New Shed Supply

If your sparkie does sign it off it will have to be 3rd party testing as far I'm aware only napit or stroma do this

 

Nothing wrong with that PaulS.

As you can see from the table for SWA cable by method D (buried in ground) 4mm2 max current capabilities are ok for 32Amp breaker but fail with length.

So lets calculate your Voltage Drop.
From the table, for a 4mm2 SWA cable the volt drop per amp per metre is 12mV.

So for 32Amp over a 31 metre run, 12x32x31= 11904 now divide this by 1000 because we were working with mV and not Volts. 11904/1000 = 11.9volts lost. Regulation states you are allowed up to 5% voltage drop and no more.

I'll save you the percentage of 5% of 230v calculations it is a value of 11.5V so you are over the allowed voltage tolerance, either reduce the max current(breaker size down to 20Amp) or increase the cable size.
Very borderline isn't it...

When we calculate the distance voltage drop to current over 31 metres clearly you cant use more than 20Amps with 4mm2 over this stretch, obviously a 20Amp because you can't buy a 25Amp breaker(in Wylex brand at least) as it goes 6, 16, 20, 32, 40 and a 63Amp.

So lets up to 6mm2:

Now the loss per amp per metre is 7.9mV so:

7.9x32x31 = 7836.8 lets call it 7837 now divide by 1000 so 7837/1000 = 7.84V rounded to 2 decimal places. Now you can run it at 32Amps but also note you have lighting.

Regulations state no more than a 3% drop allowed for lighting... 3/100 x 230 = 6.9V oh **** you can't use a 6mm2 one either because you have a lighting circuit all be it multiple lighting circuits so again drop a breaker size or up the cable size.

That means you have to run 10mm2, yikes.

Now to get creative but this is certainly not advised. If the SWA run is only 20Metre and the internal path is 11 metre. Lets place some food for thought here with run the internal circuit run with 10mm2 and the further SWA will be 4mm2 over 20 metre..:

for 10mm2 the mV loss is again look at the PDF which is 4.4mV per amp per metre:

4.4x32x11 = 1549 rounded and back to volts 1549/1000 = 1.55V rounded.

SWA of 4mm2 is 12mV loss

12x32x20 = 7680 and converted to volts 7680/1000 = 7.68V

Add them together and you get 9.23V which is still over the 3% allowed for a lighting circuit.

So 10mm2 SWA is 4.7mV per amp per metre:

4.7x32x31 = 4662 rounded 4662/1000 = 4.67V and of course this is your options.

Look at the Bright side, you could up to 40Amp breaker if you used 10mm2 as the voltage drop for 40Amp is 5.83V

But lucky for you, low voltage supply from a domestic supply to domestic circuit has higher percentage values of 6% for lighting and 8% for everything else. Now you can work out your actuals.

The dreaded burying of SWA cable without running it in a PVC conduit tube with a pull-cord left in the tube for future cable pulling...

A good install would have been to lay this stuff in the ground and the cable pulled through it and leave the pull cord in place so future pulls can be pulled through.

 

The 5% and 3% values are a recommendation and not set in stone.All the above calculations have been based on no diversity what so ever.... Realistically with the loadings you've stated your not going to be anywhere near 32A more like under 5A unless you start adding heaters etc. 4mm SWA on a 20A OCPD is adequate for what you've described.

 

Yup, post already got as long as your arm so going into diversity calcs as well would make it as long as ones leg.

Yes it is 20Amp for 4mm2 over 31 metres which may well be perfectly adequate current supply for the OP or if he wants more he will need up the cable, either way it describes how we calculate voltage drop and yes on the assumed total max of the rated 32Amp breaker current which as you say may well never be reached so dropping it to a 20Amp and sticking with 4mm2 could well be the perfect solution.

It is not for me to make the call as i not doing the job.

 

Yes it would be a 40Amp RCBO to a double pole isolator running your 31 meters on 10mm2 cable and then terminating in a CU fully populated with RCBO's for your individual circuits.

Head RCBO being a 40Amp 100mA rcbo while those in shed are 30mA RCBO's

Simple really

 

After you done all this.,

 

Good morning Fire.

Again not correct. RCDs will not discriminate on the basis of the trip current.

The most likely scenario is: a substantial earth fault at the garage end of the circuit. Lets say he's chopped through a cable. The earth fault impedance is several amps.
The RCD/ RCBO that will trip first will not be the 30mA at the garage end! It will be the RCD/RCBO that has the quicker trip time. Also it may depend on where in the AC sine wave curve the supply is, at the time of the fault.
So it could be the house RCD that trips, or the garage, or both. And that is BAD BAD design.


If there's a leaky earth fault (less than 100mA but more than 20ish milliamps) then the garage RCD may be the first to trip. But as I say, this would not be the usual fault.

The only way to discriminate RCDs is on the basis of TIME
That is why time delayed RCDs are made. So if, earth fault protection is needed from the supply end, he needs to fit a 100mA S-type (ie Time Delayed) RCD at the house end like this:



Additional benefit: This will provide double pole isolation at the house end, whereas the planned (and already installed ) RCBO will not.

