AIC ratings and available fault current...

[ron]

Bryan,
It depends on whether the system is specified by the designer as fully rated or series rated.
If the calculated SCCR is 19,000A at the line side of the service disconnecting means, then the main and branches can be rated at 22kAIC; or the main and branch can be any rating as long as the manufacturer has a series rating for them at 22kAIC (so individually they might be main=22kAIC and branch=10kAIC).

If the SCCR is 9,000A and specified to be fully rated at 22kAIC, then the main and branch must be 22kAIC. They could also have a series rating to meet the 22kAIC requirement of the designer, where the branch might be 10kAIC.

 

[bphgravity]

Thanks Ron!

 

[dahualin]

Bryan,
It depends on whether the system is specified by the designer as fully rated or series rated.
If the calculated SCCR is 19,000A at the line side of the service disconnecting means, then the main and branches can be rated at 22kAIC; or the main and branch can be any rating as long as the manufacturer has a series rating for them at 22kAIC (so individually they might be main=22kAIC and branch=10kAIC).

If the SCCR is 9,000A and specified to be fully rated at 22kAIC, then the main and branch must be 22kAIC. They could also have a series rating to meet the 22kAIC requirement of the designer, where the branch might be 10kAIC.

Are you guys talking about the Main Circuit Breaker and Branch Circuit Breaker in same panel?

David

 

[bphgravity]

Yes..........

 

[jim dungar]

Are you guys talking about the Main Circuit Breaker and Branch Circuit Breaker in same panel?

David

Most series ratings apply even if the upstream OCPD is located in a different enclosure than the down stream devices.

 

[dahualin]

If I want fully-rated system and the Main Distribution Panel is rated 65KAIC, does it mean the light panel and other power panels shall be rated 65KAIC? Thanks.

David

 

[ron]

Fully rated means that the device/equipment is rated on its own for the amount of fault current calculated at the location of the equipment in the distribution system.

 

[Jraef]

Let me ask this, if the main breaker in a panel is rated for the available fault current, now are the individual breakers in the panel required to be rated the same and every thing downstream or can they be of a lessor aic? the reason for this question is a few years back I was working for a mfg company, They expanded there operation and installed a bunch of large molding machines most using about 300 to 400 amps, it was determined that the disconnects on the machines did not have a rating for the afc, The contractor was allowed to install a junction box in each conduit run coming from the main switchgear and install a fuse block with fuses rated for the afc,

There are 2 issues at hand here, AIC ratings of interrupting devices, and AFC as it relates to equipment such as disconnect switches.

In a panelboard, AIC ratings of breakers downstream from a main breaker technically do need to have an equal AIC rating, unless the entire panel board has a series rating. That series rating will be based on the AIC rating of the main, the let-through of the main and the AIC rating of the downstream breakers. So for instance, if the AFC is 63kA, the Main CB is rated 65kAIC and it is a current limiting breaker that has a maximum let through of 25kA, then the downstream breakers can be 25kAIC rated. This series rating is typically not something that you can do in the field however, it is done by testing at the factory when they go for UL listing. Some jurisdictions will accept a PE's certification of a coordinated system, but most are no longer doing that, they only accept factory listings done by NRTLs (Nationally Recognized Testing Laboratories such as UL, ETL etc.), which require destructive testing and all possible combiinations of devices.

The situation you described may be different though. Non-interrupting devices such as non-fused disconnect switches can't have an "AIC" rating because they do not "interrupt" a fault. But they do need to have a withstand rating equal to or greater than the AFC of the system. If your switches were only rated for 25kA withstand and your breakers were rated for 65kAIC because the AFC was 63kAIC, then you would need to either demonstrate to the AHJ that you had a PE do a system coordination study to certify that the let-through of the breakers was under 25kA (assuming he would accept that), or use disconnects that were equal to or greater than the AFC, i.e. 65kA.

Just so you know, current limiting fuses are typically used in situations such as yours because they usually have much lower let-through energy levels than CL breakers, so AHJ's are more willing to accept them without factory series testing.

 

[mshields]

I wholeheartedly agree with all of the above

I wholeheartedly agree with all of the above

Especially the part about asking the PE on the job. However, if you've got a 600A service and you're at 208V, you probably have a 112KVA or 150KVA service transformer. If you're talking about pole mounted transformers they could be very low impedance; like 2 to 3 percent. That being the case, you could have a fault current in excess of 20,000. If your equipment is a standard 10kAIC, you got a problem.

They key is the size transformer and it's impedance. But again, you need to have some face time with an engineer to understand the pertinent principles.

 

[W6SJK]

X/R Ratio

X/R Ratio

One more requirement... The calculated X/R ratio at the fault point cannot exceed the X/R ratio used when the CB was tested or the CB AIC rating must be derated to account for the higher asymmetrical current.