MCB sizing

[Jako13!]

Is there any issues in having 10amp breaker if the circuit is pulling just under 6 amps? I know the breakers there to protect the cable and the cable is 2.5 3 core armour. Just curious.

 

[Taylortwocities]

You have answered your own question. For 2.5mm SWA you could have an MCB of up to 25amps. If you need to limit the current at the delivery end of the SWA - maybe because there’s lights there using smaller wiring, or because the instructions state a fuse is needed - then fit an FCU with an appropriately lower rated fuse.

 

[Wilko]

If you know the load is 6A then with 2.5mm conductors the 10 MCB may be a reasonable choice .

 

[Dustydazzler]

Day one of electrical skool

cable bigger than device , device bigger than load

 

[Lucien Nunes]

Otherwise written Iz>=In>=Ib or Ib<=In<=Iz

 

[Zerax]

Or could be... Iz≥In≥Ib or Ib≤In≤Iz

(just showing off as I've just discovered how to do those symbols)
(still working on how to do subscript on here)

 

[radiohead]

Fixed loads do not necessarily require overload protection, as long as fault current protection is met by the device a device rating may exceed the CCC of the cable and still be compliant.

 

[telectrix]

Or could be... Iz≥In≥Ib or Ib≤In≤Iz

(just showing off as I've just discovered how to do those symbols)
(still working on how to do subscript on here)

show us how. us plebs can only do the < and >. and i can sometimes enter "a" without activating the caps lock.

 

[Taylortwocities]

show us how. us plebs can only do the < and >. and i can sometimes enter "a" without activating the caps lock.

Yes, and put a couple if smiley emoticons on thd post as well!

 

[Jako13!]

So when is over load protection needed? In theory you could run a lighting circuit of a 16amp breaker then as long as the cable is correct for 16 amps?

 

[Risteard]

So when is over load protection needed? In theory you could run a lighting circuit of a 16amp breaker then as long as the cable is correct for 16 amps?

BS 7671 specifically stated that ceiling roses etc. can be connected to a circuit with a protective device rating not exceeding 16A, so yes - it's in black and white that you can do it.

 

[Electrics]

So when is over load protection needed? In theory you could run a lighting circuit of a 16amp breaker then as long as the cable is correct for 16 amps?

1.5mm cable on a 16a breaker will work perfectly fine. It's how they do it in some other countries.

 

[Dustydazzler]

1.5mm cable on a 16a breaker will work perfectly fine. It's how they do it in some other countries.

some lighting accessories, switches and connections in the U.K. are only rated at 5/6amps...

that said even running at 15-16 Amos I doubt they would over heat

 

[Electrics]

some lighting accessories, switches and connections in the U.K. are only rated at 5/6amps...

that said even running at 15-16 Amos I doubt they would over heat

Ah i didn't take into account any potential difference in fitting ratings. I continue to learn !

Having said that i'm now a bit confused as to your comment on them 'running at 16a' - as far as i thought i knew (which could be and probably is wrong) the breaker size doesn't make anything 'run' at a certain amperage it's simply there to catch a fault that spikes the amperage?

ie if you ran a kitchen extractor on a 40a breaker it would never detect the fault because the breaker is rated too highly and that's when you'd have problems?

(Just checking my understanding is right - very happy to be corrected)

So i'm presuming a fault on 6a stuff wouldn't spike sufficiently to allow the 16a breaker to detect it and therefore could cause fires? We were always taught (France) that 10a for lights but 16a if you wanted to make certain there wouldn't be any nuisance tripping.

 

[Lucien Nunes]

So when is over load protection needed

a) Where the circuit load is not limited to below the cable CCC by design, i.e. the user could plug in or dial up more load on the equipment until the cable is overloaded.

b) Where equipment manufacturers specify it. Or rather, they will sometimes specify a maximum MCB or fuse size, not necessarily as overload protection but as short-circuit protection for the wiring and components inside the equipment. E.g. you might choose a 32A MCB to protect a 6mm² cooker circuit, where the oven is connected by 2.5mm² flex, and so long as the adiabatic is satisfied the flex will be protected against short-circuit. The flex won't handle 32A but the oven can't draw 32A for long enough to damage it, even in the event of an element failure. But the oven instructions might well specify a maximum 20A MCB because the oven internal wiring splits into multiple 1.0mm² cables, and for those the 32A MCB does not offer adequate short-circuit protection.

IMO the extractor fan 3A fuse business is just daft, though!

So i'm presuming a fault on 6a stuff wouldn't spike sufficiently to allow the 16a breaker to detect it

What governs the level of fault current available to trip the MCB is not so much the normal load rating of the 'stuff' but the resistance of the wiring supplying it. It takes a higher fault current to trip the larger MCB, which puts a lower ceiling on the loop impedance, and might require heavier cables. A long lighting circuit might have to be wired in 2.5mm² simply to enable a 16A MCB to trip fast enough, which is pointless if the total load is less than 6A.