Discuss SWA Sub Main advice/comments in the Commercial Electrical Advice area at ElectriciansForums.net

TheCodboy

-
Arms
Reaction score
102
Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Hi all, looking for a bit of advice and confirmation on my proposed plans for a SWA sub main to a ISO metal storage container. I've calculated the max demand in the ISO will be about 20A (300w of led lighting and 2 double sockets for chargers etc) It will be fed from a 3ph DB in another building. The cable run is 100 meters, I'm planning on using 16mm 2 core swa ( Volt drop calcs allow about 35A with 16mm) and installing a rod at the container (the vast majority of the cable run is at height, clipped to the building). The container will have a small 4 way board with RCD main switch.
The 16mm will be fed from a C32 breaker in the 3Ph board. Im installing double insulated 16mm tails and a 16mm earth from the 3ph DB to a meter (needs a meter installed) then from the meter to an adaptable box where I will terminate the SWA. Due to the fact that I'm using a rod at the other end, I'm planning on a 100ma S rcd for fault protection for the SWA (inside the adaptable box).
Can anybody see any glaring errors in my method? I have spoke to Napit and they were fine about my proposed plan, but they didn't mention the 100mA rcd.
Thanks in advance!
 

pc1966

Esteemed
Arms
Reaction score
6,232
What is the Zs/Ze of the supply board?

If you go with 16mm 2-core then you have R1+R2 = 1.15 + 3.7 = 4.85 ohm/km so 0.485 ohm and for a C32 MCB you would need no more than 0.55 ohm end of circuit Zs so supply Ze has to be 0.06 ohm or less. Unlikely.

Your VD limit would be met for 10mm, so going 3-core there your R1+R2 is 1.83 + 1.83 (ignoring armour's 6.0 in parallel for now) = 0.366 for 100m so Ze <= 0.18 ohm, which is possible.

Is this a TN-C-S supply? If so the 16mm 2-core will not meet the 10mm copper equivalent for bonding an extraneous part (armour is 42mm steel, so about 5.3mm copper equivalent), so either it has to be TT'd or you go for 10mm 3-core.

If using the 100mA RCD for avoidance of sane limits on supply Ze it will have to be at the source end, so a cable fault still meets 5s or less disconnection, and ideally a delay one so it has selectivity with the 30mA RCD for the socket outlets. No one is going to be happy walking 200m round trip to reset it!

Other options for Ze avoidance would be a 32A switched-fuse as then you would meet 5s disconnection on 1.25 ohm end-of-circuit Zs (OSG Table B4), but obviously no selectivity with the 32A C MCB, and probably little with the socket circuits (probably fine for post-13A fuse fault, not so one a 20A B RCBO or whatever and fixed wiring fault).
 
Last edited:
OP
T

TheCodboy

-
Arms
Reaction score
102
What is the Zs/Ze of the supply board?

If you go with 16mm 2-core then you have R1+R2 = 1.15 + 3.7 = 4.85 ohm/km so 0.485 ohm and for a C32 MCB you would need no more than 0.55 ohm end of circuit Zs so supply Ze has to be 0.06 ohm or less. Unlikely.

Your VD limit would be met for 10mm, so going 3-core there your R1+R2 is 1.83 + 1.83 (ignoring armour in parallel for now) = 0.366 for 100m so Ze <= 0.18 ohm, which is possible.

Is this a TN-C-S supply? If so the 16mm 2-core will not meet the 10mm copper equivalent for bonding an extraneous part (armour is 42mm steel, so about 5.3mm copper equivalent), so either it has to be TT'd or you go for 10mm 3-core.

If using the 100mA RCD for avoidance of sane limits on supply Ze it will have to be at the source end, so a cable fault still meets 5s or less disconnection, and ideally a delay one so it has selectivity with the 30mA RCD for the socket outlets. No one is going to be happy walking 200m round trip to reset it!

Other options for Ze avoidance would be a 32A switched-fuse as then you would meet disconnection on 1.25 ohm end-of-circuit Zs, but obviously no selectivity with the 32A C MCB, and probably little with the socket circuits (probably fine for post-13A fuse, not so one a 20A B RCBO or whatever).
It will be TT
 

timbobelfast

-
Arms
Reaction score
204
Hi all, looking for a bit of advice and confirmation on my proposed plans for a SWA sub main to a ISO metal storage container. I've calculated the max demand in the ISO will be about 20A (300w of led lighting and 2 double sockets for chargers etc) It will be fed from a 3ph DB in another building. The cable run is 100 meters, I'm planning on using 16mm 2 core swa ( Volt drop calcs allow about 35A with 16mm) and installing a rod at the container (the vast majority of the cable run is at height, clipped to the building). The container will have a small 4 way board with RCD main switch.
The 16mm will be fed from a C32 breaker in the 3Ph board. Im installing double insulated 16mm tails and a 16mm earth from the 3ph DB to a meter (needs a meter installed) then from the meter to an adaptable box where I will terminate the SWA. Due to the fact that I'm using a rod at the other end, I'm planning on a 100ma S rcd for fault protection for the SWA (inside the adaptable box).
Can anybody see any glaring errors in my method? I have spoke to Napit and they were fine about my proposed plan, but they didn't mention the 100mA rcd.
Thanks in advance!
I take it the Earthing System for the upstream supply is a TN-C-S???
 

pc1966

Esteemed
Arms
Reaction score
6,232
Haven't had a dig yet but I've been looking at bentonite if I'm struggling for a good reading.
Clay cat litter is cheaper and the same sort of stuff!

