700A feeder cable to 800A MCB.

[Designer69]

Is this allowed? I know there are provisions somewhere in the NEC that you can undersize the cables to some breakers (with the next lowest available size) but I thought that is only for up to 400 or 500Amps.

loading isn't an issue.

Thank You

 

[infinity]

700 amps is a standard size so you cannot go up to 800 amps.

 

[Designer69]

700 amps is a standard size so you cannot go up to 800 amps.

Thanks. They show 4#2/0 feeding the 800A Main Disc. If they up it to 4#3/0 then that would be acceptable, do you agree?

or if the reduce to Main Disc. to 700A with the 4#2/0

 

[infinity]

Yes I agree. For the 4 sets of #2/0 you end up with 4*175 amps = 700 amps which is a standard size in 240.6(A) so you cannot go up to the next standard size. That means that if you're still using 4 sets then you'll need to go to the next standard conductor size of #3/0 (4*200 = 800 amps). Which of the two conductor sizes you choose will tell you the OCPD size.

 

[kwired]

Is this allowed? I know there are provisions somewhere in the NEC that you can undersize the cables to some breakers (with the next lowest available size) but I thought that is only for up to 400 or 500Amps.

loading isn't an issue.

Thank You

For over 800 amps the conductor ampacity must be equal or greater than the overcurrent protection.

Common misapplication is to assume 500 copper is good for 400 amps (is actually only good for 380) and to parallel 500's for 1200, 1600 or 2000 amp overcurrent device applications

 

[Adamjamma]

I know this is going to sound stupid, but... could the 800 be for inrush current on a motor or such that has a lower current fuse or breaker at its end? I have seen that before but do not do many motor circuits so just wondering.

 

[Greg1707]

Is this allowed? I know there are provisions somewhere in the NEC that you can undersize the cables to some breakers (with the next lowest available size) but I thought that is only for up to 400 or 500Amps.

loading isn't an issue.

Thank You

If this is a feeder, what is the over current protection ahead of the feeder?

 

[kwired]

I know this is going to sound stupid, but... could the 800 be for inrush current on a motor or such that has a lower current fuse or breaker at its end? I have seen that before but do not do many motor circuits so just wondering.

That 800 amp general rule is in art 240.4.

240.3 says to see art 430 for overcurrent protection for motors. I'm not aware of any 800 amp rule existing in 430, so I would think it is very possible to have less than 800 amp conductor for a motor circuit and over 800 amp overcurrent protection.

 

[Designer69]

For over 800 amps the conductor ampacity must be equal or greater than the overcurrent protection.

Thanks. You say "For OVER 800 amps"

What about exactly 800 amps? I thought the cutoff where you can do this was 400A

 

[charlie b]

What about exactly 800 amps? I thought the cutoff where you can do this was 400A

Take a look at the article previously cited as the location of this rule.

By the way, I think you are approaching this situation from the wrong direction. The design sequence should go like this:

• Step 1: Calculate the load.
• Step 2: Select a conductor that has sufficient ampacity for that load.
• Step 3: Select an overcurrent device that is capable of protecting the chosen conductor.

 

[kwired]

Thanks. You say "For OVER 800 amps"

What about exactly 800 amps? I thought the cutoff where you can do this was 400A

No it is OVER 800 amps, conductor ampacity must be equal or greater than overcurrent protection. As I mentioned I think 500 copper being mistaken to be good for 400 amps is a common reason for confusion. You can protect single 500 at 400 amps, two 500's at 800 amps, but three or more 500's cant be used for 1200, 1600, 2000... In all cases the load can't exceed 380 amps on a single 500, so if you have 390 amps of load you can put it on 400 amp breaker but you would need larger than a 500 copper conductor.

3/0 copper ampacity is right at 200 and this particular conductor doesn't really get the confusion

4/0 aluminum being assumed to be good for 200 amps can be misleading also. It works out that 2-4 4/0 aluminum work out for next size up rule as long as you don't exceed 180 amps of load per conductor. But once you go over 800 amps you have too small of a conductor because no more next size up rule above 800.

 

[infinity]

Thanks. You say "For OVER 800 amps"

What about exactly 800 amps? I thought the cutoff where you can do this was 400A

As Kwired stated the requirements change when you exceed 800 amps.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met: