Maximum demand

robsparx · Jun 26, 2009

Hi, sorry i have found a few other threads on this, but they dont quite answer my query,

I am working out the maximum demand for a small 1 bed cottage type thing. I am installing the following circuits:

40A for 9kw electric combi boiler
40A for cooker
32A kitchen ring (7 sockets)
32A downstairs ring (8 sockets)
32A upstairs ring (3 sockets)
6A upstairs lighting
6A downstairs lighting
6A smoke detectors

Do i work out the maximum demand from the mcb rating, or from what is going to be connected/plugged in to the circuits? the only circuit i have wattage ating for is the 9kw electric boiler. I have worked it out to be over 100A, but the upstairs ring for example only having 3 sockets, isnt going to draw anywhere near 32A.

I think i am going to be using low energy downlights (11w) if that helps

am i right in saying i cant allow any diversity for the 9kw boiler?

Mark.W · Jun 26, 2009

You work it from what will be connected/plugged in.

Diversity is as follows:

for final circuits it's 100% of current demand of largest circuit and 40% of every other circuit!

lighting is 66% of total current demand! With lighting use 100W for every bulb, that way if the customer wanted to uprate the bulbs from 11W to 100W in the future, you know the circuit will be safe.

What type of boiler is it (instantaneous or thermostatically controlled) ?

robsparx · Jun 26, 2009

thermostatically controlled.

you say final circuits is 100% of largest and 40% of the rest, but then say thta lighting is 66% of demand. which one do you use. surely going by the first method, you would apply 40% to the lighting circuits?

for final circuits it's 100% of current demand of largest circuit and 40% of every other circuit!

using this method i have got to 100.2 amps. very borderline!

but then this is using the mcb rating, not the estimated demand according to whats conected! how do i know the current demand without knowing ratings of appliances! AAAARRRRRGGGGHHHHHH!. this is driving me mad

basically i just need to know that i will be ok with a 100A main fuse, and wont need 3 phase

Mark.W · Jun 26, 2009

No you have 3 rings, so you'd use 100% for the ring with the largest demand and then 40% for the other 2 rings.

Lighting is 66%

You look at what the final circuits purpose is.

No diversity on the boiler as it is thermostatically controlled.

Looking at your OP, and seeing that is a domestic assuming that is going to be used to live in. 100A will be fine!!

robsparx · Jun 26, 2009

ah ring final circuits, ok. I did think it would be ok at first, but then looked again and thought 40a for boiler and 40a for cooker doesnt leave much ampage for the rest of the circuits.

Do you see what i'm getting at though regarding the current demand for rings? I have no idea what theyre going to be plugging into it, apart from the kitchen i guess, cos thats normally always the same appliances