New circuit design question

[RyanAspire]

Hi guys,

Have a job on next week for a friend, my usual industry is maintenance so want to be sure I am checking all the boxes here.

He intially measured the run out at 122m, which would require a 70mm cable due to volt drop, however its changed slightly and I have other queries...

He now has the main kiosk that UKPN have put the supply into with the 100A fuse, the meter company are coming to install next week. From there, there will be another 2 kiosks on the way to the last. So see below for the first query...

Main Incoming kiosk
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20m
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Kiosk number 2. Will have ring main circuit, lighting circuit and then circuit that carries on to Kiosk 3.
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20m
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Kiosk 3. Ring main circuit, lighting circuit and then circuit that carries on to Kiosk 4.
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80m
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Kiosk 4. Final kiosk, will have ring circuit and lighting circuit. Will also be powering a mobile home/static caravan that has power shower, washing machine, elec heaters etc (no gas here). There is a barn that will likely have power in the future for some tools.

My question here is with these various runs, each 'break in the total run will be protected by an RCD and MCB. would the cable have to be 70mm at all? Could it be 35mm all the way? as it is now a 20m run, then via an MCB, it would be another 20m run and then from there via another MCB it would be an 80m run to the final kiosk.

Another question, I will likely look to install a 30mA RCD in the first kiosk which will cover the whole system, is it worth installing further RCD's in the following kiosks as they will all be protected by the initial RCD. There is also a 30mA RCD already in the caravan consumer unit. I would prefer to avoid the nuisance trip of this initial RCD but obviously installing further 30mA RCD's in the other kiosks won't necessarily reduce the chance of unnecessary tripping, and time delay and greater RCD's aren't an option as each kiosk has final circuits.

Also, I'm sure i know what the answer is to this one....the supply is TNCS for reference, but I won't need an earth rod for caravan will I? I'm not looking to create a TT system or PME unless it is beneficial/regulation.

I don't want to sound stupid in this at all, just covering my tracks to ensure I am keeping to best practice. Thanks in advance guys!

Ryan

 

[Spoon]

What are the kiosks for?
What will they be powering?
I'm presuming each kiosk will have it's own small CU... What size breakers/RCBO's are you thinking of fitting in the CU?
If you are using armoured cable to feed these CU's in these kiosks then the cable will not need RCD protection. Therefore you can have a RCD for each kiosk CU or fit RCBO's.
If you just had the one RCD at the source then if there is a fault in any on the kiosks this will take down all the other kiosks.....

 

[RyanAspire]

Thanks for the response!

The kiosks are to house DBs yes, it is all on an acre or so of land and across it there is a log barn that will need lighting and sockets for general use, charging equipment etc, same for the second kiosk and the final one will have a DB in it as a fixed point for the caravan to connect too.

RCBOs is a more costly route, although I much prefer them, im trying to minimise cost. And RCDs my only thinking is for example, if the first RCD in the system is slightly more sensitive than the last than it may still trip despite where the fault may be. I will likely put RCDs in each DB but just trying to see if there is another plausible route is all.

Thanks for getting back ?

 

[dodger421]

My question here is with these various runs, each 'break in the total run will be protected by an RCD and MCB. would the cable have to be 70mm at all? Could it be 35mm all the way? as it is now a 20m run, then via an MCB, it would be another 20m run and then from there via another MCB it would be an 80m run to the final kiosk.

The volt drop from supply to end of run is still going to be the same, even if the distance between each point is shorter so you’ll still need to use 70mm all the way down to the last kiosk I believe.

 

[RyanAspire]

This is where my query comes in as I agree. But say if I had installed both runs of 20m (main kiosk to kiosk 2 and kiosk 2 to kiosk 3) now in 35mm, that would be fine. And then at a later date another electrician came to add to the install, he would be basing his volt drop calculations on 80m (kiosk 3 to the final kiosk) and not considering the 2x20m runs installed already? Am I right in thinking that way?

The volt drop from supply to end of run is still going to be the same, even if the distance between each point is shorter so you’ll still need to use 70mm all the way down to the last kiosk I believe.

 

[dodger421]

This is where my query comes in as I agree. But say if I had installed both runs of 20m (main kiosk to kiosk 2 and kiosk 2 to kiosk 3) now in 35mm, that would be fine. And then at a later date another electrician came to add to the install, he would be basing his volt drop calculations on 80m (kiosk 3 to the final kiosk) and not considering the 2x20m runs installed already? Am I right in thinking that way?

