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Ring main question?


spider

Question

Hi everyone,

Just a quick question can't seem to find this information any where even in books that ive read if someone could clear it up for me it would be great.

standard 2.5mm twin and earth cable is rated at being able to handle 18.5a max right so why when the single cable is used in a ring main setup its now capable of handling 32a???. Thanx for any advice given.

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15 answers to this question

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Hi everyone,

Just a quick question can't seem to find this information any where even in books that ive read if someone could clear it up for me it would be great.

standard 2.5mm twin and earth cable is rated at being able to handle 18.5a max right so why when the single cable is used in a ring main setup its now capable of handling 32a???. Thanx for any advice given.

27A

The Boys

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Remember a ring has 2 legs so theoretically half the load is down each leg.

My basic understanding is that in certain installation methods 4mm may be required for a ring on a 32A breaker.

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thanx for the quick replies i think i understand now because the hot wire both enters and leaves the 32a MCB its classed as having two points for the load to travel down where as in say a radial circuit there's only one hot wire leaving the MCB.

Whats it like terminating 4mm in a standard double socket is 4mm hard to work with? 2.5mm should be fine but 4mm might give more peace of mind.

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A little difficult at times to bend if its a cold ambient temperature.

AndyGuiness Drink

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4mm

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Whats it like terminating 4mm in a standard double socket is 4mm hard to work with? 2.5mm should be fine but 4mm might give more peace of mind.

4mm cost a lot more also it may be a bit tight getting 2 4mm cables in to a socket, though i have never tried.

the problem with 2.5mm is if a cable come out of a socket, you get 2 radials that can only carry 27A on a 32A mcb, so best to make sure all terminals are tight.

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If I have to do more than one socket of a ring I tend to wire in 4mm and never have had problems getting them in a socket.

Batty

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Current carrying capacity of each leg of the ring has got to be minimum of 20A [after applying any derating factors etc]....if you like regs try 433.1.5

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Learn something new everyday, cheers andyc.

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All that extra insulation in timber framed new builds pretty much means 4mm for the rings now.

With 25mm back boxes mounted almost flush it's a bit tight, so use deeper boxes is my tip.

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Another point with ring mains is that you can spur off them, but only with 1 point without a switch fused spur. If the spur is a double socket the potential load is 26amps, maximum current carrying capacity of 2.5mm cable is 27amps without derating factors. With derating factors the cable could be overloaded.

Is this why some of you are talking about rings wired in 4mm cable?

Never come across a ring main wired in anything bigger than 2.5mm. Heard of the 4mm 32amp radial socket circuits though.

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BS1363 double sockets are only designed for a max of 13A between both outlets, although better makes will take more

a ring main may need to be wired in 4mm

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I have wired rings in 4mm when at Heathrow Airport, BAA deliberately overspec and over-engineer everything.

AndyGuiness Drink

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I'v wired a ring main in 4mm(not my choice), waste of time I think. Much better to have a radial or two, easier to test, never any problems with ring con and overall more capacity You could use 2.5 or 4mm using 16 or 20 amp mcb's. So much easier and less faults when upgraded in the future

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