65ka SCCR

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shay.jackson

Member
Location
IOWA
Occupation
PANELS SHOP OWNER
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
 
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.
 
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.
 
petersonra said:
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.
Click to expand...
petersonra said:
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 6
Click to expand...
The spec says

Short-circuit current rating design of the fuse link for short-circuit protection of the main circuit required10kA@600V (Class H or K); 100kA@600V (Class R or J)design of the short-circuit trip Thermal magnetic circuit breaker maximum short-circuit current breaking capacity (Icu)● at 240 V 14 kA● at 480 V 10 kA● at 600 V 10 kA certificate of suitability NEMA ICS 2; UL 508; CSA 22.2, No.14

Does that mean it can never be higher than 14ka? or does it mean with class R or J fuses it can be 100Ka?
 
I don't know what it means. I would have to look at the installation sheet for the part to figure out that mess of words. Looks like a poor translation to English.

The thing is that it does not help you with the contactors unless you somewhere in the circuit have a CL fuse that is listed to increase the SCCR of the assembly. And the contactor instructions will tell you what fuses you can use. Most likely only class CC and/or class J depending on rating required.
 
What rules are you playing by? Are you dealing with the MCC in UL 845 or are you dealing strictly with the separate (control) panel under UL 508a, or some other standard?
 
Ok, then you don't really need to worry about what the MCC ratings are, for the purposes of determining the SCCR on your 508a panel. You can modify the branch circuits in the panel by putting a current limiting fuse in the feeder with a let-through current rated at- or below your weakest branch circuit device's SCCR. If that weakest device is the starters at 10ka, then you would need a current limiting fuse with a 10ka (or less) let-through current. That's going to limit you to a 60A fuse, though. See the chart.
1736543886529.png
If 60A doesn't give you enough current for the 14 motor loads, you will have to consider another approach, such as placing a transformer in the feeder circuit (but don't forget the OCPD requirements for the transformer.) You can get quite a lot of available rated current with a very low let-through current from a transformer in the feed. (See the transformer table.) Just keep in mind that neither the current limited fuse (or breaker) nor the transformer in the feeder will make any difference on the branch circuit OCPD interrupt rating.1736544334137.png
 
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