Lighting circuit rating

[dunny]

Bit of a how long is a piece of string question but.... what is the typical load for a domestic lighting circuit?? 4 bed house 5 downstairs rooms plus kitchen, large hallway and 3 - 4 landings.

Also will have to have fairly substantial outside lighting.

I could probably work it out myself if I knew how to take wattage to amps. If I do this do I have to assume that all lights could be on at the same time??

D
PS - This is related to my thread on home automation yesterday.

 

[davelx]

Watts = Volts * Amps

Re-arrange as required to find the one you don't know.

(power factor assumed to be 1!)

Diversity for a lighting circuit allows you to design for 66% of maximum demand on at any one time ( http://www.tlc-direct.co.uk/Book/6.2.3.htm ).

Outside lighting is probably best give its own, suitably rated, circuit as a normal 6A lighting circuit will not supply very many 500W floods!

 

[dunny]

So a 12v halogen running at 25w = 300 amps. Doesnt sound right to me!! Not questioning your formula but I cant be interpreting it correctly.

What am I missing??

D

 

[sm1thson]

Watts = Volts * Amps

Watts / Volts = Amps

Watts / Amps = Volts

25w/12v=2.1A

Also will have to have fairly substantial outside lighting.

if you are at the design stage put that on its own circuit, not only wil it keep the current on each circuit down it minimises disruptance if you geta problem (which is common with outside circuits)

 

[dunny]

Schoolboy error on my part...

Thanks

D

 

[Steve]

to add, 25 watts at 230v is 0.108A.

For a given power, the current varies inversely with the voltage. So the wiring on the LV side of a transformer is usually much chunkier than on the HV side, to account for the higher current.

 

[RF Lighting]

So the wiring on the LV side of a transformer is usually much chunkier than on the HV side, to account for the higher current.

This is the very reason that electricity is distributed at high voltage.

It means the grid / DNO can use smaller, lighter cables.

 

[ricicle]

And as the current is lower the I²R losses are less

 

[dunny]

Sort of with that but which would you use for calculating the amps you would need for lighting?? Despite knowing what many of you think about halogen downlighting I am to have quite a large amount as the primary source of lighting.

So if I have for example eight 25w halogen lights in the living room on its own circuit and they are low voltage (which I may have mistakenly thought they are) this would draw a lot of amps??

I think I might have answered my own question.... The low voltage ones are usually used in bathrooms etc arent they?? Using them all over the house would soon eat up the amps wouldnt it??

D

 

[Steve]

8 x 25 = 200 watts of lighting.

200 watts at 12 volts is 17 amps.

Therefore you'd need 2.5mm² cable on the LV side of the transformer, OR you could run the lights as a star with the transformer in the middle. The star is preferred, a volt drop is an issue with long runs of cable at low voltage. You could end up with a few lights dimmer than the rest if you daisychain.

200 watts at 230 volts is 0.9 amps. Call it an amp on your 230v lighting circuit.

The easiest way to work out circuit loading is to use wattages. add up all the wattages and divide by 230. A watt is a watt regardless of the voltage.

 

[dunny]

Right I got it. In that case if I use a MCP panel (automated lighting gadgetry) that can support 72A total load (4 relay modules s at 16A max load, individual relay 8A max load) I should have plenty of capacity. Thought I might have needed 2 but that would have given me more than the main fuse is rated for !!

D

PS - when I say if I use a MCP panel I really mean my spark uses a MCP panel !!

 

[RF Lighting]

PS - when I say if I use a MCP panel I really mean my spark uses a MCP panel !!

No, you use the MCP panel.

Your spark just installs it

 

[dunny]

Well spotted.... as long as I dont have to install it!!!