Hope this makes sense.

EDIT: For the OP's benefit. In that case the RCBO in the house consumer unit needs to be changed to an MCB of the appropriate rating for the cable to the outbuilding. The time-delayed RCD would go in a separate enclosure. That is assuming you think that the supply out needs RCD protection, which - in my opinion - it does not.

 

It would be best to have the upfront OCPD as a fuse that's not protected by an RCD and an RCBO board in the shed. Having the fuse upfront and RCBO's down stream will provide better discrimination than an MCB/MCB or RCBO/RCBO combo. The other problem with having two RCBO's in series even if the upfront is time delayed is that most are single pole solid neutral. In the event of a neutral earth fault the downstream RCBO will trip and a few milliseconds later the upfront will trip due to the 1st RCBO leaving the fault connected as the first RCBO doesn't break the neutral.

What make is your existing consumer unit in the house? The reason I ask is because you might get a cartridge fuse carrier that fits that particular make/model.

If the loadings you've stated are true I would go 20A Fuse>Rotary isolator/DP switch>utilise the 4mm SWA from board to shed and a board in the shed that has RCBO's or RCD/s and circuit breakers.

 

I've seen this mentioned a couple of times now. My NICEIC electrical contractor, who normally works on new builds for the big house builders has just installed my CU and the SWA that will run to my utility and onwards to a garage. He intends a smaller CU in the utility and the SWA from the main CU is connected directly into an MCB which is not RCD protected. However, no double pole isolator. I don't have the knowledge to query this. Right? Wrong?

 

Must say I would not do it that way Mr Rusty. At cut out position I would Henley off and run into DP switch fuse, and run the SWA off of that to outbuilding DB.

 

why are you having a cu in the utility?

 

Ditto.

There was an interesting discussion ...somewhere...IET forum maybe.... showing that you'll have a hard job getting two MCBs of any value to discriminate unless (again) the upstream MCB has a time delay element - not general domestic installation fodder.
http://www.electrical-installation.org/enwiki/Coordination_between_circuit-breakers
easier reading at
http://www.voltimum.co.uk/files/gb/attachments/mmu/l/circuit_breaker_protection.pdf
EDIT
Found the IET topic
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=48458

Really it needs a fuse at the source if MCBs are downstream.

I've also had another look at the OP's "design" (post #1)
He has a 32A RCBO at the origin and 32A MCB for the sockets in the garage. What's the point of that?
A further example of someone installing something without any/proper knowledge of what they are doing.

 

low voltage supply from a domestic supply to domestic circuit has higher percentage values of 6% for lighting and 8% for everything else.

Is that right, thought is was 3% and 5%

 

Leaving it to my sparks to design. The utility is what would have been the separate "wash-house" and will have sockets + lights, and a further feed out to a separate garage. I presume he will take the SWA into a smaller CU

I won't be telling him he's doing it wrong - need him to come back and do the second fix in due course.

 

I am not saying he is doing it wrong Mr Rusty, after all he is the qualified sparks and I am only diy. However as the supply is to an outbuilding from db mcb I would suppose it would be classed as a sub circuit and not a sub main proper..however that aside - the lack of dp isolation doesn't cut the ice with me, although it must be ok within the reg parameters. I always thought that dp isolation was a requisite when running supply's to outbuildings etc be it a sub circuit or sub main, but hey ho I must be wrong, however I will always use dp isolation whatever happens.

 

The DP isolation for circuits that leave the house came in with Amendment 3.

What is considered power away from the house which has the main power feed can be tricky, for instance if you screw a flood light to the wall outside your house but it is screwed to the house. Is this considered away from the house, not really as it is outside but still on the house.
If you have lights that then light up your driveway well there is no dispute that now it is not on the house any more and is away from the home containing the supply and requires it to be isolated it a DP isolator.

Amendment 3 changed that so all individual circuits even a flood lights fastened to a wall outside needs to be isolated.

Section 714 Outdoor Lighting Installations
In 2008 some major changes to the requirements for outdoor lighting installations were introduced, covering requirements for:

• car parks;
• gardens;
• parks;
• places open to the public;
• illumination of monuments; and
• floodlighting.

These will be retained in Amendment 3, along with the recommendations for additional protection by a 30 mA RCD for telephone kiosks, bus shelters, advertising panels and town plans.

Amendment 3 will make only minor changes to outdoor lighting installations. One important change will be that individual circuits will be required to be isolated.

So this means that not only does circuits that power an outbuilding need a DP isolator but so does any individual circuits like floodlights.

 

That option would require a registered spark to pull the DNO cut-out and add it into the tails. Seal fairies and all that again

The only proper way a DIYer can get this done is to ring up their power company and have them fit a double pole isolator between the meter and the consumer unit so the power can be isolated without needing to tamper with the seals.
It is a very wise choice to have a isolator fitted by the power company anyway, allows you to work on your installation safely with a lockout on the isolation switch so no one else can energise the circuit until it has been made safe.

 

I put it in as a food for thought trap.
Your answers can be found here and explains problems which can occur if these values are reached..:

http://www.electrical-installation.org/enwiki/Maximum_voltage_drop_limit