How to supply a 20A Distribution cct?

I haven't done any calcs yet until i meet cust. on Friday to determine exact cable route and anticipated load. From the couple of times I've spoken to him over the phone, he's one of those 'do you really need to do that' and 'keep it cheap' and 'we'll have practically nothing in there'. Huh, yeh right, been there, done that, got called back when four times the stated load is turned on.


System  is TN-S, and distr cct doesn't need rcd protection but will be provided in the shed unit. My original thought was to use 25m of T&E from the CU, in trunking indoors (subject to how it works out for shock protection with the reduced cpc csa, so may rethink that bit to an equal size), then when outside use e.g. Hituff, which will all be clipped direct. No water pipes involved - at present.


I was going to use a modular unit with main sw and fuse carrier with 20A fuse for better selectivity with downstream cb's, probably 16A and 6A, although i don't have graphs to confirm that by analysis. I called the trade assoc tech dept because i know that the preferred way in a dom setting would be to use a spare way in the CU (GN7, 2018) but wasn't sure if that way could be used in a commercial setting, if the client insisted on it to keep the price down, but then got the conflicting views from them amongst subsequent conversations.


Hager were referring to the stated fused switch disconnecter, not just an isolator, because he did not think their DIN rail mounted main sw and 20A fuse carrier (even though rated at 16kA and being on a single phase supply) was appropriate (it was perhaps the commercial application that influenced him).


I'm being influenced at present by the perhaps misguided assumption that the 20A fuse won't pop before the 16A cb trips. If anyone can state that the fuse could well go at the same time as the cb in a high or low impedance Earth or short cct fault condition, i may as well go with the cb in the CU option, as there's a spare 20A cb already there.


Thanks


F

I haven't done any calcs yet until i meet cust. on Friday to determine exact cable route and anticipated load. From the couple of times I've spoken to him over the phone, he's one of those 'do you really need to do that' and 'keep it cheap' and 'we'll have practically nothing in there'. Huh, yeh right, been there, done that, got called back when four times the stated load is turned on.


System  is TN-S, and distr cct doesn't need rcd protection but will be provided in the shed unit. My original thought was to use 25m of T&E from the CU, in trunking indoors (subject to how it works out for shock protection with the reduced cpc csa, so may rethink that bit to an equal size), then when outside use e.g. Hituff, which will all be clipped direct. No water pipes involved - at present.


I was going to use a modular unit with main sw and fuse carrier with 20A fuse for better selectivity with downstream cb's, probably 16A and 6A, although i don't have graphs to confirm that by analysis. I called the trade assoc tech dept because i know that the preferred way in a dom setting would be to use a spare way in the CU (GN7, 2018) but wasn't sure if that way could be used in a commercial setting, if the client insisted on it to keep the price down, but then got the conflicting views from them amongst subsequent conversations.


Hager were referring to the stated fused switch disconnecter, not just an isolator, because he did not think their DIN rail mounted main sw and 20A fuse carrier (even though rated at 16kA and being on a single phase supply) was appropriate (it was perhaps the commercial application that influenced him).


I'm being influenced at present by the perhaps misguided assumption that the 20A fuse won't pop before the 16A cb trips. If anyone can state that the fuse could well go at the same time as the cb in a high or low impedance Earth or short cct fault condition, i may as well go with the cb in the CU option, as there's a spare 20A cb already there.


Thanks


F