Just sorting out the calculations for the rewire now that the Part P
route is agreed with building control, and have come across a
problem(?) with specifiying the CU. Basically given that I'm not
supposed to allow for diversity at the CU I can't wire the house on a
single consumer unit. Any electrical installation experts advice on
reasonable cost solution very welcome. The reality is that I'm never
going to exceed the 100A supply as no electric showers and immersion
heater is emergency only if the boiler (in fact both boilers) breaks
down (pretty unlikely) and no electric heating apart from comfort
undertile heating in the cellar.

I want to wire the house in 4 'zones' with the following circuits:

Zone 1 = 2nd floor - 1x socket ring (32A) + 1x lighting (6A)
Zone 1 = 38A total

Zone 2 = 1st floor - 1xsocket ring (32A) + 1x lighting (6A) + central
heating control (6A) + immersion (13A)
Zone 2 = 57A total

Zone 3 = Ground floor RHS - 1x socket ring (32A) + 1x lighting (6A)
Zone 3 = 38A total

Zone 4 = Ground floor LHS - 1x socket ring (32A) + 1x socket radial
(20A) + lighting (6A) + range cooker (32A) + refridgeration circuit
(6A)
Zone 4 = 96A total

Zone 5 = Basement - 1x socket ring (32A) + 1x lighting (6A) + kiln
(16A) + tile heater (13A) + smoke alarms (6A)
Zone 5 = 71A total

Nominal Installation Total = 300A

Clearly this is ridiculous given that the total number of circuits is
18 the realistic diversity is very high. The only way I can see to get
the nominal total down is by combining circuits but this still won't
get me all the way as the house is 270sqm so I need at least 3 ring
circuits.

My original plan was to use an MK sentry CU with 21 ways (19 usable)
and use separate RCBOs for the socket circuits to avoid the risk of
nusiance trips from earth leakage on electronic kit (PCs etc.). This
would have the required number of ways and is in reality adequate for
my needs.

To meet the supposed design requirements I would need to use 3 CUs fed
from a Henley block which I can do but this seems like overkill. The
cost difference isn't huge it's just the hassle factor and having to
run meter tails over longer distances to the 3 dis boards.

Any advice on what is reasonable/acceptable in practice.

Sorry the post is so long but it's a big installation!

Fash

Zone 5

"Fash" wrote in message
oups.com...
Just sorting out the calculations for the rewire now that the Part P
route is agreed with building control, and have come across a
problem(?) with specifiying the CU. Basically given that I'm not
supposed to allow for diversity at the CU I can't wire the house on a
single consumer unit. Any electrical installation experts advice on
reasonable cost solution very welcome. The reality is that I'm never
going to exceed the 100A supply as no electric showers and immersion
heater is emergency only if the boiler (in fact both boilers) breaks
down (pretty unlikely) and no electric heating apart from comfort
undertile heating in the cellar.

I want to wire the house in 4 'zones' with the following circuits:

Zone 1 = 2nd floor - 1x socket ring (32A) + 1x lighting (6A)
Zone 1 = 38A total

Zone 2 = 1st floor - 1xsocket ring (32A) + 1x lighting (6A) + central
heating control (6A) + immersion (13A)
Zone 2 = 57A total

Zone 3 = Ground floor RHS - 1x socket ring (32A) + 1x lighting (6A)
Zone 3 = 38A total

Zone 4 = Ground floor LHS - 1x socket ring (32A) + 1x socket radial
(20A) + lighting (6A) + range cooker (32A) + refridgeration circuit
(6A)
Zone 4 = 96A total

Zone 5 = Basement - 1x socket ring (32A) + 1x lighting (6A) + kiln
(16A) + tile heater (13A) + smoke alarms (6A)
Zone 5 = 71A total

Nominal Installation Total = 300A

Clearly this is ridiculous given that the total number of circuits is
18 the realistic diversity is very high. The only way I can see to get
the nominal total down is by combining circuits but this still won't
get me all the way as the house is 270sqm so I need at least 3 ring
circuits.

My original plan was to use an MK sentry CU with 21 ways (19 usable)
and use separate RCBOs for the socket circuits to avoid the risk of
nusiance trips from earth leakage on electronic kit (PCs etc.). This
would have the required number of ways and is in reality adequate for
my needs.

