First Problem!! Cable calc for lighting circuit.

[Matty G]

Here goes.....Im doing the 2357 C&G course. For one question I have to do a cable calculation for power and lighting circuits.

I am having issue with the ground floor lighting circuit, in a domestic dwelling, as in I am way off the 3% tolerance for volts drop.
Details as follows:
Plan view of ground floor provided.
x2 Fluorescent fittings. 2x100wx1.8/230v = 1.57A
x5 Ceiling pendants. 5x100w/230v = 2.17A
x6 Internal Wall fittings. 6x100w/230v = 2.6A
x4 External wall fittings. 4x100w/230v = 1.74A
x8 50W 12v downlighters. 8x50w/230v = 1.74A ( as I have assumed a transformer at each fitting)

I have calculated length of run at 102mtrs from dist board through the whole ground floor (as I am assuming that switch drops, and strappers are included?)

So here I am so far.... Assumed 100w for all light fittings except the downlighters.

Total current demand is @ 9.82A, allow for diversity of 66% = 6.5A. After correction factors...Table 4D2A = 1mm.

Volts drop @ 44. mv/a/m x IB x length/1000 = 44 x 6.5 x 102/1000 = 29.2V!!!!

Firstly Have I made a mistake so far? Have I missed something obvious?

I do have an idea on how this can be changed....I'm sure somewhere I read rule of thumb x10 100w fittings per circuit?
If so shall I break this down into 2 circuits? Please point me in the right direction!

 

[Octopus]

Just a thought but all new installations need to comply with Part L so is the 100w per fitting relevant any more?

 

[Matty G]

I suppose I could make an "assumption" using Part L, and not go for 100w per fitting. The pulled that figure from Table A1 on the OSG. So just assumed that was the case!

 

[Octopus]

I suppose I could make an "assumption" using Part L, and not go for 100w per fitting. The pulled that figure from Table A1 on the OSG. So just assumed that was the case!

Just goes to show how out of touch these "important" documents are......

So you could make an assumption that you need to use the 100w figure as the customer might change the fittings...

 

[telectrix]

your calcs. seem OK. what would be donein this situation would be either:

1. split int 2 or more circuits. or:

2. run the first section in a larger cable, say, 4mm or 6mm, then continue with a smaller cable/s, calculating the VD from point to point.

with option 2, you could end up with 6mm from MCB to 1st point, then 2.5mm, then 1.5mm and so on. remember, though, to size breaker for the smallest cABLE IN THE CIRCUIT.

 

[ImpededLoop]

Hi,

Firstly you've explained your question very well and clearly have a good understanding of what you're doing.

I've calculated the volt drop using your figures and the result matches your calculation.

This question is probably deliberately designed to make you think outside of the box and shows how cable sizes can become larger than we'd instinctively use when they are actually calculated.

I would look again at the cable length and see if you can make it any shorter.

I'd also look at whether you can replace any of the filament lamps with low energy luminaires (light fittings).

With the number of lamps on the circuit, it looks like you may need to split into two circuits as you've already suggested.

 

[Matty G]

your calcs. seem OK. what would be donein this situation would be either:

1. split int 2 or more circuits. or:

2. run the first section in a larger cable, say, 4mm or 6mm, then continue with a smaller cable/s, calculating the VD from point to point.

with option 2, you could end up with 6mm from MCB to 1st point, then 2.5mm, then 1.5mm and so on. remember, though, to size breaker for the smallest cABLE IN THE CIRCUIT.

Thankyou. I undestand the breaker is there to protect the cable.

Out of the 2 is their a preferred method? I suppose it depends on the size of the consumer unit, and max demand?

Also, am I right about the rule of thumb? for 10 fittings @100w per circuit? If so is stated or hinted at anywhere in the OG or BS7671?

 

[telectrix]

i always allow 100watts/point for lighting. ( unless it's,say, downlights, then allow for 50watts/lamp if they're not integrated LED)

 

[Lucien Nunes]

Have I missed something obvious?

You have put all the lights at the end of the circuit 102m away from the DB. If they are distributed along it, the volt drop per metre will fall off with the current, until it is negligible near the far end.

Next prob, 6.5A load requires 10A breaker, can you get inside Zs for B10 on 102m of 1.0mm²?

