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

 

[chocolate]

That's why diversity is NOT a final judgement. It's up to you. If you feel, after your calculations, that having all those lights on one circuit is too much, then you split them. You're the boss. You decide. Some people will disagree - but you should have the final say. Argue your corner. Always go for safer option - the less inconvenient one for customers too.

 

[wildgoose]

A previous reply was correct.

Rule on thumb is to allow 100w per light.

Eg: 10 lights = 1000w

1000/230= 4.34Amp.

So your 6A MCB should be fine

 

[SirKit Breaker]

You can have a 10A lighting circuit but the limiting factor is usually the pendant flex which is usually 0.75mm2, hence the 6A breaker.

 

[Mac]

I use 11w low energy bulbs in my house.Times that by 10 and you have 110W

 

[Lou7]

That's why diversity is NOT a final judgement. It's up to you. If you feel, after your calculations, that having all those lights on one circuit is too much, then you split them. You're the boss. You decide. Some people will disagree - but you should have the final say. Argue your corner. Always go for safer option - the less inconvenient one for customers too.

I do agree with you,but it seems to me that alot of people use the 66% diversity without actually thinking about it.

A previous reply was correct.

Rule on thumb is to allow 100w per light.

Eg: 10 lights = 1000w

1000/230= 4.34Amp.

So your 6A MCB should be fine

Thats right but 4 lights were added which comes to just over 6A, plus in the first post the guy said he wasn't told what the breaker size was we're all assuming 6A

 

[mags1972]

I too allow the 100W max for a light fitting but these days you have to consider a 50W max for all the god damn downlights people are having fitted!!!

 

[Bright Sparks]

Diversity is fine unless you have women or Kids in the house. You come home and every damned light is on OR it's completely dark as all the fuses have blown !

 

[mags1972]

Diversity is fine unless you have women or Kids in the house. You come home and every damned light is on OR it's completely dark as all the fuses have blown !

Lol, we must share the same house