No, you have to consider volt drop from the supply in all cases.

So the later electrician has to either connect directly at the supply for his 120m run or do a bit of work to calculate (or measure) the impedance of the existing 40m of 35mm and then work out what size cable he needs to ensure his final circuits can meet the 3% or 5% volt drop if he’s looping from the last kiosk. That may mean connecting say a 90mm to the existing run to keep the volt drop sensible at the end of the line.

At least that is my understanding of it, that the volt drop calculation includes all of the wiring from the supply to the furthest point of the final circuit.

 

[ChipFanDave]

This is where my query comes in as I agree. But say if I had installed both runs of 20m (main kiosk to kiosk 2 and kiosk 2 to kiosk 3) now in 35mm, that would be fine. And then at a later date another electrician came to add to the install, he would be basing his volt drop calculations on 80m (kiosk 3 to the final kiosk) and not considering the 2x20m runs installed already? Am I right in thinking that way?

No you need to take in the whole length of the run for the voltage drop calculation.

 

[westward10]

For a submain you should consider a vd of 2% to give some leeway to the final circuits. To install an rcd at source seems a crazy thing to do.

 

[Lucien Nunes]

Most of the load seems to be at the remote end of the cable so the VD is nearly as high as if it were a single run of the full length. Obviously if the load were more evenly spread along the length you could assess it as being centred at the halfway point and halve the VD result. If there is significant design load at the other points it might pay to calculate each cable run separately with the total load it carries, then add the VDs of the three runs for the final answer.

No idea why you are wanting an RCD at the intake. Surely you just put whatever protection is appropriate at each DB for the circuits it supplies?

 

[RyanAspire]

Sorry guys, I mistyped at work! The RCD won't be at the intake, it will be at the first DB and then the circuit will continue from there via an MCB which will be downstream from this RCD.

Just for clarity I completely agree that 70mm should be the case all the way, i was just wondering if I was misinterpreting the regs or not.

The majority of the load will be at the furthest end of the install yes, but i am trying to future proof too incase they should want to add any other equipment at the other DBs.

Thanks everyone for the feedback though ?

 

[GBDamo]

What load is going on the final kiosk? 70mm SWA, buried is good for 70ish amps @ 120m

All this 70 Amps may not be needed at the final kiosk, how many?

You may find you can run 70mm to the first two kiosks then 50mm to he final.

(Numbers not calculated, just to show the principle)

 

[RyanAspire]

Sorry guys, I mistyped at work! The RCD won't be at the intake, it will be at the first DB and then the circuit will continue from there via an MCB which will be downstream from this RCD.

Just for clarity I completely agree that 70mm should be the case all the way, i was just wondering if I was misinterpreting the regs or not.

The majority of the load will be at the furthest end of the install yes, but i am trying to future proof too incase they should want to add any other equipment at the other DBs.

Thanks everyone for the feedback though ?

However I'm considering the option of no RCD at the first and second DBs and use RCBOs and then one RCD at the final kiosk

 

[RyanAspire]

What load is going on the final kiosk? 70mm SWA, buried is good for 70ish amps @ 120m

All this 70 Amps may not be needed at the final kiosk, how many?

You may find you can run 70mm to the first two kiosks then 50mm to he final.

(Numbers not calculated, just to show the principle)

Yeah i considered that too as I think i calculated at 100m that 50mm would be fine. I had said this but they decided to go ahead and purchase the whole run in 70mm

 

[Lucien Nunes]

it will be at the first DB and then the circuit will continue from there via an MCB which will be downstream from this RCD.

Why do you want another MCB in line with the run of the distribution circuit? Its size is likely to be dictated by VD not current rating, so it won't need fusing down anywhere and therefore won't be affected by the RCDs in the DBs.

 

[RyanAspire]

Why do you want another MCB in line with the run of the distribution circuit? Its size is likely to be dictated by VD not current rating, so it won't need fusing down anywhere and therefore won't be affected by the RCDs in the DBs.

The size is definitely determined by the VD. I don't want an issue downstream i.e cable damage to take out the intake fuse?

If there is an issue at the end of the mobile home on the SWA that isn't protected via MCBs along the way then I don't want the fault path to go all the back and blow the 100A fuse that UKPN will then need to come and replace at cost and downtime without power too. At least via this route the power upstream should remain intact and also help to identify the location of a fault too