Sizing Conducter To overcurrent Device

[Flight987]

What is the Mim. size THW, feeder we ca install on the following overcurrent device.

1-350 ampere 500kcm/350kcm
2-300 ampere 350kcm/250kcm
3-125 ampere #1/#2
4-400 ampere600kcm/500kcm
5-500 ampere 900kcm/700kcm
2-wires per phase 250kcm/ 4/0
6- 90 ampere #3/#4
The first Answ. our mine and that Sec. numbers are test key Answ.
First thing I messed up, was forgot about 310.4 when Con. got smaller than 1/0.
Thought this will be a piece of cake to get right.
The round up rule can fool you I found out.
110.14(C)(1) for some reason has a way to get me confused. Then we put in 240.4(B).
Where does the round up start? I see how the correct Answ. work after looking closer at 310.16.
Because we aren't given a load, there asking what the code allows for Min. size. So you assume
that the load your working with is higher tha the next lower size rating. Is that correct?
110.14(C)(1) makes my mind spin. I get confussed (some what), on the 100 Amp. or below load's and
the Temp. rating's and that stuff. Everybody say's THAT'S THE WAY WE DID IT FOR YEAR'S. Well,
that doesn't make it right. Am I alone on 110.14(C)(1)

 

[ron]

Luckily in real life, I haven't seem a 60 deg termination in a long time (even for equipment <100A), so for the most part, you just stick to the 75 deg col.

 

[Flight987]

I still need

I still need

I still need some help to explain my first theard. I have talked to guy's that took the AJ test in Min., and they said
that there were a number of Ques. regarding this type of thing. If it was simple, it wouldn't be on the test.
Take Care.

 

[david luchini]

I still need some help to explain my first theard. I have talked to guy's that took the AJ test in Min., and they said
that there were a number of Ques. regarding this type of thing. If it was simple, it wouldn't be on the test.
Take Care.

110.14(C)(1) has nothing to do with the question. This is strictly a 240.4(B) (and 240.6(A)) question. 240.4(B) allows conductors to be protected by the next higher standard overcurrent device.

Take the first example, a 350A OCPD. The next smaller size standard OCPD rating is 300A, from 240.6(A). So the smallest THW feeder we can install is the first THW conductor size that has an ampacity greater than 300. You can see from the ampacity table that 350mcm has an ampacity of 310A, so it would be the smallest allowable THW feeder allowed on a 350A OCPD. (If you look at 300mcm, you will see it has an ampacity of 285A, so the next standard size up from that is 300A. Therefore 300mcm is NOT permitted on a 350A OCPD.)

Applying the same method to the other OCPD sizes in the question will give you the same answers as the answer key.

 

[Dennis Alwon]

The problem with this question is that one tends to think that a 350 amp overcurrent device assumes a 350 amp load. Given a load of 310 amps then 350KCM is the smallest size possible.

 

[Dennis Alwon]

Remember that we can use the next higher size on ocpd up to 800 amps. The answers would be different for a 1000 amp ocpd since you cannot use the next size.

Back to the question-- I could argue that 250KCM is the smallest conductor size based on ambient temp of 50F or less.

 

[augie47]

The only answer that troubles me is #6.
In book terms, the 90 amp breaker would be linited to 60? rating per 110.14.
For that reson I would have said a #3 also.

 

[Flight987]

I thought twice

I thought twice

I thought twice before I posted this thread. Like I was the only person that was not getting it. I did not think
that anyone would respond to it. I have not had time to read what is there yet.

 

[Dennis Alwon]

The only answer that troubles me is #6.
In book terms, the 90 amp breaker would be linited to 60? rating per 110.14.
For that reson I would have said a #3 also.

Although I agree with you, I don't think most tests take 60C into consideration since most terminals are 75C unless it is old pieces of equipment.

 

[Flight987]

That why I asked

That why I asked

That is why I asked? Go back to my first thread. I get so messed up in that 90 to 100 amp range. I had
a range Cal. back some time and the senior Mod. said somthing like it should be a # so and so because it is
below 100 amp's. Ok, With that, I beleve him! But it came out to be whatever it was, and if remember
it was like a #4 75c , and that is to be under 100 amp's, so do we use 60c collium. I addmit that having
working with older people in the trade, I tend to go by what thay say. That doesn't make them wrong or
right. So, Take Care

 

[Dennis Alwon]

Curt, we don't have an answer for you simply because they are bad questions where they don't give that info. If only you could give both answers based on 75C and 60C. The question can mean one thing on one test and another thing on another test. There isn't enough info. I would ask before the test whether to assume 60C from 100 amps and below or not. I a sure they won't have an answer.

 

[Flight987]

That is what I don't get. How it work's

That is what I don't get. How it work's

The only answer that troubles me is #6.
In book terms, the 90 amp breaker would be linited to 60? rating per 110.14.
For that reson I would have said a #3 also.

That is what I don't get?

 

[Flight987]

It's not what we think

It's not what we think

How does it applay to the code? I know that when sizing the wire to the OCPD. we can do it as I understand at
this point. Take a given lower to make a next higher fuse device. Old new's!

Now! Back to 110.14(C)(1). Go back to thread. I have looked at this sec. for a long time (year's). I
can make more sence of 220.55 (Range Cal.) than this! As we can see from the response to the thread
people have different Anser. I don't know, that is why I ask.

 

[Flight987]

This is where I get confused

This is where I get confused

Different part of practice test.
The Ques. For each THW copper feeder conductor listed below, provide the maximum standard size overcurrent
device that the code allows you to use o fht egiven conductor:

I know that it is 75*c, they don't tell me that there 60*c. I did all the other Par. Ques., no problem.

Q#37. #1: 150 amps
Q#38. #2: 125 amps
Q#39. #3: 100 amps
Q#40. #4: 90 amps
Q#41. #6: 70 amps
Q#42 #8: 50 amps

I did great, tell I hit Q#40. Here is a good example, to where I get confussed on the 100 amp's or less rule.
These are the An. key numbers. We have to use these smaller Cond. on the job all the time. I have work
in the trade for along time with my father and am going for AJ test. I allway's look at this and go Huh!
No, Dad and I have been guessing and I admit it. Our guess must have been right. Further back on
on another thread that I sent in, the Ques. is there to see and I said #3 was the size needed and the
Answ. was #4. You may look back and see that.

 

[Dennis Alwon]

I believe we already determined that this test is looking at 75C no matter what amperage. #4 is 85 amps at 75C so a 90 amp breaker is okay