ring mains - balancing load
 

Hi,

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

As part of some minor building work I'm planning to install a new
ring main for the kitchen. As is often the case dishwasher and
washing machine are next to each other and would be relatively close
to the start of the ring. I would estimate about 4 m in from the cu
with a total length of circuit of around 17m. If the two appliances
were on together this might be around 6.5 kW giving a current of 27A
of which I estimate 76% would travel down the shorter leg. This would
be getting fairly close to the cables capacity. An additional load
near this point could take the cable to above its rating (~24A for
2.5mm2) and not trip the 32A mcb. I would have thought this situation
to be just about acceptable, as at least some of the loads will be
fairly short lived (minutes rather than hours).

Alternatively I could lose some cable under the floorboards to
lengthen the tail or put the wm/dw on a separate radial circuit.

The existing ring main is much much worse - serves the whole house
with a potential load of 40 A (helped by an illegal spur) at the
second socket 7m from cu on a 70m ring circuit! not good.

Thanks

Jim

On Tue, 4 Oct 2005 19:14:39 +0100, "Jim Ingram"
scrawled:

Hi,

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

Alternatively I could lose some cable under the floorboards to
lengthen the tail or put the wm/dw on a separate radial circuit.

Well, 'losig some cable under the floor' isn't the way to balance a
ring.

Easiest thing to do if you're worried is put in 2 16A radials for the
WM and TD, or 1 32A radial for both.
--
Stuart @ SJW Electrical

Please Reply to group

Jim Ingram wrote:

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

The essential requirements are in Reg. 433-02-04 and can be summarised thus:

- min. conductor size is 2.5 mm^2 (with one exception: 1.5 mm^2 is
permitted for 2-core MI cables);

- the as-installed cable rating (Iz), taking into account the
installation method and grouping factors, etc., must not be less than 20
amps;

- under the intended conditions of use, the load current in any part of
the ring should be unlikely to exceed, for long periods, the actual
as-installed value of Iz.

The last few words are significant. Short term loads such as kettles
and toasters (assuming normal domestic use) can be disregarded. The
actual Iz can vary between 20 A (the minimum allowed) and 27 A (clipped
direct, no grouping).

As part of some minor building work I'm planning to install a new
ring main for the kitchen. As is often the case dishwasher and
washing machine are next to each other and would be relatively close
to the start of the ring. I would estimate about 4 m in from the cu
with a total length of circuit of around 17m. If the two appliances
were on together this might be around 6.5 kW giving a current of 27A
of which I estimate 76% would travel down the shorter leg. This would
be getting fairly close to the cables capacity. An additional load
near this point could take the cable to above its rating (~24A for
2.5mm2) and not trip the 32A mcb. I would have thought this situation
to be just about acceptable, as at least some of the loads will be
fairly short lived (minutes rather than hours).

In this example there's probably nothing to worry about, unless you have
other long-term loads. Even dishwashers and washing machines aren't
particularly long term loads: thermostats and diversity will conspire to
reduce the 6.5 kW quite quickly.

Alternatively I could lose some cable under the floorboards to
lengthen the tail or put the wm/dw on a separate radial circuit.

If you are worried, other design options might be to take off the DW &
WM feeds from nearer the mid-point of the ring, running unfused spurs
back to the kitchen (but watch the grouping factors and circuit
impedances), or to wire the whole ring in 4 mm^2.

HTH
--
Andy

In article ,
Andy Wade writes:
If you are worried, other design options might be to take off the DW &
WM feeds from nearer the mid-point of the ring, running unfused spurs
back to the kitchen (but watch the grouping factors and circuit

or spur one of the loads directly from the MCB as it's close by.

--
Andrew Gabriel

Andrew Gabriel wrote:

or spur one of the loads directly from the MCB as it's close by.

Good point.

--
Andy

On Tue, 04 Oct 2005 22:29:21 +0100, Andy Wade
wrote:

Jim Ingram wrote:

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

The essential requirements are in Reg. 433-02-04 and can be summarised thus:

- min. conductor size is 2.5 mm^2 (with one exception: 1.5 mm^2 is
permitted for 2-core MI cables);

- the as-installed cable rating (Iz), taking into account the
installation method and grouping factors, etc., must not be less than 20
amps;

SNIPPED

This is all well and good in theory but what happens when new
occupants move into a house, with various load appliances ?
Are the regs soon going to tell us where we can place our appliances?
Have there been cases of houses being damaged by misplaced appliances
on properly installed ring mains ?
This issue seems on the surface, to be getting out of hand.

Andy

On Tue, 4 Oct 2005 19:14:39 +0100 someone who may be "Jim Ingram"
wrote this:-

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

In most cases, test that the ring actually is a ring.

As is often the case dishwasher and
washing machine are next to each other and would be relatively close
to the start of the ring. I would estimate about 4 m in from the cu
with a total length of circuit of around 17m. If the two appliances
were on together this might be around 6.5 kW giving a current of 27A
of which I estimate 76% would travel down the shorter leg.

