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A room contains electrical devices which has a max demand of 10A (may be more than that in the future).

The mains cable from the electric panel (distribution board) is 4 mm2 which can handle current up to 23A.

The sub-cables from the main cable to the wall outlets are 2 mm2 which can handle current up to 15A.

Each outlet is rated for 16A.

What should be the current of the automatic circuit breaker? and Why?

If the panel contains 5 breakers and each breaker is rated for 20A.

Is the value of the main breaker 5*20A = 100A?

Edit:

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ A small diagram with circuit names would make the question more clear and easier to discuss. As it stands it's not very clear where you want to put a CB and how many CBs there will be in total. \$\endgroup\$ – Dmitry Grigoryev Jul 4 '16 at 7:54
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    \$\begingroup\$ 15A to insure the smallest cables are not a fire hazard. Did I get it right? What do I win? The main breaker is probably labelled. Unless, wait a minute, is this a test question? Let me know if I got the answer right. \$\endgroup\$ – mkeith Jul 4 '16 at 8:02
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    \$\begingroup\$ All kidding aside, the sizing of breakers is determined by the wire diameter of the downstream wire. For the main, it depends on the wire diameter inside the electrical box that connects to all the breakers. But I am not an electrician. \$\endgroup\$ – mkeith Jul 4 '16 at 8:05
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    \$\begingroup\$ @DmitryGrigoryev I'm sorry about the unclear question. I added the diagram. \$\endgroup\$ – Michael George Jul 4 '16 at 9:11
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    \$\begingroup\$ @mkeith No, It is not a test. I would like to know how to design a panel practically. \$\endgroup\$ – Michael George Jul 4 '16 at 9:12
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As a rule, each CB must be dimensioned to protect the smallest unprotected wire in its circuit. In your schematic, the first CB has to protect the 4mm2 wire going to Room 1, but also 2mm2 wires going to outlet 1, outlet 2 etc., because those wires have no individual protection. Because of this, CB1 should be dimensioned to 15A or less. Otherwise, a 20A load on outlet 1 will not trip the protection, and the 2mm2 wires will overheat and catch fire.

The same reasoning has to be applied to circuits in Room 2 - Room 5.

The main CB is there to protect the wire of electrical company and not your house, since all the wiring inside your house is already protected by individual CBs. Therefore, the main CB is not supposed to be dimensioned to the sum of your individual CBs. In my experience it has to be dimensioned (and sometimes even installed) by the electrical company, and you don't have the right to touch it afterwards. Also in my case the price of electricity depends on the main CB setting: if you want more peak current, you have to pay more.

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  • \$\begingroup\$ I don't know how main breakers are chosen in general. But the wire or bus bar after the main breaker, but before the subcircuit breakers must be large enough to carry the rated breaker current. It could also be larger. In the US I see the boxes referred to as, for example, "200 Amp service." Maybe the box itself has a current rating? \$\endgroup\$ – mkeith Jul 4 '16 at 21:31
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The CB protects the wires connected to it. At each node where the thick wire is distributed to thiner wires there should be placed CBs. For your example, the 1.5mm^2 wires are protected with 10A, 2.5mm^2 with 16A. The incoming 4mm^2 is protected before, at point where it was taken form thicker wire.

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  • \$\begingroup\$ Thank you for your answer but I feel like it is not a practical solution. The 4mm wire is protected before by a 20A breaker that exists in the panel. \$\endgroup\$ – Michael George Jul 4 '16 at 9:15
  • \$\begingroup\$ What about the other wires that are connected to the wall outlets? We can not install a circuit breaker at each wire. \$\endgroup\$ – Michael George Jul 4 '16 at 9:15
  • \$\begingroup\$ Not clear what you are asking, but those wires: Outlet 1 to 3 should be protected with 16A CB. Or if you can't install CB for eaach wire, then you have to change th 20A CB of Room1 to 16A (2.5mm^2 or less if you really have 2mm^2). Always, the thiner wire dictates the CB rating. \$\endgroup\$ – Marko Buršič Jul 4 '16 at 9:51
  • \$\begingroup\$ @MichaelGeorge Which country you live? what is the voltage? Where did you get 2mm^2 wires? In EU we have 1.5mm^2, 2.5mm^2, 4mm^2, 6mm^2,...so on for residential buildings, in US they use AWG not mm^2, in both scenarios there is no 2mm^2 wire. \$\endgroup\$ – Marko Buršič Jul 4 '16 at 10:11
  • \$\begingroup\$ Egypt, 220V, 50Hz,The best company that produces cables in Egypt called Elsewedy. elsewedyelectric.com/BE/ImageFiles/SubProducts/Pdf/… \$\endgroup\$ – Michael George Jul 4 '16 at 13:26

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