Another outbuilding query...

[jubee]

Afternoon all,

Been reading lots and now starting to doubt myself with the route I've taken...

I've ran 18m 10mm 3 core SWA to an outbuilding come art studio. Supply is TN-S and had no option but to put the SWA on an RCD protected breaker (unless of course I Henley blocked it). 17th edition Ammendment 3 CCU at house end, metal sheath glanded to CCU. TBH although the SWA has mechanical protection and my understanding no need for it to be RCD protected I still preferred this route of it being so. It's been laid underground at the correct depth and in ducting also.

Okay so regarding the CCU at the studio end do I?

Q1) terminate the SWA in a Wiska type box rather than terminating in the CCU (metal)
Q2) provide a non RCD CCU in studio as circuits protected at house end

The studio is timber built, no extraneous conductive parts. Water main to studio is in MDPE as is the house up to point of external stopcock at street. There's a kitchenette and seperate toilet with sink laid all in plastic.

Circuits will consist of

-32A ring main, serving 9 DSO. All low draw stuff; radio, heat gun, phone charger maybe a TV. At most an oil heater. Reason there's so many is cause my wife will be working at different stations and needs (I prefer 'would like' ;0)) power near by.

-6A lighting circuit

-40A undercounter instantaneous 9.5w water heater or 2no 3kw water heaters to serve both sinks.

Heaters will only be run for short time to wash hands and fill up sink for the odd bit of washing up.

Q3) SWA protected with a 50A MCB oringinally a 40A until introduction of water heaters as hot water was going to come off the boiler at the house. Is 50A okay?

Q4) What size main breaker at the studio CCU? Was thinking 40A?

I'm hoping to use this along with the new install of the house to use as my assessment. Nervous is an understament!

Cheers all, hopefully I've got it all down, all the best.

 

[Richard Burns]

TNS with no extraneous parts I would earth the armour at both ends.
Because of the 30mA RCD at the house the one at the studio would be redundant (but might provide a second chance).
The distribution circuit design current is likely to be over 50A (with the 9.5kW water heater) but you cannot go up another level as that would go over the CCC of the cable.
What main breaker at the studio?, you said you were putting in a consumer unit, so it will have individual MCBs for each circuit.
I would avoid the single water heater if possible as this would need a 50A MCB as well.

You will have no discrimination between the MCB at the house and those at the studio, but this should not be critical as the RCD is at the house as well.

 

[jubee]

Sorry that was meant to read 'Main switch' not 'breaker'. My understanding would be that the CCU at the studio wouldn't be an RCD one?

Understand about the 9.5 kW heaters putting me over. I'll be going with the two individual units.

To clarify 50A MCB at house RCD protected and 40A main switch with individual MCB'S in studio.

 

[Dave OCD]

There are occasions when 40 amps could possibly be exceeded, but in any case I'd use a 63a main switch as a minimum at the studio end as it needs to be rated to at least the current of the OCPD at source.

 

[Pete999]

Afternoon all,

Been reading lots and now starting to doubt myself with the route I've taken...

I've ran 18m 10mm 3 core SWA to an outbuilding come art studio. Supply is TN-S and had no option but to put the SWA on an RCD protected breaker (unless of course I Henley blocked it). 17th edition Ammendment 3 CCU at house end, metal sheath glanded to CCU. TBH although the SWA has mechanical protection and my understanding no need for it to be RCD protected I still preferred this route of it being so. It's been laid underground at the correct depth and in ducting also.

Okay so regarding the CCU at the studio end do I?

Q1) terminate the SWA in a Wiska type box rather than terminating in the CCU (metal)
Q2) provide a non RCD CCU in studio as circuits protected at house end

The studio is timber built, no extraneous conductive parts. Water main to studio is in MDPE as is the house up to point of external stopcock at street. There's a kitchenette and seperate toilet with sink laid all in plastic.

Circuits will consist of

-32A ring main, serving 9 DSO. All low draw stuff; radio, heat gun, phone charger maybe a TV. At most an oil heater. Reason there's so many is cause my wife will be working at different stations and needs (I prefer 'would like' ;0)) power near by.

-6A lighting circuit

-40A undercounter instantaneous 9.5w water heater or 2no 3kw water heaters to serve both sinks.

Heaters will only be run for short time to wash hands and fill up sink for the odd bit of washing up.

Q3) SWA protected with a 50A MCB oringinally a 40A until introduction of water heaters as hot water was going to come off the boiler at the house. Is 50A okay?

Q4) What size main breaker at the studio CCU? Was thinking 40A?

I'm hoping to use this along with the new install of the house to use as my assessment. Nervous is an understament!

Cheers all, hopefully I've got it all down, all the best.

No pictures so I'll ask why can't you make the SWA off straight into the outhouse CU? and do away with the Wiska box

 

[jubee]

Hi

No pictures so I'll ask why can't you make the SWA off straight into the outhouse CU? and do away with the Wiska box[/QUOTE

The idea behind not making off the SWA at the studio CCU was through forums I've been trawelling but more for TN-C-S and TT systems.

This is where the doubt crept in... Originally I thought it would've been better to make it off at the CCU and not use a wiska box. Haven't yet done anything at the studio end. Just want to get it 100% right for assessment. But moreso to understand why.
Cheers

 

[Richard Burns]

If you were to not gland the SWA to the CU at the studio end then this would be acceptable as the armour is not being used as the earth and so it only needs to be earthed as an exposed conductive part and this could be done at the house end through that gland. The third core in the SWA is providing the connection to the supply earthing system. Earthing the armour at both ends could lead to circulating current in the armour but I would prefer the security of earthing at both ends.

If you were planning on having the studio as a TT system then you would need to also divorce the third core from the supply earth and provide a local earth, via rod or mat at the studio. However there is no point in doing this for a TNS supply as the risk of a disconnected neutral and earth without disconnection of the line is remote.

 

[Taylortwocities]

Sorry, I do not understand the logic of this question. Even if you substitue the words "main breaker" for "main switch".

Q4) What size main breaker at the studio CCU? Was thinking 40A?

Main switches do not have any over-current characteristics. The rating is just how much current the contacts of the switch can handle.
If the OPD at the house end is 50amp then the switch should be rated at >50amp. I would just use a 100A DP switch, they are cheap enough.


PS Have you notified the local authority, before you started work?

 

[haptism]

Just to add It might be acceptable to have a thermoplastic CU installed in the outhouse as it may be part P exempt (depending on distance away from dwelling). Timber structures may cause condensation that could degrade a metal CU over time.

 

[richy3333]

Theres loads in the original Question I dont get.

Why not split the tails and use a DB to supply this building?

Why did you feel the need to RCD protect the SWA supply cable?

Why do you need a Wiska box if the CU is metal clad?

Why are you asking if it's OK to swap out a 40A for a 50A OCPD. What do your calcs show?

As others have said, you are confusing the rating of an MCB with a switch?

With all your questions are you ready for an assessment?

 

[rhitey]

Well I know I'd prefer the RCD protection in the outbuilding
Putting just the one at the supply end isn't really "minimising inconvenience" should it trip
Especially if you've got to traipse back in the house to reset it
Plus now that we're picking up RCBO's (Lewden) for £12 a pop thats pretty much all we fit

 

[robmiller69]

Why not just split the tails, install a single pole c/unit.
And install an rcd board at the outbuilding, all refs met and no additional load on the existing rcd.