Not a problem. These things were thought through some time ago.

I have a vague recollection of a figure of 20% going the "long way"
being the worst case. That is for a standard circuit, which has a
maximum floor area for a number of reasons one of which is to do
with loads.

This would
be getting fairly close to the cables capacity. An additional load
near this point could take the cable to above its rating (~24A for
2.5mm2) and not trip the 32A mcb. I would have thought this situation
to be just about acceptable, as at least some of the loads will be
fairly short lived (minutes rather than hours).

Cable ratings depend on the installation. They are also time
dependent. 2.5mm2 cable will safely carry many hundreds of amps for
a short period, which is what happens in a short circuit. In an
earth fault, in flat twin and earth wiring, the smaller protective
conductor carries the same current as the live. A 32A MCB will take
around 30s to operate at 100A, which the circuit wiring will take if
it is designed, installed and tested correctly. A lot of thought has
gone into the Wiring Regulations over a long time and co-ordination
of protective devices with cable capacity. 100% safety is not
possible, don't use electricity if you want that, but it is a good
compromise with the competing factors.



--
David Hansen, Edinburgh | PGP email preferred-key number F566DA0E
I will always explain revoked keys, unless the UK government
prevents me by using the RIP Act 2000.

"Andy Pandy" wrote in message
...
On Tue, 04 Oct 2005 22:29:21 +0100, Andy Wade
wrote:

Jim Ingram wrote:

Could someone please expain to me what the requirements are for
ensuring a balanced road on a ring main.

The essential requirements are in Reg. 433-02-04 and can be summarised
thus:

- min. conductor size is 2.5 mm^2 (with one exception: 1.5 mm^2 is
permitted for 2-core MI cables);

- the as-installed cable rating (Iz), taking into account the
installation method and grouping factors, etc., must not be less than 20
amps;

SNIPPED

This is all well and good in theory but what happens when new
occupants move into a house, with various load appliances ?
Are the regs soon going to tell us where we can place our appliances?
Have there been cases of houses being damaged by misplaced appliances
on properly installed ring mains ?
This issue seems on the surface, to be getting out of hand.

Andy

I find it surprising, will the IEE and authorities hell bent on reducing
risk - Part P and all that, that a ring main based on 2.5mmT+E is still
allowed. In most environments 2.5mm is not up to passing 32A which is an
every day possibility. Excusing this on the basis of diversity really isn't
good enough unless the MCB rating suits the rating of the cable, ie worst
case scenario.

The very fact that this is not seen as a problem gives me opinion it's quite
acceptable to exceed a cable working temperature of 70 deg C.

Hi,

Thanks to all for the replies. I don't think I need to worry as I
haven't got any other large loads.

Cheers

Jim

Andy Pandy wrote:

This is all well and good in theory but what happens when new
occupants move into a house, with various load appliances ?

Unless they alter the kitchen layout, similar appliances will end up in
similar positions - no problem. If the kitchen is radically altered
then wiring work is likely and the Part P registered electrician will of
course re-check the design :-)

Are the regs soon going to tell us where we can place our appliances?
Have there been cases of houses being damaged by misplaced appliances
on properly installed ring mains ?
This issue seems on the surface, to be getting out of hand.

There are certainly cases where cables have been damaged due to failure
to consider the requirement to avoid the likelihood of long-term small
overloads [433-01-01]. Hence the clarification added to 433-02-04 in
respect of ring circuits in Amendment 1 to BS 7671:2001.

Nothing's getting out of hand, but more emphasis is now placed on
ensuring that the regs are actually complied with, and on making
installers realise that there is a design process (however trivial)
involved in wiring work.

--
Andy

On Wed, 5 Oct 2005 09:13:33 +0100, "Fred" scrawled:

I find it surprising, will the IEE and authorities hell bent on reducing
risk - Part P and all that, that a ring main based on 2.5mmT+E is still
allowed. In most environments 2.5mm is not up to passing 32A which is an
every day possibility.

When? You sure you've not got ring and radial mixed up?
--
Stuart @ SJW Electrical

Please Reply to group

"Lurch" wrote in message
...
On Wed, 5 Oct 2005 09:13:33 +0100, "Fred" scrawled:

I find it surprising, will the IEE and authorities hell bent on reducing
risk - Part P and all that, that a ring main based on 2.5mmT+E is still
allowed. In most environments 2.5mm is not up to passing 32A which is an
every day possibility.

When? You sure you've not got ring and radial mixed up?
--
Stuart @ SJW Electrical

Please Reply to group

From a current perspective, what's the difference. Suppose we have two
sockets close to one end, say nominally 1/10 into the ring. 9/10 of the MCB
breaking current will pass through the shortest path. So yes a radial
circuit can then take 10% less current.

I accept that probability is small, nevertheless it's still there, and in
our present nanny state I'm surprised it's still allowed. Wait until the
17th Edition! It'll be the 18th or 19th before they insist all light
fittings be 12V or to have "finger proof" fittings. After that 115V
balanced around earth.

