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Topic Title: Garage Suppy
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Created On: 14 November 2017 09:00 AM
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 14 November 2017 09:00 AM
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I am a retired electrician and its been a few years since i did the 17th edition , am i right in my understanding of the on site guide with regards to a 6MM SWA that feeds my sons garage it has a 32amp mcb at the supply end and it is 22meters long run underground (500mm) with a Zs 0.75 as i read it (but i may be wrong) the table says i can up the supply mcb to 40amp as long as the zs is within limits but i am going to fit a RCBO so would 40amp be ok.He is looking to fit some woodworking machines in garage to start a business if it takes off we will run a 3phase supply in from the house .
 14 November 2017 11:34 AM
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Table 4D4A says 46A for a 2-core SWA (probably the same for two loaded cores of a 3-core too), since you'll probably need overload protection there's an addition 0.9 factor to take into account, which brings it down to 41.4A - so 40A OPD should be fine. (You could probably squeeze a few more amps out of the cable safely too, as the tables are a bit conservative for typical UK soil conditions, but there doesn't seem to be much need for that complication at the moment).

If it's going to be a workshop you might want to think about discrimination - it may not be ideal to loose all the lighting at the same moment that some cutting machine develops a fault and your son is left in the pitch dark as the blades spin down... A single 30mA RCD (RCBO) at the house end is far from ideal from that point of view - indeed feeding MCBs (or RCBOs) from another MCB (or RCBO) upstream doesn't fare much better either (MCBs generally don't discriminate with MCBs at all on faults). Better would be to feed the submain from a HBC fuse. Alternatively an emergency light or two might be adequate.

- Andy.
 14 November 2017 11:55 AM
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Lighting more generally may be worth a bit of thought, depending what is already there. For machine work, EM fittings and either HF ballasts or LEDs as stroboscopic effects with rotating machines under traditional florries can give nasty illusions about how fast things are really moving.
If you can convince yourself the cable is armoured and does not need a fast disconnection time, by dint of being a sub-main, then RCBO or two at the load end may be much more suitable, and allows for an 'always on' supply.

regards Mike
 15 November 2017 01:36 AM
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I would agree on the RCBO's at the load end if the armouring is okay. (This is assuming the workshop situation)

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