wire size

[burbsele]

I am installing a 1600 amp 480v 3 phase service, is it allowed by code to install 4 parallel sets of 750 mcm xhhw alu 90C conductors ?

 

[augie47]

Depends....
Main OCP device or MLO (or 6 circuit rule )
Caluculated Load ?

 

[burbsele]

main OCP , do not know the load, the feeders tying into this are 1200 amp rated.

 

[augie47]

If it's a service panel and the main is over 800 amps, the conductors must have an ampacity equal to or greater than the main and
the temperature limitations of 110.14 must be taken into account.
Unless you main is 90? rated (somewhat rare), you must terminate at the 75? ampacity so you 750 would not be adequate.

Can you eloberate on the 1200 amp feeder comment.

 

[burbsele]

The conductors leaving the 1600 amp switchboard are ending in a tap box approximatley 50' away and getting extended by another party to a tenant 1200 amp switchgear.

 

[augie47]

Just for clarification was your original post referencing actual service conductors feeding your 1600 amp service ?

The 1200 amp conductors would be governed by the tap rules in Art 240 unless the overcurrent device protecting them is 1200 amps or less.

 

[burbsele]

This installation is in a mall electrical room, there is a 2000 amp switch board with the load conductors of that switch board ending in a wireway about 50' away, my conductors are tapping into the wireway and feeding a 1600 amp switchboard. My tap lenght is 10'

 

[augie47]

Wow. Now I'm totally confused.
In post 1 we had a 1600 amp service
In post 3 we have a 1200 amp panel
In the last post we have a 2000 amp switchboard.

Simple answer. With few exceptions, when dealing with over 800 amps, the conductors must have an ampacity equal to or greater than the overcurrent device protecting them. Normally the terminated ampacity will be at the 75? rating.
There are exceptions.

 

[burbsele]

Sorry for the confusion, what code article is that in ? why do we have conductors rated at 90C if we can't use that rating because the breakers are at 75C ?

 

[Dennis Alwon]

Sorry for the confusion, what code article is that in ? why do we have conductors rated at 90C if we can't use that rating because the breakers are at 75C ?

We can use that rating for the conductor but the final ampacity must not be higher than the weakest link. The 90C rating comes in handy since we can use it for de-rating.