6mm^2 T&E in conduit

[Fred]

Thanks; that's worth thinking about. Would 10mm^2 fit into 25mm conduit?

I was interested in RCBOs until I saw they are about £40 each! At that price I think it would be cheaper to buy two RCD'ed CUs and fit them side by side in the garage: one for the external lights and sockets and the other for the freezer so that if the RCD trips on CU1, CU2 supplies power to the freezer uninterrupted. Is this technically and legally possible?

In the future I would like to add a motorised door to the garage. There is no other door into the garage so if the RCD trips I will have to wind the door by hand! Would it be better to put the RCD outside the garage, i.e. under the stairs? Notwithstanding the freezer problem, why can't I have the RCD inside the house, rather than inside the garage? The advantage of that would be that the whole 6mm^2 (or

10mm^2) cable run would be protected from house to garage.

[Fred]

To try and answer my own question: is the downside to locating the RCD in the house that in the event of a trip you will have to check two separate CUs: one in the house and one in the garage? OTOH if I went with plan A and just had a fuse in the house, I suppose I would have to still visit both boxes to see whether the fuse had blown or an MCB tripped.

I suppose that I also need to measure the length of the cabel run to make sure that if the RCD is in the house, all the disconnect times are still within limits?

[John Rumm]

With the RCD in the house, you make it difficult to avoid having the feed to the freezer sharing a RCD with other devices, hence the implications of a trip are worse.

The fuse in the house is also highly unlikely to trip if sized appropriately for the submain.

The only time is it likely to be a factor is if your earth loop impedance is marginal in the garage. In which case you would be better off making the garage a TT install in its own right.

Can't remember if you have posted the type of earthing in your house, or the length of the run to the garage. If you let us know those it should be easy enough to give a worst case estimate.

[Fred]

Thanks. It seems that there are pros and cons to each way.

The advantage of having the RCD in the garage is that you can wire to keep the freezer on if something else trips. However, I wonder about moving the freezer inside when I get round to decorating the kitchen and hbow often do RCDs trip anyway?

The advantages of having the RCD in the house are:

1 the whole cable is RCD protected in case of accident 2 if there is a trip, I would not have to manually wind up the garage door to reset the RCD... of course, that assumes I am not in he garage when it happens. If I were in the garage with the door shut when it tripped, that would be different again!

The fuse would be 30A for 6mm^2T&E but I would connect that to a 2-way CU in the garage with smaller MCBs, only the garage CU would not have a RCD, so the garage circuits would still be fused appropriately.

I can't remember how long the cablet o the garage would be exactly but definitely less than 10 metres. It's just I like the idea of that cable being RCD protected in case someone knocks/drills/drives into the conduit carrying it. How resilient is heavy duty plastic conduit anyway?

Thanks again.

[John Rumm]

On a properly installed setup without any faulty appliances - almost never. But you can't always rely on the no faulty appliances bit.

If you want this (or need it due to poor earth fault loop impedance) then the solution is to include a 100mA trip RCD with time delay at the head end as well as the more sensitive RCD at the destination. That way you have earth fault protection for the cable[1] and personal shock protection in the garage.

[1] Note that if you use SWA with earthed armour, you have this anyway in most cases with TN head ends.

In the garage RCD but not protecting the door motor would seem to be the optimum arrangement.

With 6mm^2 you could fuse at 40A if required. Ought to leave ample power available for anything you are likely to do in the garage.

At 10m, even with a TN-S head end you really don't have a problem. The Live to earth round trip resistance of 6mm^2 is 10.49 mOhm/m so about

0.1 ohms total. Add the worst case assumption for your main earth impedance of 0.8 ohms and you get 0.9 total. Lets say you nail right through the end of the cable, you will get a fault current of 230/0.9 = 255A. Referring to fig 3.3 in BS7671 (16th) that shows a time of