Cable Sizing and adjustment factors

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Electriman

Senior Member
Location
TX
Friends,

I am confused. Should the cable ampacity be more than 125% of continues load after adjustments due to (temperature or more than three current caring cables) or before adjustment?

For example, if I have 100 A continues load and I have .94 derating factor due to high temperature, can I use #2 cable that handles 115 A and due to temperature I have to derate it to 115*.94= 108 A? (in this case cable ampacity is less than 125% of the load after and before adjustment)

or Do I have to use #1 which handles 130 which grater than 125% of (100A) and if I derate it 130*.94=122A? (in this case cable ampacity is less than 125% of the load after adjustment but it is more than 125% before adjustment)


or Do I have to use 1/0 which caries 150 A in normal condition and when I derate it 150*.94=141 A which is grater than 125% of 100A. (in this case cable ampacity is more than 125% of the load before and after adjustment)



Considering in all of these cases my breaker is 125 A magnetic breaker.

Thank you for your help.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
#1 would be your minimum conductor size. 100A*1.25 = 125A, you need a conductor with an ampacity of at least 125A BEFORE and adjustment or correction factors.

Applying your correction factor of 0.94 gives the #1 an ampacity of 130A*0.94 = 122A. An ampacity of 122A is large enough to carry the load of 100A, so the #1 would be acceptable in your example.
 

Smart $

Esteemed Member
Location
Ohio
I agree with David... but will add to it for your future endeavors.

Factoring a continuous load (or that portion of the calculated load) by 125% only applies to the minimum conductor SIZE and the minimum OCPD rating... so these must be not less than 100% noncontinuous plus 125% continuous.

After adjustment and correction the conductor AMPACITY must be not less than 100% noncontinuous plus 100% continuous, or simply the calculated load.

And lastly, the conductor AMAPCITY after adjustment and correction must be a value that is considered protected by the OCPD per 240.4(B). In your example, the conductor must have an adjusted and corrected ampacity of at least 111A, because it has to be higher than the next lower standard OCPD rating is 110A.
 

Electriman

Senior Member
Location
TX
I agree with David... but will add to it for your future endeavors.
Thanks,

Factoring a continuous load (or that portion of the calculated load) by 125% only applies to the minimum conductor SIZE and the minimum OCPD rating... so these must be not less than 100% noncontinuous plus 125% continuous.

OK

After adjustment and correction the conductor AMPACITY must be not less than 100% noncontinuous plus 100% continuous, or simply the calculated load.

Interesting point.

And lastly, the conductor AMAPCITY after adjustment and correction must be a value that is considered protected by the OCPD per 240.4(B). In your example, the conductor must have an adjusted and corrected ampacity of at least 111A, because it has to be higher than the next lower standard OCPD rating is 110A.

Can you support this with an NEC article no?

 

Electriman

Senior Member
Location
TX
240.4 general statement, then 240.4(B) as cited in my earlier post.

Smart, But no where in 240.4B talks about adjustment factors. Here is what it is saying:
" The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes."

How did you conclude that means after adjustment? is it because of 240.4(B)(2)?
 

Smart $

Esteemed Member
Location
Ohio
Smart, But no where in 240.4B talks about adjustment factors. Here is what it is saying:
" The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes."


How did you conclude that means after adjustment? is it because of 240.4(B)(2)?
Yes, at least the after adjustment and correction part. One of the nuances of Code is where there use the terms ampacity and allowable ampacity. The former means after adjustment and correction while the latter means before. This meaning can only be extratxed via context as it is not defined anywhere in the Code.

FWIW: If 240.4(B) as a whole cannot be satisfied, the adjusted and corrected ampacity must be not less than the OCPD rating.
 

Electriman

Senior Member
Location
TX
Yes, at least the after adjustment and correction part. One of the nuances of Code is where there use the terms ampacity and allowable ampacity. The former means after adjustment and correction while the latter means before. This meaning can only be extratxed via context as it is not defined anywhere in the Code.

FWIW: If 240.4(B) as a whole cannot be satisfied, the adjusted and corrected ampacity must be not less than the OCPD rating.

Thanks. But this should be applicable only for non motor loads. For motor loads since cables can be protected by both over load relays and over current device then the breaker can be greater than cable ampacity, am I right?
 

Smart $

Esteemed Member
Location
Ohio
Thanks. But this should be applicable only for non motor loads. For motor loads since cables can be protected by both over load relays and over current device then the breaker can be greater than cable ampacity, am I right?
Yes. Anytime you get into Specific-Purpose Branch Circuits under Table 210.2, you could be looking at a different set of conditions that can cascade up to feeder and service level. Motor circuits are a prime example of that.
 
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