Hence, need larger cable &/or split into more circuits.

E2A, what do you mean 'all the switch drops included?' Each one only carries the current of the luminaires it feeds, so if you have added them all in i.e. your 102m is the amount of cable you've pulled off the drum to do the job, you're way over the top! Calculate the drop in each leg of cable separately for a precise answer!

 

[Matty G]

You have put all the lights at the end of the circuit 102m away from the DB. If they are distributed along it, the volt drop per metre will fall off with the current, until it is negligible near the far end.

Next prob, 6.5A load requires 10A breaker, can you get inside Zs for B10 on 102m of 1.0mm²?





Hence, need larger cable &/or split into more circuits.

E2A, what do you mean 'all the switch drops included?' Each one only carries the current of the luminaires it feeds, so if you have added them all in i.e. your 102m is the amount of cable you've pulled off the drum to do the job, you're way over the top! Calculate the drop in each leg of cable separately for a precise answer!

Regarding your last paragraph I am struggling to understand what your saying....Lets see if i can make sense...For example one of the rooms has x1 ceiling light and two way switching, switches at both entrances to room.

Therefore I assume Cable wired into ceiling rose (from room 'A' ceiling rose/junction box), then the switch cable is terminated at the ceiling rose, and fed to one of the two way switch, then from that switch a 3 core 'strapper' cable to other switch. So (want for a better phrase) switch/control circuit complete.
(hope that makes sense?)

So I have included the length of each switch cable (from ceiling rose to the first switch). As well as the length of each strapper cable. Is this wrong?

Also when you say 'Calculate the drop in each leg of cable separately'... I need to break it down..

When you say leg do you mean....Say the ground floor has 4 rooms. Do you mean leg as in light circuit for each room. Therefore 4 legs? ie point of origin (for room B) could be ceiling rose/junction box at room A, ( then off to the lamps and switches) and then that leg/circuit ends at ceiling rose/junction box at room b, and so on? I do hope Im making sense!!

 

[Matty G]

Hi,

Firstly you've explained your question very well and clearly have a good understanding of what you're doing.


This question is probably deliberately designed to make you think outside of the box and shows how cable sizes can become larger than we'd instinctively use when they are actually calculated.

I would look again at the cable length and see if you can make it any shorter.

I'd also look at whether you can replace any of the filament lamps with low energy luminaires (light fittings).

With the number of lamps on the circuit, it looks like you may need to split into two circuits as you've already suggested.

Thankyou, you can never be too sure if you make sense to other people when typing!!

I must admit the thought has crossed my mind where they are looking to see if you can understand what your calculating, rather than just go through the motions!

I have decided to leave the filaments alone and use the OSG suggested rating at 100w.

I have re drawn the circuit route on the plan already and split it into 2 circuits. (which took some time!!) So far the volts drop is within spec.

 

[hasel5]

Simples

 

[Flanders]

You need to break down the calcs for each part of the circuit here is my example you have 4 lights 100 watts each on one circuit 10 meters apart
40 meters to furthest light
Calc 1 would be 100 watts from on cable from light 3 to 4 t 10 meters
Calc 2 would be 200 watts on cable from light 2 to 3 at 10 meters
Calc 3 would be 300 watts on cable from light 1 to 2 at 10 meters
and Calc 4 would 400 watts on cable from DB to light 1 at 10 meters
each switch drop would take 100 watts but you would only have to work this out for the furthest light so do
Calc 5 at 100 watts for Switch wire (from Light to switch and back to light )

Now add all 5 Calcs together and you will get your true volt drop

I think this is what ImpeededLoop was saying

 

[Hawkmoon]

your calcs. seem OK. what would be donein this situation would be either:

1. split int 2 or more circuits. or:

2. run the first section in a larger cable, say, 4mm or 6mm, then continue with a smaller cable/s, calculating the VD from point to point.

with option 2, you could end up with 6mm from MCB to 1st point, then 2.5mm, then 1.5mm and so on. remember, though, to size breaker for the smallest cABLE IN THE CIRCUIT.

6mm in first ceiling rose?...that should be fun

 

[Matty G]

Thankyou all for your help and info, much appreciated.

 

[Octopus]

Thankyou all for your help and info, much appreciated.

Good man, nice touch.