Peak Let-Through Current

[dcheser]

I have been handed the task to fix an issue where someone has designed, built and installed a control panel. When the inspector came in to inspect he would not approve the installation. I was first informed that the breaker feeding the control panel had a short circuit rating of 36KA and the main breaker in the panel has a short circuit rating of 18KA and this was the problem. This made absolutely no sense and upon further questioning I finally got that the inspector wants to see the calculation showing the Peak Let-through Current of the 18KA Main breaker and whether or not the branch circuits downstream and starter assembly's (rated 42KA) are capable of withstanding the let through current.

The panel consists of a Main breaker (300A, Isc=18KA) with three branch breakers ((100A, Isc=18KA), (100A, Isc=18KA), (20A, Isc=18KA)) and a set of fuses (10A, Class J)

To make this short and simple I have no idea how to determine the peak let-through current and I am looking for a little help and/or guidance on how to go about this.

Any help is greatly appreciated.

 

[steve66]

My best suggestion is to read how to do fault calculations in the Cooper SPD handbook. Its available for download here:

http://spd.cooperbussmann.com/

I don't think you need to calculate let through current - that usually applies to current limiting fuses.

You will need to figure out how much fault current is available at the 36 KA breaker, then see if the feeder to the 18KA panel limits the fault current enough to get it below 18KA. (Its very possible.)

The formulas are a little complex, but they can be entered into excel, so you don't have to repeat them every time you need to do a calculation.

 

[dcheser]

Thanks, Steve. Now this is making a little more sense. I have gone through the short circuit calculations before but in this instance I have a lot of unknowns that I will need to obtain unless there is a way around it that I don't know about. The starting point for the calculations would have to be the transformer secondary feeding the Main Distribution Panel that this panel is being fed from....which I don't have. If there is away around that please let me know but at present the only thing I know about anything upstream is that the Feed Breaker has a Short Circuit Rating of 36KA.

 

[jim dungar]

Stop.

You must know the available fault current at your panel.
The AIC ratings of devices is absolutely immaterial except it must not be lower than the amount of available fault current at their line side terminals.

 

[Jraef]

Stop.

You must know the available fault current at your panel.
The AIC ratings of devices is absolutely immaterial except it must not be lower than the amount of available fault current at their line side terminals.

Right. As I interpret that, your inspector was not so much interested in the peak let-through as he was directing you towards the main issue, the SCCR rating of your control panel. When you don't have a UL (or NRTL) label that states the SCCR, then it's up to you to prove all of the ratings are sufficient.

But there is hope. IF you used all components from the same manufacturer, you may find that the SPECIFIC combination of devices will allow a higher SCCR rating than the individual components. If so, all you need to show is that UL SCCR file from the mfr.; most of the majors have it readily available by download as an Excel spreadsheet. If you mixed and matched to get the cheapest price on each individual component, yolu are in for a rude awakening. Without the series listing you will likely find that your OL relays are only rated at 5kA, not 18kA, and as the weakest link they will be the maximum rating for the entire system regadless of the let through. You said the starters were rated 42kAIC so maybe that covers it, but you need to check specifically.

However back to Jim's point, if the AFC is higher than even that series combination rating, you're dead at the starting gate. You have to start there.