310.16 column 75C or 90C for calculating feeder siize

[steve holt]

Which column on 310.16 do I use for calculating a feeder size? The feeder is above 800A so I know I need to size the feeder to at least meet the overcurrent protection, but am I limited by the rating of the lugs at the termination points (breaker 75C) or can I use 90C column?

 

[Smart $]

75?C column value is the maximum. You can use the 90?C column values for adjustment and correction. Final ampacity the lesser of the two.

 

[Dennis Alwon]

The reason why you must use the 75C instead of the 90 is because the temperature limitation on the terminals is generally 75C not 90C so you must size to the weakest link.

 

[templdl]

The only other place that I can think of with rererence to 90degC rated cable is with the applications n of 100% rated breakers. When apply 100% rated breakers at 100% you must use 90degC rated cable but sized for 75degC.

 

[steve holt]

Thanks for the info guys. Unfortunately you confirmed what I already figured.

 

[Gaffen99]

Why can't they make a lug rated for 90 degrees?

 

[Dennis Alwon]

Why can't they make a lug rated for 90 degrees?

They can and there are some I have been told but I am guessing they don't want that much heat on the panel.

 

[Smart $]

Why can't they make a lug rated for 90 degrees?

A lot of [most?] compression lugs are rated 90?C... but when you use them, they usually get terminated to equipment that is not rated for 90?C operation.

 

[Dennis Alwon]

Here is a nice graphic on temp. of lugs

 

[templdl]

Sorry, this ended up to be a duplicate post.

 

[templdl]

Why can't they make a lug rated for 90 degrees?

But it is the device not the terminal that determines how the cable is actually applied and that would be commonly 75degC.

 

[GoldDigger]

And just to confuse things more, there are some situations where the device terminal requires 90C or higher wire, but you are specifically prohibited from using that temperature for ampacity calculations.

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