To meet the supposed design requirements I would need to use 3 CUs fed
from a Henley block which I can do but this seems like overkill. The
cost difference isn't huge it's just the hassle factor and having to
run meter tails over longer distances to the 3 dis boards.

Any advice on what is reasonable/acceptable in practice.

Sorry the post is so long but it's a big installation!

Fash

Zone 5

Takeing a look at The Electrcians guide to wireing regs by John Whitfield

You have 5 rings 5 lighting ciruits one radial 20A one cooker one killn one
tile heater and one imersions

so rings are you realy going to have 12 fan heaters pluged at the same time
??

The book suggests 85A for power
16A for the cooker
the water heater kiln and tile heater can not have diversity applied too
them so 42A what is the wattage of these appliances???
19.8 for lighting (is this a light house?)
1A for fire and smoke alarms
the Fridge must not be on a 6A breaker, it will trip due to the starting
current, run the circuit in 2.5mm and fit a standard 13 socket and 16A
breaker ( so you can plug the kettle in when the RCBO has taken the kitchen
out)

SO

Suggested total is 163.8A however this is a little excessive you are
unliklly (very!) to have a main fuse that is over 100A

I would suggest that you take a look at what you are going to use and say
put the cooker kiln tile heater and imersion in one 80A CU and the other in
the rest and fit a 100A fuse switch to keep it legal
http://www.electrium.co.uk/download_...Company=8&Id=3 page 26
part 110M

When you starts blowing the electrcity companies fuse then think of
upgradeing, this will cost £££££££££


total

From a quick scan of the IEE On Site Guide:

p.86/87

Allowance for diversity,
1. Lighting - 66% of total current demand.
9. Standatad arrangements of final circuits - 100% of largest circuit +
40% of every other.

Thanks for the advice, so you reckon it is worth going with 2 CUs
rather than 1? Would you just put them next to each other? If I put
them next to each other then this gets round the problem of running
long tails and needing switch-fuse units next to the meter for the
remote CU, plus then the fact that the CU loads are not split
geographically doesn't matter.

As I said before the reality is that the diversity on the socket
circuits is much greater than the rule of thumb in the regs and since
this is guidance it's not really an issue.

Since I haven't blown the company fuse yet and in the current
installation I do have an electric shower I'm not particularly worried
about this. Certainly wouldn't want to get a bigger supply in since it
took me 18 months to negotiate to get rid of my 1920's overhead TT
service and have it replaced with an underground TN-C-S!

Take your point about the fridge, I was always going to run as a 2.5mm
radial but wasn't sure on the breaker size since they could all be hard
wired in which case it wouldn't be a socket circuit at all. Some other
comments/questions below.


Takeing a look at The Electrcians guide to wireing regs by John Whitfield

You have 5 rings 5 lighting ciruits one radial 20A one cooker one killn one
tile heater and one imersions

so rings are you realy going to have 12 fan heaters pluged at the same time
??

The book suggests 85A for power
16A for the cooker
the water heater kiln and tile heater can not have diversity applied too
them so 42A what is the wattage of these appliances???
Kiln is 3.2kW so 14A and recommendation is for 16A supply. Clearly not
diversified but pretty low usage.
Tile heater depends on whether it's just comfort heating or whether I
spec it to provide full heating given that I will need to apply some
heat all year round when the rest of the house won't be heated. The
manufacturers suggest it can be run off a fused spur (provided it's
13A) in which case it can be diversified but I'm not keen on this.
19.8 for lighting (is this a light house?)
No, the number of circuits is geographical rather than for load, but
there are a lot of rooms, so if you apply the recommendation of 100W
minimum per fixture it clocks up. Reality is that most bulbs are low
energy CFLs but I don't want to put in 4 pin fittings as pain in the
neck.
1A for fire and smoke alarms
Haven't checked this yet but Fire Brigade recommended putting in 8
smoke alarms (interlinked)! Sounds excessive but is because the
property was originally 2 houses (until 1862) so ceiling heights don't
match across floors leaving dead areas that require additional alarm
points.
the Fridge must not be on a 6A breaker, it will trip due to the starting
current, run the circuit in 2.5mm and fit a standard 13 socket and 16A
breaker ( so you can plug the kettle in when the RCBO has taken the kitchen
out)
Fair enough as discussed above.

