Hi,

Apologies if this has been posted previously. I am currently working
on the outline plan for rewiring a 3 bed semi and have worked through
the calculations for cable size but have clearly missed something as
the resulting cable sizes are far too large. I understand the standard
ring main cable size would be 2.5mm^2 or 4mm^2 depending on load and
whether run through thermal insulation.

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89) and grouping 3 cables
(0.7) this gives a requirement of 51.36A - using the IEE cable size
tables this requires at least a 6mm^2 cable depending on installation
- this can't be right.

Firstly, should I be factoring in the grouping if the cables are only
being grouped for short run in conduit through joists and walls.
Secondly, I must have made a mistake somewhere of not applying some
other factor for it being a ring main - can anyone advise where I
have gone wrong?

Thanks in advance, Simon

On Nov 12, 1:36 pm, slogical wrote:
Hi,

Apologies if this has been posted previously. I am currently working
on the outline plan for rewiring a 3 bed semi and have worked through
the calculations for cable size but have clearly missed something as
the resulting cable sizes are far too large. I understand the standard
ring main cable size would be 2.5mm^2 or 4mm^2 depending on load and
whether run through thermal insulation.

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89) and grouping 3 cables
(0.7) this gives a requirement of 51.36A - using the IEE cable size
tables this requires at least a 6mm^2 cable depending on installation
- this can't be right.

Firstly, should I be factoring in the grouping if the cables are only
being grouped for short run in conduit through joists and walls.
Secondly, I must have made a mistake somewhere of not applying some
other factor for it being a ring main - can anyone advise where I
have gone wrong?

Thanks in advance, Simon

Not sure where you've gone wrong but don't forget that a ringmain
wired using 2.5mm twin/earth actual provides a total conductor size of
5mm to each socket on the cct.

Andy

slogical wrote:

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89) and grouping 3 cables
(0.7) this gives a requirement of 51.36A - using the IEE cable size
tables this requires at least a 6mm^2 cable depending on installation
- this can't be right.

It's not. The as-installed rating required for a standard ring is 20 A,
not 32 A - see Reg. 433-02-04. This is subject to a check that the
likely load distribution won't cause long-term overload in any part of
the circuit, compared to the actual as-installed rating (Iz) - this is
only likely to be a problem if you have a kitchen or washroom at one end
of a ring.

Applying your factors, the It rating needed is, by coincidence, 32 A -
which would mean 4 mm^2 cable. With a bit more thought and
re-arrangement you could probably get that down to 2.5. The Cg factor
you're using is for bunched cables; can you not use the single layer
figures with T&E? Can you space the cables where they go through the
insulation, so that the the Cg and Ci factors don't apply simultaneously?

Firstly, should I be factoring in the grouping if the cables are only
being grouped for short run in conduit through joists and walls.

How short is short? I wouldn't worry too much about 50 mm through a
joist, but 300 mm through a wall is a different matter.

--
Andy

"Andy Wade" wrote in message
...
slogical wrote:

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89) and grouping 3 cables
(0.7) this gives a requirement of 51.36A - using the IEE cable size
tables this requires at least a 6mm^2 cable depending on installation
- this can't be right.

It's not. The as-installed rating required for a standard ring is 20 A,
not 32 A - see Reg. 433-02-04. This is subject to a check that the likely
load distribution won't cause long-term overload in any part of the
circuit, compared to the actual as-installed rating (Iz) - this is only
likely to be a problem if you have a kitchen or washroom at one end of a
ring.

Applying your factors, the It rating needed is, by coincidence, 32 A -
which would mean 4 mm^2 cable. With a bit more thought and re-arrangement
you could probably get that down to 2.5. The Cg factor you're using is
for bunched cables; can you not use the single layer figures with T&E?
Can you space the cables where they go through the insulation, so that the
the Cg and Ci factors don't apply simultaneously?

Firstly, should I be factoring in the grouping if the cables are only
being grouped for short run in conduit through joists and walls.

How short is short? I wouldn't worry too much about 50 mm through a
joist, but 300 mm through a wall is a different matter.

I fully admit to not being an electrician so obviously I'm already donning
my flameproof jacket )

However, I do know four different electricians (as a teenager I could always
earn extra by labouring for any of them at weekends) and when rewiring a
3-bed semi such as the OP is doing, none of them ever did any calculations
for anything. It was always 1.5mm T&E for lighting (splitting into upstairs
lights and downstairs lights with a 5A fuse/6A MCB for each) and 2.5mm T&E
for ring mains (splitting into upstairs ring, downstairs ring and kitchen
ring with a 30A fuse/32A MCB for each) as long as each ring didn't serve a
floor area of more than 100sq m.

I got the impression that it was a sort of "rule of thumb" if you like that
(in domestic situations like 2 and 3 bed houses) that 2.5mm T&E was used for
ring mains but from what I'm reading here, that no longer applies does it -
if it ever did?

