Wire sizing

[Brandon Loyd]

Have you ever heard of de-rating wire (THHN) down from the 90 degree chart in 310.16 to the 75 degree chart because the lugs on a breaker are rated at 75 degrees? I my mind, a 225 amp breaker with 3/0 THHN is perfectly accepted and the wire is protected even if that breaker happened to load up to 225 amps.

 

[infinity]

Where would you find a CB with a 90? C rating for it's terminals and use? #3/0 THHN is only good up to 200 amps @ 75? C. You need #4/0 or larger for a 225 amp load.

 

[roger3829]

The 90 degree rating is for derating purposes only. I belive that 75 degrees is the maximum rating you will find on a circuit breaker or lugs. So 3/0 is only good for 200amps

 

[bphgravity]

Brandon,

I suggest you review Section 110.14(C) of the NEC...

 

[Brandon Loyd]

Clarify

Clarify

If I am understanding you right, a 225 amp breaker with 3/0 THHN is not legal because the lugs on the breaker are rated at 75 degrees, which means the 3/0 THHN is only good for 75 degrees at 200 amps. So the 3/0 THHN would not be protected properly for overcurrent?

 

[roger3829]

If I am understanding you right, a 225 amp breaker with 3/0 THHN is not legal because the lugs on the breaker are rated at 75 degrees, which means the 3/0 THHN is only good for 75 degrees at 200 amps. So the 3/0 THHN would not be protected properly for overcurrent?

Yes. That is correct.

 

[bphgravity]

The byproduct of electrical current through a resistance is heat. A conductor loaded to its 90 degree rating will rise the temperature of the conductor terminations to greater than their maximum rating of 75 degrees. This will result in the failure of the connection point.

So, the code only allows conductors to be loaded to a value so that its temperature under load will not exceed the rating of any equipment, devices, or terminations connected to the circuit.

 

[Brandon Loyd]

Clarify detrating

Clarify detrating

If I have to use the 75 degree chart for all breakers and disconnects, were would I ever be able to use the 90 degree chart. Someone mentioned it is for derating. What derating? Ambient temp, multiple conductors?

 

[ron]

If I have to use the 75 degree chart for all breakers and disconnects, were would I ever be able to use the 90 degree chart. Someone mentioned it is for derating. What derating? Ambient temp, multiple conductors?

Yes

 

[squaredan]

If I have to use the 75 degree chart for all breakers and disconnects, were would I ever be able to use the 90 degree chart. Someone mentioned it is for derating. What derating? Ambient temp, multiple conductors?

Hey Brandon watch this clip it will help alot..

http://www.youtube.com/watch?v=WL81zOzNagM

 

[kwired]

If I have to use the 75 degree chart for all breakers and disconnects, were would I ever be able to use the 90 degree chart. Someone mentioned it is for derating. What derating? Ambient temp, multiple conductors?

Yes you can derate conductors for those things at 90 deg but if the allowed size at 90 deg is smaller than the size required for terminal temperature rating you must use at least whatever size is required for the terminal temperature.

 

[Little Bill]

Yes you can derate conductors for those things at 90 deg but if the allowed size at 90 deg is smaller than the size required for terminal temperature rating you must use at least whatever size is required for the terminal temperature.

To add to that or make it a little simpler, if the terminals you are sizing for are 60 deg. you can never use wire size from the 90 deg. column (after derating) that is smaller than what is called for in the 60 deg. column. Same thing for the 75 deg. column. You're not using the 90 deg. column so that you can use a smaller wire, but maybe it will keep you from using a larger wire when derating.

 

[brother]

I will say at least the guys here helped you out. At other websites, they would question your profile (listed as a FOREMAN) running a job and do not know how to use the derating 90 degree column etc..