65ka SCCR

[shay.jackson]

A customer has an mcc bucket with a 1200amp adjustable breaker (480v). They will adjust this breaker down to 400amps and it has a 65ka SCCR. They want to feed a separate panel that will house 14 motor starter circuits for pumps in the field. They say they don't want a main breaker in the panel, just a disconnect. from there we are going to distribution blocks, then breakers (not mpcb to achieve the 65ka) then to the starters/overloads. The starters have a 10ka SCCR. to avoid my panel having a 10ka SCCR, My UL rep told me that the way to do this is to series protect the starter with a fused terminal rated at 65ka peak let through, then the breakers rated at 65ka peak let through, then the starters for every branch.

I have never seen this done and i have the following questions.

1. How would sizing be done to ensure they are coordinated to trip in the correct sequence based on the fault current?

2. If i used a main breaker as a disconnect that was rated for 65ka SCCR would i still need the fuses at all?

Thank you for you time

 

[MyCleveland]

Am I understanding this correctly....

400A 65k AIC CB to new 10k MLO panel with 14 Branch CBers? You said 480 so the lowest rating is 14K SCCR for your new panel.
Then 14 branches going to a distribution block....then to a fuse block with CL fuses...then to a 10k Starter....then ending at load?

Your starter literature will tell you if a CL fuse can be used to give you a 65k AIC at the starter. If nothing is stated then you are at the 10K number and your fusing does nothing to adjust the value.
This fuse will do nothing for the new panel SCC rating.
Your SCC calc will tell you what SCC rating you need at the new panel...it will be less than what the available SCC is at your upstream serving panel based on feeder wire size and length.

 

[petersonra]

First off it would not have anything to do with peak let thru.

Most contactors these days are listed for 65kA if fed from appropriately sized current limiting fuses. Check the manual. It is unlikely you will find CL fuses that will fit into terminal strips although fuse blocks are available DIN rail mounted.

Check the manual on the distribution blocks too. It may not be rated for 65 kA.