help please

[JohnW2]

After all you can have two double sockets on a RFC next to each other with high wattage appliances on at the same time such as kettles, toasters, irons,etc.

Actually, you may not - read the last sentence in 433.1.103

Whilst I agree with everything else you have written, all 433.1.103 says is the designer has to be satisfied that it is "unlikely" that the Iz of the cable will be exceeded for "long periods". We all know that multiple spurs originating at essentially the same point on a ring are pretty common - particularly in the context of grid switch setups in kitchens (and, as I recently illustrated, even that is OK unless the point of origin is fairly close to one end of the ring - particularly if the cable is clipped direct, the difference between In and Iz is so small that one has to be very close to the end of a ring to overload the cable, even with a load equal to In)

"Long periods" is obviously not defined, but I would imagine that things like kettles, toasters etc. etc. would not be regarded as presenting a load for a "long period" (which is more than one can say for some of the things supplied by those kitchen grid switches ).

Kind Regards, John

 

[eveares]

Then how about just extending the RFC to the shed with two 2.5/4mm cables depending on specs and requirements.

 

[JohnW2]

Then how about just extending the RFC to the shed with two 2.5/4mm cables depending on specs and requirements.

That's a theoretical possibility if the shed is very close, but in most situations it would probably make little sense. Also, I'd personally want the outdoor supply to be either on a dedicated circuit or some sort of 'spur' off an existing circuit, so that it could be DP isolated - otherwise, a fault in the garden/shed would take out the whole of the ring in question (and quite probably a lot more, given RCDs).

Kind Regards, John

 

[eveares]

Actually, you may not - read the last sentence in 433.1.103

433.1.103

The current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit.


So that would mean for 2.5mm with a maximum current rating of 27A (without derating factors), that would mean a typical RFC wired in 2.5mm, the current causing operation of the MCB cant exceed 39.15 Amps. Don't see a problem unless the MCB is over 32A.

 

[JohnW2]

433.1.103 .... The current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current-carrying capacities (Iz) of any of the conductors of the circuit.

I'm not sure where you found that - it's not in 433.1.103 in my BGB! Whatever....

So that would mean for 2.5mm with a maximum current rating of 27A (without derating factors), that would mean a typical RFC wired in 2.5mm, the current causing operation of the MCB cant exceed 39.15 Amps. Don't see a problem unless the MCB is over 32A.

The I2 of a B32A MCB is ???

Kind Regards, John

 

[eveares]

433.1.103 (17th edition BS7671 - Page 73)

To be honest Zebdog, I would just contact a qualified electrician and tell him your requirements and needs.

 

[JohnW2]

433.1.103 (17th edition BS7671 - Page 73)

No. What you are quoting is 433.1.1 of the original ('red') version of BS7671:2008, which didn't even have a 433.1.103. That latter reg exists only in the currents ('green') regs (Amendment 1, 2011, of BS7671:2008), and is the virtually unchanged equivalent of 433.1.5 in your 2008 version.

Whatever, you didn't respond to my question, so I'll tell you the answer. The I2 of a B32 MCB is 46.4A, hence invalidating your argument. At least for MCBs, the regulation you quoted was using unnecessarily contorted language. They could simply have said that In of the MCB must not be greater than the Iz of the cable. Instead, they chose to say that the I2 of the MCB (aka 1.45 times its In) must not be greater than 1.45 times the Iz of the cable!

Kind Regards, John

 

[sparkwright]

I suppose it is reasonable to use an over-sized SWA to the shed, as the cable could later be extended if an opportunity to bung it on it's own circuit arose, without digging up the garden again.

Always a good idea to put SWA in a duct if you can, so you can draw a new cable in. However, the need for this is often rare, so it rarely gets a duct.

 

[JohnW2]

I suppose it is reasonable to use an over-sized SWA to the shed, as the cable could later be extended if an opportunity to bung it on it's own circuit arose, without digging up the garden again.

Indeed, as has been said that would be one (probably the only one) reason for using oversize cable. If, at least for the time being, it is wanted to run it from a ring final then, despite attempts at argument, to be 'compliant' it would have to be fed via an FCU.

Kind Regards, John