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All else being equal circuit breakers can have different "breaking capacity" - it's the maximum current which the breaker can disconnect and it's typically several thousand amperes. The idea is that after that the electric arc can start inside the breaker and it will continue conducting current even with contacts moved apart.

Now how important is the actual value? Suppose I'm shopping for a 20 amperes breaker for my apartment. The retailer offers models with breaking capacities of 1,3 kiloamperes, 3,5 kiloamperes and 5 kiloamperes and higher breaking capacity comes for higher price. Do I always choose the highest breaking capacity? How do I make my choice?

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I get the feeling this will only make a difference when you are struck by lightening. And even then, it depends on how close that lightening strike is. On a side note, for an apartment, I'd suggest an AFCI breaker. –  BMitch Jun 23 '11 at 11:48

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If you get a complete short between live and earth or live and neutral a lot of current will flow until the breaker operates. If the current that is flowing is more then what the breaker can cope with, the breaker may explode or catch or file, or just fail to trip out. Hopefully your main fuse will blow before too match damage is done.

(A wired main fuse takes a lot longer to burn out then a working breaker takes to trip.)

The current that flows when you get a short is dependent on how far the short is from the breaker and the resistance of the wire. So an overrated length of wire close to a breaker could stop the breaker working!

There is also the issue of how quickly a breaker will operate under different conditions. I think it is assumed in the UK you will not get a short within the first few feet from the breaker.

In the UK you would look in the “IEE On-site Guide”, if you had to know. However there will be a standard value of breaking current that is good enough for a normal residential situation, I would be amazed if you could buy a breaker that did not have “good enough” "breaking capacity" for normal cases.

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Typically the Prospective Short Circuit Current is governed by your supply, not the circuit downstream of the breaker. (Think of it like this: a very short but otherwise normal circuit would be as "overrated" as a longer one using thicker wire. The breaker has to work regardless of where the short is.) The governing PSCC may be vastly greater than what your worst short could practically pull due to distance from substation etc. Local codes will say what minimum PSCC to assume. –  Bernd Jendrissek Jul 15 '12 at 23:08

I am not a physicist, but i don't think the larger more expensive breaker is going to make much difference in your residential situation. If you draw 1,300 amps, not only will the breaker arc over, but your wiring will be arcing, burning , not to mention your main breaker and service entry cable from the utility, which is probably only 4/0 alu rated at 200 amps with a max out of 400amps. Keep in mind that excessive current will heat up conductors and cause them to melt ,burn etc, thus allowing shorting between conductors. It is voltage that supplies the potential to arc. All wire and devices for residential use are rated for 600VAC min, unless the insulation is compromised. At 1,300 amps, every wire and devise will become a fuse! Call 911

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I agree, but you should probably check your local building codes as well. –  electricsauce Jun 23 '11 at 20:17
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1,300 for 0.01 of a second may not damage the wireing, a wired main fuse takes a lot longer to operate then most breakers. –  Walker Jun 24 '11 at 21:30
    
Except that more than 1.3kA will arc across an open breaker (with 1.3kA or lower breaking capacity) and into the wiring. There will be a voltage drop across that gap as the electricity has to do work to jump it, but the wiring is probably still being juiced with 1.5x the peak amperage rating of even a 12AWG wire. –  KeithS Jun 29 '11 at 15:28
    
Yea they are reated 600VAC - but at voltage changes the maximium Amperage changes. The 600VAC is only for safety of insualtion as for each 1000VOLTS can penetrate 1mm of insulation!!!!! Your answer is a bit poor -1 –  ppumkin Jul 2 '11 at 16:50

If you're worried about a power surge strong enough to arc across an open circuit breaker, you're pretty much worried about a lightning strike. It could theoretically also happen in the case of a catastrophic failure of the delivery transformer, which would send several thousand volts' potential into your service panel. Transformers are designed with many fail-safes to prevent this, but who knows...

