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Anonymous Poster #1

Regarding MCBs Rating

02/15/2015 9:32 PM

If we have branch MCBs rating of 16A, 16A, 6A, 6A, 16A - then the rating of the main parent MCB above the branch MCB will be the sum of all of them? Or, it should be at least 20% above the sum of value of those ratings? Their sum would obviously be 60 Amps - and the next available rating is 63 Amps. So can 63 A be selected - as it is the highest available MCB rating? But as far as I know we also have MCB ratings upto 125 A. But if we allow 20% of room then - it is 60*1.2 = 72 Amps; so next higher available is 80 Amps MCB.

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RAMConsult
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#1

Re: Regarding MCBs Rating.

02/15/2015 9:37 PM

"...as far as I know..." if you have to ask the question you shouldn't be doing the work...

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lyn
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#2

Re: Regarding MCBs Rating.

02/15/2015 10:14 PM

If you have to ask, find other work.

Or, buy a copy of BS 7671 or the NFPA 70: NEC®
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pnaban
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#3

Re: Regarding MCBs Rating

02/15/2015 11:57 PM

consider discrimination between breakers as well as load and diversity factors etc

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Tornado
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#4

Re: Regarding MCBs Rating

02/15/2015 11:59 PM

If diversity is taken into account, the main breaker could be somewhat less than the sum of the sub-breakers. OTOH, if future expansion is possible, it could be larger. No single answer is possible.

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pnaban
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#5

Re: Regarding MCBs Rating

02/16/2015 12:23 AM

In addition to maximum simultaneous load consider maximum starting current too.

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#6

Re: Regarding MCBs Rating

02/16/2015 12:55 PM

So, are all your sub-circuits' breakers all passing their rated current all the time? And, if you intend changing breakers for a higher rating, are you certain that the main feeder can take it? Your ball.

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#7
In reply to #6

Re: Regarding MCBs Rating

02/16/2015 11:55 PM

If connected load increases,upgrading of MCBs is necessary. Also selecting correct type(B,C,D etc) of MCB is important.

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pnaban
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In reply to #7

Re: Regarding MCBs Rating

02/16/2015 11:58 PM

Bear in mind that the OP need a solution. If this arises in your work place what an engineers should do without beating round the bush.

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KeepItSimpleStupid
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#9

Re: Regarding MCBs Rating

02/17/2015 12:54 AM

If you had say a 60 A (sized properly) feed to a sub-panel, you could have a 200 A breaker in the sub panel and it would be OK.
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Oomborrie
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#10

Re: Regarding MCBs Rating

02/17/2015 2:21 AM

The Main MCB max rating is a reflection of the current capacity of the feeding/supply cable from the Supplier. I will assume that you do not know how to find the rating of that cable if you do not know how to calculate combined rating. Legaly you are commiting an offence working in a db without qualifications and it is better to get a qualified person to do this. Electricity have laws because people can not see the danger you expose them to doing things you know nothing of. The regulations is to protect the people arround you and the buildings you can fry/burn down with the workers inside. Be responcible and stand back untill you finish your education and apprentice period. If you are in training or an apprentice you will be reporting to somebody with the answers and dtailed explinations.t

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#11

Re: Regarding MCBs Rating

02/17/2015 2:47 AM

Breakers are sized to protect the conductors.

Your main breaker should have a rating that suits your feed cable/drop line size.

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silverfox
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#12

Re: Regarding MCBs Rating

02/17/2015 3:10 AM

No. THe breakers are sized to protect the wiring and installation not each other. It is useful to get discrimination between the breakers and as you say a 6A should trip before a 10A. There are tables and curves from which you can determine this. Agree with the other posters - there are too many questions you have not asked to be able to continue with this safely. Suggest handing this over to someone else. Good if this someone else isn't the fire brigade.
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Users who posted comments:

Crabtree (1); KeepItSimpleStupid (1); lyn (1); Oomborrie (1); pnaban (4); RAMConsult (1); silverfox (1); Tornado (1); Wal (1)

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