Can 2-400A breakers be paralleled to feed a temporary 800a load ?

[studentofthecode]

I have to set up a feeder that provides 250 to 600 amps for temporary connections of Emergency power for shutdown work. There are only 2 spare 400A breakers to supply this load. The main of the panel they are in is 1200 amp and can easily supply the current we need to use for our shutdowns. The wiring will be in identical lengths and terminated in a 800A power block in a j box.( What if it terminated in an 800A switch or C Breaker instead?)We are planning on running generator grade flexible cables to feed the gear temporarily. I'm thinking that the load will divide equally on the 500 Mcm phase conductors and will carry 1/2 of the load equally, according to Ohm's law. I know this is unorthodox, but it appears to work , in theory. I have not been able to find a code section that I can cite as a violation. Again, this is only for a temporary feeder that will be removed when repair work is complete. Thanks

 

[Dennis Alwon]

Temporary or not I don't believe you are compliant. There are exceptions but I don't believe they apply to you

408.36 Overcurrent Protection. In addition to the requirement
of 408.30, a panelboard shall be protected by an overcurrent
protective device having a rating not greater than that of
the panelboard. This overcurrent protective device shall be
located within or at any point on the supply side of the panelboard.

 

[kwired]

It may "work" but violates 240.8.

 

[charlie b]

Sorry, but I have to agree with Dennis. I will add that 240.8 very explicitly forbids what you are suggesting.

 

[LarryFine]

Run the two sets of conductors, but not in parallel, and divide the load in half as well as you can.

 

[studentofthecode]

I think that may be the only way. I was trying to temp in the entire board with the least amount of work. There is no load study for the individual loads, only the gear as a whole. I apparently have more work to do to get my own load readings.
Thanks to all who responded

 

[Adamjamma]

so in this case the code is requiring an 800 amp breaker and the two 400 amp breakers cannot be paralleled? But the wires can be?

 

[Andy Delle]

It's interesting though that they do this on residential panels. Many have a quad 100a breaker as the main. Of course this breaker is probably listed for this purpose.

 

[GoldDigger]

It's interesting though that they do this on residential panels. Many have a quad 100a breaker as the main. Of course this breaker is probably listed for this purpose.

Yes, it is labelled and listed for the purpose, because it has been designed for the purpose.
A small difference in contact resistance can cause a much larger current imbalance.

Sent from my XT1585 using Tapatalk

 

[petersonra]

I have to set up a feeder that provides 250 to 600 amps for temporary connections of Emergency power for shutdown work. There are only 2 spare 400A breakers to supply this load. The main of the panel they are in is 1200 amp and can easily supply the current we need to use for our shutdowns. The wiring will be in identical lengths and terminated in a 800A power block in a j box.( What if it terminated in an 800A switch or C Breaker instead?)We are planning on running generator grade flexible cables to feed the gear temporarily. I'm thinking that the load will divide equally on the 500 Mcm phase conductors and will carry 1/2 of the load equally, according to Ohm's law. I know this is unorthodox, but it appears to work , in theory. I have not been able to find a code section that I can cite as a violation. Again, this is only for a temporary feeder that will be removed when repair work is complete. Thanks

Come off the bus directly, go to an 800A fuse or MCCB and then run the wires down to the power block.