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Anonymous Poster #1

MCB Calc.

03/28/2017 7:28 PM

Hi,

I am given a work where I have to find the type and size of upstream protection device. Downstream protection device would be:

1. 5 sets of 16 Amps SP MCB -

2. 6 sets of 32 Amps DP MCB -

Assuming that MCBs are 25% over-rated for sf, estimated max loading of no 1 and no 2 would be:

1. = 5*16*0.75 = 60 amps.

2. = 6*32*0.75 = 144 amps.

Is this process correct?

Regards,

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#1

Re: MCB calc.

03/28/2017 8:35 PM

No.

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#2

Re: MCB calc.

03/28/2017 9:18 PM

OCPD size = 100% of noncontinuous load + 125% of continuous load.

OCPD can be loaded to only 80% of its rating for continuous loads.

http://ecmweb.com/basics/sizing-circuit-breaker

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#4
In reply to #2

Re: MCB calc.

03/29/2017 4:23 AM

I think that's for individual OCPDs; for aggregate combination loads, as I recall, there is a factor of 1.15 involved. (My code version is old, so things may have changed.)

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#3

Re: MCB calc.

03/29/2017 4:10 AM

No, it isn't.

The procedure in British Standard 7671 is correct.

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#5

Re: MCB calc.

03/29/2017 4:37 AM

Hint: it's interesting that nothing has been said about the cables! What are they - 7/.029 (rhetorical question - NNTR)?

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#6
In reply to #5

Re: MCB calc.

03/29/2017 11:54 AM

7/029? When was the last time you had anything to do with an electrical installation?

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#7
In reply to #6

Re: MCB calc.

03/29/2017 12:26 PM

The second question is best addressed to the original poster.

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