Using 75 and 90 degree rules

[dm9289]

i think this video with the example at the 16 minute mark may be incorrect. 300amp continuous load from breaker to junction box 300x1.25= 375amp wire 500kcm wire at 75 degrees 380amp agreed. Splice box to splice box load at 100% of continuous so 300amp wire required not 380 so my opinion is at 90C 300kcm is adequate. The video shows 400kcm at 380amps required. please help with what im missing

Dave








REPLY

 

[Tulsa Electrician]

Look correct to me.
Once you hit the 90 c blocks you can use the 90 c table. 400 mcm 380.
Then before and after 90 c blocks you use the 75c table and 500 mcm required. Based to termination temp.

 

[dm9289]

Look correct to me.
Once you hit the 90 c blocks you can use the 90 c table. 400 mcm 380.
Then before and after 90 c blocks you use the 75c table and 500 mcm required. Based to termination temp.

But the load is only 300 amps. If you look at bottom of graphic and code ref. 300Kcm at 90c is good for 320.
In the video the 125% was because breaker cannot be run at 100% but the splice can be run at 100%

 

[winnie]

This is all caused by the fact that the breaker is only 80% rated, and as you correctly point out _conductors_ are 100% rated and absent breaker issues could be used at 100% of their ampacity.

However whatever breaker used must also protect the conductors. A 400A breaker is generally not permitted to protect a 320A conductor.

-Jon

 

[david luchini]

But the load is only 300 amps. If you look at bottom of graphic and code ref. 300Kcm at 90c is good for 320.
In the video the 125% was because breaker cannot be run at 100% but the splice can be run at 100%

300kcmil at 90c is good for 320A...which cannot be protected by a 400A c/b

350kcmil at 90c is good for 350A...which cannot be protected by a 400A c/b

400kcmil at 90c is good for 380A...which is permitted to be protected by a 400A c/b.

edit: Or what Jon said.

 

[dm9289]

300kcmil at 90c is good for 320A...which cannot be protected by a 400A c/b

350kcmil at 90c is good for 350A...which cannot be protected by a 400A c/b

400kcmil at 90c is good for 380A...which is permitted to be protected by a 400A c/b.

edit: Or what Jon said.

Thank you so much now it makes sense. I omitted the breaker size from the equation

 

[winnie]

A question that I had trying to answer this: what happens if you have a 300A continuous load, and a conductor with an ampacity of 365A (say because of ambient temperature)?

Are you still good with a 400A breaker?

In this case the conductor ampacity exceeds the load, but not the load * 1.25. However the ampacity is high enough that the 'round up' rule comes into play and the conductor is considered protected by the 400A breaker.

-Jon

 

[david luchini]

Are you still good with a 400A breaker?

Yes...(as long the conductor is not smaller than 500kcmil in this case.)