Multiple cables in place of one

[DNS1]

Under what (if any!) circumstances is it permitted to run two separate cables to make up the conductor surface area of a larger cable (which, in this case is impossible to install...

Need to change 2 6mm radials for a single 10mm, but it's going to be near enough impossible to draw through a new cable due to lack of access to the route....

Are there any loop holes in the regs which will allow me to use the existing cables?

Excuse what's probably a really stupid question, I mainly do temp installations these days... Festival sites etc, so never have to overthink access and making do with what's already there!

 

[telectrix]

what's the cable/s feeding? it's common to use 2 cables in parallel for distribution circuits. If they are final circuits, could yo make them into a 6mm rfc?

 

[SJD]

I like the idea of a ring final circuit in sizes other than 2.5mm2 !

 

[Wilko]

Hi - To make use of 2 conductors for 2x the current carrying capacity it’s important they both be the same length so the current is equally shared. In your case it may just be easier to run your new cable as 10mm and be done, just saying.

 

[DNS1]

Cheers for the help everyone.

In this case, it's removing two 8.5kW electric showers (in adjacent bathrooms, being fed off the same unit, via the same route) and replacing with a single high powered unit in one of the bathrooms.

It's in T+E and both runs will be almost exactly the same length.

Considering the hassle of replacing the cables, if needs be, I'd be happy to put a small CU in the cupboard next door, use the existing cables as a distribution circuit and run a final in 10mm from that to the shower itself.

The whole RFC idea isn't something I'd considered earlier... I've never done one in anything other than 2.5mm!

 

[SJD]

If your calculations (accounting for installation method etc) showed that two identical cables of 5mm2 would be sufficient (for example), then if you had two cables of 6mm2 in parallel, they would be slightly overrated.

If the cables are not quite exactly the same length, the current would be shared inversely proportional to their resistances. It should be possible to work out what imbalance in lengths was permissible before one of the cables started to be overloaded due to taking too much of the current.

I'm not offering to do the calculations, but my guess is the odd few percent difference in lengths would be perfectly OK.

 

[telectrix]

Cheers for the help everyone.

In this case, it's removing two 8.5kW electric showers (in adjacent bathrooms, being fed off the same unit, via the same route) and replacing with a single high powered unit in one of the bathrooms.

It's in T+E and both runs will be almost exactly the same length.

Considering the hassle of replacing the cables, if needs be, I'd be happy to put a small CU in the cupboard next door, use the existing cables as a distribution circuit and run a final in 10mm from that to the shower itself.

The whole RFC idea isn't something I'd considered earlier... I've never done one in anything other than 2.5mm!

what's the rating of the new shower? and the reference method that the 6mm cables are installed

 

[Lucien Nunes]

I'd be happy to put a small CU in the cupboard next door, use the existing cables as a distribution circuit and run a final in 10mm from that to the shower itself

You don't need a CU, just a junction box. Preferably locate it so that the lengths of the 2x 6mm² runs are equal, but don't bust a gut over the last inch or two!

 

[Andy78]

Look up parallel supplies in the regs. As far as I know the rules are same CSA cable, sample cable type, same length, same route.

 

[davesparks]

Technically it can be done, but the question is should it be done?

With parallel conductors there is always a risk that a fault on one conductor will leave the other conductor(s) overloaded. With two T&E in a house which may, or may not follow the exact same route, there is a higher chance of damage occurring to one cable than there is with cables installed for the purpose of being parallel conductors.

I personally wouldn't be inclined to do it as it is unconventional, and unconventional in a domestic installation often leads to trouble down the line.

 

[static zap]

Technically a shower on a ring ,just feels wrong .
Frugal.
(long term reliance on it's perfection -makes it dangerous)

 

[telectrix]

if it saves a ball-ache installing a 10.0mm cable, then, although unconventional, I can see no problems as long as installed and terminated correctly, esp. to avoid overload on 1 leg, I can't see why not. notmuch different than a RFC for sockets.

 

[davesparks]

Technically a shower on a ring ,just feels wrong .
Frugal.
(long term reliance on it's perfection -makes it dangerous)

if it saves a ball-ache installing a 10.0mm cable, then, although unconventional, I can see no problems as long as installed and terminated correctly, esp. to avoid overload on 1 leg, I can't see why not. notmuch different than a RFC for sockets.

It wouldn't be a ring, it would be a radial circuit made up of two parallel cables.

 

[static zap]

...It wouldn't be a ring...

It still needs testing like one ( with access to measure loop !)
(or current clamp individuals to check balance ?)

{ "individuals" is there a proper term ..Load circuit conductors, L-N leg ends !)

 

[davesparks]

It still needs testing like one ( with access to measure loop !)
(or current clamp individuals to check balance ?)

No it doesn't, you can't test it like a ring with only one point in it as far as I can see.
I've never tested parallel cables like I would a ring, and can't see how you could?

 

[static zap]

, you can't test it like a ring

Can we agree on it needs more testing than a single cable ?
(& that must be repeated through-out its useful life )

 

[Mct.tr]

Just reread the initial post.
Other than checking the book, my worry would be the next person who makes it unsafe as they cant work out what's done.