Shed Supply Maths

[Onoff]

I'm looking at an existing shed electrical supply that currently has a 25m run from the house. It's a bit unusual so thought I'd "do the maths" here.

More interested at the mo in seeing the max load I can have at the shed. I'm alright doing this really just one question really, me being pedantic!

The 25m total run is roughly this:

- Supply is TN-S

- Small cu next to the main cu currently has a 16A breaker feeding the shed.

I think this is preceded by the main cu 30mA RCD but can't be sure until I take lids off. It might be the small cu is simply fed from the main switch in the main cu. Immaterial at the mo as it will be "done right" now.

- The unusual bit comes next. There's two pieces of 2.5mm2 t&e in parallel coming off the 16A shed breaker. These run through two ducts under the stairs that has the floor concreted.

Under the dining room suspended floor is an adaptable box (galv?) that I can't quite get to to inspect. (The floor is coming up there soon so I'll get to see properly). I don't know if the swa is glanded etc. Guessing it's all in a choc block.

At the adaptable box it changes to a 4-core 2.5mm swa that runs to the shed. 3 cores used I think, L,N & E with one left spare.

At the mo I can't change the swa as it's buried in the wall / floor of the outhouse.

I'm hoping the best I can do is replace the 2 bits of 2.5mm2 t&e with a bit of 2.5mm2 swa. Properly jointed under the dining room.

So I've then a "consistent" 25m of 2.5mm2 swa. It's a combination of direct in the ground, ducted and clipped direct so looking at table 4D4A a mix of Reference Method C & D.

It's actually a 4x2.5mm2, 3-phase swa being used on single phase.
Looking at the table I'm thinking to treat it as "1 two core cable single phase AC or DC" which says 29A. That's the question, 29A?

Volt drop then is 18mV/A/m.

Doing the volt drop calc I get 11.25V volt drop if I assume a max load of 25A at the shed which is usable.

VD = (mV/A/m) x lb x l
---------------------------
1000

So:

11.25 = 18x25x25
---------------
1000

Assuming 5% max volt drop is 11.5V all's good.

I can put a 25A MCB in to feed the shed then a 6A & 16A in the shed.

Cheers

 

[gaz1]

personaly id drop the earth and connect 2 wires 2.5 to L and 2 wires to neutral then fit an earthing bar at the shed

 

[sparkysy]

Yes 25a will be the maximum you could put down a 2.5mm, but if you have 2x 2.5mm from the Mcb and you have no exposed conductive parts at the shed e.g. don't need bonding to it then I would double up the swa cores so 2x live 2x neutral and use the swa armor as the earth, then you could up the Mcb at the house end to 32a as long as ads is still met!
I don't like using mcb's in series as they don't give much selectively on instant trip between them, so a big fault at the shed may well take out the 25a or 32a at the house end. However it is done more often than not!
Sy

 

[hermetic]

You could (when you get the floor up) double up the 2.5 4core swa to the shed and use the swa as the earth conductor, you could then swap the two 2.5 t/e for one 4.0mm t/e and up the MCB to 25a. You would need to ensure earth continuity from the Consumer unit to the shed. Is your shed end protected by an RCD? it should be!
Phil

 

[hermetic]

Sy types faster than me!!

 

[AdventuresInMetal]

I would also double up the conductors to give you 5mm2 to play with, I would replace the twin and earth with more 4 core SWA back to DB1 so it is clear to any future electrician what is going on just by looking at the DB. I would use the SWA of the cable as the CPC for just for the cable and make the shed TT with its own DB. Then i would put a 32A Type C in the house as the feed and use a 6A for lighting, a 20A for sockets and a 16A for a 16A socket if you use one for your welder.

 

[Pigeon_Droppings2]

While I had the floors up I asked the electrician to fit 10mm^2 cables to the garage and also the front of the house...has come in very handy over the years...when I bought a compressor I had no problems powering it....and everything else!

While you have the access definitely worth thinking about future needs.

My cable was TE under suspended floors to accessory boxes on the outside wall...SWA from there to the garage.

10mm^2 ain't half difficult to handle though!

