Cable Size for 100m Shed Run

[Creamy Goodne5]

Can anyone recommend a good Cable Size calculator to use online?

The old man wants me to order some 6mm 3 SWA to feed his shed that will go on to supply some sockets, fairy lights around the garden and other stuff, no power tools or high load activities.

Cost is the mitigating factor here because the cable run is (his estimate) 100m (my estimate is 70 meters) from Consumer unit to shed running through the full length of his bungalow attic, down one side of the house and that ran along the kick board or bottom of his fence around the side of his garden.

Im thinking it might have to be 10mm which is going to put the job past his budget.

 

[buzzlightyear]

Can anyone recommend a good Cable Size calculator to use online

Cleveland cables .

 

[telectrix]

work out your load current. probably be no more than 20A if that. then calculate the VD for 6.0mm.

 

[westward10]

As Buzz said this one is pretty straightforward.

Cable Calculator

Cleveland Cable Company - Cable Calculator

www.clevelandcable.com

 

[Creamy Goodne5]

As Buzz said this one is pretty straightforward.

Cable Calculator

Cleveland Cable Company - Cable Calculator

www.clevelandcable.com

Cheers,

Sorted now thanks.

 

[i=p/u]

Ib>In?

 

[pc1966]

Also Doncaster have a similar one:

Welcome to Doncaster Cables - Technical Help

Doncaster Cables is the largest British owned general wiring manufacturer in the UK. We have been manufacturing cables since 1984 and among our current staff alone we have over 1000 years direct cable manufacturing experience. Throughout this time we have continued to develop in to an...

www.doncastercables.com

 

[Julie.]

Be careful, on long runs the R1+R2 could take the Zs over the limit for the breaker, it's not just load current, and voltage drop, the loop impedance could be an issue.

 

[pc1966]

If looking at, say, 10A max load and a 16A B-curve MCB then Zs max is 2.2 ohm, assuming TN-S max of 0.8 ohm then 1.4 ohm for R1+R2 or 14mOhm/m for a 100m cable run.

Looking at the 2-core SWA for mucho cheapness, then

• 6mm is 3.1+7 = 10.1 mOhm/m
• 4mm is 4.6+7.9 = 12.4mOhm/m

So either those should be fine. Assuming 16A B-curve MCB/RCBO of course!
Also assuming the SWA is properly glanded to get a good and reliable connection to the armour!

 

[pc1966]

If you work with 10A and 95m then the calculators come back with 4mm. Yes, below a 13A socket but for the expect use-case not a big deal. Seems also for 13A and 70m.

Of course you would use LED lights now so I don't think the 3% drop traditionally applied there is anything to worry about.

 

[Dustydazzler]

Once you decide how much load capacity you require , the rest is simple using the voltage drop calculation and the tabulated values in the OSG

 

[buzzlightyear]

If looking at, say, 10A max load and a 16A B-curve MCB then Zs max is 2.2 ohm, assuming TN-S max of 0.8 ohm then 1.4 ohm for R1+R2 or 14mOhm/m for a 100m cable run.

Looking at the 2-core SWA for mucho cheapness, then

• 6mm is 3.1+7 = 10.1 mOhm/m
• 4mm is 4.6+7.9 = 12.4mOhm/m

So either those should be fine. Assuming 16A B-curve MCB/RCBO of course!
Also assuming the SWA is properly glanded to get a good and reliable connection to the armour!

please could you in light use how you got to this equation .

 

[pc1966]

please could you in light use how you got to this equation .

Starting point was the light load, so assume enough for a 13A socket and not much more. So Table B6 of the OSG has Zs limit at 2.2 ohms for 16A B-curve MCB.

Usual assumption for supply Ze are 0.35 ohm max for TN-C-S and 0.8 ohm max for TN-S, from OSG page 63. So for a probably worst case (and ignoring the TT elephant for just now) we need Ze+R1+R2 to be less than Zs

R1+R2 = Zs - Ze = 2.2 - 0.8 = 1.4 ohm

The OP had the length as 70-100m so assuming 100m case then the max cable R1+R2 = 1.4 / 100 = 0.014 ohm/m or 14 mOhm/m (or ohm/km if you prefer).

Next went for the Prysmian SWA data sheet here:

https://uk.prysmiangroup.com/sites/default/files/atoms/files/BS5467.pdf

Page 5 (last page) has a table of conductor and armour resistances. As the OP was worried about the cost, cheapest is going to be 2-core SWA assuming the armour as CPC is acceptable. So looking down the table we have:

• 4mm conductor = R1 = 4.61, 2-core armour = R2 = 7.9
• 6mm conductor = R1 = 3.08, 2-core armour = R2 = 7.0

Rounding to 1 digit we have the R1+R2 values as 4.6+7.9 = 12.4 and 3.1+7.0 = 10.1 ohm/km (or mOhm/m) for those cables types respectively.

Since both are less than the 14 mOhm/m from the first half, both are going to meet the Zs requirement for 100m or less length. Or you could work it around another way using length = (max R1+R2) / (cable R1+R2) so:

• 1.4 / (12.4/1000) = 113m max for 4mm 2-core
• 1.4 / (10.1/1000) = 138m max for 6mm 2-core

I would add a bit of margin though as those are DC resistance values, for 50Hz AC they would be slightly higher, but in both cases they look fine for 100m & 16A B-curve MCB.

 

[TimGemm]

Can anyone recommend a good Cable Size calculator to use online?

The old man wants me to order some 6mm 3 SWA to feed his shed that will go on to supply some sockets, fairy lights around the garden and other stuff, no power tools or high load activities.

Cost is the mitigating factor here because the cable run is (his estimate) 100m (my estimate is 70 meters) from Consumer unit to shed running through the full length of his bungalow attic, down one side of the house and that ran along the kick board or bottom of his fence around the side of his garden.

Im thinking it might have to be 10mm which is going to put the job past his budget.

 

[pc1966]

@TimGemm
Most cable size calculators just do the current and volt-drop limits, for example:

Welcome to Doncaster Cables - Technical Help

Doncaster Cables is the largest British owned general wiring manufacturer in the UK. We have been manufacturing cables since 1984 and among our current staff alone we have over 1000 years direct cable manufacturing experience. Throughout this time we have continued to develop in to an...

www.doncastercables.com

Proper design is a lot more complicated (see above) as you also have to consider fault-clearing times and any minimum CPC size if you are bonding "extraneous conductive parts" when the supply is TN-C-S (most common for new homes in UK).

Generally if in doubt get a local spark in to sort it out. many of them will be happy for you to do the donkey work of trench digging and cable installation so long as they have oversight on the choice of cable and the installation at both ends.