help please

[zebdog]

i have an electrician coming to wire my shed.
he is going to spur from a socket to the shed using 6mm 3 core armoured cable to a garage consumer box then to 2 double sockets and a strip light.

i will be using a chest freezer and tumble dryer and the odd saw / drill

will this be ok

 

[Taylortwocities]

If he spurs from a socket, he will need to put a fused connection unit at the house end. This will limit the maximum current to 13amps.

There is no point using 6mm cable unless the distance to the shed is a long, long way from the house.

 

[ban-all-sheds]

i will be using a chest freezer
.
.
will this be ok

Quite possibly not.

How cold does it get in your shed in the winter? What's the minimum temperature the freezer will work in? You must check with the manufacturer if you can use it in your shed.

a strip light.

i will be using ... the odd saw / drill

will this be ok

Yes, as long as you specify a light with a high frequency ballast. If you don't then you might get a strobe effect which leads you to think that your rotating tool isn't rotating - this can be a Bad Thing.

 

[ban-all-sheds]

There is no point using 6mm cable unless the distance to the shed is a long, long way from the house.

It would have to be about 50 - 70m away.

Or the OP has seen here what is often advised re future proofing and trench digging.

 

[eveares]

If the spur he is coming from is 2.5mm/4mm radial socket circuit, then possibly yes providing the voltage drop and Zs measured from the meter all meets the required specs.

However;

If he spurs from a 2.5mm ring final circuit socket with 6mm 3 core SWA, also possibly yes as 6mm can carry more current than 2 x 2.5mm what make up a ring final circuit. Note again voltage drop and Zs again have to comply with standards of course.


Most importantly:

Now the smallest cable used anywhere on its own in a spur configuration (either from RFC/Radial circuit) before a fused spur or suitable rated mcb in the shed to protect the smallest cable used before protection, will limit the amount of current you can draw in the shed.

In other words if you use 2.5mm T&E over 6mm T&E to spur from a socket on a 32A RFC circuit, then your shed can't draw any more current than the 2.5mm T&E can handle, regardless of the 6mm SWA.

And of course the light in the shed, will have a fused spur with a 3 amp fuse in it or be on a 6A mcb assuming voltage drop is not greater than 3% of 230v.

 

[ericmark]

Asking will it work then yes. Asking if good design then no. There is no point in fitting a consumer unit coming from a FCU with a maximum fuse size of 13 A.

When the whole supply is limited to 13 A then all that's needed in the shed is a switched FCU to work the lights with 3 A fuse.

Lights should be HF type not just because of strobe effect but also they will better cope with the volt drop.

Tumble drier is likely over 2 kW as is too heavy to be considered portable so it should be on a dedicated supply. I know often they are not but supplying a shed from a FCU in the house when it is intended to run for an extended time an item using over 2 kW is rather a poor design. The supply should come directly from the consumer unit.

 

[Lectrician]

A HF light starts better in the cold too.

A small CU in the shed is pointless.

If you are coming from an FCU on the socket circuit, I would run 2.5mm SWA (depending on length) into a metal clad socket, and from there on wards to other sockets and a switched FCU for the light.

This all of course depends on your existing sockets having RCD protection.

 

[Taylortwocities]

If he spurs from a 2.5mm ring final circuit socket with 6mm 3 core SWA, also possibly yes as 6mm can carry more current than 2 x 2.5mm what make up a ring final circuit.

Not sure which "ring final" you are meaning here?
Not the house ring final, as the connection from the house ring final to the SWA be protected by an FCU which limits the maximum current to 13amps.
and you surely aren't suggesting those two sockets in the shed are wired as a ring?

 

[ban-all-sheds]

I wonder where the OP found this "electrician"?

 

[JohnW2]

... If he spurs from a 2.5mm ring final circuit socket with 6mm 3 core SWA, also possibly yes as 6mm can carry more current than 2 x 2.5mm what make up a ring final circuit. Note again voltage drop and Zs again have to comply with standards of course.

I'm not clear what you are suggesting here. If it were a fused spur, then from the point of view off CCC, there clearly would be no need for 6mm² (or even 4mm²) cable - such larger sizes would only be used for reasons of VD/Zs or for 'future proofing'.

However, if you are suggesting that it is acceptable to have an unfused spur from a ring final supplying more than just one socket, provided that the effective CSA of the spur cable is at least 2 x 2.5mm², then that's certainly not something which is 'compliant' with the regs - which are as much about avoidance of large point loads on a ring as the adequacy of protection of the spur cable. Compliance with BS7671 is, of course, not compulsory, but if anyone wanted to have such an unfused spur, they would have to produce their own convincing argument as to why it was acceptable - which could be difficult.

