Motor Protection Breaker SCCR Sizing
Motor Protection Breaker SCCR Sizing
(OP)
For a motor circuit were you install a a breaker for short circuit protection in series with a thermal overload. How do you come up with the SCCR Short circuit Current rating of the breaker.
In other words how do you calculate the expected short circuit current for the circuit. eg for 40hp squirell cage motor 550Vac. Motor is installed 20 meters from the mcc panel.
In other words how do you calculate the expected short circuit current for the circuit. eg for 40hp squirell cage motor 550Vac. Motor is installed 20 meters from the mcc panel.





RE: Motor Protection Breaker SCCR Sizing
Starter has a short circuit rating based on the combination of the circuit breaker, starter and overload relay, based on UL test requirements. The UL test standard is a whole other story.
BTW, don't confuse the terms "Rating" and "Duty". The rating is what the equipment is rated to interrupt. The duty is the actual fault current for this installation.
RE: Motor Protection Breaker SCCR Sizing
So for example, if you look at a Siemens contactor, Siemens OL, and a Square D breaker, the breaker is (let's say) 65kAIC, the contactor has a 10kA SCCR and the OL relay has a 5kA SCCR. The entire unit, if you build it that way, can only have a 5kA SCCR label applied. If you used a Siemens breaker and it was on the list as approved to use with that contactor and overload, the entire lineup could be labeled at 65kA SCCR. Conversely f you used a Square D breaker and contactor and OL relay in their tested listed combination, the same is likely true.
* In some cases, device manufacturers who do not make circuit breakers will pay for the testing of their devices with someone else's circuit breakers, but you still must use the EXACT combination as tested. There s no extrapolation allowed. This is often true of big ticket power devices such as VFDs and Soft Starters.
RE: Motor Protection Breaker SCCR Sizing
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Protection Breaker SCCR Sizing
RE: Motor Protection Breaker SCCR Sizing
But if the fault is at the load side terminals of the MCCB, this combination rating is just not valid.
RE: Motor Protection Breaker SCCR Sizing
RE: Motor Protection Breaker SCCR Sizing
RE: Motor Protection Breaker SCCR Sizing
I know, I was just pointing out that the rules have a bit of leeway, usually based on what is know from the real world, even though you can't apply the same principals everywhere.
RE: Motor Protection Breaker SCCR Sizing
RE: Motor Protection Breaker SCCR Sizing
Limited but certainly not impossible. Especially with someone's big mitts sticking voltage probes in the starter to check voltage. I've seen quite a few burned out buckets.
RE: Motor Protection Breaker SCCR Sizing