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Johnsly
Active Contributor
India - Member - Member working in Qatar

Join Date: Oct 2008
Location: Kottayam, Kerala, India. Now at Qatar
Posts: 10

MCCB Sizing Calculation

12/02/2008 11:39 AM

My consultant gave me 160A MCCB as incomer to a 4-way SMB for a connected load of 77.6 KW. Also they have provided one 100A MCB as spare in the SMB. So total 3 o/g and 1 spare. Now we have to connect a 20 KW load to this 100A spare. But then this 160A MCCB will not be enough as incomer, am i right? Then my question is how they calculated this rating of incomer, means 160A. Has they made any mistake in calculation? Why it is not able to connect this load to this SMB? How we are usually calculating the rating of the incomer? How much we will be providing as spare, means how much % we will be considering as spare while calculating incomer rating?
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wareagle
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1604
Good Answers: 63
#1

Re: MCCB sizing calculation

12/02/2008 4:06 PM

Can not answer your question without the voltage. Is the load 3 phase? What is SMB mean?
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Anonymous Poster
#5
In reply to #1

Re: MCCB sizing calculation

12/07/2008 11:36 AM

SMB means Sub Main Board. Before DB, we will be installing this. The load is 3 ph
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Anonymous Poster
#11
In reply to #1

Re: MCCB sizing calculation

01/18/2011 12:04 AM

when you want to enter MCCB room what u need to check 1st before u start your job.
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Anonymous Poster
#2

Re: MCCB Sizing Calculation

12/03/2008 12:26 AM

what are 3 0ut goings now beside the spare one?
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Diris
Active Contributor

Join Date: Aug 2008
Posts: 22
#3

Re: MCCB Sizing Calculation

12/03/2008 1:01 AM

You can get guidance from NEC codes.
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babybear
Guru
United Kingdom - Member - Not a new member!

Join Date: Jun 2008
Location: USA/Europe
Posts: 4547
Good Answers: 68
#4

Re: MCCB Sizing Calculation

12/04/2008 12:37 AM

Hello Johnsly:

Transformer calculations will automatically populate selected fields based ... conductor size between secondary main Molded Case Circuit Breaker (MCCB) and ...
www.eaton.com/EatonCom/Markets/Electrical/Tools/Calculators/SelectiveCoordinationCalculator/index.htm - 70k

This is the page explaination: SELECTIVE COORDINATION CALCULATOR INSTRUCTIONS

Achieving selective coordination is straightforward when using the following selection calculator. Follow these steps to ensure that selective coordination is in place:

Step 1a:When the Utility Fault Current is provided by the local utility enter the value here. Transformer KVA and impedance data is not required to continue.
Step 1b:If the Utility Fault Current is not provided, enter the transformer kVA and % impedance data.
Step 2:Select Primary, Secondary Voltage & Phase data from the dropdown boxes.
Transformer calculations will automatically populate selected fields based upon embedded formulas in the calculator.
Step 3:Select Conductor Type and Raceway Characteristics from the drop down box.
Step 4:Select conductor size between secondary main Molded Case Circuit Breaker (MCCB) and branch MCCB from AWG drop down box.
Step 5:Enter the length (feet) of one phase conductor between secondary main MCCB and branch MCCB or the length of conductor between the step-down transformer and panelboard with built-in secondary main MCCB and branch breakers. [Note: See NEC 240.21 regarding tap rules for transformer secondary conductors.]
Step 6:Enter the number of conductors per phase between secondary MCCB & branch MCCB.
Step 7:Select secondary transformer MCCB type from drop down box.
Step 8:Select the desired branch MCCB type from downstream branch breaker drop down box.
Step 9:Calculator will display coordination level and indicate if there is selective coordination between selected breakers. Note: Selected coordination is based on utilizing appropriate adjustable trip settings for all selected breakers. If the calculator indicates no selective coordination, then vary parameters such as conductor length, secondary, branch breakers to create a coordinated system.

Notes: Selective coordination will be achieved with the indicated frames WHEN utilizing appropriate adjustable trip settings. In some cases the calculator shows large frame branch breakers almost as large as or the same as the main breaker frame size. The long delay and the short delay adjustments must be properly set to coordinate for low-level overloads/faults.

I know this will be helpful.

Good luck and let me know if and when you use it to solve your problem.

Thank you..............

__________________
 Take it easy, bb. >"HEAR & you FORGET<>SEE & you REMEMBER<>DO & you UNDERSTAND"<=$=|O|=$=>"Common Sense is Genius dressed in its Working Clothes"<>[Ralph Waldo Emerson]
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KKK
Associate

Join Date: Aug 2009
Location: India
Posts: 25
#6

Re: MCCB Sizing Calculation

09/12/2009 1:13 AM

Pl. specify voltage & load power factor
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therock9211
Participant

Join Date: Feb 2010
Posts: 3
#7
In reply to #6

Re: MCCB Sizing Calculation

02/03/2010 1:11 AM

I if I have p.f 0.8 and total load of 70 KW , and voltage is 230/400 KV in three phase and the fault duty is 40 KV , how to calculate the MCCB rating in AMPS?
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Anonymous Poster
#8
In reply to #7

Re: MCCB Sizing Calculation

02/03/2010 3:48 AM

P = 1.732*V*I*pf

Here, P = 70 kW

V = 400V.

pf = 0.8

From the above formulae, you can calculate the total load current, which works out to be 126.3A. Considering safety factor of 25%, incomer MCCB rating shall be 158A.
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hammouda
Participant

Join Date: Jul 2010
Posts: 1
#9

Re: MCCB Sizing Calculation

07/19/2010 3:40 AM

See your total load 77.6+20=97.6KW then the total connection current at 0.9PF is 151A so 160A is sufficient but its recommended to raise it to 200A MCCB in the same time the rate of the spare should be changed to 63A because the load is 20W then the current at 0.9 PF is 31A so 63A MCCB more than enough after that you will have balance connection
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Anonymous Poster
#10

Re: MCCB Sizing Calculation

07/19/2010 3:40 AM

See your total load 77.6+20=97.6KW then the total connection current at 0.9PF is 151A so 160A is sufficient but its recommended to raise it to 200A MCCB in the same time the rate of the spare should be changed to 63A because the load is 20W then the current at 0.9 PF is 31A so 63A MCCB more than enough after that you will have balance connection
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