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We have one circuit breaker (40 ampere) in entry point of electricity line in our house, than we divided it into two separate line in two levels and put one circuit breaker( 32 ampere) in level one and other other circuit breaker (25 ampere) in level two.The main circuit breaker ( the 40 ampere) become automoticaly off every several hours, but other two circuit breakers ( the 32 ampere and the 25 ampere) don't become off.I am thinking that the distribution circuit breakers 32+25=57 is larger than 40 ampere, and it make the problem. Am I right? Should total of different circuit breakers in one system be less than the main circuit breaker?

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40amps seems pretty small for household service. At least in my locaiton, almost all new detached houses have 200amp service, and even back in the 70's, 60-100amp service was common. I have a blender that alone can pull 10amps! –  Steven Feb 4 '12 at 15:43
    
Where in the world are you, Ghulam Mohammad? Most people on this site are in the USA or UK, and may be assuming you have something similar to their home systems. –  Jay Bazuzi Feb 4 '12 at 19:28
    
What is the voltage on your system? In the USA, the main panel gets 240V across 2 hot wires, which is typically distributed as 120V on 1 hot wire. @Steven's blender is 120V @ 10A, so that would be 1/8th, not 1/4th of the total power available on a 40A service in the USA. –  Jay Bazuzi Feb 4 '12 at 19:30
    
@JayBazuzi for most areas yes. There are some exceptions, like rural areas where they might only get a single phase –  Steven Feb 4 '12 at 20:37
    
@Steven: Our 1950's house (USA) had a 30 amp service panel when we bought it. We had it professionally upgraded to 200 amp before moving in, though. –  BlueRaja - Danny Pflughoeft Sep 8 '13 at 0:48
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4 Answers

Typically your master breaker is sized to be less then the sum of all of the sub breakers. The reason for this is in a typical house, you don't pull the maximum current from all branches at once. However, this setup does allow for the situation that you are running into where your master breaker can trip even though none of the sub breakers have tripped. There is nothing unsafe about this setup, but as I am sure you are aware, it can be very annoying to have the whole house loose power.

In order to deal with this, you should look into what exactly it is that is happening to trip the main breaker. For example, is it the AC kicking on while a refrigerator is running? Or is there some other circumstance that causes the breaker to trip? Once you figure out what this situation is, you will know if it is possible to just avoid pulling too much power.

If you do need to be able to pull more power, you will need to look into the specs of your distribution panel. It could be that it is not capable of handling more than 40 amps. Or potentially, depending on your location, the main service lines can't handle more than 40 amps. It is difficult to tell you what your situation can handle, but this is the path you will need to look into.

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The two branch circuit breakers will trip if the load on their protected circuit is greater than 32 amperes, and 25 amperes respectively. However, since the main breaker trips at 40 amperes. If both branch circuits are pulling a full load the branch circuit breakers will not trip, but the combination of the loads will trip the main breaker (32 + 25 = 57 > 40 as you have said).

It's a very common situation to have the branch circuit breakers total more than the main breaker, but this is almost never a problem since the circuits usually don't pull a full load. Remember, circuit breakers are there to protect the wires. They shut off the power if you are drawing enough power to damage the wire, so installing a larger main breaker is likely not an option (unless you contact your power company and have the service upgraded, which will include upgrading the feeder cable).

Contact your local power company, and speak to them about a service upgrade. Other than plugging less stuff in, or cycling loads, there is not much you can do yourself in this situation.

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There are 2 situations where the main breaker could trip before the branch breakers:

  • As others have mentioned, the main is sized less than the sum of the branches; if all the branches pull heavy loads, you can exceed the main without exceeding the branches, and the main will trip. If this is the cause, you can;

    • Accept the overload, and be ready to flip your main breaker when it happens.
    • Decrease your load, including managing loads between the two floors so you don't overload.
    • Have the power company upgrade your service. In my area, they would replace the wires to the street, and I would put in a larger breaker, and they would charge me a higher monthly fee.
    • Upsize the main breaker (say, to 50A) without changing the service. This is dangerous (it can allow overcurrent on the feeder wires, leading to a fire), inefficient (heavy loads will cause a more loss to heat on the feeder wires), and (in my area) illegal.
  • In a short circuit, multiple breakers are overloaded at the same time. Which one trips first has nothing to do with the load rating of the breaker. AFAIK, it's not normally predictable, although in hospitals they carefully select breakers so that branches will trip before main breakers in the case of a short, since a main breaker tripping is much worse than an annoyance.

If you are experiencing a short in the system, you should address the cause of the short.

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Heres is the easy answer. Breakers and fuses are used to protect wiring from overheating and starting fires. They have nothing to do with protecting the load. Match your breakers to what your wire is rated for and you will never go wrong. If your circuits still trip then you need to increase wire capacity and put bigger breakers on it or decrease the load on the circuit. Do not use breakers that will allow more current to run the wire than the wire is rated for, a fire will result or at the very least your wires will melt.

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