Exam Prep- Min. Conductor size

6

64th_Chamber

Member
Location
Downey, CA
Occupation
Inside wireman
Hello all,

I'm prepping for my up coming journeyman test and I missed a prep test question that I thought I understood.
I chose 1 awg because 110.14(1)(a)(2) states conductors higher temperature ratings provided the ampacity of such conductors is determined based on the 60c ampacity of the conductor size, when terminating circuits that are 100 amps or less. Can some one please clarify the code to me.

Thanks

https://imgur.com/a/jGudUng
 
The problem with the question is it doesn't state what the rating is of the equipment, ie. the panel. I think they are saying that since the breaker is 60/75 then the panel is rated 75C so they answered with the 75C rating. This would be correct. The wire size only matters if we don't know the rating of all the equipment. Today almost everything out there is 75C rating.

This is the part in 110.14 that is telling

Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.12.
Click to expand...
 
There are several factors to consider in the question:

1) THWN which is a 75° C conductor.
2) 60/75° C rated terminal on the circuit breaker.
3) EMT, they're telling you that this is not a wiring method like NM cable that must be sized at 60° C.
4) 100 amps, this is the minimum conductor ampacity.

When you take all four of those factors and compare them to the answers you'll find that No. 3 AWG is correct because you're looking for a 100 amp THWN conductor and a 75° C ampacity.
 
infinity said:
There are several factors to consider in the question:

1) THWN which is a 75° C conductor.
2) 60/75° C rated terminal on the circuit breaker.
3) EMT, they're telling you that this is not a wiring method like NM cable that must be sized at 60° C.
4) 100 amps, this is the minimum conductor ampacity.

When you take all four of those factors and compare them to the answers you'll find that No. 3 AWG is correct because you're looking for a 100 amp THWN conductor and a 75° C ampacity.
Click to expand...
I think Mike Holt would have answered this at 60C without knowing the panel rating. He does this alot on his tests.
 
IMO the pertinent information has been given. If the answer is based on assumptions (missing information) it is a trick and the test writer sucks.
 
Dennis Alwon said:
The problem with the question is it doesn't state what the rating is of the equipment, ie. the panel. I think they are saying that since the breaker is 60/75 then the panel is rated 75C so they answered with the 75C rating. This would be correct. The wire size only matters if we don't know the rating of all the equipment. Today almost everything out there is 75C rating.

This is the part in 110.14 that is telling
Click to expand...
Thanks. This clears it up a little.
Them clearly stating 75C rating allows you to use 110.14(C)(1)(a)(3)

Thank you everyone for the input
 
Dennis Alwon said:
All the equipment must be rated at 75C to take advantage of the 75C rating. We have the wire insulation, the breaker but not the equipment (panel) . If the panel doen't have a 75C rating then we must assume 60C for 100 amps or less.
Click to expand...
The question does not mention a panel but I see that you're considering 110.14(C)(1). Even if it is a panel it does not say that the feeder is terminating on some piece of equipment that is rated 100 amps or less. Just because the feeder size is 100 amps doesn't automatically indicate that the equipment at the end of the feeder should be considered to be only rated for 60° C.
 
infinity said:
The question does not mention a panel but I see that you're considering 110.14(C)(1). Even if it is a panel it does not say that the feeder is terminating on some piece of equipment that is rated 100 amps or less. Just because the feeder size is 100 amps doesn't automatically indicate that the equipment at the end of the feeder should be considered to be only rated for 60° C.
Click to expand...
My point is if you don't know the temp rating for the all the equipment then you must consider 60C for under 100 amps. Where does it say anything different.

I agree that it is a trick question if they wanted 60C however technically that would be the correct answer.
 
Dennis Alwon said:
My point is if you don't know the temp rating for the all the equipment then you must consider 60C for under 100 amps. Where does it say anything different.
Click to expand...
I hear what you're saying but the question says that the feeder is 100 amps not the panel. You're assuming that the panel is rated for 100 amps but there is nothing in the question to indicate the amp rating of the panel. What if the panel is rated for 125 amps with a 100 amp feeder, the question doesn't say that it is 100 amps or that it isn't 125 amps? The question was written (poorly) but appears to use the 75° C ampacity of every component so that's how they've arrived at the answer of #3 AWG. Assuming that the panel rating is only 100 amps so that you can apply 110.14(C)(1) is just an assumption and would be adding wording to the question that doesn't exist.
 
Dennis Alwon said:
I hear you also but aren't you assuming the panel isn't 100 amp?
Click to expand...
No. The question actually says lighting feeder so there might not even be a panel involved. We all agree that the question is poor. They appear to be asking to find the conductor ampacity when everything is 75°.

Your point is valid for these types of questions when you have a panel at the other end with a 100 amp rating. The question makes no mention of such a panel. If the question used 150 amp feeder instead of 100 amp feeder our discussion would be moot.
 
infinity said:
No. The question actually says lighting feeder so there might not even be a panel involved. We all agree that the question is poor. They appear to be asking to find the conductor ampacity when everything is 75°.

Your point is valid for these types of questions when you have a panel at the other end with a 100 amp rating. The question makes no mention of such a panel. If the question used 150 amp feeder instead of 100 amp feeder our discussion would be moot.
Click to expand...
I hear you but I am confused. How can you have a 100 amp feeder and not have a panel where the feeder originates from?
 
Dennis Alwon said:
I hear you but I am confused. How can you have a 100 amp feeder and not have a panel where the feeder originates from?
Click to expand...
The feeder originates at the circuit breaker that the question tells us is rated 60/75°C. What we don't know is what's on the other end of that feeder or it's amp rating. Without the question stating the amp rating there is no way to apply 110.14(C)(1).

The assumption is that since the feeder is rated for 100 amps that you would use the 60°C conductor value due to 110.14(C)(1) but that is not part of the actual wording of the question. It says a 100 amp feeder it doesn't say a 100 amp feeder feeding a 100 amp panel.

Lets see if we can get on the same page.
Scenario 1:
100 amp breaker 60/75° terminals>THWN lighting Feeder in EMT. According to the question parameters it's ends there without making some assumptions.
For scenario 1 everything is rated for 75° C so the 75° C ampacity can be used to determine the minimum conductor size.

Scenario 2:
100 amp breaker 60/75° terminals>THWN Feeder in EMT>100 amp rated panel (assumption).
For scenario 2 the panel at the end is only 100 amps so we must use the 60° C ampacity to determine the minimum conductor size. {110.14(C)(1)}

What's ahead of the 100 amp circuit breaker at the origin of the feeder is irrelevant.
 
Last edited:
You must log in or register to reply here.
Top