power to garden shed

[b21playa]

planning on building a shed the full width of the house right at the back of the garden (subject to planning)

anyway would be good to have at least a flourescent light in there.

thing is it would be about 120ft away from the end of the house and about 150ft away from the consumer unit

now i know swa cable will be needed but what size and what is the best way to get it down there?

will it best to connect to CU or use the meter tails splitter and install another CU for the shed and then run the swa cable from there to another CU in the shed.

 

[Mosez]

planning on building a shed the full width of the house right at the back of the garden (subject to planning)

anyway would be good to have at least a flourescent light in there.

thing is it would be about 120ft away from the end of the house and about 150ft away from the consumer unit

now i know swa cable will be needed but what size and what is the best way to get it down there?

will it best to connect to CU or use the meter tails splitter and install another CU for the shed and then run the swa cable from there to another CU in the shed.

If you do not intend to have any extraneous conductive parts
( Water/Gas Pipes, Radiators, Structural Metal work etc which are
earthed), and the shed is to be built from wood with wooden floor,

then you could connect 4.0mm2 T&E wire from a 16A breaker from the
30mA RCD side in the CU to a double pole 20A fused isolator switch in a
convenient position in the house to allow isolation of the shed electrics.

Run the 4.0mm2 T&E cable from this 20A Isolator to a metal junction box inside the house.

Then terminate a 4mm2 SWA in this Metal box with the SWA gland and banjo, and feed the other end to the Shed.

Install a Metal J/B in the shed and terminate the SWA.

Run 2.5mm2 T&E to a Double pole Isolator in the shed and an other
2.5mm2 cable to a 3A FCU.

Run a 2.5mm2 Cable to the sockets from the 20A Iasoltor.

Run a 1.5mm2 cable for the lights( internal and a 500W Flood) from the
3A FCU.




For Only the internal Lights+500W Floodlight the SWA cable size will be
1.5mm2 and a 6A breaker. But I would suggest that for a small extra
amount, you should run a 4mm2 cable (500mm under Grass, 750mm
under plants with warning tape 200mm above the cable) to allow for
future sockets.

 

[b21playa]

cheers, makes perfect sense

will i not need an earth rod or anything on the shed end, i dont know if it will be wooden or brick/concrete built

 

[Mosez]

cheers, makes perfect sense

will i not need an earth rod or anything on the shed end, i dont know if it will be wooden or brick/concrete built

If it is built in brick/conrete,

then Run a 4mm2 cable from the non-RCD side of the CU.

As before you must maintain the continuity of the CPC via the Armour of the SWA, but now the SWA is terminated in a Plastic J/B in the shed
( with the Armour being terminated in this J/B and going no further in to the shed).

Run a 4mm2 T&E from the Plastic J/B to a standalone 30mA RCD and then on to the Socket Isolator & 3A FCU for lights.

OR

Run a 4mm2 T&E from the Plastic J/B to a 4 way CU with 30mA RCD as the main switch and then a 16A breaker for the sockets and a 6A breaker for the lights.

You then need to install an earth rod in an inspection chamber outside the
Shed ( prferably in a place with Clay soil where the rain can get at it), and run a 10mm2 cable in a protective PVC duct in a safe route with
warning tape if burried, back to the RCD.

 

[dingbat]

Don't run shed electrics off the RCD side (if you have one) of your consumer unit. And don't use so many joints.

If you have a spare (non-RCD) way in your consumer unit you can use a 20A MCB to feed SWA directly to an insulated garage consumer unit with its own RCD main switch. This adequately protects the cable and avoids discrimination problems.

Three-core 4 sq mm SWA will not be compliant on volt drop using 20A and 45metres, giving a VD of 9.9V. You need to use 6 sq mm. The cable should be glanded at each end, with the gland suitably earthed to the earth terminals of each consumer unit.