Obviously check for any buried services as far as you can. There are sites like this for main stream power & gas, but not everyone is on there:

Also remember that the main factor is soil conductivity is moisture, so if you can go deeper (say 8' = 2.4m rod, typically in two sections) you will generally do better than two short ones in parallel as more chance of the soil remaining moist.

I have used M16 stainless studding as an earth rod driven in like a giant drill (SDS adaptor and a 1/2" drive stud runner), but obviously you get purpose made ones and matching bit like access boxes, etc. For example:
 
OP
T

TheCodboy

-
Arms
Reaction score
102
Clay cat litter is cheaper and the same sort of stuff!

Obviously check for any buried services as far as you can. There are sites like this for main stream power & gas, but not everyone is on there:

Also remember that the main factor is soil conductivity is moisture, so if you can go deeper (say 8' = 2.4m rod, typically in two sections) you will generally do better than two short ones in parallel as more chance of the soil remaining moist.

I have used M16 stainless studding as an earth rod driven in like a giant drill (SDS adaptor and a 1/2" drive stud runner), but obviously you get purpose made ones and matching bit like access boxes, etc. For example:
Just did a job today where a house had no main earth. Did a Zs at the sockets and it was 290 Ohms. Did as bit of digging out, probably a couple of feet and got the 4ft rod pretty much all the way in. Ze of 28 ohms. That'll do for me!
 

Reply to SWA Sub Main advice/comments in the Commercial Electrical Advice area at ElectriciansForums.net

Similar Threads

Z
Hello all. I have visited a domestic property with two consumer units, one downstairs and then a separate one for upstairs. The upstairs is fed by a 16mm twin and earth and the downstairs is fed by 25mm tails. The cu downstairs is a duel RCD. The question I want to ask is, since the cable...
Replies
3
Views
233
freddo
freddo
W
Been retired for a number of years and not kept up with the industry hence I'm well rusty, so apologies for that. I recently moved home and found that the sub-main cable (10mm x 3 core SWA) feeding the garage/outbuilding is connected to the electricity boards meter & service fuse (cutout) via...
Replies
5
Views
273
wobbly2
W
U
Hi Fitting a consumer unit in garage . Unfortunately the 16MM SWA is too short to reach consumer unit So i have done as follows . Glanded SWA into a metal adaptable box . earthing nut earthed with 10mm crimped and run to earth block . 2 cores of SWA Brown and Grey ( oversleeved BLUE) run to...
Replies
46
Views
2K
brianmoooore
B
S
Hi all, I am new to this forum so firstly HI ?? I am looking for some advice after buying a property I have a TT system feeding a split load RCD protected DB in main house, I then have 10mm SWA sub main coming off that board protected by a 50A MCB This then supply's an extension with another...
Replies
6
Views
384
loz2754
loz2754
J
Hi everyone, A local pub is changing one of there function rooms in to a cafe, there plan was to have all gas equipment so i done the cable calcs and installed a 16mm 3 core swa as the submain cable from the main 3 phase db in the pub to what was going to be a single phase db in the cafe...
Replies
3
Views
229
pc1966
P

Electrical News from Electricians Forums and Friends

E
  • Article
Want to become a qualified electrician? Look carefully at the training courses on offer to avoid wasting time and money, is the advice in a new campaign from electrical industry body The Electrotechnical Skills Partnership (TESP). With thousands of searches each year on terms such as...
Replies
49
Views
6K
Marty92
Dan
  • Article
You can now link your account with social media by going to the following page. https://www.electriciansforums.net/account/connected-accounts/ We had this feature some time ago but I removed it when Facebook was found to be selling the data they collected without informing the users of what...
Replies
0
Views
2K
Dan
Dan
  • Article
There was a dodgy advert, typical clickbait, showing "how to save 90% on your electricity bill" which was not the case and it was actually showing some dangerous wiring tactics. Read the actual threads here:- What should be the code for this - C2 or C3 -...
Replies
37
Views
8K
Jurassic Sparks
Jurassic Sparks
James
  • Article
The Go Plus hot tub is being recalled because you could get electrocuted taking a dip, according to press releases from the Chartered Trading Standards Institute (CTSI). An Amazon spokesperson said: "Safety is a top priority at Amazon. "We require all products offered in our store to comply...
Replies
6
Views
3K
Dave Appleby
D
Dan
  • Article
Posted on behalf of @CKI:C.K-Tools Carl Kammerling International Ltd, also known to as ‘CKI’, are the company behind the C.K Tools brand. Founded in Germany in 1904 and with roots stretching back to 1790, we provide a wide range of hand tools, tool storage, power tool accessories and...
Replies
2
Views
4K
telectrix
telectrix

Search Forum

Top