2.5mm armoured cable for shed - sanity check please

[camel307]

I intend to run 2.5mm armoured cable (underground) to my shed and then use a Garage Kit from B+Q (1 RCD and 2 MCBs) in the shed. 1 MCB for light circuit and 1 for a double plug socket.

I then intend to ask a Sparky to connect the armoured cable to the fusebox in my house.

I am no expert but i assume this will form a 'Radial Circuit' and I have read this set up would require a 4mm cable?

Will i be okay with 2.5mm considering there will be a single double socket and a light?

Thanks guys

 

[davelx]

I intend to run 2.5mm armoured cable (underground) to my shed and then use a Garage Kit from B+Q (1 RCD and 2 MCBs) in the shed. 1 MCB for light circuit and 1 for a double plug socket.

I then intend to ask a Sparky to connect the armoured cable to the fusebox in my house.

I am no expert but i assume this will form a 'Radial Circuit' and I have read this set up would require a 4mm cable?

Will i be okay with 2.5mm considering there will be a single double socket and a light?

Thanks guys

Much more info needed to answer that question!

What is the type & rating of the OPD (MCB) in the house CU that will protect the SWA run to the shed?

What are the ratings of the OPDs (MCBs) in the shed CU?

What is the total length of the SWA run?

What is the greatest length of the cabling IN the shed to (a) lights and (b) socket.

What type of earthing system do you have in the house?

Are their any metallic service (water, gas, oil ) pipes entering the shed?

All of these may influence the size and type of cable needed, and the possible need for an earth rod at the shed.

 

[Risteard]

What is the type & rating of the OPD (MCB) in the house CU that will protect the SWA run to the shed?

Two circuit breakers in series will not provide selectivity between the distribution circuit and final circuits.

 

[securespark]

Depends on their size, but on 2.5, it is highly unlikely.

 

[Risteard]

Depends on their size, but on 2.5, it is highly unlikely.

Not really. Consider a fault of negligible impedance on a final circuit. This is almost certain to cause an upstream BS EN 60809 circuit breaker to operate. Better with a fuse (or MCCB) upstream.

 

[davelx]

What is the type & rating of the OPD (MCB) in the house CU that will protect the SWA run to the shed?

Two circuit breakers in series will not provide selectivity between the distribution circuit and final circuits.

And? (i.e - you might explain so that the OP understand the problem).

 

[Spark123]

If the energy let through of the downstream breaker is greater than the pre-arcing energy of the upstream breaker then they won't discriminate hence an overload or fault on the shed socket circuit may trip the MCB in the house as opposed to the one nearest the problem i.e. in the shed, cutting off power to the lights as well as the socket circuit.

 

[winston1]

You don't need a garage kit. Run your 2.5 from a 20A MCB in the house directly to your double socket in the shed.
Also run it to a switched fused connection unit with a 3A fuse for the light.

I assume you have an RCD in the house. if not put one there.

 

[camel307]

The current consumer unit in the house is an old style fuse box with wire fuses so no RCD. This will be replaced for the latest type of consumer unit, with an RCD.

The consumer unit I have for the garage has a 40A 30mA RCD with both a 6amp and 32A MCB.

I just really need to understand whether the 2.5mm cable will be sufficient to supply possibly 2 lights and 1 double socket?

Thanks

 

[camel307]

So.......

I have done plenty of reading on the topic and I think I am right in thinking the following:

I) As my circuit is radial I should ideally have a 4mm cable for a 32A MCB (in the garage). If I use 2.5mm I should only have a 20A MCB.

Would it be safe to have a 20A MCB and a 6A MCB (Socket and light) with the 2.5mm or should it be combined 20A?

Also, would an electrician sign off the first set up or not?

Would the RCD trip before a fault occurs in either of the proposed set ups?

 

[EFLImpudence]

What is the total length of the SWA run?

What is the greatest length of the cabling IN the shed to (a) lights and (b) socket.

What type of earthing system do you have in the house?

Are their any metallic service (water, gas, oil ) pipes entering the shed?

All of these may influence the size and type of cable needed, and the possible need for an earth rod at the shed.

 

[camel307]

Length of SWA is 15 metres.

Longest length of cable to lights and sockets in shed is 2 metres.

Not sure about the earth in house.

I have no metal service pipes into the shed.

Thanks

 

[seasickstevie]

So....Would the RCD trip before a fault occurs in either of the proposed set ups?

An RCD should never trip before a fault occurs.
What would be the point?
And how would it know?

 

[camel307]

So....Would the RCD trip before a fault occurs in either of the proposed set ups?

An RCD should never trip before a fault occurs.
What would be the point?
And how would it know?

Apologies, I think my question should have been will the RCD trip before the MCBs.

ie if i am using what is technically an OVER rated MCB, will the RCD pick up any smaller faults, acting as a safety barrier?

 

[aesmith]

ie if i am using what is technically an OVER rated MCB, will the RCD pick up any smaller faults, acting as a safety barrier?

The RCD won't trip on an overload condition, only on a fault to earth.