stuck on a Cable Calc

[BAYEUX]

Afternoon chaps. I'm new here, hope i've posted in the right place.

My tutor has asked a cable calculation question i just can't get my head round.

senario is this, there are already 10 lighting points in a house when a customer asks for 2 x twin wall lights to be added. i have to show with calculations if it would be necessary to install a seperate circuit.

The size of the existing cable or MCB haven't been given

My understanding is that a max of 1300w is to be used for a standard lighting circuit, 1.5 twin and earth, with a 6A MCB.

any ideas greatly appreciated

 

[Mac]

Afternoon chaps. I'm new here, hope i've posted in the right place.

My tutor has asked a cable calculation question i just can't get my head round.

senario is this, there are already 10 lighting points in a house when a customer asks for 2 x twin wall lights to be added. i have to show with calculations if it would be necessary to install a seperate circuit.

The size of the existing cable or MCB haven't been given

My understanding is that a max of 1300w is to be used for a standard lighting circuit, 1.5 twin and earth, with a 6A MCB.

any ideas greatly appreciated

Cable Size Selector Calculator and online cable selector for electricians Hope this helps.You will need to apply diversity to the lighting circuit mate.

Lighting - 66% of total current demand for each circuit.1400x66%=924. 924/230=4 Amps.6 Amp breaker is fine mate.

 

[BAYEUX]

thanks, i think my tutor is having a laugh, I don't even have a cable lenth to work anything out.

all i can do is

1400w/230 = 6.09 Amps

if the rating of the current circuit is less it needs upgrading if its over it doesn't.

Sorry for ranting a bit but i really think he's pulling my plonker!

 

[ezzzekiel]

tell him your just gonna use 10mm for everything as the customer has plenty cash

 

[Mark.W]

Max power for a lighting circuit is 1200W. Rule of thumb says 10 x 100W lights per circuit.

 

[Lou7]

You can have a 10amp lighting circuit if you want, who say's it has to be 6amp ?
In the OSG it even has 16A ltg cir.

 

[chocolate]

Assume all lighting points use a minimum 100W lamp. So we have 10x100W plus 4x100W = 1400W. This is the minimum power consumption if all lights are on, and have a minimum 100W lamp installed. In reality, the chances of all lights being on, and all being rated at 100W is smaller. Hence for diversity in the regs.

As table 1B ( 17th Eg. OSG ) states, for a domestic property, we can presume a 66% diversity of load.

So 66% of 1400W is 924W.

At a nomoinal voltage of 230V, then the current = 4.017 amps.

We can therefore assume that taking diversity into account, then a 6A MCB would be suitable for this circuit - taking diversity into account for a domestic location.

As stated by MAC.

 

[Lou7]

I thought diversity was used to work out Max demand not fuse size.

 

[chocolate]

Not that straight forward. When planning, you work out the load you have and hence current demand. Then apply diversity ( at your own risk ) to determine the most probable current loading on that circuit. This allows you to then decide on the fuse rating, from which you can calculate the most suitable cable size.

 

[Lou7]

I know what you're saying,but by using this method you could have 20 lights on a 6A breaker,you wouldn't do it would you.

thats 2kW