Ampacity & ambient temperature question

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Ajr188

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electrician
Hey all,

new to the site, I have been studying for my master Electrial license exam out of the 2017 nec code book. I came across these two questions and I am not sure how to pull up the correct answer. can anyone help me out if you have time.

Question 1

The ampacity of a #12 THW-2 conductor, when there are six conductors in a conduit and the temperature is 30° C, would be __ amps.

Question 2
What is the maximum current allowed on a #12 THWN-2 copper conductor in an ambient temperature of 122° F with a total of eight current carrying conductors in the conduit?


I have the answer but it does me no good because I have no idea how to come up with them
 
Question 1& 2 look up in the table 310.15(B)(16) amp of #12... Use the correct column..... Then T. 310.15(B)(2)(a) and then use T. 310.15(B)(3)(a)
 
Here is the answer to #1 when you have tried it

#12 wire in the 90C is rated 30 amps.

30C has a correction factor of 1 so that doesn't come into play.

Assuming 6 current carrying conductor's then we have an 80% factor

30 amps x 1 x .8 = 24 amps however in most cases you cannot use #12 at an ampacity of 20 amps except for derating. See the double asterisks at the bottom of T. 310.15(B)(16)
 
There is no keyword. You need to know where to look for conductor stuff as T. 310.15(B)(16) is used all the time in the trade. I suppose you could try ampacity adjustments.
 
The first question cannot be answered as written as it does not specify how many of the six conductors are current carrying.

In the printed version the key word in the index, would be "correction factors"
 
don_resqcapt19 said:
The first question cannot be answered as written as it does not specify how many of the six conductors are current carrying.

In the printed version the key word in the index, would be "correction factors"
Click to expand...


Yes, that is why I replied assuming they are current carrying conductor's. Not a great question but based on question 2 I am guessing that is what they meant.
 
Dennis Alwon said:
Question 1& 2 look up in the table 310.15(B)(16) amp of #12... Use the correct column..... Then T. 310.15(B)(2)(a) and then use T. 310.15(B)(3)(a)
Click to expand...
Can you please help me with the key words you use to find this answer
Dennis Alwon said:
Yes, that is why I replied assuming they are current carrying conductor's. Not a great question but based on question 2 I am guessing that is what they meant.
Click to expand...
Your first question the answer is correct any idea about the second?
 
Ajr188 said:
Can you please help me with the key words you use to find this answer

Your first question the answer is correct any idea about the second?
Click to expand...

How about trying the second one and see what you get.

The calculation is similar so follow the demand factor and then multiply by the ampacity of the #12.

Table ampacity of #12 at 90C x demand factor for temp x demand factor for # of current carrying conductor's.
 
Dennis Alwon said:
How about trying the second one and see what you get.

The calculation is similar so follow the demand factor and then multiply by the ampacity of the #12.

Table ampacity of #12 at 90C x demand factor for temp x demand factor for # of current carrying conductor's.
Click to expand...
Ok great
 
Dennis Alwon said:
Dennis Alwon said:
Here is the answer to #1 when you have tried it

#12 wire in the 90C is rated 30 amps.

30C has a correction factor of 1 so that doesn't come into play.

Assuming 6 current carrying conductor's then we have an 80% factor

30 amps x 1 x .8 = 24 amps however in most cases you cannot use #12 at an ampacity of 20 amps except for derating. See the double asterisks at the bottom of T. 310.15(B)(16)
Click to expand...
Click to expand...
Where do I find 30c correction factor… and the rest of them for that matter
 
Dennis Alwon said:
How about trying the second one and see what you get.

The calculation is similar so follow the demand factor and then multiply by the ampacity of the #12.

Table ampacity of #12 at 90C x demand factor for temp x demand factor for # of current carrying conductor's.
Click to expand...
30x1x.7

21 amps
Is what I got
 
30x.82x.7= 17.22 amps

thanks so much. Sorry for my ignorance

i’m still a little confused how you are coming up with 90°C for your table reference
 
Ajr188 said:
30x.82x.7= 17.22 amps

thanks so much. Sorry for my ignorance

i’m still a little confused how you are coming up with 90°C for your table reference
Click to expand...
It's in the wire type. At the top of each column in Table 310.16 it lists the wire type (insulation type, actually) that determines whether it is 60, 75, or 90 degree wire. The wire types are in the question statements.
 
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