Thread: Minimum conductor size

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

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Wendell,

I don't follow what you are saying/asking above, however, let's start with the proper way to do it and see if that answers your question.

(code references are from the 2006 IRC)

Table E3605.5.3 shows that the maximum overcurrent protection allowed for various conductors, with 14 AWG copper having a maximum overcurrent rating of 15 amps. (Note that the two main column headings are split wrong, that the dividing line between "copper" and "aluminum or copper-clad aluminum" is in the wrong place - it is over the right side of the first column and should be over the right side of the second column, reading the chart, you will see that the chart only makes sense if you correct that 'editorial error' ... and it may have been corrected in printings or in newer CDs than I have.)

Thus, 14 AWG copper is allowed to have a maximum of 15 amps overcurrent protection, thus, you can use 14 AWG copper on a 15 amp protected circuit.

However, you now need to go through the derating requirements for ambient temperature (which is why they threw the attic temperature in there) and for bundling (if any). And, you need to do that derating from the conductor insulation temperature rating/ampacity rating (and they gave you a guide to that too).

You cannot exceed 15 amps overcurrent on 14 AWG, but, if the derating does not work out, you might need 12 AWG to make 15 amps with the derating factors applied.

In the question, the 14 AWG insulation rating derating worked out to still allow 15 amps for that 14 AWG NM-B cable.

Did that help?

Thanks, Jerry. I understand your approach. Let's try my question this way.

When do you apply E3605.4.1 Conductors rated at 60 degrees C ?
I intrepret 3605.4.1 to mean that where the temperature rating on the termination of the device is 60 degrees or less, you need to use the 60 degree (first column) of Table E3605.1 (allowable ampacities).

Sooooo....going back to my first post, because in the question it states that the terminals are rated for 60 degrees C, I used the first column (60 degrees C) of Table 3605.1. Because there is no rating for 15 Amps in the 60 degrees column, I went to the next highest rating which was 20amps. 20 amps corresponds with 14 AWG (answer A).

It seems your approach and mine gets the correct answer. But am I applying E3605.4.1 correctly? The answer ICC provided uses the 90 degree column in Table E3605.1 which is allowed by E3605.5.5.

Thus, again brings me back to, "When do you apply E3605.4.1 Conductors rated at 60 degrees C"? To me in my first post, the 90 degree column doesn't apply because the terminals are rated 60 degrees.

Did I just confuse everything again?

Originally Posted by Wendell Swedberg

Thanks, Jerry. I understand your approach. Let's try my question this way.

When do you apply E3605.4.1 Conductors rated at 60 degrees C ?
I intrepret 3605.4.1 to mean that where the temperature rating on the termination of the device is 60 degrees or less, you need to use the 60 degree (first column) of Table E3605.1 (allowable ampacities).

For the "maximum" allowable ampacity.

Sooooo....going back to my first post, because in the question it states that the terminals are rated for 60 degrees C, I used the first column (60 degrees C) of Table 3605.1. Because there is no rating for 15 Amps in the 60 degrees column, I went to the next highest rating which was 20amps. 20 amps corresponds with 14 AWG (answer A).

You are ignoring (from my previous post) "Table E3605.5.3 shows that the maximum overcurrent protection allowed for various conductors, with 14 AWG copper having a maximum overcurrent rating of 15 amps.", and you cannot ignore that.

You have several conditions which take precedence all at the same time: a) "maximum" allowable ampacity rating for the conductor with a given temperature insulation; b) "maximum allowable ampacity rating for a conductor in a given ambient temperature; c) "maximum" allowable ampacity rating for a conductor in a bundle or without maintaining spacing; d) "maximum" allowable overcurrent device rating for a given conductor size.

First, you need to look up what is the "maximum" allowable overcurrent device rating for that conductor size.

Then, you need to look up the "maximum" allowable ampacity rating of the conductor for its given insulation - a) above -, then you need to address - b) above -, then you need to address - c) above -, all of which confirm that a given conductor size with the given insulation temperature rating with the given bundling, will meet the "maximum allowable overcurrent device rating" - if they do, then you can go by the "maximum allowable overcurrent device rating" for the breaker size.

But, if they do not meet that (i.e., the maximum allowable rating are derated below the maximum overcurrent rating size), then the overcurrent rating would need to be reduced or the conductor size increased.

Sound complicated?

That's because it is.

But am I applying E3605.4.1 correctly?

No. See the above explanation.

Basically, you need to find the starting point (which is at the bottom of the list) then work through the list to see if the starting point still applies, and, if not, you need to change to a different starting point (conductor size and maximum allowable overcurrent protection rating).