ambient temperatures other than 30 degrees C (86 deg. F), mutiply your table 3605.1 allowable ampacity by th correction factor in table 3605.2 (0.87).

30 * 0.87 = 26.1 ......

Gunnar Alquist · #3 (**permalink**) 04-23-2008, 02:32 PM

If I remember correctly, there is a line in the NEC that states that #14 cannot exceed 15 amps and #12 cannot exceed 20 amps. Even though the tables indicate otherwise. I never understood what that was all about.

Michael Larson · #4 (**permalink**) 04-23-2008, 02:48 PM

Quote:

Originally Posted by Gunnar Alquist

If I remember correctly, there is a line in the NEC that states that #14 cannot exceed 15 amps and #12 cannot exceed 20 amps. Even though the tables indicate otherwise. I never understood what that was all about.

That is true for residential. I don't have the code reference handy.

Jerry Peck · #5 (**permalink**) 04-23-2008, 04:20 PM

Quote:

Originally Posted by Gunnar Alquist

If I remember correctly, there is a line in the NEC that states that #14 cannot exceed 15 amps and #12 cannot exceed 20 amps. Even though the tables indicate otherwise. I never understood what that was all about.

Gunnar,

Wendell asked, and Bob correctly gave, the ampacity rating of the conductor in that condition.

You are referring to the maximum allowable overcurrent protection allowed, and the ampacity of the conductor in all derated conditions must exceed that for that overcurrent protection to be used.

Thus, for the question, Bob is correct, and, for overcurrent protection of that conductor in that condition, you are correct.

It is not separated 'residential' or 'non-residential', that applies to all installations.

Michael Larson · #6 (**permalink**) 04-23-2008, 05:40 PM

My error. I was thinking of overcurret protection not circuit ampacity.

Gunnar Alquist · #7 (**permalink**) 04-23-2008, 07:14 PM

JP,

Um... right. I knew that.

This would be why I am a home inspector and not a "code inspector". Guess I am not going to be taking the ICC exam any time soon.