Garage circuit

[adam10]

Hi,
I need to supply electricity to a garage which will be ~20m from the main house CU. It will need to supply power for a freezer, lighting, a pond pump and a few sockets for a workshop (although it is unlikely that more than 2 power tools could be used at the same time!!). The cable will run through a 40mm duct buried ~600mm underground.
In my last house the garage was supplied by a single 4mm2 armoured cable which was usually fine but the garage CU mcb would trip whenever I used a big 9" grinder (no problem from the main house sockets). So the question is what should I do this time? should I use a bigger cable eg 6mm2 (which might be rather difficult pulling through the duct!) or run a ring circuit from the main house CU? Any advice gratefully received.

 

[ericmark]

There is no point in a ring circuit for the supply. 20 meters of cable on a 25 A supply at 4mm sq will give around volt drop of around 5.5 volt and same length at 32 A with 6mm sq will give around 4.7 volt with ELI of 0.22 or 0.15 ohms plus what ever the supply is so either way no real problem.

Using a B type then for 25A you need 1.84 ohm and for 32A you need 1.4375 ohm and for a type C then 0.92 and 0.71875 ohms depending on the earth loop impedance of incoming supply all could be achieved.

By enquiry or measurement you need to find the figures and work it out just like I have done. You have not given earthing arrangement or readings so we can only guess.

The grinder will take an inrush amps and the Type B, C, and D MCB will allow 5 times, 10 times, and 20 times the rated value as a peak value before tripping the magnetic part of the trip. So a type D MCB at 16A would allow 320A surge and a type B MCB at 32A just 160A and it is this value which will trip the MCB with a grinder not what it draws in use.

The difference between 14.7 mm and 15.9 mm (4mm and 6mm 2 core SWA overall diameter) is so small I would not see any real difference in pulling in the two cables. In fact over a length it may be easier to push the thicker cable then the thinner one.

But it is not a case of just putting a cable in you do need to calculate what you are doing. Or of course get some one to do it for you. With Part P likely the latter is cheapest option anyway.

 

[sparkticus]

Hi,
I need to supply electricity to a garage which will be ~20m from the main house CU. It will need to supply power for a freezer, lighting, a pond pump and a few sockets for a workshop (although it is unlikely that more than 2 power tools could be used at the same time!!). The cable will run through a 40mm duct buried ~600mm underground.
In my last house the garage was supplied by a single 4mm2 armoured cable which was usually fine but the garage CU mcb would trip whenever I used a big 9" grinder (no problem from the main house sockets). So the question is what should I do this time? should I use a bigger cable eg 6mm2 (which might be rather difficult pulling through the duct!) or run a ring circuit from the main house CU? Any advice gratefully received.

As ericmark has suggested there is no point in a ring final circuit.

You would be better off putting in a 3 core 6mm SWA armoured cable with perhaps a 25amp type C MCB for the sockets, a 6amp type B for the lighting and maybe a 6amp type B for the pond pump. But the calculations need to be done and that involves taking impedance measurements of the external supply. The work is notifiable and frankly it would be cheaper to get an electrician in who is registered for self certification.