 

[Robotstar5]

When I moved into this house the garage was fed off a fused spur on the downstairs ring main, 1.5mm SWA which just about coped with lights and 13A socket.
Now replaced with 6mm SWA fed off a non-rcd way in the CU.

 

[gaz1]

Yes 25a will be the maximum you could put down a 2.5mm, but if you have 2x 2.5mm from the Mcb and you have no exposed conductive parts at the shed e.g. don't need bonding to it then I would double up the swa cores so 2x live 2x neutral and use the swa armor as the earth, then you could up the Mcb at the house end to 32a as long as ads is still met!
I don't like using mcb's in series as they don't give much selectively on instant trip between them, so a big fault at the shed may well take out the 25a or 32a at the house end. However it is done more often than not!
Sy

its not connected to the main cu as theres a juction box under the floor

theres no harm in fitting a earth bar at the shed as it would improve the connection to earth on both sides of the consumer units

the piece under the floor would also need seperating with another junction box for the swa wire to go into with the house earths whilst the other handles the live and neutrals

 

[dobbslc]

If you can get a new bit of swa to the junction under the floor use a resin joint kit so you can forget about it. If it has screw type terminals it needs to be accessible for inspection, Wago's are considered as a permanent connection not needing inspection if that helps.
If you need a bigger supply double up on the swa cores keeping the swa as cable protection and make a TT earthing system at the shed.
If you only use 3 of the 4 cores make sure the spare one it grounded to prevent induced voltages floating around.
RCD' where needed obviously.

 

[Brad93]

We had MCBs in series as per OP.

Pain in the butt walking up the house because of a tripped breaker in the workshop.

When electricians fitted 3 phase we took supply for single phase CU from that so no more problems

 

[Onoff]

This is the house end. Main cu with the shed cu below. As yet I have no idea how the shed cu is fed i.e from the 30mA RCD or not.

https://flic.kr/p/2p2tFBa

https://flic.kr/p/2p2o83g

 

[sparkysy]

Are you swapping that board too?
I would.

If you do you can either run it from the new board or get an isolator installed on the incoming supply tails and split the supply in Henley (100a service) blocks to the new board and a separate one for the shed.

If you changed the twin and earth section of cable to swa or the twin and earth is not less than 50mm within the building fabric or is surface mounted then the separate one could be a switch fuse due to the shed supply no longer (not) needing RCD protection.
Then you could use RCBOs at the shed end for the final circuits and have better discrimination if a fault occurred.
If not and the twin and earth isn't installed as mentioned above it will require 30ma RCD protection at the house end.
Sy

 

[mtt.tr]

The regs book has a section related to parallel conductors.

 

[Dave-sparks]

theres no harm in fitting a earth bar at the shed as it would improve the connection to earth on both sides of the consumer units

An extra earth bar won't change anything about the earthing, it's just a metal terminal block screwed to the wall.

 

[Dave-sparks]

I would use the SWA of the cable as the CPC for just for the cable and make the shed TT with its own DB.

Why? It's a TN-S supply, you'll never get a TT earth that is better than a TN-S earth.

 

[Onoff]

An extra earth bar won't change anything about the earthing, it's just a metal terminal block screwed to the wall.

I think he might have meant an earth electrode aka rod?

 

[Onoff]

As it's TN-S my thoughts are an RCBO in the new house board and just a couple of breakers in the shed.

 

[MetalMonkey]

Whether or not there are any extraneous conductive parts in the shed will dictate the earthing arrangements.

 

[DAPPH]

While I had the floors up I asked the electrician to fit 10mm^2 cables to the garage and also the front of the house...has come in very handy over the years...when I bought a compressor I had no problems powering it....and everything else!

While you have the access definitely worth thinking about future needs.

My cable was TE under suspended floors to accessory boxes on the outside wall...SWA from there to the garage.

10mm^2 ain't half difficult to handle though!

Ran my 6mm STILW to the log cabin in solvent welded rain water pipes no water ingress and no silting up inside the pipe .. expensive but like you say think of the future when you want heating & three phase VFD etc .in the mancupboard .