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Hi, as I've said I'm not an electrician but studying C&G in my own time and have worked as a mate.
I've just had a qualified sparky upgrade my CU to a split board 10 way dual RCD. He has run a 4mm2 cable (from a 40A MCB protected by one of the RCDs) to my hut and will be coming back to wire up a sub main for a radial with a couple of double sockets and a light circuit. He says he is fitting a sub board whose main switch will have no RCD (since the circuit is already protected by the RCD form the main board?) I don't understand that. Can someone help? BTW, I couldn't find the DIY section to post this.
 
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rcd sub main
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littlespark

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If any circuit has an rcd at its origin, it doesn’t need another one.
2 rcd’s in-line like that can cause a lot of head scratching if there’s a fault....

4mm to a hut? What’s in there? Sounds like a lot of power for a potting shed
 

Andy78

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If you're studying then performing a cable calculation for the hut feed could be a useful and enlightening experience.
A said, the RCD affords it's protection for all circuits downstream of it, it will detect imbalances in the hut distribution circuit and all the associated final circuits so another RCD in the hut is not necessary.
 

buzzlightyear

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yes you are correct ANDY,but if a fault in the shed or their abouts the RCD will fail and what ever that is on that rcd in house will go off has well.but i did not design it .
 
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  • #10
If any circuit has an rcd at its origin, it doesn’t need another one.
2 rcd’s in-line like that can cause a lot of head scratching if there’s a fault....

4mm to a hut? What’s in there? Sounds like a lot of power for a potting shed
Thanks for the prompt reply. Potting shed......I have great plans :)
Post automatically merged:

Wow, that was quick and thanks for the prompt responses. I agree that the 4mm cable (rated at 37A) is overkill. By my calculations on a load of say 4KW gives approx 18A and since there is little need to allow for derating factors (wooden framed man cave) then 2.5 mm (rated at 27A) would be enough, but if this is wrong please let me know.
 
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davesparks

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Thanks for the prompt reply. Potting shed......I have great plans :)
Post automatically merged:

Wow, that was quick and thanks for the prompt responses. I agree that the 4mm cable (rated at 37A) is overkill. By my calculations on a load of say 4KW gives approx 18A and since there is little need to allow for derating factors (wooden framed man cave) then 2.5 mm (rated at 27A) would be enough, but if this is wrong please let me know.
The problem is that if the cables maximum rating is 37A then the largest standard size ocpd it could be connected to is 32A, it is not suitable for a 40A ocpd unless overload protection can be omitted, which it can't for a submain.

I think you have some terminology confused in your OP, 'submain' is the common term for a distribution circuit. So a submain generally refers to the cable and associated ocpd which forms the circuit feeding a distribution board.
 
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The problem is that if the cables maximum rating is 37A then the largest standard size ocpd it could be connected to is 32A, it is not suitable for a 40A ocpd unless overload protection can be omitted, which it can't for a submain.

I think you have some terminology confused in your OP, 'submain' is the common term for a distribution circuit. So a submain generally refers to the cable and associated ocpd which forms the circuit feeding a distribution board.
Thanks for taking the time to reply. After studying a bit more I can see what you mean. We have Ib = 18A, In = 40A and Iz = 37A. We need to have Ib<=In<=Iz so these numbers fail. Should have used 32A MCB - am I correct?
 

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