To guard against those cases, I would recommend a "whole-house surge protector", like this one. It goes between the main breaker and the panel switches, and works like an industrial-strength surge protector strip covering all the circuits in the house. The first stage when a surge happens is to absorb the charge into a capacitor bank, which will then release the charge safely over time. If the capacitors overload, circuitry in the surge protector itself will safely fail open. When that happens you will lose power as if the circuit breakers tripped, but unlike a normal breaker, the surge suppressor has a staggering 50kA "breaking capacity"; 10 times anything you're looking at for circuit breakers.

These are actually a pretty good idea in general; they don't replace the everyday surge protector strips for delicate electronics like your TV, computer, etc, but think of your microwave, stove, refrigerator, HVAC, etc. Those can blow out from a strong surge too, and if they do you could be out thousands of dollars to replace them.

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You are looking at the wrong suppliers! Anything rated at kiloamps is meant for a power station not your home! Are you sure he is not quoting you milliamps?

Home breakers come in ratings around 1,2,5,10, 20 and 60 Amperes. In the UK the main breaker is usually a 60 amp one and feeds a couple of circuits with lower rating breakers ( lights, wall plugs, oven, etc ). A circuit breaker is intended to protect the wiring not the device at the end of it.

If you want to protect someone touching the wiring then you need to look at RCD ( Residual Current Devices ) or Earth Leakage devices. Typically these are rated at 20 milliamps or less ( the amount of current required to give you a fatal shock ) These devices detect an imballanced discharge between phases or to earth and trip out the main current.

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Yes, I'm totally sure I quote the right figures - it can be a 20 amperes breaker that has 5 kiloamperes breaking capacity and it would be offered at a local shop somewhere between gardening stuff and ceramic tiles. –  sharptooth Jun 24 '11 at 11:38
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I think your are thinking of the specs of what we call a Ground Fault Interrupter or Arc Fault breaker/outlet here in the USA. Sharptooth is referring to the internal arc over/ max peak or spike current handling capacity of a circuit breaker in the main distribution panel. These ratings have nothing to do with fault current tripping ability as regular breakers do not respond to ground fault or arc fault conditions. –  shirlock homes Jun 24 '11 at 13:36

Usually the electrician that installs the ciruit breakers will know what ratings to use, because usually after an electrical installation it needs to be CERTIFIED so that in case of electrical fires they know the circuits were properly installed- otherwise any home insurance be be invalid because it was installed by an unqualified person.

You need to know where each branch goes to. Lighting usually does not need more than 4AMpere. Why? Lighting fixtures are rated at 100Watt Max for typical home installation. Commercial lighting uses more wattage. On one line you can have upto 3 light switches and assuming you can have 3 lights per swtich at 100watt maximim. (3 X 100Watt) * 3 = 900Watt Maximum On a 1.5mm solid core cable.

Now how do i know a 4.0Ampere is good?

900Watt / 240Volt = 3,75

So thats the worst case scenario

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Except that in the U.S. single-phase voltage is 110-120 not 240, so worst-case in America is about 8.2A. However, Sharptooth's limited profile info suggests he's from Russia, where single-phase voltage is 220-240, so you're correct in context of the OP. –  KeithS Jun 29 '11 at 15:35

Where are you located? In the U.S. residential circuit breakers typically have an Ampere Interrupting Capacity (AIC) of 10,000 Amps. I notice that you use commas instead of decimal points, so you are probably not in the U.S., and will be subject to different requirements.

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I'm not in the US and we have 220 volts, so I guess 10KA you quote is equivalent to 5KA here. –  sharptooth Jun 28 '11 at 5:47

Your question...How important is circuit breaker ampacity? can be answered very easily. How important is your life? Exceeding ampacity can be directly related to risk of fire. Do you want to take the chance? I hope not

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The question was actually "How important is circuit breaker breaking capacity?", the maximum current that the breaker can disconnect in a short circuit, not "ampacity", which is the maximum sustained current it can carry. –  Niall C. Apr 18 '13 at 2:34
    
"Can be directly related" is a buzzphrase to me. Can I be directly hit by a lightning? Yes, I can. Am I scared to death? No, I'm not. –  sharptooth Apr 22 '13 at 6:44

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