Changing the main fuse

[SPECIAL LOCATION]

Ok, this is where I'm getting confused...how is it decided what size the Main Fuse should be then? You obviously don't just add up the breakers that are in the installation, 'cos 9 times out of 10, the total would always exceed the MF size

That's why I was allowing for a 50A mcb in the main CU to cover the circuits in the garage CU:

32A - ring main

10A - rear garden lights

6A - front garden lights

6A - garage lights

Doesn't the mcb for the garage (in main CU) have to be rated higher than the highest mcb in the garage CU? <- hope that makes sense to you!!!

Obviously I was WAY off the mark, which I've now realised, but TBH, if I was to do this again (which I hope I will at some point!), I'd still be a bit stuck as to the size of fuses to use.

What additional info is required? Sorry to go on about this, but it's doing my head in and I really need to understand [thoroughly] what I'm doing, otherwise I don't see the point...

You've all been a great help so far - much appreciated.

Nat

Hello NAT!

This I dont think will be a "quicke" sounds like we need a "Longie" here to get a few basics to gel a bit more in your mind?

GARAGE:

What ACTUAL items of equipment are you going to connect in your garage???

why do you pick a 32A ring main?

Are you running around 2.5Kilowatts of lighting on you rear light circuit? (10a x 230v =2300watts!!!)

HOUSE:

How big is the property

e.g. No or rooms approx floor space?

How many people are living there ?

Are there any specific/unusual requirements.?

e.g. I have a customer who does ceramic design work with a small electric Kiln!! which ramp the old max demand up a bit!!!

DIVERSITY:

You said in an earlier post you had calculated demand with "diversity" to be 159amps???

How did you calculate this??

It may help us to explain a bit where you are going wrong.

I will add a bit more latter rather than making a ridiculously long post??

 

[SPECIAL LOCATION]

To try and give an example of the concept of diversity..

look at it from a different angle.. lets forget electrics for a minute..

Consider instead Traffic on the road network!

If you need to know the number of cars per hour travelling down a particular road into a town centre..

numerous variables will be brought into the equation

No of cars.

No of drivers.

Width of road.

Time of day. etc.. etc..

1) you can't just add up the number of cars parked outside the houses in the adjoining roads.

2) you can't add up the number of drivers who live in the houses in the adjoining roads

3) you can't just add up the the number of lanes in the road.

4) you can't just say it will always be constant at all times of the day.

e.g. some houses may have 3 cars but only 2 drivers. 1 driver may go out once in the morning to go to work and back again in the evening, another driver runs a pizza delivery service and is driving about left right & centre!

also there will be peaks and troughs during rush hours etc.. & mid day or late in the evening!

The only accurate way to establish traffic flow is to measure it

{on the side of the road with clip board counting the cars over a long period during the day! / one of those counter wires across the road}

OR..

make a rough statistical guess based on experience in other towns..

Electrics is a bit like this ..

You need to take into account the total load of the number of appliances..

How many people could actually be using them simultaneously..

what times of day usage will increase etc.. etc.. etc..

as I said before a family of 7 will have a higher max demand than a single elderly widow.. even if the property's & the number of socket outlets & ring mains are identical!!

For your own house you could do a rough measurement by sticking a clamp meter around the incoming tails and seeing what actual current is flowing!!

do a sample reading once and hour on a typical weekday & typical weekend.

note the figures and work out an average!!!

for example we have a whole bunch of lights on at the moment (@ 10:15pm) currently TOTAL LOAD is 6.2 amps on the whole house..

although there are 3 x 6a lighting circuits which is way less than the 66% guidance.. there are six of us in the house at the moment.. inc t.v Pcs etc..

If you haven't got a clamp meter you can get a good value one from maplins

 

[avinalarf]

Hello NAT!This I dont think will be a "quicke" sounds like we need a "Longie" here to get a few basics to gel a bit more in your mind?

GARAGE:

What ACTUAL items of equipment are you going to connect in your garage???

why do you pick a 32A ring main?

Are you running around 2.5Kilowatts of lighting on you rear light circuit? (10a x 230v =2300watts!!!)

Hi SL

Nothing out of the ordinary ie: lawnmower, supply for garage door, power tools etc. TBH, I picked a 32A ring because I was informed that I couldn't run 2 radials from the same mcb (wanted to consider this option due to not wanting to run cable around the complete garage, but needed sockets at opposite ends!) I read in the Red Book [433.1.5] that a "ring final circuit is connected to a 30 or 32A opd.

Rear lighting: think this misjudged allowance was pointed out to me in a previous post. I hadn't allowed for LV lighting (need to look at plan again!) There will be around 20 lights in the garden though.

