I’m a trainee electrician and would really appreciate the benefit of someone’s skill and experience on the following:
Our house is a TN-C-S (100A fuse) with a main DB (100mA RCD).
This DB feeds a sub DB in a garage outbuilding via a 6mm SWA with all 3 conductors used, by a 63A MCB. The armour is also earthed for mechanical protection.
The sub DB (30mA RCD) is also a TT system with 10mm earth cable to electrode. (I’ve tested the electrode and it’s at 83 Ohms so well inside the 200 Ohms limit.
The sub board feeds the following in garage and outside:
Outside
32A hot tub via 6mm SWA
16A power radial
6A lighting
Inside garage:
32A ring
6A lighting
I’ve checked all cable runs for voltage drop and current capacity, all fall within BS7671.
The issues I’m unsure of is:
1.The sub DB is taking the PME from main DB, whilst also benefiting from an earth electrode. AFAIK having 2 x systems of earthing may conflict and lead to main DB RCD tripping, and not the sub board, but I could be wrong?
2.The 6mm SWA has a current capacity of 53A (ref method D), with a 32A hot tub and diversity applied to both power and lighting I’m coming in at 36.92A + 32A = 68.92A. In reality the ring circuit in garage is rarely used above a freezer and occasional drill etc. Certainly nothing with a load that would require a ring final circuit for a new instal.
My initial thoughts were to change the ring final to a radial, with diversity applied will be 13A therefore bringing the total to 60.9A but this is still too much. The next choice would be to upgrade the feed SWA to 10mm giving me 71A.
What’s your options on this? And the best way to make things compliment, both for SWA size and earthing issue?
Thanks in advance.
Our house is a TN-C-S (100A fuse) with a main DB (100mA RCD).
This DB feeds a sub DB in a garage outbuilding via a 6mm SWA with all 3 conductors used, by a 63A MCB. The armour is also earthed for mechanical protection.
The sub DB (30mA RCD) is also a TT system with 10mm earth cable to electrode. (I’ve tested the electrode and it’s at 83 Ohms so well inside the 200 Ohms limit.
The sub board feeds the following in garage and outside:
Outside
32A hot tub via 6mm SWA
16A power radial
6A lighting
Inside garage:
32A ring
6A lighting
I’ve checked all cable runs for voltage drop and current capacity, all fall within BS7671.
The issues I’m unsure of is:
1.The sub DB is taking the PME from main DB, whilst also benefiting from an earth electrode. AFAIK having 2 x systems of earthing may conflict and lead to main DB RCD tripping, and not the sub board, but I could be wrong?
2.The 6mm SWA has a current capacity of 53A (ref method D), with a 32A hot tub and diversity applied to both power and lighting I’m coming in at 36.92A + 32A = 68.92A. In reality the ring circuit in garage is rarely used above a freezer and occasional drill etc. Certainly nothing with a load that would require a ring final circuit for a new instal.
My initial thoughts were to change the ring final to a radial, with diversity applied will be 13A therefore bringing the total to 60.9A but this is still too much. The next choice would be to upgrade the feed SWA to 10mm giving me 71A.
What’s your options on this? And the best way to make things compliment, both for SWA size and earthing issue?
Thanks in advance.
