understanding the "de-rating of feeders"

Klockopotomis

Member
Location
Parker CO
I am not understanding 310.12 NEC 2020 and the corresponding table.
If I understand it, you are allowed to derate up to 83 % the feeder or service conductors supplying a single or multi-family dwelling.
Example:
New overhead service for 200 amp main breaker panel fed from utility meter.
the supply side side conductors aka service entrance conductors can be derated according to 310.12 by 83%
If you look at Table 310.12 and follow 200 amp service or feeder rating it tells you 2/0 copper or 4/0 aluminum.
However, without derating, Table 310.16 lists 2/0 copper for 195 amps in the 90 deg column and 4/0 aluminum is listed for 205 amps.
So nothing changes...
I assumed if you could derate it then 83% or 200amps =166 Amps. which would be sized at 1/0 copper and 3/0 Aluminum
thoughts?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I am not understanding 310.12 NEC 2020 and the corresponding table.
If I understand it, you are allowed to derate up to 83 % the feeder or service conductors supplying a single or multi-family dwelling.
Example:
New overhead service for 200 amp main breaker panel fed from utility meter.
the supply side side conductors aka service entrance conductors can be derated according to 310.12 by 83%
If you look at Table 310.12 and follow 200 amp service or feeder rating it tells you 2/0 copper or 4/0 aluminum.
However, without derating, Table 310.16 lists 2/0 copper for 195 amps in the 90 deg column and 4/0 aluminum is listed for 205 amps.
So nothing changes...
I assumed if you could derate it then 83% or 200amps =166 Amps. which would be sized at 1/0 copper and 3/0 Aluminum
thoughts?
The ratings are based 75deg conductor temperature. So 1/0 copper is rated for 150A, and is too small for the 83% of 200.
 

Klockopotomis

Member
Location
Parker CO
gotcha. Okay bear with me here bc I guess I have been using the most common chart in the whole code book wrong.
If we use XHHW-2 for all overhead lines it is listed in the 90 degree column, does that change anything?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
gotcha. Okay bear with me here bc I guess I have been using the most common chart in the whole code book wrong.
If we use XHHW-2 for all overhead lines it is listed in the 90 degree column, does that change anything?
No it's still used at its 75° C ampacity because the equipment that it's connected to is only rated for a maximum of 75° C.
 

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
And there used to be a note in the code that XHHW conductors do not have to be derated due to sunlight. Don't know if that is still in the 2020 code
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
IMHO a language nit: I would not call the use of the 83% rule 'derating'; that term is specifically used elsewhere in the code to describe a change in the ampacity of a conductor.

The 83% rule says that particular feeders rated at X amperes and protected by OCPD rated at X amperes may be carried on conductors of ampacity 0.83X or greater.

The rated ampacity of the conductors is not changed, the rating of the feeder is not changed. But because of the expected diversity characteristics of the type of feeder in question the ampacity of the conductors is permitted to be smaller than the rating of the feeder.

Jon
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
IMHO a language nit: I would not call the use of the 83% rule 'derating'; that term is specifically used elsewhere in the code to describe a change in the ampacity of a conductor.

The 83% rule says that particular feeders rated at X amperes and protected by OCPD rated at X amperes may be carried on conductors of ampacity 0.83X or greater.

The rated ampacity of the conductors is not changed, the rating of the feeder is not changed. But because of the expected diversity characteristics of the type of feeder in question the ampacity of the conductors is permitted to be smaller than the rating of the feeder.

Jon
IMO the opposite of derating because you're taking a conductor and increasing it's allowable ampacity by ~21%.
 
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