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Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
Originally Posted by Tesla
Craft skills taught in school/ apprenticeship are lost when never used. Most of the j-men I know stumble when confronted with even "AIC." They don't even know what exactly that means... for the very same reason.

Their lives are filled with issues of execution -- not design.


And I thought it was because the instructors spent too much time discussing ground-up versus ground-down. smile
Every time an electrician adds equipment they need to consider 110.9 and 110.10, not just at the design stage.

I am in the process of creating work scopes to replace breakers that were added to every building in a school district by some company installing in Classroom Instructional Centers. The breakers they put in were bought on the secondary market (some are over 30 years old) and are only rated 10kAIC even though some breakers they are installed next to are labeled 18kAIC and others 65kAIC.

I worked for a distributor for 16 years, during which time I never remember a contractor asking me for 'rejection clips' for safety switches, even though they are required for UL Listed ratings above 10kA. Eventually we stopped stocking them.

How many times have you heard an electrician say something like 'just use current limiting fuses then we don't have to worry about that 5kA SCCR on the equipment label'?

And no, I do not believe that every EC must be able to perform complicated SC calculations. Not everyone needs to be an expert on everything. But they do need to know to ask.

Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
Member
Quote

208Y/120V 3PH 4W 150 KVA transformer located close to the building. The available fault current at the secondary of the transformer will be over 40K.


Abbreviations:
"SCA" = Short-Circuit Amperes,
"Z" = Impedance,
"LRA" = Locked-Rotor Amperes,
"WCS" = Worst-Case Scenario

Not trying to be an A$$ here, but when I saw the SCA -vs- the Transformer Size, I had to run a quick Calculation; as the SCA seemed too high.

Per the Commercial use of a 150KVA PMT (Pad-Mount Transformer), installation for So. California based Utilities (SCE, SDG&E, LADWP, APU, Etc... the EUSERC crowd) would normally be as follows:

  • Primary: 12,470V, Secondary: 208Y/120V 3PH 4W,
  • Z = 4.0% (no less than 3.0%),
  • Secondary Feeders = (2x) 4 #300MCM AL; 3" PVC 40 (depends on Load Calcs) - at least 40 feet of Conductor length,
  • Typically Feeds into 600A Switchboard Section, with 600/3 Main as Service Disconnect,
  • Service Disconnect rated min. 35KAIC@240VAC, Bus Kit braced for 42K.


I ran a quick calculation using a 150KVA 208/120V Transformer with 2.0% Z (WCS rated to 1.955% Z), and came up with a SCA of 20,819A at the Secondary Terminals.

With 50% KVA Capacity for LRA (Motor Contribution), the SCA at the Terminals increased to 21,651A (added 75KVA fault contribution).

With 100% KVA Capacity for LRA, the SCA at the Terminals increased to 23,733A (added 150KVA fault contribution).

Then figuring Feeder distance = 40 Feet, using Copper Conductors in Magnetic Ducts (2 sets of 300MCM CU), The SCA at the Service would be:

w/o LRA Contribution: 16.9KA,
w/ 50% LRA Contribution: 17.6KA,
w/ 100% LRA Contribution: 19.7KA.

Utility would likely quote the SCA at the Line Side of the Service Disconnect to be 22KA.

By the time the Fault Level has traveled to the First Panelboard, it may easily be down to 10KA - or even less... depending on the length of run and Panelboard Feeder Size.

That's my 2¢ worth for today... wink

Leaving the Soapbox... tired

--Scott (EE)


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: May 2005
Posts: 984
Likes: 1
G
Member
If you ever see an OCP that doesn't have a fault current rating on it, that means it's only rated at 5kAIC.

You can make a rough check using various websites for most of the jobs...and write a "please confirm that this is correct" RFI for the Engineered ones.


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