sizing overcurrent device for parralled conductors

[Matt_hndrx]

Have 3 parrallel runs of 75 degrees C rated, 350 kcmil in three parrallel runs of Emt. how would you figure out the maximum standard overcurrent device. My guess is you would need a 1000 amp?

 

[Pierre C Belarge]

240.4(B) permits the rounding up of overcurrent protection of conductors for up to 800 Amps.
Since 350 Kcmil is rated at 310A in the 75 degree column, one adds that up = 930Amps.
930A is not a standard OCPD size as per 240.6, so you will need to round down to 900Ampere OCPD.

 

[Matt_hndrx]

Woudn't you round up to 1000 amp?

 

[benaround]

Woudn't you round up to 1000 amp?

Not if you read the first sentence in Pierres' answer.

 

[mistermudd]

Not for over 800 amps.

 

[infinity]

The OCPD would have to be 930 amps or less. You may be stuck with an 800 amp OCPD since 900 amps is not a standard size.

 

[Pierre C Belarge]

The OCPD would have to be 930 amps or less. You may be stuck with an 800 amp OCPD since 900 amps is not a standard size.

As per 240.6, 900A OCPD does not exist, but in the field ("real life") one can procure a 900A rated OCPD.

 

[charlie b]

In those circumstances (i.e., under 800 amps) that you are allowed to round up, you are still not required to round up. The only requirement is that you not round up beyond the next higher standard rationg.

When you are over 800 maps, you don't have to round down to a standard size. You just have to protect the conductors at or below their ampacity. So in this instance, if you can get an adjustable trip breaker and set it to 930 amps, that will be acceptable.

 

[jimday]

Don't forget that if you use an adjustable trip breaker with a max avail setting higher than the feeder ampacity, it needs to have restricted access per 240.6(C)