New sockets on RFC

[Poy]

After a bit of advice on adding new sockets to a ring final circuit in garage please, existing ring is all wired in 2.5mm with a 32A MCB.

At the moment the ring travels across the top of a 4m long workbench, I would like to add a line of sockets underneath the bench for equipment that is permanently plugged in (mitre saw, belt sander, pillar drill, vacuum etc nothing too powerful) in order to free up the sockets above for more mobile equipment (worklamp, battery chargers, dremel, radio etc)

Attached image shows my thoughts and just wondered what people thought was the best and safest way to do this:

Option 1: Drop a FCU spur down from each socket using 2.5mm cable, keep MCB at 32A - least liked solution, means lots of holes thru top of bench.

Option 2: Drop a spur down form each socket, keep 32A MCB, use 4mm cable, means avoiding needing each FCU? Lots of holes still.

Option 3: Drop a spur from each socket, downrate MCB to 20A, use 2.5mm cable, still lots of holes.

Option 4: This one I'm not sure of at all but would mean the least amount of work - come off the ring at a junction box or socket, drop below the workbench, through some sockets and then rejoin the ring above at the other end of the workbench, creating a ring off a ring, would this in theory be acceptable?

Option 5: Just split the ring at one end of the bench, go under, through sockets to other end, and return to the same point thereby just extending the ring - simple enough and shouldn't involve any worrying about cable or MCB sizes?

Thinking about this too much now, head hurts!

 

[EFLImpudence]

1. FCUs are not necessary because protected by plug fuses

2. 4mm² (although it would be ok) is not necessary even though 32A because protected by plug fuses.

3. Yes, that's fine.

4. Definitely not, for testing reasons although electrically it would work.
You could go down and up to next socket to maintain the ring.

 

[Lectrician]

 

[Taylortwocities]

As Lectrician above, but the MCB in teh garage CUcan be 32A, assuming it is a ring and the cable size is at least 2.5mm²

This does beg the question as to what is feeding the garage and what protective device is protecting it?

I always laugh when a customer shows me his workshop, wired up with dozens of sockets and lights, with a snazzy "garage" consumer unit. Especially when it is revealed that the whole shebang is fed from a spur off the downstairs ring final through a fused spur with a 13amp fuse in it.
Totally pointless.

 

[Poy]

Thanks for the quick replies, seems splitting the ring is the way to go.

Garage is fed from main CU in house, RCD protected 16A MCB, 6mm through house and ancient 6mm SWA about 4m under garden.

Garage CU is also RCD protected with 5 ways:

MCB1 20A 2.5mm RFC (thought it was 32A but ho hum)
MCB2 16A Outside sockets (unused, apart from 'emergency' socket under CU labelled 'NOT ON RING, FED FROM MCB2'
MCB3 6A Outside Lights 2.5mm - 500W PIR flood at front and 60W PIR bulkhead at back door
MCB4 6A Front interior lights 1.5mm - 8 6ft tubes ~ 35w each
MCB5 6A Back interior lights 1.5mm - 8 6ft tubes ~ 35w each

MCB in the house could probably be increased in size but it's never tripped out so will leave it at that - garage MCB1 could go up to 32A, but never tripped either - and unless I'm wrong cables are beefy enough so MCB will defo go before cables melt Dual RCDs are a bit redundant too I guess but no reason to remove them?

 

[ban-all-sheds]

With a 16A MCB supplying it the entire garage CU is redundant, and could be got rid of, with 1 or 2 FCUs used for the lights.

A CU on a 16A submain?

RCDs in series?

32A and 16A devices downstream of a 16A?

A garage with 5 final circuits?

Whoever installed all that was an idiot, and you'd better hope that his ignorance, incompetence and general stupidity has not left you with anything dangerous somewhere.

I can't urge you too strongly to do some comprehensive testing before and after your changes.

 

[Taylortwocities]

I always laugh when a customer shows me his workshop, wired up with dozens of sockets and lights, with a snazzy "garage" consumer unit. Especially when it is revealed that the whole shebang is fed from a spur off the downstairs ring final through a fused spur with a 13amp fuse in it.
Totally pointless.

Or a 16A MCB

 

[SimonH2]

Consult a qualified electrician to assess whether your cable to the garage will a) suit a larger MCB (actual current carrying capacity, and hence protection requirements vary with both cable type and installation method), and b) is installed in a such a manner that it can be on a non-RCD protected circuit.

As BAS points out, you've two RCDs in series - both will trip if you have a problem in the garage, and when the one in the house trips, it will plunge the garage into darkness as the lights will go out. I assume it's just a "split board" so you'll also take out part of the house which may reduce the WAF somewhat

Plus, you've a 20A RFC, plus other circuits, and all feed from a 16A breaker. If you overload things (such as by using a welder), then the 16A breaker in the house is more likely* to trip than the 20A one in the garage - and again it'll plunge you into darkness.

I'd suggest that being plunged into darkness while using power tools is "something of a safety hazard". For that reason, I'd suggest at the minimum a split board, preferably RCBOs on each circuit. At the very least, having to grope your way out of a dark garage (tripping over whatever project you are working on) and find your way in the dark to a dark house to reset the RCD is "not fun".


* More likely - though it's quite possible to trip both of them.