Cable sizing for 800 amps serivce circuit breaker

[AmmarAbdou]

Hi,

Given:
recently, the local electricity company provided our factory with 800 amps circuit breaker ( service, if i may say so) see attached photo 1.jpeg
before that, we used to power up our factory with generators feeding our Main Distribution Board (MDB) which has a 800-1600 amps adjustable MCCB as a main breaker. see attached photo 2.jpeg and photo 3.jpeg.

Requirements:
We want to connect a change-over switch (1-0-2) which will allow us to switch between either the normal power source or the generator source.
so, the new connection will be as following:
from the 800 service breaker to the change-over switch to Our MDB.

My question is:
as per the NEC table 310.15(B)(16) I have found that for 800 amps, I have to use 2 sets of 500 kcmil copper cables as per the 75C column.
am I right?
any suggestion will be appreciated,
Regards,
Ammar

 

[kwired]

Yes, assuming your calculated load is not over 760 amps. 500 copper has ampacity of 380 amps x 2 conductors = 760.

You can use next higher standard overcurrent device up to 800 amps to protect those conductors, as long as load doesn't exceed ampacity of the conductors. Over 800 amps the conductor ampacity must be equal or greater than the overcurrent protection.

That is what NEC rules are, whether or not they apply where you are may be a different issue.

 

[AmmarAbdou]

Yes, assuming your calculated load is not over 760 amps. 500 copper has ampacity of 380 amps x 2 conductors = 760.

You can use next higher standard overcurrent device up to 800 amps to protect those conductors, as long as load doesn't exceed ampacity of the conductors. Over 800 amps the conductor ampacity must be equal or greater than the overcurrent protection.

That is what NEC rules are, whether or not they apply where you are may be a different issue.

Thanks for your reply,
But the current calculated load will not exceed 400 amps on full load, because the factory is still in the early stages.

May be in far future we will reach the 800 amps in the future expansion.

I am thinking of installing 400 amps circuit breaker with appropriate cable sizing to feed the MDB to save money as we will use cheaper cable.

What do you think?

Regards,
Ammar

 

[kwired]

If/when you get to 760 amps of load you are likely considering upgrades or additional supplies anyway, I'd just use the 500's and 800 amp breaker for now, especially if you already have it or know the load will be increasing soon.

 

[AmmarAbdou]

the 800 amps service Circuit breaker is already installed!

the 800 amps service Circuit breaker is already installed!

If/when you get to 760 amps of load you are likely considering upgrades or additional supplies anyway, I'd just use the 500's and 800 amp breaker for now, especially if you already have it or know the load will be increasing soon.

Please note that the 800 amps service breaker is already installed (already installed by the local electricity company), i do not have a choice to use it or not.

so am i right to say that 2 sets of 500 kcmils cables shall be used regardless of the calculted load?? is that okay!

Regards,
Ammar

 

[oldsparky52]

Please note that the 800 amps service breaker is already installed (already installed by the local electricity company), i do not have a choice to use it or not.

so am i right to say that 2 sets of 500 kcmils cables shall be used regardless of the calculted load?? is that okay!

Regards,
Ammar

If the calculated load is more than 760-amps, then (2) 500's are insufficient (per the NEC), if the calculated load 760-amps or less then (2) 500's are okay.

If I'm understanding your situation correctly, I would install the (2) sets of 500 and leave the existing breaker as is. Later if the load increases above 760-amps as the expansion happens, upgrade the system then.

Personally, I would rather use (3) sets of 250 if the conditions allow (I hate trying to bend large copper conductors).

 

[infinity]

Since you have the possibility of exceeding 760 amps in the future just use 2 sets of 600 kcmil now and make it a full 800 amps.