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Anonymous Poster

Circuit Breaker Sizing

09/22/2010 4:51 PM

Dear all contributors,

Can you help me how to determine the size of Circuit Breaker (VCB) for distribution Transformer protection both at incoming & outgoing line. Let say the transformer Specs are: 3.2 MVA, 13.8 KV/400 volts. the load of this transformer are Water Chillers, Various Pump and air compressors.

Is their any Factor use in determining the size? Is 1.25(Iprimary) or 1.5(Ipri) is safe enough?

any help are highly appreciated

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Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Piping Design Engineering - New Member

Join Date: May 2009
Location: Richland, WA, USA
Posts: 21017
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#1

Re: Circuit Breaker Sizing

09/22/2010 6:14 PM

If there is a local electrical code in force, it will prescribe the ratings of fuses or circuit breakers. In the U.S., the NEC (National Electrical Code) allows the secondary protection to be 1.25 times FLC, or the next higher standard size. The multiplier for the primary protection depends on the size of the transformer; I don't have a copy of the Code handy and don't remember the exact numbers.

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Associate

Join Date: Nov 2009
Posts: 42
#3
In reply to #1

Re: Circuit Breaker Sizing

09/23/2010 7:56 AM

thanks very much

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Active Contributor

Join Date: Aug 2010
Posts: 22
#2

Re: Circuit Breaker Sizing

09/23/2010 12:04 AM

For sizing a CB, the factor 1.25 or 1.5 for main (primary) or feeder (secondary) is not required for safety or magrin, for feeder there are not any fator for sizing if protection curve of CB is suitable for this feeder, for main CB a factor maybe required due to the biggest motor in feeder (if any) operation in acceptable overload condition (1.25 as per motor protection relay setting).

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Anonymous Poster
#4

Re: Circuit Breaker Sizing

09/23/2010 7:58 AM

thank you very much.

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