Conductor Derating and 90 degree column?
- Thread starter doof
- Start date
- Status
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
My understanding is that you have to use the _lowest_ temperature rating of everything connected in the region being considered.
If you have 90C wire connected to a 75C terminal, then at the point of that connection you need to use the 75C rating to determine ampacity
If somewhere else along the length of the wire it is subject to different surrounding conditions (say higher ambient temperature or derating for other current carrying conductors), then you use the temperature ratings of the components in that region. So the 90C conductor _in the conduit_ is derated from 90C, even though elsewhere it is connected to a 75C component.
Since the current is the same everywhere along the wire, the minimum ampacity determined at every point along the wire is the one that holds.
There is clearly a hole in this understanding, and I don't see an answer in the code: how much separation is required between 'regions'. Copper is a good conductor of heat; if the wire is at 90C inside the conduit (it likely is not, but it could get to that temperature), then how much wire is required before you hit the 75C termination.
-Jon
If you have 90C wire connected to a 75C terminal, then at the point of that connection you need to use the 75C rating to determine ampacity
If somewhere else along the length of the wire it is subject to different surrounding conditions (say higher ambient temperature or derating for other current carrying conductors), then you use the temperature ratings of the components in that region. So the 90C conductor _in the conduit_ is derated from 90C, even though elsewhere it is connected to a 75C component.
Since the current is the same everywhere along the wire, the minimum ampacity determined at every point along the wire is the one that holds.
There is clearly a hole in this understanding, and I don't see an answer in the code: how much separation is required between 'regions'. Copper is a good conductor of heat; if the wire is at 90C inside the conduit (it likely is not, but it could get to that temperature), then how much wire is required before you hit the 75C termination.
-Jon
110.14(C)(1) is the equipment provisions I think is being discussed here.
It reads, in part, "...Conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6)."
If you are trying to derate conductors, start with the value given in T310.16 and modify them.
It reads, in part, "...Conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6)."
If you are trying to derate conductors, start with the value given in T310.16 and modify them.
Last edited:
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
No. You start with the 90C column only if you are using conductors rated for 90C. Look at the second sentence of 110.14(C), the sentence located just before subparagraph (1) begins.doof said:
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
doof said:
You can start at the 90C for derating even though the terminations are rated 75C, however the OCPD cannot be any higher than the 75C-- art.110.14
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Dennis Alwon said:
The same would apply if you were using NM cable which contains THHN conductors, only that the final adjusted ampacity cannot exceed the 60 degree C ampacity.
- Status