Meeting Short circuit current rattings.

[arnettda]

Is it ever a issue to meet the short circuit current ratting of a system. I am waiting on engineering from POCO to see actual fault current but the designer gave me a rough number of 86000? My system is 120/240 3 phase with a high leg. The fault current he gave me was from the larger of the three transformers
I have both a single phase and three phase panel off this service. My 1phase breaker is rated 22K ,100K for a main lug. I need a main breaker
What are my options for meeting this requirements?

 

[David Castor]

86,000 A of fault current at 240V is extremely high unless this is some type of secondary network system. This seems unlikely with a wild leg system. My first guess is that this number is incorrect. Maybe 8600 A? How big is the transformer?

Whatever the actual number turns out to be (get it in writing), the breaker or fuse has to be rated for it. While 100 kA breakers are available in some sizes and type, you can almost certainly use a fused disconnect with a current-limiting fuse and get 100 kA or 200 kA short circuit rating.

 

[petersonra]

86,000 A of fault current at 240V is extremely high unless this is some type of secondary network system. This seems unlikely with a wild leg system. My first guess is that this number is incorrect. Maybe 8600 A? How big is the transformer?

Agree

Whatever the actual number turns out to be (get it in writing), the breaker or fuse has to be rated for it. While 100 kA breakers are available in some sizes and type, you can almost certainly use a fused disconnect with a current-limiting fuse and get 100 kA or 200 kA short circuit rating.

Only if it is listed that way.

 

[texie]

I also agree that this number sounds too high. Most open delta xformer banks are not large enough to produce this much fault current.

 

[electrofelon]

That may not be unrealistic. A few jobs ago we had a 3x 166 bank with 1.7%z at 208 volts. That's nearly 82,000 (infinite primary). POCO will also provided very bloated numbers. This one they quoted 105k.

You might be below 65k by the time you get to your service equipment, run the numbers.

Even if you have to go 100k, that shouldn't be a problem. What you likely WILL want to do is keep your mains at 250A or under to get a series rating with standard 10k branches. Once you get into 400A frames, there are likely not any series ratings to 10k branches (this is for Siemens breakers, I can't speak of the top of my head for other brands or fuses).