SO

Suggested total is 163.8A however this is a little excessive you are
unliklly (very!) to have a main fuse that is over 100A

Main fuse is 100A and all nice and brand spanking new, including tiny
digital meter with customer accessible tails (to CU not main fuse
obviously). So I can turn it all off back to the meter without needing
my friendly electicity board to remove their fuse. I do have a spare
main fuse for the first time I blow it anyway!
I would suggest that you take a look at what you are going to use and say
put the cooker kiln tile heater and imersion in one 80A CU and the other in
the rest and fit a 100A fuse switch to keep it legal
If I put them next to each other and run to each from the Henley block
(about 500mm away) can I ignore the switchfuse?
http://www.electrium.co.uk/download_...Company=8&Id=3 page 26
part 110M

When you starts blowing the electrcity companies fuse then think of
upgradeing, this will cost £££££££££

Never going to happen.

total

Fash wrote:
Just sorting out the calculations for the rewire

/snip/

The reality is that I'm never
going to exceed the 100A supply

/snip again/


I want to wire the house in 4 'zones' with the following circuits:

Zone 1 = 2nd floor - 1x socket ring (32A) + 1x lighting (6A)
/snip/

Nominal Installation Total = 300A

Clearly this is ridiculous given that the total number of circuits is
18 the realistic diversity is very high. The only way I can see to get
the nominal total down is by combining circuits but this still won't
get me all the way as the house is 270sqm so I need at least 3 ring
circuits.

The underlying problem here is that a UK ring circuit depends on
statistical chance [aka 'diversity'] for security against overload.

Each socket has an uncertainty as to its load - it may be over or under
norm. String the sockets together in a ring (ie in series) & the
uncertainties (over or under) tend to cancel out, keeping the circuit
maximum load within bounds.

Divide the single ring into 2 separate ones however and loading
uncertainty no longer cancels out in quite the same way. Hence a large
number of rings, being in parallel, tends to escalate the nominal total
load even if the same applicances are going to be used either way.
Which is as you have found.

One way out of the dilemma is to consider statistical loading from a
higher viewpoint - ie that of the whole house rather than individual
rings. The basis of this is already in the diversity rules (100% of
1st ring + then 40% of subsequent circuits). You need to justify
reducing the 40% and perhaps the 100%. IIRC there is a clause
somewhere (maybe in the On-Site guide?) allowing a suitably qualified
elec engineer to make that sort of allowance in design calculations.

IIRC the topic is covered partly in P Cook's book " Commentary on BS
7671 (IEE Wiring Regulations)" (see my other post).

In the case of high power fittings such as showers, washing machines,
and so on, it may be that you can argue they are only used for short
time spans intermittently over 24 hours & substantial diversity
reductions can be made. Cook's book explores something of diversity
from this angle. Also try Googling this group on the subject of cooker
wiring and fusing. IIRC there's been a few queries on this & the
related subject of diversity needed.

I struggled in this area, so would be most interested to hear how you
resolve the issues: please do post the details of your solution.

HTH

ironer wrote:
Fash wrote:
Just sorting out the calculations for the rewire

/snip/

The reality is that I'm never
going to exceed the 100A supply

/snip again/


I want to wire the house in 4 'zones' with the following circuits:

Zone 1 = 2nd floor - 1x socket ring (32A) + 1x lighting (6A)
/snip/

Nominal Installation Total = 300A

Clearly this is ridiculous given that the total number of circuits is
18 the realistic diversity is very high. The only way I can see to get
the nominal total down is by combining circuits but this still won't
get me all the way as the house is 270sqm so I need at least 3 ring
circuits.

The underlying problem here is that a UK ring circuit depends on
statistical chance [aka 'diversity'] for security against overload.

Each socket has an uncertainty as to its load - it may be over or under
norm. String the sockets together in a ring (ie in series) & the
uncertainties (over or under) tend to cancel out, keeping the circuit
maximum load within bounds.