HOUSE:How big is the property

e.g. No or rooms approx floor space?

How many people are living there ?

House is 5bed, 3.5 bath with double garage - approx 230m2 with 4 habitants.

Are there any specific/unusual requirements.?

Nothing unusual apart from very large marine fish tank which KME said would only equate to around 6/7A.

DIVERSITY:You said in an earlier post you had calculated demand with "diversity" to be 159amps???

How did you calculate this??

It may help us to explain a bit where you are going wrong.

OK, here goes - don't fall asleep on me

upstairs lighting 1100/230 = 4.78 x 66% = 3.15A - I was told that I had to allow 100w for each lamp, hence the high initial figure.

downstairs lighting 5.16A (after diversity)

Smoke alarm - 6A

Lighting circuit for extension & 5th Bed - 9.46A - I couldn't add the bed/ensuite to the upstairs circuit due to it only being wired in 1mm. Also, I have subsequently changed my calculation to allow for lamps which are low energy, thus reducing to 6.03A.

Downstairs RM - 32A (OSG says "no diversity allowed for RM with most usage)

Upstairs RM - 40% of 32 = 21.8A

You must be sleeping by now, but if not...

There's:

1 x kitchen RM

1 x radial for oven - 16A

1 x radial for microwave/oven - 16A

1 x radial for induction hob - 40A

Then I added the bits for the garage.

I hope you're getting the gist of how I reached my estimated total demand and where I've gone wrong.

DIVERSITY:I will add a bit more latter rather than making a ridiculously long post??

Sorry it was so long, but there was no way of shortening it embarrassed smiley

 

[SPECIAL LOCATION]

Hi SLNothing out of the ordinary ie: lawnmower, supply for garage door, power tools etc. TBH, I picked a 32A ring because I was informed that I couldn't run 2 radials from the same mcb (wanted to consider this option due to not wanting to run cable around the complete garage, but needed sockets at opposite ends!) I read in the Red Book [433.1.5] that a "ring final circuit is connected to a 30 or 32A opd.

.....

[xxxxxxxx cut a bit xxxxxxxxxxxxxxx]

...

Sorry it was so long, but there was no way of shortening it embarrassed smiley

Hi Nat..

taking a little bit at a time

It may not be considered good practice to run two radials from the same MCB...

BUT!

A radial can be branched wherever you feel appropriate.. e.g. lighting "Radial" circuits.. so...

1) you could take one cable to a j/box then branch up either side of garage!

2) you could wire as if you were doing your ring.. but don't come back to the fuse box! thus a radial!

3) if you were going to do option '1' not a fat lot of difference making branch point at MCB in CU rather than j/box???

If you keep stuff suitably labelled & clearly identified cant see to much of a problem.. IMO!

the MOST IMPORTANT bit is that the correct MCB rating is used to protect the cable size..

from what you have said about loading a 20A.. or even a 16A MCB would probably do the garage sockets!

433.1.5 Is NOT saying that all rings MUST BE 32A...

The topic is talking about overload protection

and IMHO

it is saying that accessories connected via ordinary 13amp plugs (BS1363), see appendix 1, may be connected to a ring circuit rated at 32A provided it is connected via cable with min CSA 2.5mm.

as a side note and no practical use whatsoever..

if you were that way inclined you could wire a 40A ring with 6.0mm cable..

OR a 10A lighting ring wired in 1.0mm it would still be electricaly safe!!!

the cable size MUST be big enough for the fuse rating!

 

[SPECIAL LOCATION]

Smoke alarm - 6A

The smoke alarm is such a negligibly small current..

it could almost be ignored on your max demand calculations.

REMEMBER

the smoke probably have battery back up!

they will operate quite happily of a 9v DC PP3 battery..

with a typical lifespan of at least 12months,

PP3 9v battery will not be pumping out 1.38kilowatts (6a x 230v) of juice!!!

I normally stick my smoke circuit via switched fused spur with neon, with a 3amp cartridge fuse stuck in the spur..

even 3Amps is well over what is needed..

Me thinks milliamp more likely!!! so can be discounted for calculations..

Another quick thought regarding the installation as a whole...

which you must also remember..

if it is all through ONE consumer unit..

The main switch will have a maximum current rating.. normally 100amps!

so even if you could reach a max supply demand of 100amps..

that would be you max before sticking a second board OR a higher rating switch!!!

 

[avinalarf]

am reading with interest, but must go to bed...

Hope you are able to provide more info. for me to read tomorrow SL.

Many thanks and I bid you goodnight!

