UL508 panel branch protection for multiple motors

[Kiljoy]

I am designing a UL508 panel for industrial machinery. I’m starting two 50HP 230/3/60 motors with FLA of 116A each. I typically use Siemens Type E self protected starters, but due to the larger current rating I’m exceeding the highest amp rating of the series. I’d like to use a solid state Overload relay (Siemens 3RB10).
The specs in the catalog say that I will need to add extra short circuit protection for these overloads. Of course, I could add two fuse blocks with dual element J-fuses sized at 175%. I was wondering if it was OK to use one set of fuses to protect the two motor branches as per group installation rules. The UL508 code has section 66.7.5 that covers industrial machinery, but I found it a bit confusing. I’m thinking of 300A J fuses protecting the two motor branches. Other suggestions would be welcome.

 

[jraef]

The rules for doing that are complex and full of pitfalls that can trip you up, but it is technically possible. Basically, you would need to size the conductors going to each motor starter based on the main SCPD (Short Circuit Protective Device, i.e. fuses or circuit breaker) because those conductors are not technically protected by the OL relays any longer. Then the SCPD ratings of the individual motor circuits cannot be exceeded either, folowing the rules in the NEC article 430.52. Then you have to follow the "Group installation requirements" of the OL relays, i.e. max. fuse or circuit breaker size. Then you have to make sure you meet any requirements for lock-out / tag-out with regards to having the ability to lock out both motors or one at a time. Lastly, now you will need to check the SCCR coordination issues with the devices you may end up with from the above, to see if they have been tested together. If not, you will have to put a label on it that says it is not suitable for use in a circuit capable of delivering more than 5kA fault current.

I may have left something out, it's been a while now since I've done it and it seems to me that there were a total of 6 criteria that needed to be met, but I can't think of any more off the top of my head.

 

[Kiljoy]

And that's why I gave up trying to decipher it and posted my question here.

I'm embarrassed to admit it, but we have some 300A fuses and a fuse block from a canceled order that I am trying to use up somewhere. I thought, “Just maybe…”