Maximum Demand - Please help.

[WHJOHNSON]

Hello.
I am planning to have my cottage rewired and have done some initial planning. I have already called out the electric board to come and uprate my fuse in advance but they want a Max Demand figure. There is a sticker on the present fuse which says it is only rated at 60 amps and I think I need it to be of a higher rating. My brother's house has a 100A fuse.

This is all still in the planning stage and I would like to get the electricity board to come and fit an isolator as well as fit a higher rated fuse.
Then I can ask the electrician to come in and make a start on rewiring.

My planned circuits are as follows

Ring 1 - no of points served = 6 B32A

Ring 2 - no of points served = 7 B32A

Radial 1 - no of points served = 9 B32A

Radial 2 - no of points served = 4 B32A

Cooker point - no of points served = 2 (1 is the built-in socket outlet) B32A

Radial 3 (Central HTG) no of points served = 1 B6A

Radial 5 Lights down - no of points served = 17(inc 12v Bathroom fan)B6A

Radial 6 Lights up - no of points served = 7 B6A

Radial 7(6.0mm sub main to garage) B32A

Garage fusebox

Garage will comprise of
2 x lights on another B6A
2 x double sockets , each on it's own B16A trip switch

What would be the Max Demand figure for the whole lot and how do you work it out.

Thank you in advance.

William

 

[oharaf]

Try this it may help. Maximum demand needs to be calculated in ampereshttp://www.voltimum.co.uk/consult.p...universe=consult.index.questions&fullsize=yes

 

[JohnD]

despite what you might think, sockets don't use power

It's what you plug into them that matters.

You have an electric cooker? On Christmas day it will occasionally be running at max demand, most of the time it will be no more than 25%

An electric shower or immersion heater will run at max demand

You ceiling lamps will occasionally all be on at the same time. If you are profligate you will have 100w or 150W or even chandeliers or downlighters. If not you will have CFLs at 12-20W each

your tumble drier wil run at max load for long periods. You dishwasher and washing machine will run at max load while they are heating.

Your kettle and toaster will run at max load but only for short periods.

You TV, hifi, PC, phone charger, table lamps, smoothie maker, blender, raio alarm etc are trivial.

If you have electric fan heaters then on cold winter evenings they might all be running at peak load at the same time.

If you have storage heaters they will all be running at max load for long periods.

 

[oharaf]

25 lights @ 100W = 2500

2500 x 0.66 = 1650W

1650 / 230 = 7.17amps

1 x 32a ring = 32amps

3 x 32a x 0.4 = 38.4amps

Total = 70.4 amps

COOKER (for example 13.6 kW

= (13.6 x 1000) / 230 = 59amps

First 10 amps + 30% of remaining + 5amps for socket

= 10 + 14.7 + 5 = [b29.7amps[/b]

Garage

2 x 100W = 200W

200/230 = 0.9 amps


sockets 16a




Maximum Demand (assuming that your cooker is 13.6kW)

= 124Amps


You will however have to check your cooker rating and adjust.



BAS, RF, anyone want to check my calc

 

[JohnD]

hee hee!

I'll bet you max load never reaches half that!

 

[SeanT]

Maplin have a clamp meter for 15 quid, assuming you already have most of the stuff you'll be using after the rewire?

 

[WHJOHNSON]

Hi again,
Thank you very much for taking the time and trouble.
If it helps, the cooker will be half-gas and half-electric if that makes sense!
Hob will be gas, and oven will be something like 2.2KW.

The kitchen will have two 4-spot wavy 'S' type halogen light fittings. I think that each bulb is rated at 50W.
The Utility will have one three-lamp fitting with 3 halogen spots (50W each).
The bathroom has a 4 spot fitting. (50W each)
The Dining room has one 4 spot (50W each) fitting and two 40W wall uplighter things.
The Lounge has four wall uplighters, each with a 40W halogen tube type bulb.
The Landing has one wall-mounted twin 40W halogen tube lamps.
Beds 1, 2 & 3 have traditional ordinary pendents.
There are 2 outside lights. One is an old Coughtree type light on a corner bracket. The other is a globe type thing with a 40W bulb inside.
The stairs light will be one of those energy saving 4 pin type PL lamps
There is to be another one of these on the Landing too.
There are 2 mains rated extractor fans downstairs. ONe in the utility and one in the downstairs toilet.
There is one 12 volt fan in the bathroom.

The heating is gas central heating - no electric storage heaters.

I don't know if this extra information makes any difference to the final figure?

Will.

The lights in the garage are two 60W flourescent tubes.

 

[JohnD]

I run lots more than that on a 60A main fuse though I have a 100A fuseholder.

Let's suppose you have the oven, washing machine, dishwasher and tumble drier on. That's about 52A, intermittent. You can add the kettle and toaster on, they won't be running long enough for the fuse to notice them. The lighting load is trivial but changing to CFLs will save you money. You can ignore the fans.

If you buy an electric shower and an electric hob you can think again and if you ever blow the main fuse (most unlikely) the supplier will whiz round and fit a new one, and probably upgrade it to save themselves the trouble of having to come round again. Someone will have the load/time curve for a BS88 fuse, but it takes a big overload for a long time.

I can't see any problem.

Why are you asking?

 

[oharaf]

JohnD was me maffs rightish then?

I thought b****x there is no way you could even thing of attempting to draw that!!!

But I checked again and got the same, with his 2.2kW cooker it drops nicely anyway.

I used the basis that each lamp was 100w so with your piffly little 60 and 40 your well within my Max Demand Calc

I really should find better things to do on my day off than number crunch

 

[plugwash]

The diversity guidelines in the regs tend to overestimate. Plenty of houses come out well over 100A per those calcuations yet 60A service fuses very rarely blow in domestic installations.