overcurrent devices

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micary

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Please identify area/article of the NEC that sizes conductors in relation to overcurrent devices.
Example: I have sized the conductors based on 215.3. Now I'm looking for a table or formula to size the overcurrent device. Had always heard the conductor capacity is suppose to be 125% of the overcurrent device but would like to find it in print. If I'm wrong in this procedure, please point me in the right direction.

Thank you,
micary
 
micary said:
Please identify area/article of the NEC that sizes conductors in relation to overcurrent devices.
Example: I have sized the conductors based on 215.3. Now I'm looking for a table or formula to size the overcurrent device. Had always heard the conductor capacity is suppose to be 125% of the overcurrent device but would like to find it in print. If I'm wrong in this procedure, please point me in the right direction.

Thank you,
micary
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The 125% does not apply to all loads. As far as wire sizing look in the tables in article 310.15 310.16
 
I should add there are many things that affect what wire size and type you use , Please get a qualified electrician to help you understand what these tables mean and how to read them.
 
I am closing this thread, in accordance with the Forum?s rules. This Forum is intended to assist professional electricians, inspectors, engineers, and other members of the electrical industry in the performance of their job-related tasks. Although your member profile shows that you may be a member of this industry, or of a related industry, your question suggests that you are planning to perform electrical work at your own home, and that you are not an electrician. We are not permitted to provide ?how-to? assistance to a ?do-it-yourself? person.

If I have misinterpreted the situation, if this is a task that you are performing on behalf of your employer, then send me Private Message to explain the circumstances. I would gladly reopen the thread, and post an apology for the delay and the inconvenience.
 
It appears, alas, that I have mis-interpreted the situation. In a PM, I was informed that this is a work-related project, and not a home installation project that the originator is planning to perform himself. Therefore, I am reopening this thread. I apologize to the originator for the inconvenience and the delay.
 
Here is a bit of information that the OP sent to me via PM. I think it will help explain the nature of his question.
micary said:
The contracted electrician for one of my many projects bumped the size of a current limiting device from 100 amps to 125 without increasing the wire size. The 90 deg wire, rated at 130 amps seems undersized, yet when confronted, I cannot find a definitive statement/table/article in the NEC where it addresses this. I'm almost sure I have studied it somewhere in my early years.
Click to expand...
 
I think you will find your answer in 110.14(C), including the sub-paragraphs that follow. The simple answer is that you cannot take advantage of the 90C rating of a wire's insulation system, unless all components of the circuit are also rated for 90C. It is commonly presumed, and I think it is a fair presumption, that the terminations to which the wire is attached or connected will not be rated higher than 75C. Thus, the rating of a #2 (which I infer is the size used in your project) can be no higher than 115 amps.

As to whether this installation is acceptable, it depends on the calculated load. If, for example, the calculated load is 115 amps, with that value having already taken into account an extra 25% for any continuous loads and an extra 25% for the largest motor, then a #2 has sufficient ampacity for that load. Furthermore, in accordance with 240.4(B), you would be allowed to use the next higher standard size overcurrent device (i.e., 125 amps).

So more information would be needed, in order to tell for certain if you have a code violation on your hands.
 
charlie b said:
I think you will find your answer in 110.14(C), including the sub-paragraphs that follow. The simple answer is that you cannot take advantage of the 90C rating of a wire's insulation system, unless all components of the circuit are also rated for 90C. It is commonly presumed, and I think it is a fair presumption, that the terminations to which the wire is attached or connected will not be rated higher than 75C. Thus, the rating of a #2 (which I infer is the size used in your project) can be no higher than 115 amps.
Click to expand...

It is a fact that there are no UL489 Listed breakers that are rated for wire sized per the 90?C column. This includes all molded case circuit breakers, 600V max.

There are medium voltage devices which are rated for MV 90?C cable. There are 600V devices which require 90?C insulation but the wire must still be sized per the 75?C column.
 
There is one case where upsizing the breaker is acceptable it is on motors and air conditioners, this is covered in articles 430- (430.52) & 440. This is to allow for the motor to start without tripping the breaker. But it does not sound like this is you situation.
 
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