SERIES RATINGS

[PE (always learning)]

Hey Everyone,

Ok, so this is a weird scenario to describe so bear with me here. I have a 2000 amp main distribution panel MDP that feeds a main lug only panel down stream. The breaker that feeds the MLO panel from the main distribution panel is rated for 65kA, but the main distribution panel itself has an SCCR rating of 42kA. The breakers in the downstream MLO panel are shown to series rate for 65kA with the upstream breaker in the main distribution panel. My question is: Do the breakers in the MLO panel series rate for 65kA with the upstream breaker in the main distribution panel as shown on the series rating table or do they only series rate for the maximum SCCR rating of the main distribution panel which is 42kA? Obviously they are supposed to series rate for 65kA as shown on the tables, but I'm wondering if the SCCR rating of the main distribution panel brings this series rating down. Again, I know this is kind of confusing, so keep your mind open here.

Best Regards

 

[PE (always learning)]

To summarize again, I have a higher rated breaker (65kA) in a main distribution panel with a lower short circuit current rating (42kA), feeding a main lug only panel with breakers that are supposed to series rate up to 65kA. Does the short circuit current rating of the main distribution panel reduce my series rating down to 42kA?

 

[Jraef]

Moot point. If the MDP is rated for 42kA, you cannot connect it to a system that has 65kA available, so whether or not the sub-panel gets rated for 65kA is irrelevant. Technically, I suppose it would however, for whatever that's worth. I would just consider it rated for no more than 42kA since that is the max rating of the upstream equipment.

 

[PE (always learning)]

Moot point. If the MDP is rated for 42kA, you cannot connect it to a system that has 65kA available, so whether or not the sub-panel gets rated for 65kA is irrelevant. Technically, I suppose it would however, for whatever that's worth. I would just consider it rated for no more than 42kA since that is the max rating of the upstream equipment.

Awesome, thanks for the response. That's exactly what I thought too, you cannot go higher than what the upstream equipment is rated for. Maybe the breaker would survive a fault current up to 65kA, but the main distribution panel might become damaged because it is not rated to withstand that fault current.

 

[Russs57]

Okay, time to fess up as I must have a huge misunderstanding. See, I thought a bare circuit breaker panel didn't have a SCCR.

I thought the SCCR of the completed panel was equal to the lowest interrupting capacity of a branch or main SCCR circuit breaker installed in the panelboard.

For example, I could buy an I-Line with factory installed breakers and their I-Limiter main and get a 200 kA rating. I could also populate that same panel with my own breakers and bring that rating down to 42 kA.

 

[topgone]

A circuit diagram could speak a million words, IMO.

 

[electrofelon]

Okay, time to fess up as I must have a huge misunderstanding. See, I thought a bare circuit breaker panel didn't have a SCCR.

I thought the SCCR of the completed panel was equal to the lowest interrupting capacity of a branch or main SCCR circuit breaker installed in the panelboard.

For example, I could buy an I-Line with factory installed breakers and their I-Limiter main and get a 200 kA rating. I could also populate that same panel with my own breakers and bring that rating down to 42 kA.

Yes that is common language that appears in most/all panelboard literature. Siemens says, regarding the SCCR of their P1-P5 panelboards "200 KAIC max. symmetrical or equal to the lowest rated device installed unless a series rating is indicated." (100k for P1 @480/277). I dont really like that statement, as I feel it doesnt make any sense. The SCCR is what it is and doesnt change. Yes you cant use that panelboard above the rating of a lower rated breaker that is installed, but the SCCR of the panelboard itself doesnt change. Im a math guy, I just dont like the logic there.

 

[Jraef]

A lot of circuit breakers are now current limiting, so a series rating, TESTED and listed with SPECIFIC combinations, can be higher than the downstream individual beakers. The basis is that the let-through of the current limiting breaker is less than the IC or downstream breakers (or withstand of downstream gear).