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soon1
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Join Date: Jun 2009
Posts: 23

CB KA Selection

04/04/2016 2:15 AM

Hi all,

i have incomer CB of 100A three phase at 415v.

Outgoing CB at 63 A single phase.

what is the short circuit KA for this distribution board?

thanks

chew
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spades
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Location: Port Macquarie N.S.W. Australia
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#1

Re: CB KA selection

04/04/2016 2:38 AM

Are you asking for the short circuit current rating of the circuit breaker/s or the short circuit current that might flow in the event of a short circuit?

Short circuit current is determined by the size of the upstream supply transformer and the impedance of the cable run to the switchboard.

Your main breaker has to be able to withstand that current and any subsequent breakers need to be able to handle a slightly lesser current as outlined below..

You need to know - firstly the short circuit current rating of the transformer, and secondly the impedance of the current path from said transformer to your circuit breaker.

Knowing those figures, you can then determine the correct kA rating of the breaker.

If the short circuit is downstream of the second breaker, then the first circuit breaker will reduce the level of short circuit current to the second breaker, and so the second breaker is not required to have as high a kA rating as does the first - this is called cascading, and generally the specs of the relevant breakers will have cascade charts which will inform you which ones can be cascaded for your specific circumstances.
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lyn
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Join Date: Oct 2008
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#2
In reply to #1

Re: CB KA selection

04/04/2016 3:55 AM

This information can be found in the applicable electrical codes. Probably either NFPA 70 or BS 7671 and any regional and local codes as may be in force.
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PWSlack
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#3

Re: CB KA selection

04/04/2016 5:23 AM

It is the voltage on the board divided by the resistance of the conductors connected to the short circuit in question.

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Ali18
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#8
In reply to #3

Re: CB KA selection

04/05/2016 8:26 AM

There's a minor issue, which is related to the nature/concept of AC circuits and their calculations, So it'd be: << It is the voltage on the board divided by the "impedance" of the conductors connected to the short circuit in question>>.

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TonyS
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#4

Re: CB KA selection

04/04/2016 5:33 AM

The quick and easy method is use a loop impedance meter. You don't say where you are in the world, in the UK almost every electrician could carry out the test for you.
You mention a 100A incoming supply so you're not that close to the distribution transformer to give a MPFC in the 10's of KA.

The incoming supply to my flat has a measured MPFC of 3.4KA.

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spades
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Location: Port Macquarie N.S.W. Australia
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#5
In reply to #4

Re: CB KA selection

04/04/2016 7:09 AM

I understood him to mean that his incoming main circuit breaker is rated at 100 amps (incomer CB of 100A), no reference to actual circuit capability.
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TonyS
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#6
In reply to #5

Re: CB KA selection

04/04/2016 12:17 PM

So the incoming circuit impedance doesn't enter in to the equation?

The world is for once lagging behind the UK. Even for a domestic installation we are required to measure the actual MPFC and not rely on assumed values provided by the DNO (PoCo). A thoroughly certified installation will give the MPFC for each circuit.

For industrial situations I use "infinite source" for my MPFC calculations. The reason being it is the worse case scenario.

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ronches
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#7

Re: CB KA Selection

04/05/2016 4:07 AM

The information provided is insufficient. Give more details please
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moons
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Join Date: Jun 2014
Location: South Korea
Posts: 18
#9

Re: CB KA Selection

04/06/2016 12:14 AM

Simply, you may use CBs which has a same kA rating CB of upstream feeder.

If you wanna in detail, you need to check that the Selectivity is required or not.

Selectivity is required, you may use smaller kA CB for your DB incomer. But not required, the same kA rating would be applied.

And also, max. kA current can be considered. there are some cases to flow huge current on DBs but normally Phase-Phase fault current is the biggest value on it.

Then you can calculate it with impedance of your DBs.

Hope it help you.
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Ali18 (1); lyn (1); moons (1); PWSlack (1); ronches (1); spades (2); TonyS (2)

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