Divide the single ring into 2 separate ones however and loading
uncertainty no longer cancels out in quite the same way. Hence a large
number of rings, being in parallel, tends to escalate the nominal total
load even if the same applicances are going to be used either way.
Which is as you have found.

One way out of the dilemma is to consider statistical loading from a
higher viewpoint - ie that of the whole house rather than individual
rings. The basis of this is already in the diversity rules (100% of
1st ring + then 40% of subsequent circuits). You need to justify
reducing the 40% and perhaps the 100%. IIRC there is a clause
somewhere (maybe in the On-Site guide?) allowing a suitably qualified
elec engineer to make that sort of allowance in design calculations.

IIRC the topic is covered partly in P Cook's book " Commentary on BS
7671 (IEE Wiring Regulations)" (see my other post).

In the case of high power fittings such as showers, washing machines,
and so on, it may be that you can argue they are only used for short
time spans intermittently over 24 hours & substantial diversity
reductions can be made. Cook's book explores something of diversity
from this angle. Also try Googling this group on the subject of cooker
wiring and fusing. IIRC there's been a few queries on this & the
related subject of diversity needed.

I struggled in this area, so would be most interested to hear how you
resolve the issues: please do post the details of your solution.

HTH

I've spoken to a number of CU manufacturers and advice is pretty
similar from all. Since the supply fuse is 100A and a 21 way/18way CU
is also 100A there's no point having 2 since it just moves the
bottleneck. Also clear that it's not more dangerous to have more rings
given that it has no real effect on total load in the house. The way
forward is to justify increasing the diversity based on the fact that
the installation loads are the same as if I had only 2 rings. Since the
diversity is advised not absolute provided it is justified it is OK. In
my case the absolute minimum rings I could have is 4 based on floor
area (unless I started mixing floors which would get it down to 3).
Advice is that it's better to be correct on the ring circuits and worry
less about the diversity issue at the CU. So it's back to a 21 module
MK Sentry. They all validated my decision to use single module RCBOs
rather than a split load board as with 5 rings I would only need
6mA/ring to get a nuisance trip on the RCD. THis isn't a problem now,
but once the kids all get playstations and PCs and other things with
switch mode supplies the chances of tripping a single RCD over all the
circuits is quite high.

Fash

Fash wrote:




I've spoken to a number of CU manufacturers and advice is pretty
similar from all. Since the supply fuse is 100A and a 21 way/18way CU
is also 100A there's no point having 2 since it just moves the
bottleneck.

TFT info: illuminating.

Also clear that it's not more dangerous to have more rings
given that it has no real effect on total load in the house.

Not sure I'd say it was more dangerous as such. Provided suitable MCBs
are in place each ring will current limit safely. But what is
happening is that as the no of parallel rings increase, the chance of
one or more ring becoming overloaded statistically increases. Which
means more chance of an overload trip. String the sockets out in
series and the individual risks tend to cancel out.

However despite the statistics I agree that multiple rings are
preferable. This is especially so if the effect is to limit the
average current in each ring. That does 2 things -it keeps I^2R losses
down & reduces voiltage drop so you get to use more of the KW-hrs you
pay for. Downside is that it may increase the length of cable used
which will have the opposite effect.

When I did my rewire I kept a check (via a spreadsheet) on in-circuit
I^2R losses (and annual cost) as well as voltage drop. In some cases
it can pay to use a larger cable to cut the loss, OTOH cable larger
than 2.5mmsq is hard to fit in most ring sockets and fittings.

Owain wrote:

Having 2 CUs has the advantage that if max demand is too high for the
100A incomer it is now fairly straightforward (perhaps not cheap) to
bring in a second phase for the basement.

plus it gives you a power source for maintenance on the other CU &
circuits & vice versa.

"Fash" wrote in message
oups.com...

ironer wrote:
Fash wrote:
Just sorting out the calculations for the rewire

/snip/

The reality is that I'm never
going to exceed the 100A supply

/snip again/


I want to wire the house in 4 'zones' with the following circuits:

Zone 1 = 2nd floor - 1x socket ring (32A) + 1x lighting (6A)
/snip/

Nominal Installation Total = 300A

Clearly this is ridiculous given that the total number of circuits is
18 the realistic diversity is very high. The only way I can see to get
the nominal total down is by combining circuits but this still won't
get me all the way as the house is 270sqm so I need at least 3 ring
circuits.