You will get an (R1 + R2) value of around 0.28 ohms, discounting the parallel path via the cable armour. You will need to consider your Ze value - provided that the Zdb at your shed is low enough there is no need to provide additional earthing.

The cable should be buried, appropriately protected from mechanical damage and marked with danger tape.

From the shed consumer unit you can run whatever circuits you need, in cabling appropriate for the conditions - T & E, singles in conduit, SWA, whatever.

This work is notifable under the building regulations.

 

[Mosez]

Cable length = 50m
Cable size = 4mm2 SWA ( 2-3 Core)
Power Supplied by Circuit = 3680W
Installation Method = 1
Breaker protecting Cable = 16A
CPC minimum 'S' = 1.90mm2 Steel or 0.51mm2(CU Equivelant)
Volt drop = 7.81V
Assuming a TN-C-S earthing system then Zs = 0.78 Ohms
Fault Current = 306A
CB Tripping Time <= 0.1Sec
Cable Max Length = 52.27m
Ambient Temp = 30C
Cable Operating Temp = 36.1C

 

[DESL]

This work is notifable under the building regulations.

Include all the electrical work you've described in the planning application and building control notification as then LABC are obliged to check it at no extra cost to you.

 

[dingbat]

Mosez,

Table 4D4B gives VD as 11 mV/A/m for 70 deg C and table 4E4B gives a VD of 12 mV/A/m for 90 deg C 4 sqmm multicore SWA cable.

At 50m and 16 A this gives a VD of 8.8V for 70 deg C cables or 9.6V for 90 deg C cable. Presumably your software is basing its calculation on 3 or 4 core 3-phase for its calculations?

In any case, I'd design a circuit from first principles with due regard for both protection and discrimination.

 

[Mosez]

Mosez,

Table 4D4B gives VD as 11 mV/A/m for 70 deg C and table 4E4B gives a VD of 12 mV/A/m for 90 deg C 4 sqmm multicore SWA cable.

At 50m and 16 A this gives a VD of 8.8V for 70 deg C cables or 9.6V for 90 deg C cable. Presumably your software is basing its calculation on 3 or 4 core 3-phase for its calculations?

In any case, I'd design a circuit from first principles with due regard for both protection and discrimination.

Circuit VD depends on the operating temp of the cable at the designed load, and not the on the maximum working temp of the cable.

Cable protection is based on the Max working temp of the cable.

 

[dingbat]

Circuit VD depends on the operating temp of the cable at the designed load, and not the on the maximum working temp of the cable.

Maybe so, but you would never actually work this out, would you? It is usual practice to use the VD figure given in the appropriate BS 7671 tables, which is different for cables of different max operating temperatures.

 

[Mosez]

Circuit VD depends on the operating temp of the cable at the designed load, and not the on the maximum working temp of the cable.

Maybe so, but you would never actually work this out, would you? It is usual practice to use the VD figure given in the appropriate BS 7671 tables, which is different for cables of different max operating temperatures.

My Dear DB

The VD figures given in BS 7671 are the worst case figures, at maximum
cable working temp, which will be lower at lower cable working temps.

With simple cable calculations, based on the BS 7671 tables, without cable
working temp corrections, you would end up using a larger sized cable every time.

With cable working temp correction, on the tables, the same circuit could
be designed with a smaller sized cable and would do the job perfectly
within the spec of the cable, reducing cost, especially now that copper has
rocketed in price.

We have not been doing the cable temp correction as the cable used to
be 'cheap'. But I think we will have to start thinking more and more about
this, especially on larger projects, which could save 1000's of pounds!!!

 

[Adam_151]

If you were using the current carrying capacities for 70degree cable (which you should for 90 Degree cable, unless you are sure that the switchgear will handle them being at 90 degree) then it makes sense to also use the VD tables for 70Degree cable

AFAIK there is a way to compensate for under-run cables but its complicated and not worth it unless you have a long and huge cable thats seriously under-run and you stand to save a lot of copper if you can get the VD compliant on a smaller size