LV switch board short circuit rating

[sramesh]

Good day to you

We are planning to add a Sub switch board (415V)using 800Amps ACB feeder from a main LV switch board of 4000Amps, 65kA 1 sec.
The sub switch board(800A) will be located about 300mts from the main Switch board and connected using 3 runs of 630Sqmm Cable. The consultant is suggesting short time rating for the switch board as 25kAmos for 1 sec. Consultant says it is reduced due to cable impedence. Will it be enough? All our existing sub switch boards in the old plant are rated for the same main switch board rating of 65kA 1 sec irrespective of distance from the main switch board.

It will be helpful if any of you can advise whether we can accept the new sub switch board with lower Short time rating.

Thanks in advance.

 

[alehman]

25kA at the panel may be reasonable. If you don't trust him, ask to see his calculations.

Is that three 630sq mm per phase? That seems excessive for 800 amps.

 

[sramesh]

Thanks for the suggestion, I will ask the calculation from the consultant. 630Sqmm is chosen asthe Voltage drop requirement is 2%.

Regards

 

[ScottyUK]

Why is the volt drop requirement so tight? The old UK standard when we had 415V distribution was [±]6% at point of use.


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If we learn from our mistakes I'm getting a great education!

 

[pugwash]

Yup. 25kA/1sec should be OK with normal motor loads, etc. Agree with Scotty re volt drop. Wiring Regs (see Appx12) now much more relaxed for private supplies, unless you have a specific voltage drop problem?

 

[alehman]

The U.S. NEC "recommends" 3% voltage drop for feeders.

Sometimes it's better to use more smaller conductors. Two 250 sq mm copper (is yours copper or aluminum) per phase would be about 7.5% for 800 amps at 85% pf. It would take five 250 sq mm to get 3%. For 500 sq mm, you would still need four per phase for 3%.