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I work as and E/E intern for a industrial panel company. A customer wants a 30kA SCCR panel. The input power is 3 phase 460V, an unknown distance from the main panel in the customer factory. Inside the NEMA 12 enclosure is a 2 phase transformer for 120v control voltage, fuse blocks, Motor protectors, motor controllers and a GFCI for the 120v power. The only circuit breakers in the box are used post ac-dc converters. Everything else is either fused or behind a motor protector.

My initial understanding of SCCR was that the last component that needed the SCCR was the current protection device. I have recently been educated on that mistake as I recently found out that the wiring is also a factor.

The smallest panel I work on is a 460v 60A up to a 460v 400A panel. For simplicity sake lets say all wires from current limiting devices are 10 feet long with the only amp load for sizing.

I have found the part in the UL508A about transformers and I think I understand that part. The power from the transformer mostly stays inside the panel but does go out to multiple sensors and switches nearby.

Most of our components from the disconnect, primary fuses, and distribution block are already in the 100kA+ range.

Finally my question, I have a contactor that is a 5kA rating, controlling a 47A resistive heater load that is 10 feet away from the fuse block. What do I need to look at / calculate to see if it will meet the required 30kA for the box and wiring?

Jeff A
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  • What is this: SCCR? – Andy aka Apr 02 '21 at 18:29
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    I believe SCCR is "Short Circuit Current Rating" based on the context here. If you are an intern, aren't there any actual engineers at your company who are available to help steer you in the right direction? – jwh20 Apr 02 '21 at 18:35
  • @jwh20 I am under the impression I know the same info as any other E/E in the office. I watched a webinar by Siemens about SCCR last week and I spent most of the time picking my jaw up off the floor because of how wrong we have been doing it. Probably loosing money for the company in the process by over engineering the panel and possibly shorting the customer on the rating for not getting the wiring right. – Jeff A Apr 03 '21 at 16:40
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    I'd be very surprised if your company isn't required to have a P.E. on staff to approve what I think you are building there. – jwh20 Apr 03 '21 at 22:15

1 Answers1

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The first thing to understand is that an SCCR rating and calculation generally follow a "weakest link in the chain" approach. No one will be able to answer without a single line of the panel power but there are a lot of possibilities.

First I'll share a couple of links here. The first, from Mouser, is a great white paper on completing an SCCR calculation. The second, from Eaton, is what I personally followed when I needed to complete this.

Finally my question, I have a contactor that is a 5kA rating, controlling a 47A resistive heater load that is 10 feet away from the fuse block. What do I need to look at / calculate to see if it will meet the required 30kA for the box and wiring?

If your branch circuit is a properly rated fuse followed by the 5kA SCCR contactor, and then the load; using the methodology from those two links, the SCCR rating of the fuses and contactor are looked at as a pair (a component rating) and it would pull the branches rating up to 100kA SCCR. If that's not the case and the branch circuit is just the contactor and the fuses down the line, the rating for the branch would be 5kA and, subsequently, the panel rating would be 5kA.

In the end, the panel's overall SCCR rating is the lowest rating of any individual branch so I can't say what it would be for sure. There are a few methods to fix and raise the branch circuits' ratings so this generally isn't an issue.

Also, of note, you're calculating the SCCR rating for the panel, not for the panel plus auxiliary equipment. If it is a fuse to a contactor that's in the field, you would ignore that. Any time I've needed to get a panel certified, it was while it was sitting in the shop, completely disconnected from anything else.