John

In article ,
Owain wrote:
2.5mm is the *minimum* for a 32A ring circuit wired in T&E or conduit
singles, or 1.5mm in MICC ('pyro'). It is not guaranteed to be
compliant, but a smaller cable size may not be used even if it were
compliant.

This is the difference between a professional and an amateur job - an
amateur can take his time and do the job properly.

I'd say if the calculations show 2.5mm is inadequate in the average house
you want to go to two rings for that area anyway.

--
*The statement below is true.

Dave Plowman London SW
To e-mail, change noise into sound.

On Mon, 12 Nov 2007 05:36:26 -0800 someone who may be slogical
wrote this:-

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89)

For what distance? A short length of insulation means that the heat
can escape on either side, a long length means the heat can't
escape.

and grouping 3 cables (0.7)

Grouping factors only apply if the cables are heavily loaded. If
they are not then they don't matter for grouping. Typically the
cables in lighting circuits can be ignored for grouping, though this
depends on individual circumstances.

Both are explained in the usual guides.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Andy,
Could you explian the issue here ?

This is subject to a check that the likely load distribution won't
cause long-term overload in any part of the circuit, compared to the
actual as-installed rating (Iz) - this is only likely to be a problem
if you have a kitchen or washroom at one end of a ring.

thanks
Andy

ac1951 wrote:
Andy,
Could you explian the issue here ?

This is subject to a check that the likely load distribution won't
cause long-term overload in any part of the circuit, compared to the
actual as-installed rating (Iz) - this is only likely to be a problem
if you have a kitchen or washroom at one end of a ring.

A 32A circuit can notionally supply over 7kW of power. However one needs
to ensure that the current loading on any individual cable does not
exceed the cable's rating. In the case of a ring circuit, the current is
carried by two conductors in parallel, the proportion of the current
split between them will be dictated by the ratio of the resistances of
the cable runs. So for example, a 30A load at one end of the ring would
overload the shorter cable leg since it would end up taking the bulk of
the current, whereas three 10A loads positioned at the start, middle and
end would not load any one leg at more than 15A.

So basically with ring circuits, distributed loads spread about the ring
are ok, but big concentrations of load at one end of the other are not
so good.



--
Cheers,

John.

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On Nov 12, 8:07 pm, John Rumm wrote:
ac1951 wrote:
Andy,
Could you explian the issue here ?

This is subject to a check that the likely load distribution won't
cause long-term overload in any part of the circuit, compared to the
actual as-installed rating (Iz) - this is only likely to be a problem
if you have a kitchen or washroom at one end of a ring.

A 32A circuit can notionally supply over 7kW of power. However one needs
to ensure that the current loading on any individual cable does not
exceed the cable's rating. In the case of a ring circuit, the current is
carried by two conductors in parallel, the proportion of the current
split between them will be dictated by the ratio of the resistances of
the cable runs. So for example, a 30A load at one end of the ring would
overload the shorter cable leg since it would end up taking the bulk of
the current, whereas three 10A loads positioned at the start, middle and
end would not load any one leg at more than 15A.

So basically with ring circuits, distributed loads spread about the ring
are ok, but big concentrations of load at one end of the other are not
so good.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Ok, understood thanks for the clarification..

John Rumm wrote:

[...]
So basically with ring circuits, distributed loads spread about the ring
are ok, but big concentrations of load at one end of the other are not
so good.

Thanks for answering that, John. There's also an IEE /Wiring Matters/
article on the subject he
http://www2.theiet.org/Publish/WireR...c ircuits.pdf

--
Andy

On Mon, 12 Nov 2007 23:29:31 +0000 Andy Wade wrote :
Thanks for answering that, John. There's also an IEE /Wiring Matters/
article on the subject he
http://www2.theiet.org/Publish/WireR...c ircuits.pdf

My ring at home probably contravenes - there's about 12m of cable
one side of the kitchen, 50m the other. What's irritating is that
not so long back I was moving cables around and it would have been
easy to split it into two.

Having said this, I think the IEE paper you cited is rather
pessimistic in terms of today's appliances - washing machine and
dishwasher each drawing 2.5kW for a solid half hour, so I think
it more a theoretical problem than real.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk

On Tue, 13 Nov 2007 12:32:34 GMT someone who may be Tony Bryer
wrote this:-

http://www2.theiet.org/Publish/WireR...c ircuits.pdf

Having said this, I think the IEE paper you cited is rather
pessimistic in terms of today's appliances - washing machine and
dishwasher each drawing 2.5kW for a solid half hour, so I think
it more a theoretical problem than real.

It is certainly pessimistic in terms of "old" appliances. My washing
machine is over 15 years old. However, the heating element
(nominally 3kW) is not on for half an hour solid, even for a 95C
wash. It may be on for as much as 10 minutes, perhaps even 15
minutes, but no more.

Of course this is an "old" machine with hot and cold fill. What one
of the "modern" "improved" washing machines, with cold fill only
reduced water consumption and a puny 2.something kW heater, does in
the way of heating I have no idea, but I suspect that the heating
element is on for at least as long as in my "old fashioned" washing
machine.