The underlying problem here is that a UK ring circuit depends on
statistical chance [aka 'diversity'] for security against overload.

Each socket has an uncertainty as to its load - it may be over or under
norm. String the sockets together in a ring (ie in series) & the
uncertainties (over or under) tend to cancel out, keeping the circuit
maximum load within bounds.

Divide the single ring into 2 separate ones however and loading
uncertainty no longer cancels out in quite the same way. Hence a large
number of rings, being in parallel, tends to escalate the nominal total
load even if the same applicances are going to be used either way.
Which is as you have found.

One way out of the dilemma is to consider statistical loading from a
higher viewpoint - ie that of the whole house rather than individual
rings. The basis of this is already in the diversity rules (100% of
1st ring + then 40% of subsequent circuits). You need to justify
reducing the 40% and perhaps the 100%. IIRC there is a clause
somewhere (maybe in the On-Site guide?) allowing a suitably qualified
elec engineer to make that sort of allowance in design calculations.

IIRC the topic is covered partly in P Cook's book " Commentary on BS
7671 (IEE Wiring Regulations)" (see my other post).

In the case of high power fittings such as showers, washing machines,
and so on, it may be that you can argue they are only used for short
time spans intermittently over 24 hours & substantial diversity
reductions can be made. Cook's book explores something of diversity
from this angle. Also try Googling this group on the subject of cooker
wiring and fusing. IIRC there's been a few queries on this & the
related subject of diversity needed.

I struggled in this area, so would be most interested to hear how you
resolve the issues: please do post the details of your solution.

HTH

I've spoken to a number of CU manufacturers and advice is pretty
similar from all. Since the supply fuse is 100A and a 21 way/18way CU
is also 100A there's no point having 2 since it just moves the
bottleneck. Also clear that it's not more dangerous to have more rings
given that it has no real effect on total load in the house. The way
forward is to justify increasing the diversity based on the fact that
the installation loads are the same as if I had only 2 rings. Since the
diversity is advised not absolute provided it is justified it is OK. In
my case the absolute minimum rings I could have is 4 based on floor
area (unless I started mixing floors which would get it down to 3).
Advice is that it's better to be correct on the ring circuits and worry
less about the diversity issue at the CU. So it's back to a 21 module
MK Sentry. They all validated my decision to use single module RCBOs
rather than a split load board as with 5 rings I would only need
6mA/ring to get a nuisance trip on the RCD. THis isn't a problem now,
but once the kids all get playstations and PCs and other things with
switch mode supplies the chances of tripping a single RCD over all the
circuits is quite high.

Fash


There is still the small issue of if you stuff all those MCBs into one CU of
how you will provide overload protection to the CU. You should not rely on
the DNO cutout (meter fuse) as it is not your property..

James Salisbury wrote:

There is still the small issue of if you stuff all those MCBs into one CU of
how you will provide overload protection to the CU. You should not rely on
the DNO cutout (meter fuse) as it is not your property..

Two points:

1. For a given number of final circuits and given set of diversity
assumptions the ADMD (after-diversity max. demand) of the installation
will not depend on whether one, two or N consumer units are used. For
= 100 A 1-ph installations the decision to use multiple CUs is
dependent on things like the building layout, wiring convenience, and
circuit lengths. If the ADMD exceeds 100 A you are beyond the rating of
a standard consumer unit (BS EN 60439-3), so will need either to use
more than one CU on a 3-ph or split-phase supply, or to use a "Type B"
3-ph dis-board.)

2. It _is_ accepted that the DNO's fuse can be used to provide both
overload and fault protection to the meter tails, provide that same are
reasonably short (interpreted as 2 m or 3 m by different DNOs).
There are of course millions of installations where the sum of the
fuse/MCB ratings in the CU exceeds the rating of the main fuse, with no
other fusing on the consumer's side of the meter. In all these the
DNO's fuse is providing overload protection.

--
Andy