Kind Regards, John

 

[zebdog]

thanks for advice had another electrician who would surface mount 6mm twe from socket to a twin external weatherproof socket
then 6mm armoured cable from that to shed into a metal twin socket add another twin socket and fix light via a switched fused spur in the shed.
shed is 5 meters from house.

is this a better idea.

 

[ban-all-sheds]

Is your shed 50 - 70m away?

 

[eveares]

However, if you are suggesting that it is acceptable to have an unfused spur from a ring final supplying more than just one socket, provided that the effective CSA of the spur cable is at least 2 x 2.5mm², then that's certainly not something which is 'compliant' with the regs - which are as much about avoidance of large point loads on a ring as the adequacy of protection of the spur cable.

That was what I was suggesting, although not sure why it would not comply with regs. 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.. so not sure how this would not also class as a high point load what might normally occur.

Just to clarify, what I was originally suggesting was to spur directly from a double socket that is part of a RFC (and not already a spur!) with 6mm T&E, and then through the wall to a suitable box with the 6mm 3 core SWA.

In the shed I would of then used either 16A or 20A mcb for the sockets , and a fused spur or 6A mcb (6A mcb if you don't care about 3% v drop for lighting).

Might not comply with regs, but can't see why it would not be safe?

 

[JohnW2]

However, if you are suggesting that it is acceptable to have an unfused spur from a ring final supplying more than just one socket, provided that the effective CSA of the spur cable is at least 2 x 2.5mm², then that's certainly not something which is 'compliant' with the regs - which are as much about avoidance of large point loads on a ring as the adequacy of protection of the spur cable.

That was what I was suggesting, although not sure why it would not comply with regs.

Simply because the regs do not recognise (even in the 'informative' Appendix 15 - which is where most detail about ring finals exist) the concept of an unfused spur (of any CSA) supplying more than just one socket.

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.. so not sure how this would not also class as a high point load what might normally occur.

Yep, I often cite that myself, but it doesn't alter what the regs (at least App 15) say about unfused spurs. As I said, there is no compulsion to comply with BS7671, so one is free to justify any 'non compliant' approach for oneself - but that would be quite likely to result in grief for the owner of the installation next time the installation was inspected.

Just to clarify, what I was originally suggesting was to spur directly from a double socket that is part of a RFC (and not already a spur!) with 6mm T&E, and then through the wall to a suitable box with the 6mm 3 core SWA.

Yes, that's what I assumed you were suggesting. Again, one can argue that it is acceptable, but such an unfused spur is not 'compliant' with the regs.

In the shed I would of then used either 16A or 20A mcb for the sockets

Why? The circuit as a whole is presumably already protected by a 32A MCB, and the 6mm² cable more than cable of carrying that current. Is this just your attempt to create a 16A/20A 'fused spur', so that you can use 2.5mm² cable, or what?

... and a fused spur or 6A mcb (6A mcb if you don't care about 3% v drop for lighting).

I don't understand that bit in brackets.

Might not comply with regs, but can't see why it would not be safe?

Certainly not 'compliant' (at least, not with App 15) and, as you say, no worse than having two or more unfused spurs connected to essentially the same point in a ring final. It's really for the designer to satisfy him/herself that no part of the circuit is likely to be overloaded for significant periods of time - plus, as above, the fact that the next person to inspect the installation will probably simply 'condemn' it as 'non compliant'.

Kind Regards, John

 

[ban-all-sheds]

That was what I was suggesting, although not sure why it would not comply with regs.

Well, whether you are sure or not it does not comply. You may have an unfused spur, and you may use whatever cable you like, but you may not use it to supply a large load on the basis that the ring final has a 32A device and your cable can cope with that.

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

In the shed I would of then used either 16A or 20A mcb for the sockets , and a fused spur or 6A mcb (6A mcb if you don't care about 3% v drop for lighting).

How does an MCB make a difference to voltage drop?

Anyway - your idea would not be allowed.

Might not comply with regs, but can't see why it would not be safe?

Because a ring final is a special case of a circuit where Iz is not ≥ In which is deemed to be safe enough as long as the nature and placement of the loads is such that the largest component of Ib does not significantly exceed Iz. They are not designed to supply sub-mains or unfused spurs serving more than 1 socket.