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

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

TheCodboy

-
Arms
Reaction score
109
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!
 
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:
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
 

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

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

YOUR Unread Posts

This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock