Wiring

Hello everyone.

I need some advice. I am wiring a shed (more like a room then a shed). I am wiring it in 6mm swa clipEd direct outdoor.  The distance between the C.U is approx. 30m. I am protecting the cable with a 40A , 30ms disconection RCBO in the C.U. At the other end of the cable, the customer wants one of the garage consumer unit (3 way C.U). The problem is that these C.U'so all ready comes with a 63A RCD as the main switch. If I leave it in this configuration , I won't be achieving the discrimination... What are your thoughts on this?

Can I just remove the 63A RCD and use the the breakers to achieve the  ADS?

self employed electrician charging a customer for their work and dont know how an RCD works... they dont have any overload or fault protection, the rating is simply what it can safely handle...

Andy™ said:

self employed electrician charging a customer for their work and dont know how an RCD works... they dont have any overload or fault protection, the rating is simply what it can safely handle...

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Hey Andy.

I am on my last leg of the NVQ ( aprenticeship) mate. (An RCD only disconnects the supply in the event of an earth leakage.) I  need some advice . because the cable is protected by a 40 A MCBO at the origin of the installation

makes less sense than the first post...

if you have suitable overload / fault protection so the submain at the start, then what exactly is the problem?

Change the rcd in the garage CU for a main switch, or just buy one with a main switch instead of an rcd if it fits the application.

My personal gripe with a lot of garage CU's is many put a poxy 40A rated rcd in. Why? the saving from putting a decent rated one is peanuts.

ProDave said:

Change the rcd in the garage CU for a main switch, or just buy one with a main switch instead of an rcd if it fits the application.

My personal gripe with a lot of garage CU's is many put a poxy 40A rated rcd in. Why? the saving from putting a decent rated one is peanuts.

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not an issue here, he has a 40a device protecting the cable the the CU has a 63a RCD...

Sorry mate, been a long day......hehe hehe

I was worring for nothing.... I just answer the question myself....in case of a fault/ overload the MCB will turn the circuit off....

Andy™ said:

not an issue here, he has a 40a device protecting the cable the the CU has a 63a RCD...

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I know. But I needed to use one to create a submain out of an existing 50A circuit, and found half the ones I looked at only had 40A rated rcd's.

Andy™ said:

not an issue here, he has a 40a device protecting the cable the the CU has a 63a RCD...

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My first job on my own....I like to make sure I do everything by the book, don' like to bodge things and hope for the best later. 

fair enough

Top Tip - DON'T let customer dictate to you about parts ....YOU are the qualified person..

What's the  CCC and VD on a 6mm swa over 30+m on a 40a ocpd ,? 

steptoe said:

What's the  CCC and VD on a 6mm swa over 30+m on a 40a ocpd ,? 

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top of my head at 30 mtrs you're looking around 30 amps with a volt drop of almost 3%, and I mean almost, personally I wouldn't be putting a twin and earth outside unprotected, for one thing it's not UV stable and for another where's the mechanical protection. Totally unsuitable in my opinion, but each to their own.

phil d said:

top of my head at 30 mtrs you're looking around 30 amps with a volt drop of almost 3%, and I mean almost, personally I wouldn't be putting a twin and earth outside unprotected, for one thing it's not UV stable and for another where's the mechanical protection. Totally unsuitable in my opinion, but each to their own.

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The OP in his first post said he was using 6mm SWA...

john..

Still, cable is not suitable for the OCPD anyway. [R1 + R2 too high]

john..

apprentice87 said:

Still, cable is not suitable for the OCPD anyway. [R1 + R2 too high]

john..

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Did I not make it simple enough in post #12,? 

Yes, but i was just saying!!

[Was awaiting a reply to your post from the OP] They are now possibly wondering how they are going to go about uninstalling this cable...

john..

apprentice87 said:

The OP in his first post said he was using 6mm SWA...

john..

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my bad, still it was late and I'd had a few pints though. and still well below 40A on a swa,

apprentice87 said:

Yes, but i was just saying!!

[Was awaiting a reply to your post from the OP] They are now possibly wondering how they are going to go about uninstalling this cable...

john..

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Easier taking out than putting in John, all he does is unscrew the screws, not like he has to undrill the holes, lol