Fault Current and CB Interrupting Capacity

[mkgrady]

I'm looking at changing a service. The available fault current from POCO at the manhole feeding the service lateral will be 15,000 amps. If I install an MCB that has an interupting capacity above this 15,000 can the down stream breakers in a new MLO panel be rated 10,000AIC?

I realize the system would be better designed if it had a real coordination study done, but this customer just wants cheap. I just want to know that the MCB will open even if the downstream CB's do not.

 

[ron]

The main would open, but if the fault was at the downstream breakers, and they were not properly rated, they may catastrophically fail. And it would be a code violation to provide underrated equipment.

You need to do a calculation to determine if the downstream equipment would be exposed to <10kA or not.

 

[petersonra]

I'm looking at changing a service. The available fault current from POCO at the manhole feeding the service lateral will be 15,000 amps. If I install an MCB that has an interupting capacity above this 15,000 can the down stream breakers in a new MLO panel be rated 10,000AIC?

I realize the system would be better designed if it had a real coordination study done, but this customer just wants cheap. I just want to know that the MCB will open even if the downstream CB's do not.

a lot depends on stuff you have not told us but many times you can get a main breaker panel series rated for less than a MLO panel.

 

[kwired]

You need to know what the available fault current is at the load end of the lateral. The longer the lateral (of same size conductor) the more resistance it will have - the less fault current will be available at the load end. Even if you have more than 10k available at the service equipment you will have to recalculate what is available at the load end of your feeder as it will be lower yet because of impedance of the feeder conductors, and it could very well be less than 10k.

 

[jim dungar]

I'm looking at changing a service. The available fault current from POCO at the manhole feeding the service lateral will be 15,000 amps. If I install an MCB that has an interupting capacity above this 15,000 can the down stream breakers in a new MLO panel be rated 10,000AIC?

Maybe.

I just want to know that the MCB will open even if the downstream CB's do not.

AIC is about a device not failing violently (i.e. exploding) when it tries to open on a bolted fault. AIC ratings have nothing to do with the sequence in which devices open.

Per 110.9, all protective devices must be rated for the fault current available at their line side terminals. If you are going to 'go cheap' and not calculate the available fault current, your only option is for the downstream breakers to be 'series rated' with the upstream device for a minimum of 15kA (for 240V equipment, a common series rating is a 22kAIC main with 10kAIC branches).

 

[mkgrady]

Thanks for all the replys. Very helpful.

It turns out the supplier that was offering 10KAIC breakers for the panel and a higher rated AIC for the MCB cost more than the supplier that offers 18KAIC breakers for the panel so it becomes a moot point. I understand it all a lot more now after getting these answers but I wish I understood "series rated" for future reference.