derating

[GoldDigger]

Mine's more effective, at the very least in the use of white space

Any good technical writer (of a certain era, anyway) is familiar with Horton's Laws of Publication, one of which states: "White space never lies."

 

[kwired]

Please let us know what you find. Thanks.

I find very good method of determining conductor ampacity, pretty much exactly the way I did it earlier, but that is all it has, and my 4/0 would be fine when it comes to determining minimum ampacity. It does not mention overcurrent protection, but I have come to the conclusion that overcurrent protection and determining ampacity are not the same thing and I simply missed the fact that overcurrent protection of this conductor that otherwise has enough ampacity for the load just doesn't work with the selected conductor. I'm guessing it doesn't work out that way too often, but at same time I wonder how many times I may have missed something similar.

As far as the motor calculation in the other thread, I did not find anything that really specifically addressed the problem I was having there. The book I have did mention same thing NEC mentions about minimum ampacity, but this book does not have any specific examples or instructions of selecting ampacity for motor circuits, just a general ampacity selection section that kind of combines the general requirements from 210, 215, 230 and 310 for ampacity selection. I still concede to what was said in that thread about what I was doing wrong in that particular calculation.

I appreciate the input though, I have learned from this.

 

[Ohms law]

I find very good method of determining conductor ampacity, pretty much exactly the way I did it earlier, but that is all it has, and my 4/0 would be fine when it comes to determining minimum ampacity. It does not mention overcurrent protection, but I have come to the conclusion that overcurrent protection and determining ampacity are not the same thing and I simply missed the fact that overcurrent protection of this conductor that otherwise has enough ampacity for the load just doesn't work with the selected conductor. I'm guessing it doesn't work out that way too often, but at same time I wonder how many times I may have missed something similar.

As far as the motor calculation in the other thread, I did not find anything that really specifically addressed the problem I was having there. The book I have did mention same thing NEC mentions about minimum ampacity, but this book does not have any specific examples or instructions of selecting ampacity for motor circuits, just a general ampacity selection section that kind of combines the general requirements from 210, 215, 230 and 310 for ampacity selection. I still concede to what was said in that thread about what I was doing wrong in that particular calculation.

I appreciate the input though, I have learned from this.

Your wrong and right....


According to Larry T. Smith's 2008 National Electrical Study guide, he states from his 5 step method that you compare steps 1 & 2 and the larger of the two is the correct conductor.

STEP 1: Calculate the load at 125% of continuous load, and 100% of the noncontinuous load.

STEP 2: Select the conductor from the temperature column that corresponds to the equipment rating.

note 1: if derating is not involved your DONE.

note 2: Derating is a completely a new calculation, start over. Use 100% of the load, and select the conductor at its maximum operating temperature. For example, THHN is rated at 90c. You are permitted to use the 90c column for derating.


STEP 3: Adjacent conductor adjustment

STEP 4: Ambient temperature correction

STEP 5: Go to the applicable Table in Article 310 and select the conductor. Compare this conductor with the one selected in Step 1 and 2. The larger of the two is the correct conductor.


So 4/0 would not be minimum ampacity. 250KCMIL would be the minimum if you compared the two, since it is the largest of the two.

 

[david luchini]

According to Larry T. Smith's 2008 National Electrical Study guide, he states from his 5 step method that you compare steps 1 & 2 and the larger of the two is the correct conductor.

Isn't this exactly the method that arrived at an incorrect answer in this thread?

STEP 1: Calculate the load at 125% of continuous load, and 100% of the noncontinuous load. 85*1.25 + 105 =211.25

STEP 2: Select the conductor from the temperature column that corresponds to the equipment rating. #4/0 AWG @ 75 C

note 1: if derating is not involved your DONE.

note 2: Derating is a completely a new calculation, start over. Use 100% of the load, and select the conductor at its maximum operating temperature. For example, THHN is rated at 90c. You are permitted to use the 90c column for derating. 100% load = 85A+105A = 190A


STEP 3: Adjacent conductor adjustment 190/0.8 = 237.5

STEP 4: Ambient temperature correction 237.5/0.96 = 247.4

STEP 5: Go to the applicable Table in Article 310 and select the conductor. Compare this conductor with the one selected in Step 1 and 2. The larger of the two is the correct conductor. #4/0 @ 90 C


So 4/0 would not be minimum ampacity. 250KCMIL would be the minimum if you compared the two, since it is the largest of the two.

