what is the answer to these questions?

S

solar man

Member
Location
california
Occupation
self emplyed
Hi all -
I am trying to figure out the answers and the logic behind the answer for the following 3 questions.

1) What's the minimum size THHN conductor permitted to terminate on a 70A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75C conductor sizing?

a) 2 AWG
b) 4 AWG
c) 6 AWG
d) 8 AWG
e) 10 AWG

2) What's the minimum size THHN conductor permitted to terminate on a 50A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 60C conductor sizing?

a) 2 AWG
b) 4 AWG
c) 6 AWG
d) 8 AWG
e) 10 AWG

3) What's the minimum size THHN conductor permitted to terminate on a 50A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75C conductor sizing?

a) 2 AWG
b) 4 AWG
c) 6 AWG
d) 8 AWG
e) 10 AWG
 
These are clearly test questions and easy ones at that. Do you have an NEC and if so where did you look? We won’t answer for you.
And what are you self employed at, are you studying for an electrical position?
 
Welcome to the forum.

We like to teach, not answer. Try to answer your own questions first.

We are happy to tell you which NEC tables to look in if needed.
 
Another hint all three questions needs to take 240.4(B) into consideration and keep in mind they are asking for the smallest conductor size allowed for each situation.

This assumes they are asking for general use situations, there are special applications where this answer can be different probably the most common ones is with motors and air conditioning equipment.
 
So, the logic is when the terminal is not rated for a higher temperature, the wire can't operate at a temperature that exceeds the terminal temperature. For the rest, I am with the other posters. I agree these are easy questions. I would refer you to 110.14 to understand the issue with the rating of the terminals.
 
Hi I appreciate you all taking the time to respond.
So here is what I think the answers should be....

Logic -
- As per 110.14 - Conductors are to be sized in accordance with the lowest temperature rating of the terminal, device or conductor of the circuit.
- Since the questions are asking for minimum size THHN conductor, we will look at the copper conductors and not aluminum conductors

1) Since the equipment and terminals are listed for 75 C we look for the needed ampacity (70A) in the 75 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (70A) will be satisfied by #4 AWG copper.

2) Since the equipment and terminals are listed for 60 C we look for the needed ampacity (50A) in the 60 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (50A) will be satisfied by #6 AWG copper.

3) Since the equipment and terminals are listed for 75 C we look for the needed ampacity (50A) in the 75 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (50A) will be satisfied by #8 AWG copper.

As mentioned in answer by Kwired - all are related to 240.4(B) - this is what I am confused about??

I understand that there is a round up rule; but isn't it for just when you are sizing the OCPD device? So, I don't know how 240.4(B) is related to these questions.. that's what I am trying to figure out. Thank you
 
solar man said:
As mentioned in answer by Kwired - all are related to 240.4(B) - this is what I am confused about??

I understand that there is a round up rule; but isn't it for just when you are sizing the OCPD device? So, I don't know how 240.4(B) is related to these questions.. that's what I am trying to figure out. Thank you
Click to expand...

240.4(B) is the 'next size up' rule. It says that when the wire ampacity is not a standard overcurrent device rating (see 240.6(A)) then the next size up standard breaker can be used (unless over 800A).

This affects your answer to question 1.
 
To be pedantic, the questions are poorly worded. Any of these breakers could be used to supply a tap conductor under 240.21(B). So the answer is really whatever the smallest conductor the breaker is labeled for.

These and other problems would be fixed by changing "THHN conductor" to "THHN branch circuit conductor not subject to 240.4(G)." In a test situation, I think it's reasonable to assume that is the test question writer's intent.

Cheers, Wayne
 
I did..

Here is repost. Thank you again for everyone's help.
I don't want to move on to other topics if I don't understand this one. I feel like everything kind of builds off of these tables ( ampacity and ocpd)
(
Hi I appreciate you all taking the time to respond.
So here is what I think the answers should be....

Logic -
- As per 110.14 - Conductors are to be sized in accordance with the lowest temperature rating of the terminal, device or conductor of the circuit.
- Since the questions are asking for minimum size THHN conductor, we will look at the copper conductors and not aluminum conductors

1) Since the equipment and terminals are listed for 75 C we look for the needed ampacity (70A) in the 75 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (70A) will be satisfied by #4 AWG copper.

2) Since the equipment and terminals are listed for 60 C we look for the needed ampacity (50A) in the 60 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (50A) will be satisfied by #6 AWG copper.

3) Since the equipment and terminals are listed for 75 C we look for the needed ampacity (50A) in the 75 C copper column.
So.. in Table 310.15.B.16 - We see that the needed ampacity (50A) will be satisfied by #8 AWG copper.

As mentioned in answer by Kwired - all are related to 240.4(B) - this is what I am confused about??

I understand that there is a round up rule; but isn't it for just when you are sizing the OCPD device? So, I don't know how 240.4(B) is related to these questions.. that's what I am trying to figure out. Thank you
)
 
solar man said:
I did..
Click to expand...
If we assume the question writer meant "pay attention to 240.4(B), but not 240.4(G), nor 240.21(B)," then the point of the comments about 240.4(B) is that the minimum ampacity you can protect with a 70A breaker is 61A, and the minimum ampacity you can protect with a 50A breaker is 46A. 1 more than the next lowest size standard breaker in each case.

Does that change your answers to the test questions?

Cheers, Wayne
 
I get it guys.. thank you.

It's basically next size up rule.
So,
For #1 - we use #6 since next size up 65 amps would be 70amps which would equal 70 amps

For #2 - we choose #6 because if we choose #8, next size up would be 45 amps and not 50 amps on table 240.(6)A

And for #3 - we choose # 8 and not #10 as next size up for #10 would be 40 amps and not 50 amps.

Is my logic now correct? or no? please let me know.. ty
 
solar man said:
I get it guys.. thank you.

It's basically next size up rule.
So,
For #1 - we use #6 since next size up 65 amps would be 70amps which would equal 70 amps

For #2 - we choose #6 because if we choose #8, next size up would be 45 amps and not 50 amps on table 240.(6)A

And for #3 - we choose # 8 and not #10 as next size up for #10 would be 40 amps and not 50 amps.

Is my logic now correct? or no? please let me know.. ty
Click to expand...
Do you have an answer key to these questions? If not, how are you determining that the writers meant the "next size up" rule?
It may just be a poorly written question and they meant to just use the table for the ampacity.
 
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