Fault Current of Main Feeders

[PM cold-n-mn]

I have a design build project that the owner has hired an outside engineering firm to review my design. I have a power company transformer feeding a 4000amp connection cabinet from there we have fed a 4000amp Main Circuit Breaker Distribution Center (85K) (120/208v 3ph. 4w.) fully rated inside the building. Due to site restraints we ended up moving the transformer/connection cabinet pad 90 feet away. We provided the power company chase between the Xfrmer and the connection cabinet and from the connection cab. to the Main we used 12 sets of 600MCM alum. The outside engineering firm wants me to install cable limiters stating that the fault current on the feeders is 72,000. The power company finally has installed the transformer which ended up being 750kva. According to my calcs. the fault current would be around 81,000 the power company list the fault current for a 750 at 90,600. I don't understand at why he is looking for cable limmters. Is this something new? As far as I can tell even if I was 10 feet away the fault current on the sire would still be less than the power company.

 

[charlie b]

I don't quite follow. What's the 72,000 number? Is that what the other company wants the max fault current to be? And is 85K the rating of the gear? What I think you are saying is that your gear is rated for the available fault current. If so, that is all that is required.

But I need to ask, did your calculation (81K) take into account any motor contribution to the fault current? The fault current supplied by the utility cannot be any higher than the value they give you, and increasing the distance between the transformer and the main gear will further reduce their contribution. But add a few large motors to the party, and all bets are off.

 

[PM cold-n-mn]

The 72K (72,000) I think is the fault current they don't want me to exceed. The 85k is the AIC rating on my gear. I did not take into account the motor loads. At the time they were not all known. We ended up with 3) 30hp elevators, 1) 50hp domestic water pump, 1) 60hp fire pump (tapped ahead of the main at the gear) and around 6) 10hp or less Misc.

 

[mayanees]

Isolation

Isolation

The engineering firm is asking for the cable limiters to provide isolation of individual conductors in the event that one conductor becomes faulted, it can isolate itself from the other conductors and let them stay energized.

As far as the fault current, this sounds like a case of the Utility using a very low impedance 750kVA transformer - on the order of 2% Z.
Using a 250MVA Utility contribution and a 2%Z transformer, 90,600 amps is available at the transformer secondary. After 90 feet of (12) 600kCM AL per phase, there is about 72,500 amps at the Switchgear.

If the Switchgear is rated at 85kA, that would allow your load to consist of as much as (30) 25HP motors connected at the switchgear before it reaches 85kA at the gear.

So the limiters are surely there for cable isolation.

John M

... just saw your last post PM. Your fault current ends up at worst-case 76,700 amps at the gear.

 

[PM cold-n-mn]

Thanks for the quick replies. I think that might be exactly what they want is to use them for isolation. I do have 24k MLO panelboards used for all the brach circuits fed from 600amp circuit breakers in the 85k rated MDP. The breakers are all fully rated at 85k also. There wouldn't be any reason this would make them think I should install cable limiters?

 

[don_resqcapt19]

Cable limiters are not intended to reduce the fault current to your gear. They are installed to maximize continunity of service where there are three or more cables per phase..