 

[kme]

Goodnight.

 

[SPECIAL LOCATION]

upstairs lighting 1100/230 = 4.78 x 66% = 3.15A - I was told that I had to allow 100w for each lamp, hence the high initial figure.

downstairs lighting 5.16A (after diversity)

Lighting circuit for extension & 5th Bed - 9.46A - I couldn't add the bed/ensuite to the upstairs circuit due to it only being wired in 1mm. Also, I have subsequently changed my calculation to allow for lamps which are low energy, thus reducing to 6.03A.

Lets have a look at your lighting a bit more..

1 x 10amp, 2 x 6amp

you calc'd 3.15a & 5.16a & 6.03a = 14.34.

This is where the notes in the o-s-g ("intended for guidance only because it is impossible to specify allowance or diversity for every type of installation and such allowance call for specialist knowledge & experience") come in to play a bit more..

I would tend to apply the 100watt per lampholder in calculating CABLE SIZE!

but use actual bulb loads to calc max demand/diversity or the circuit..

There is the rule of thumb of 66% of the total current demand!

which personally I would guess that your 10A circuit could be a 6A, so 66% or three 6amps is 12amps!

but in reality that is probably a tad high! because demand is NOT 66% of the mcb rating.

have a walk around your house on a few random sample evenings & count how many lights are on?

 

[a1]

Goodnight.

 

[SPECIAL LOCATION]

Goodnight.

Phtt!

TRAILER BOY says

goodnight..

to

collage boy!!!!!! :^O :^O:^O

 

[SPECIAL LOCATION]

POST #43

....OK, here goes - don't fall asleep on me...............You must be sleeping by now, but if not.....

POST #46

am reading with interest, but must go to bed...Hope you are able to provide more info. for me to read tomorrow SL.

Many thanks and I bid you goodnight!

phtt! :|

part timers!!!! X(

goodnight!!! ; \ :^O :^O:^O

where's Moonpig, Apache & KME when I need em??

 

[avinalarf]

Lets have a look at your lighting a bit more..1 x 10amp, 2 x 6amp

you calc'd 3.15a & 5.16a & 6.03a = 14.34.

This is where the notes in the o-s-g ("intended for guidance only because it is impossible to specify allowance or diversity for every type of installation and such allowance call for specialist knowledge & experience") come in to play a bit more..

I would tend to apply the 100watt per lampholder in calculating CABLE SIZE!

but use actual bulb loads to calc max demand/diversity or the circuit..

There is the rule of thumb of 66% of the total current demand!

which personally I would guess that your 10A circuit could be a 6A, so 66% or three 6amps is 12amps!

but in reality that is probably a tad high! because demand is NOT 66% of the mcb rating.

have a walk around your house on a few random sample evenings & count how many lights are on?

Definitely concur with you Special. I am vigilant about lights being turned off when not in use, so unless my son's gone mad upstairs, there aren't usually more than 5 lamps on simultaneously in the evenings.

So does this mean that we could probably get away with 3A mcbs [can't get lower than 6A can you???].

Look forward to your response on my other circuits...

 

[a1]

Lol @ Specs.

Was you feeling lonely, Mate? There was loads on here when I logged off.

 

[SPECIAL LOCATION]

House is 5bed, 3.5 bath with double garage - approx 230m2 with 4 habitants.

Nothing unusual apart from very large marine fish tank which KME said would only equate to around 6/7A.

Good morning all!

To continue with the ole diversity and methods of calculating max demand....

Another illustration of why we just don't add up the circuits is because in modern wiring practice we tend to put more circuits in for convenience in the event of faults or maintenance.

so there may be more physical final circuits but no more actual demand.

e.g. For many a long time your traditional house had a little 4-way wylex CU,

30amp cooker

30amp sockets

15amp immersion heat

5amp lights

possibly with an additional single CU mounted next to it with a 40amp or 45amp shower circuit.

(in fact there are still loads of houses like this at the moment)

adding all these values up = 120 - 125amps

but they are often running through a 60amp main incoming fuse!

indeed the main switch rating off the wylex box was probably only 60amp.

If this example property had your "average rewire" done..

as a minimum it may have the following arrangement..

45A shower

32A cooker

32A ring u/s

32A ring d/s

20A garden, socket,shed,greenhouse supply.

16A immersion heat

6A lights u/s

6A light d/s

6A smokes

adding all these up now is a massive 195amps worth of final circuits..

BUT there are still the same people living there & their actual demand per person hasn't changed,

(just the house has had a rewire with more convenient circuit arrangements!).

Looking at things from a per person demand.. say four people.