On Wed, 5 Oct 2005 16:25:55 +0100, "Fred" scrawled:


"Lurch" wrote in message
.. .
On Wed, 5 Oct 2005 09:13:33 +0100, "Fred" scrawled:

I find it surprising, will the IEE and authorities hell bent on reducing
risk - Part P and all that, that a ring main based on 2.5mmT+E is still
allowed. In most environments 2.5mm is not up to passing 32A which is an
every day possibility.

When? You sure you've not got ring and radial mixed up?
--
Stuart @ SJW Electrical

Please Reply to group

From a current perspective, what's the difference. Suppose we have two
sockets close to one end, say nominally 1/10 into the ring. 9/10 of the MCB
breaking current will pass through the shortest path. So yes a radial
circuit can then take 10% less current.

Hmmm, don't think it's quite as simple a calculation as that.
--
Stuart @ SJW Electrical

Please Reply to group

On Wed, 5 Oct 2005 15:25:55 UTC, "Fred" wrote:

From a current perspective, what's the difference. Suppose we have two
sockets close to one end, say nominally 1/10 into the ring. 9/10 of the MCB
breaking current will pass through the shortest path.

Oooh, I dunno....when it gets overloaded enough, its resistance will
increase and it'll even out... :-)

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

On Tue, 04 Oct 2005 22:29:21 +0100, Andy Wade
wrote:

[snip]

If you are worried, other design options might be to take off the DW &
WM feeds from nearer the mid-point of the ring, running unfused spurs
back to the kitchen (but watch the grouping factors and circuit
impedances), or to wire the whole ring in 4 mm^2.

HTH

Would it be possible to run the short length or the ring, as far as
these high loads, in 4mm^2?

Replace 'spam' with 'org' to reply

"Bob Eager" wrote in message
...
On Wed, 5 Oct 2005 15:25:55 UTC, "Fred" wrote:

From a current perspective, what's the difference. Suppose we have two
sockets close to one end, say nominally 1/10 into the ring. 9/10 of the
MCB
breaking current will pass through the shortest path.

Oooh, I dunno....when it gets overloaded enough, its resistance will
increase and it'll even out... :-)

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

The temperature coefficient of the resistance of copper is 0.4%/deg C.
Therefore for a 30 degree increase the resistance will go up by 12%. So
instead of 90% of current flowing through the short leg in the example I
gave, it would be just under 89%.

"Lurch" wrote in message
...
On Wed, 5 Oct 2005 16:25:55 +0100, "Fred" scrawled:


"Lurch" wrote in message
. ..
On Wed, 5 Oct 2005 09:13:33 +0100, "Fred" scrawled:

I find it surprising, will the IEE and authorities hell bent on reducing
risk - Part P and all that, that a ring main based on 2.5mmT+E is still
allowed. In most environments 2.5mm is not up to passing 32A which is
an
every day possibility.

When? You sure you've not got ring and radial mixed up?
--
Stuart @ SJW Electrical

Please Reply to group

From a current perspective, what's the difference. Suppose we have two
sockets close to one end, say nominally 1/10 into the ring. 9/10 of the
MCB
breaking current will pass through the shortest path. So yes a radial
circuit can then take 10% less current.

Hmmm, don't think it's quite as simple a calculation as that.
--

Short leg 10% of resistance of ring. Long leg 90% resistance of ring. The
voltage drop is the same across both legs. Simple ohms law - short leg
carries 9 x current of long leg. Add the two legs together to get (90% +
10%) = 100% Where do you think I'm going wrong?

On Fri, 7 Oct 2005 10:46:23 UTC, "Fred" wrote:

The temperature coefficient of the resistance of copper is 0.4%/deg C.
Therefore for a 30 degree increase the resistance will go up by 12%. So
instead of 90% of current flowing through the short leg in the example I
gave, it would be just under 89%.

(a) I did know that - it was tongue in cheek! (b) I was thinking of a
glowing cable!

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

"Bob Eager" wrote in message
...
On Fri, 7 Oct 2005 10:46:23 UTC, "Fred" wrote:

The temperature coefficient of the resistance of copper is 0.4%/deg C.
Therefore for a 30 degree increase the resistance will go up by 12%. So
instead of 90% of current flowing through the short leg in the example I
gave, it would be just under 89%.

(a) I did know that - it was tongue in cheek! (b) I was thinking of a
glowing cable!

--

Apologies, I don't normally see tongue in cheek until it hits me in the eye!

Fred wrote:
"Bob Eager" wrote in message
...

On Fri, 7 Oct 2005 10:46:23 UTC, "Fred" wrote:


The temperature coefficient of the resistance of copper is 0.4%/deg C.
Therefore for a 30 degree increase the resistance will go up by 12%. So
instead of 90% of current flowing through the short leg in the example I
gave, it would be just under 89%.

(a) I did know that - it was tongue in cheek! (b) I was thinking of a
glowing cable!

--


Apologies, I don't normally see tongue in cheek until it hits me in the eye!


That's not tongue in cheek, that's tongue in eye :)