Which is larger...#4/0 or #4/0. The 5 step method leaves out the 6th step which is make sure that the conductor is properly protected for its ampacity per 240.4.

 

[Ohms law]

Isn't this exactly the method that arrived at an incorrect answer in this thread?




Which is larger...#4/0 or #4/0. The 5 step method leaves out the 6th step which is make sure that the conductor is properly protected for its ampacity per 240.4.

So looking over and over has just made me confuse myself. You are right. The largest of the two ratings is 211, so that is the rating of the conductor, the conductor size is 4/0 with either answer 211 amp or 200 amp. The sum of the continuous and non continuous at 100% is 190, when you derate 4/0 from the 90c column you get 199.68. And when you take 125% of the continuous and the sum of the non continuous you get 211.25 which is still 4/0.

Your highest conductor rating is 211, 240.4 allows you to use OCPD rated at 225 amp.
So we should be covered right?

 

[david luchini]

The sum of the continuous and non continuous at 100% is 190, when you derate 4/0 from the 90c column you get 199.68.

240.4 allows you to use OCPD rated at 225 amp.
So we should be covered right?

No, the 4/0 90c has an adjusted ampacity of 199.68. 240.4 says the conductor should be protected at its ampacity, or the next standard size up (for 800A and less). The 4/0 90c in the given conditions can't be protected with a 225A OCPD.

 

[Ohms law]

No, the 4/0 90c has an adjusted ampacity of 199.68. 240.4 says the conductor should be protected at its ampacity, or the next standard size up (for 800A and less). The 4/0 90c in the given conditions can't be protected with a 225A OCPD.

200amp OCPD?
I don't ever do load calcs other than simple single motor setups, no derating ever involved, I am trying to understand them better.

 

[david luchini]

200amp OCPD?
I don't ever do load calcs other than simple single motor setups, no derating ever involved, I am trying to understand them better.

We're all here to learn:thumbsup:

No to the 200A OCPD. 215.3 requires the OCPD to be not less than 125% of the continuous load plus the non-continuous load...85*1.25 + 105 =211.25. The OCPD must be 211A or greater. 225A would be the smallest allowable standard OCPD size for the load.

And since the 4/0 has an ampacity to is too small to be properly protected by a 225A OCPD, you need a larger conductor. 250mcm 90C would meet the requirements.

 

[Ohms law]

We're all here to learn:thumbsup:

No to the 200A OCPD. 215.3 requires the OCPD to be not less than 125% of the continuous load plus the non-continuous load...85*1.25 + 105 =211.25. The OCPD must be 211A or greater. 225A would be the smallest allowable standard OCPD size for the load.

And since the 4/0 has an ampacity to is too small to be properly protected by a 225A OCPD, you need a larger conductor. 250mcm 90C would meet the requirements.

211amp being the highest rating of the two and the ampacity of what at least the wire should be rated for, correct? 75c column says 4/0 is rated at 230 amp. No derating when you take 125% of cont. and the sum of non-cont., since this is the highest rating of the two. It's driving me nuts. This is how I read the 5 step method. I think I need a break:blink:

 

[david luchini]

211amp being the highest rating of the two and the ampacity of what at least the wire should be rated for, correct? 75c column says 4/0 is rated at 230 amp. No derating when you take 125% of cont. and the sum of non-cont., since this is the highest rating of the two.

Step 1-2 defines a minimum conductor size. Size, not ampacity. Step 1 tells you that you can't use a conductor smaller than 4/0.


Step 3-5 tells you what ampacity you need to supply the load. You need a conductor with an ampacity (after adjustment/correction) of not less than 190, but the conductor size cannot be less than 4/0 (from Step 1-2.)

Step 6 is to check that the ampacity of the conductor is protected by the OCPD. If it is not properly protected, try a larger conductor adjusted/corrected for the same conditions.

 

[kwired]

Isn't this exactly the method that arrived at an incorrect answer in this thread?

I sure hope so, because the publication mentioned is exactly the same one I was referring to.

The problem is not with the ampacity selection, that works, the problem you brought up is an overcurrent protection issue for the conductor that had been selected.

 

[kingpb]

The difference reminds me of sorta like a top down, or bottom up approach. But each should lead you to the same correct answer, IF applied correctly.

 

[Ohms law]

I understand now. I feel stupid, sometimes you just have to walk away and come back to what you are doing.