How to supply a 20A Distribution cct?

If consumer unit has space for a 20A MCB or RCBO, and it is of a type readily available, do that.

If the submain is SWA, and the CU is a bit full,  I'd consider to  change to something less willful (perhaps 2.5mm T and E) for the last foot or 3 , and have a very obvious metal box, either adaptable for large SWA, or just a double socket box and a blanking plate where the outdoor cable glands in and some junction so the cable becomes something easier to work with. It can always be labelled 'supply to outbuilding, isolate at breaker no .4 ' or whatever on the lid.

IF the CU is full, then a small new one plus henley blocks may be worth considering. Or henley blocks and a switch fuse with a low value fuse in.

All the stuff about rotary handles seems a bit overkill, was this someone trying to sell you stuff ?  Just needs to be able to be turned off if there is a problem.

And the place for a thing like a main switch, is in a mini CU at the load end in the new building, as if the fan heater catches light, that is where they need to turn it off from, not running back to the main building, and also if the lights ever need changing, so you can see it is isolated.

If the supply earth is TT then if it is SWA or not, you need an RCD at the source end, otherwise, I'd suggest non RCD protected cable, like SWA, and then have RCDs at the load end, so the new building is self contained. Take care when drilling holes and siting things in the outbuilding, that rain cannot drive in or run along your cables and into the back of your new kit.


at 65m have you checked the volt drop considerations, you may need to use either 4mm or 6mm for the bulk of the length, even though it is only 20A,  depending on what you allow for volt drop in the rest of the system. (and more if you think there will be significant load creep in future)


How is that kettle filled? , do we have a metal water pipe that needs 10mm main bonding, or better, some plastic pipe, that does not.

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

If it's only a 20A supply then you wouldn't need anything bigger than 6mm² to carry that, a 16mm² is properly overkill (that's including volt drop and power requirements etc.)


I'd take it from a 20A RCBO in the existing CU on a bit of 6mm² SWA (unless it's already on an RCD incorporated into the existing CU. I'd make sure it's going onto an RCD/RCBO just to be safer, but an MCB is fine as long as all requirements are met and the board in the shed incorporates additional protection in case anyone decides to use a radio in the garden or a lawnmower etc.)


Reason I'd go for a 6mm² is that it has a capacity of 53A (ref method D) and you'd have a volt drop of only about 4%. You could use a 10m² but I think it's overkill for what you're suggesting will be required.

In my experience, whatever the calcs come up with go the next cable size up. Demand tends to grow like Topsy.

Kettle and lights? Next there'll be heating . . . .

Thank you for your replies.


So has the 20A fuse got a good chance of surviving a fault on the cb's, or as likely to pop as they are trip. I called Lawson fuses who didn't know offhand but said they would investigate but i haven't heard a peep.

Din rail fuse curriers for a 20-amp fuse for domestic consumer units are as rare as hens teeth, because nobody uses fuses in domestic installations anymore.


Andy

Believe Hager do a fuse unit for their consumer units.

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