Submain question

Afternoon folks,

We bought a 400 year old vicarage about 1 year ago now and were just starting to refurbish it, part of this involves a fair bit of joinery. As I don't get to get my hands dirty as much as Id like at work these days I'm building a small workshop so I can make a load of sash windows, a kitchen and some other furniture.

I've buried 40m of 3 core 10mm2 SWA cables at a depth not less than 600mm between our incoming supply and the location of the new workshop, as I had 30m of cable left over I've also buried a cable to a location where we will hopefully have a hot tub in the future (got to try and keep the wife happy).

I plan to split the meter tails and install a small CU along side the existing CU to protect the submain and then install a small DB locally to break out into the required circuits.

It seems to be normal practice to only provide over current but not earth leakage protection to the two sub mains at the supply side, is this correct and if so does this still stand with 18th edition?

I would if possible like to protect the submains with RCBOs and then protect individual circuits with RCBOs just to be belt and braces. I appreciate this leads to a race condition which may be unsafe or a nuisance.

1) Is there any benefit to fitting a 100mA RCBO to the cable given the guidance seems to be 30mA max for protection of life.

2) If I fit a 30mA RCBO to the supply end then 15/20mA RCBOs to the downstream DB am I likely to suffer from nuisance stripping with machinery?

3) Can you get any form of 30mA RCBO with a short time delay - I appreciate the main purpose of the RCD is to make the circuit safe as quickly as possible and this is obviously a little counter intuitive.

For info:

I believe the incoming supply was at one point TT but has since been converted to TN-C/PEN.

Cables have been installed by direct burial without use of ducting ect.

The workshop will likely require some form of three phase supply wither from a rotary convertor or VSD.

Apologies for what is probably bread and butter to you guys but its been a while since I've worked with onshore small power stuff.

Kind Regards

Dave

not sure you can run a hot tub off that cable, but your plan is basically sound. You don't need to RCD (RCBO) protect the sub-main to the workshop. If you really want to do this use a 100mA Type S RCD as the 'main switch' in your mini board for the sub-main.  Once at the workshop use RCBOs for every cct.

I read that as TWO separate submains. One to the workshop, the other to the hot tub.  Each would have their own over current protection.

Bit late now but on 'projects' oversized ducting (big enough to add 1 or two mini-ducts for low voltage)  is almost always a good investment. 

You think you've done the job now then someone says why not run ethernet down there . . oh and an extension to the front doorbell . .    oh and an alarm cable . . . oh and a CCTV camera . .   then someone says we need bigger cable for the new sauna heater . .

Sorry wasn't too clear there Dave is correct, 2 sub mains. Both under 40m 3 x 10mm2 which seems to be sufficient for 50A.

I cant imagine I'd draw anything like that in a joinery workshop most equipment seems to be around 2-3KW so probably 15A max with a machine and dust extraction.

Hot tubs seem to be less than 5KW and the demand obviously isn't constant being thermostatically controlled.

Hi Bruspark,

I had the same thought and I've run a 60mm cable duct from the house up the drive along with a 25mm water MDPE water main, I've got 500m of 32 pair multi mode fibre I rescued from being skipped which is probably more suited to a small phone exchange than enabling me to watch porn hub/YouTube but it was free :slap

I was being tight with the power cable and the incomer is on the other side of the house so I took the shortest route I could to keep the conductor sizes and costs down. I bought cable on the instruction of the boss not to ruin the lawn but realised I could get a second submain out of it if I took the shortest path.

cbuk2k2 said:

Hi Bruspark,

I had the same thought and I've run a 60mm cable duct from the house up the drive along with a 25mm water MDPE water main, I've got 500m of 32 pair multi mode fibre I rescued from being skipped which is probably more suited to a small phone exchange than enabling me to watch porn hub/YouTube but it was free :slap

I was being tight with the power cable and the incomer is on the other side of the house so I took the shortest route I could to keep the conductor sizes and costs down. I bought cable on the instruction of the boss not to ruin the lawn but realised I could get a second submain out of it if I took the shortest path.

Click to expand...

cbuk2k2 said:

It seems to be normal practice to only provide over current but not earth leakage protection to the two sub mains at the supply side, is this correct and if so does this still stand with 18th edition?

Kind Regards

Dave

Click to expand...

Welcome to the forum, There is a common misconception nowerdays that everything must be RCD protected. There is no wiring regulation stating that a sub-main needs RCD protection, (unless of course it is unprotected buried in a wall less than 50mm deep).   You are perfectly entitled to design and install to 17th edition at the moment so you have no obligation to worry if your work complies with 18th edition.  Of course if you want to work to 18th you are also at liberty to do that if you wish.  What edition are you choosing to design to? 

Doc H.