1] is doing cooking using 5500watts worth of oven.

2] is in the shower using 9000watts worth of hot water.

3] is doing some washing or tumble drying approx 2800watts worth of white goods power.

4] is feeling cold with a 3-bar electric fire one 3000watts worth of heat.

all of them have 150watts of lighting, per person, on total 600watts.

5500+9000+2800+3000+600 = 20900 (20.9KW)

20900w/230v = 90.8amps

Still under 100A, and thats assuming they are all running at the same time..

reality & diversity, actually says they are probably not!

Most of the persons in the property are probably using low power equipment..

TV's, Hi-Fi's, PC's, Games consoles... etc..

& much of the lighting would be shared i.e. some of the people are actually in the same room.

From your own descriptions your property and personal household requirements do not appear out of the ordinary..

and a 100A or even 80A will most likely be more than adequate.

Remember a fuse will not pop the minute it exceeds its rated value..

tolerances are built in for normally expected power surges.. & short spells of overload current.

(we are a family of 7, youngest 11, eldest 21, and our supply was a 100a..

but recently downgraded to 80a by supply company...

their standard procedure when changing meter!)

Hope thats helps you understand some of the principals you can apply when considering max demand!

As KME said... it comes with experience..

and if you go back & re-read Flying Scotsmans figures..

I think you will find they are a very good rough estimate of realistic loads.

 

[SPECIAL LOCATION]

Here's another way you can look at things..

All cars have an absolute maximum top speed..

For safety reasons all components with the car should be able to operate at that maximum top speed..

However 99.9% of cars are NEVER ever driven at their max top speed...

The electrical Max demand is more just a check of ensuring cables etc..

are large enough for a potential max demand which in reality Hardley ever occurs!

 

[avinalarf]

Looking at things from a per person demand.. say four people.1] is doing cooking using 5500watts worth of oven.

2] is in the shower using 9000watts worth of hot water.

3] is doing some washing or tumble drying approx 2800watts worth of white goods power.

4] is feeling cold with a 3-bar electric fire one 3000watts worth of heat.

all of them have 150watts of lighting, per person, on total 600watts.

5500+9000+2800+3000+600 = 20900 (20.9KW)

20900w/230v = 90.8amps

Still under 100A, and thats assuming they are all running at the same time..

reality & diversity, actually says they are probably not!

From your own descriptions your property and personal household requirements do not appear out of the ordinary..

and a 100A or even 80A will most likely be more than adequate.

Remember a fuse will not pop the minute it exceeds its rated value..

tolerances are built in for normally expected power surges.. & short spells of overload current.

Ok, this is making more sense now. It's not just a straightforward calculation as I was lead to believe; many more things have to be considered.

we are a family of 7, youngest 11, eldest 21

OUCH! Where do you get the time to deal with work, the forum AND the family?Applaud Smiley

Hope thats helps you understand some of the principals you can apply when considering max demand!As KME said... it comes with experience..

and if you go back & re-read Flying Scotsmans figures..

I think you will find they are a very good rough estimate of realistic loads.

Can't thank you enough SL & everyone else who contributed to this lengthy thread...

Nat

 

[SPECIAL LOCATION]

Can't thank you enough SL & everyone else who contributed to this lengthy thread... Nat

Ahhh!

But DON'T forget the Admins & Moderators for keepin this place running smooth to allow sensible discussion & debate of all sorts of complex & simple questions without resorting to name calling & derogatory statements!

Applaud SmileyApplaud SmileyApplaud Smiley

(oh and a bit of saftness now & again!)

 

[avinalarf]

Ahhh!But DON'T forget the Admins & Moderators for keepin this place running smooth to allow sensible discussion & debate of all sorts of complex & simple questions without resorting to name calling & derogatory statements!

Applaud SmileyApplaud SmileyApplaud Smiley

(oh and a bit of saftness now & again!)

Indeed! Forgive me Admins - you are definitely included in the thank yous! Applaud Smiley

 

[kme]

Aw, Bless:x:x:x

 

[kme]

Aw, Bless :x :x:x:x:x:x

n.b. it should be noted that KME is on a "guinness" mission 2nite!

Not a good day for me - too much time bending down.

Plus, I was daft enough yesterday to move something a little heavier than I should (lower back knaffered!! - i.e., disc removed!)

SO - to try and make the pain go seepies a bit - Dr. Guinness is on the case:xPray

I apologise in advance if I`m either terse or belligerent in responses. (more than normal). It`ll pass.

[i need some warmth and sunshine. 6 days & counting...................]

:x :xPrayGuiness Drink