Conductor sizing?

[Cieba]

By Mike Holt for EC&M Magazine

Q 1. I have a question on the application of the conductor ampacity adjustment factors contain in 310.15 (B)(2). When adjusting conductor ampacity, do we use the conductor ampacity based on the 90?C insulation rating (for THHN) or the 75?C temperature rating of the equipment terminals?

A 1. When adjusting conductor ampacity, use the temperature rating of the conductor, not the equipment terminals [110.14(C)

The above Q & A was extracted from the web, which appares to be an interview, or extracted from a forum.

The reason I'm displaying this is because I'm a little bit confused when it comes to sizing feeder conductors & overcurrent protection.
Now, if you compare the above Q & A with the following, can someone tell me if the conductor was sized based on the (inverse-time breakers 75*c terminal rating) or because the ampacity is over 100A, thus the ampacity must be based on the provisions of 110.14 C. which states, circuits rated over 100A or marked for conductors larger than 1 AWG, shall be used only for the following: (1) conductors rated 75* C.

Q. What size feeder conductor (THHN) and protection device (inverse-time breaker 75*C terminal rating) for the following loads: 46.2A X 1.25% + 30.8A + 30.8A + 16A = 136A

From table 310.16 the answer that was given was 1/0 AWG for the above question, I ask again, was this answer derived from the terminal rating of the device or by following article 110.14 C

I would really appreciate if someone can enlighten me, thanks!

Ceiba

 

[cpal]

Cieba
How much time do you have??


The answer to your question is a bit more extensive than a simple answer and you are probably going to get quite a few responses .

The history of the answer resideds with the manufactures (read the first few pages of the UL whitebook for panel boards)

Manufures test their fuses and breakers (lugs etc.) above 100 amps (unless listed otherwise) with conductors sized per the 75 deg col of 310.16.(this more for continuious loads)

If you use the conductors at their 90 rating (will not damage the wire) the lugs on the breakers loose their heat sink (conductor mass, ability cool themselves) and the ocpd may not function to specs.


110.14 (C) is a UL driven code requirement.

the 1/0 you described at 75 deg is listed for 150 A the breaker will need to be a 150A. (240.6)


You may generally use a conductors at it's 90 deg rating and derate from that ampacity, if derating is req but you may not load the conductor beyond it's 75 deg ampacity. (because of the rating of the lugs)



Thats just some of the answer
Charlie

 

[augie47]

response

response

excellent, cpal.

 

[kingpb]

Terminals cannot be loaded beyond their temperature rating. Cable cannot be loaded such that it's temperature rating will be exceeded.

When derating cable start with the maximum current at the cable temperature. If the derated current is more then the terminal rating, then you cannot exceed the terminal current rating.

The reason for 310.15(B) (2) is becasue multiple cables in a conduit cause heating. The heat from one cable will cause an increase in the temperature, and vice versa. By decreasing the current of the cables you are decreasing the heat. The derate is to actually lower the temperature so that the combined temperature does not cause the cables to exceed their rating. This has nothing to do with terminals.

Keep the issues separate.