Ring circuit query - could this be done?

[Bert Coules]

This is something I've wondered about:

Is it practical to wire a ring circuit by taking a single cable from the CU a short distance to a junction box, then starting the ring (in both directions) from there?

Thanks.

[charles]

If you used 4mm cable to run to the split point, it should be OK.

[Bert Coules]

I wondered if that might be the case. It's not something I'm planning to do necessarily, just an approach I've wondered about. Thanks for the reply.

Bert

[John Rumm]

Its not a standard circuit, but in theory is doable. Its similar to taking a submain to a remote point and then wiring circuits from there.

[Bert Coules]

I thought it might be: if you regard the single cable to the junction box as simply an extension to the output terminals on the CU, then the theory seems perfectly sound. Thanks.

Bert

[jgharston]

I would have thought Ok as long as the junction box is inspectable, it's effectively a sub-CU that's fused at the supply end.

And, of course, has terminals big enough for 4+2.5+2.5 cable. That may be your biggest problem.

JGH

[Andrew Gabriel]

That's what I've done here:

singles between the junction box and the breakers for the ring circuits are 6mm2. Note also that the four earth wires are commoned at both ends, so there's more than one earth path (just as there is around a ring). I would use more than one earth connection when moving the ring join away from the CU.

Closed up:

[ARWadsworth]

It's one that I have done. I used the old 10mm shower cable for a loft conversion.

[Norman Billingham]

No idea what the regulations say but as long as the cable to the start of the ring is adequately sized to carry the total current in both arms of the ring I don?t see why not. Two of the rings in our house have earths wired like that (phase and neutral are rings from their respective MCBs but both earth rings are terminated in one terminal block from which a single earth cable runs back to the CU). This setup was recently tested and certified as part P compliant and passed all the earth impedance and disconnect tests.

[Andrew Gabriel]

Here's another point. Rings don't suit high loading near one end. I put two rings in a kitchen which was a long way from the CU. In this case, I ran each end back separately to the CU in a conventional circuit arrangement. This means the load is all near the centre of the ring because there are no wiring accessories connected near the ends of the ring, which is ideal for balanced ring loading.

[dennis]

That's not a generic problem with rings, its the half baked ones where the cable is too small if all/most of the current goes one way which it can if the path is a significantly higher resistance (e.g. length) one way.

Anyway if 4 mm T&E cable is used to the split then the earth may be too small as the earth wire in 4 mm T&E cable isn't very big. It would be much larger if it were 3 core SWA using one of the cores for instance.

[Bert Coules]

Many thanks to everyone who responded. Much appreciated.

Bert