Sizing a Distribution Board (Consumer Unit)

Posters should start to be much more specific on what juristiction they are operating in, and what rules they are operating under. For example, this type of sizing exercise, for most of the United States would be done per the National Electical Code, which provides some relatively clear guidance.

My guess, based on a few of your terms used, and clues like 13A wall outlets (in the US we would never use 13 ampere wall outlets, they are unlucky), is that you are not in the US, and not looking for sizing help that is NEC based.

Judging by the useage of the term "consumer unit" and 13A sockets, I would quess that he is in BS1362 land. Either Great Britain or one of their influenced former colonies. So I would guess that British standards apply.

Yeah, am from one of the British former colonies. So i use the BS....

Dear Colleque,

The incomer rating of the distribution board depends on the total load connected to the distribution board.The calculation done by you is correct.Hoewver you may take diversity factor(DF) of load usually around 69-80% of connected total loads.In this case taking Df as 80% , the total load on the incomer works out to 0.8 x 90.8=72.64A.Taking for future expansion of loads say 10%(if required), the incomer rating works out to 79.90A.Take the nearest standard available rating of MCCB shall be 100A. The no of out going feeder MCCB will depend upon number of out going feeders if you consider that each load will be fed by independent MCCB.The rating of each out going feeder MCCB depends upon the load of that feeder.Calculate the load current required for each out going feeder.Select the standard circuit breaker with rating higher than the calculated feeder rating.

you are in calculating your main breaker, i 100% agreed with you.. you can use 100A MCCB rating.. actually the function for the main breaker is to protect the whole panel from any faults and not from the individual connection.. this 100A amp is morethan enough even for the future expansion as what because of the diversity factor or demmand factor.. but for selecting the branches.. its the other issue.. for the motor calculate it according to its rating and just multiply it 150% for its starting current.. and if you make spare.. in my own idea as an contractor engr. dont use 10A or 6A.. use 15A or 20A as standard.. coz ussually we use 4mm² wire or 10AWG which carries morethan 20A current...

I = 50.2 x 1000/1.73x400x0.8(Pf) = 90.8A

hi, im new at this. What is the name of this formula?

P=Square Root of 3 * V * I * cos (the angle between KW&KVA)

cos (the angle between KW&KVA) = Power factor = p.f

and he use it to calculate the current to know the rating of the DB Incomer.

The outgoing breakers are there to protect the circuit wiring downstream. The circuit wiring size depends on a number of factors, including circuit length, permissible voltage drop, and method of installation of the cables.

Cable sizing has to take into account the permissible voltage drop en route to the load and the ability of the cable to survive a fault current safely until the breaker operates to disconnect it.

The best recommendation possible is to deploy the electrical standards applicable in the country of installation; in the UK, for example, British Standard 7671 applies.

If in doubt, as suggested by the original post, consult a local qualified Electrician, who will be more than capable of carrying out the calculations, installing the circuits, and testing and certifying it afterwards. Expect to receive an installation certificate document covering the work at handover.