Dishwashers contain rather less water than washing machines, so the
heater is on for rather less time in one go. However, there may be
several hot parts of the cycle and thus the heater is on for longer
overall.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Tony Bryer wrote:

My ring at home probably contravenes - there's about 12m of cable
one side of the kitchen, 50m the other. What's irritating is that
not so long back I was moving cables around and it would have been
easy to split it into two.

If clipped direct conditions apply for the 12 m run it's probably
compliant. At 32 A full-load the current in the short leg will be
approx. 50/62 * 32 A which is about 26 A. The clipped direct rating for
2.5 T&E is 27 A.

Its when the short leg is run in thermally insulated walls etc. that
this becomes an issue.

Having said this, I think the IEE paper you cited is rather
pessimistic in terms of today's appliances - washing machine and
dishwasher each drawing 2.5kW for a solid half hour, so I think
it more a theoretical problem than real.

And how often, in practice, do you switch them both on from cold at the
same time?

--
Andy

Thanks everyone - very helpful - hopefully building control will be
happy!

Cheers, Simon

In article David Hansen wrote:

It is certainly pessimistic in terms of "old" appliances. My washing
machine is over 15 years old. However, the heating element
(nominally 3kW) is not on for half an hour solid, even for a 95C
wash. It may be on for as much as 10 minutes, perhaps even 15
minutes, but no more.

Of course this is an "old" machine with hot and cold fill. What one
of the "modern" "improved" washing machines, with cold fill only
reduced water consumption and a puny 2.something kW heater, does in
the way of heating I have no idea, but I suspect that the heating
element is on for at least as long as in my "old fashioned" washing
machine.

Our 2 year old cold fill washing machine consumes about 2.25 KWh on a 90 deg
full size wash. I think it's a 3 KW heater so I'd guess the heater's on for
about 35 to 40 minutes.

Dishwashers contain rather less water than washing machines, so the
heater is on for rather less time in one go. However, there may be
several hot parts of the cycle and thus the heater is on for longer
overall.

The quite elderly slimline (9 place setting) dishwasher uses just over 1 KWh
for the heavy duty cycle. I think it's a 2.5 KW heater so that'll be about
25 minutes.

--
Mike Clarke

On Monday, 12 November 2007 13:36:26 UTC, slogical wrote:
Hi,

Apologies if this has been posted previously. I am currently working
on the outline plan for rewiring a 3 bed semi and have worked through
the calculations for cable size but have clearly missed something as
the resulting cable sizes are far too large. I understand the standard
ring main cable size would be 2.5mm^2 or 4mm^2 depending on load and
whether run through thermal insulation.

Having calculated the required MCB rating of 32A, if corrections are
applied for running in thermal insulation (0.89) and grouping 3 cables
(0.7) this gives a requirement of 51.36A - using the IEE cable size
tables this requires at least a 6mm^2 cable depending on installation
- this can't be right.

Firstly, should I be factoring in the grouping if the cables are only
being grouped for short run in conduit through joists and walls.
Secondly, I must have made a mistake somewhere of not applying some
other factor for it being a ring main - can anyone advise where I
have gone wrong?

Thanks in advance, Simon

With respect. You have 3 bed semi and probably gas central heating. Allowing for the diversity factor and that you will no doubt not be using 3kw electric fires in every room (lol), you only need 2.5 cables, except for the cooker the electric shower and possibly the immersion heater. Unless you are going into cannabis production in which case you need to speak to the National Grid plc..

On 29/01/2016 23:24, wrote:
On Monday, 12 November 2007 13:36:26 UTC, slogical wrote:
Hi,

Apologies if this has been posted previously. I am currently
working on the outline plan for rewiring a 3 bed semi and have
worked through the calculations for cable size but have clearly
missed something as the resulting cable sizes are far too large. I
understand the standard ring main cable size would be 2.5mm^2 or
4mm^2 depending on load and whether run through thermal
insulation.

Having calculated the required MCB rating of 32A, if corrections
are applied for running in thermal insulation (0.89) and grouping 3
cables (0.7) this gives a requirement of 51.36A - using the IEE
cable size tables this requires at least a 6mm^2 cable depending on
installation - this can't be right.

Firstly, should I be factoring in the grouping if the cables are
only being grouped for short run in conduit through joists and
walls. Secondly, I must have made a mistake somewhere of not
applying some other factor for it being a ring main - can anyone
advise where I have gone wrong?

Thanks in advance, Simon

With respect. You have 3 bed semi and probably gas central heating.
Allowing for the diversity factor and that you will no doubt not be
using 3kw electric fires in every room (lol), you only need 2.5
cables, except for the cooker the electric shower and possibly the
immersion heater. Unless you are going into cannabis production in
which case you need to speak to the National Grid plc..

I spose he might have done it in the 9 years since he asked...

(and for those that are interested, what he was missing was the cable
need only take 21A for one leg of a ring)



--
Cheers,

John.

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