mixed 1.5 and 2.5 mm lighting - volt drop calcs

[markc17]

ok.. so any help appreciated..i want to add some Klik lighting management boxes to the end of a circuit to control 10 lights. The circuit up to the ceiling rose I wish to utilise to feed my first klik is wired in conduit using 2.5mm cable. I want to add a two way lighting switch to control the boxes, using 1.5 mm 3c & E cable rather than splash out 200 quid on 2.5 fp or the like... spanning a corridor 20 m long, so 40 if I go there and back on the strappers. The circuit is backed up by a 10 amp breaker. I have done some calcs and was wondering if somebody could look them over and see if I am correct.. its the different sizes of cable that is confusing me slightly:This is what I’m thinking to work this out.. please help if im way of the mark..
A- Closest leg (classroom 1) 2.5 m in conduit 22m from CU – 9 x 40w – 360 – 1.5 a
B – Next classroom 2.5 mm in conduit 32m from CU– 9 x 40w – 360 – 1.5 a
C – Furthest ceiling rose, including 40m strappers (there and back) in 1.5 mm 77m from CU – 10 x 40w – 1.7 a
So section c = 29(tabulated) x 45(distance between b & C) x 1.5a = 1.96 drop
B = 18 (tabulated) x 10 (distance between a & B) x 1.5a = 0.27
A = 18 x 22(distance to first point from CU) x 1.5a = 0.56
0.56 + 0.27 + 1.96 = 2.79 total volt drop using both sizes of cables
So I can use 1.5mm 3c and e to join my boxes, and do my switching.. even though it is at the end of the circuit.

 

[markc17]

doh.. already see where ive gone wrong

 

[markc17]

I did not total the power as I came closer to the CU..this is now what I came up with C = 1.96
B= 18 x 40 = 3.13 18 x 10 x 3.13 = 0.56
A = 28 x 40 = 4.9 18 x 22 x 4.9 = 1.94
1.96 +0.56 + 1.94 = 4.44 VD
So if this is right.. I can still use 1.5 mm for the last ten lights?

 

[Richard Burns]

I get 4.656 V but may have got confused about the layout. Either way it seems OK.

 

[Badged01]

From the info Richard seems to have given a comprehensive answer and a fine drawing to the question. A picture paints a thousand words and make the problem so much easier to understand!

Only point I would add is that based on your wattages Im guessing you dont have fluorescents in the circuit? If so this could adjust the cals slightly. But even if they all were fluorescents (and assuming that magical factor of 1.8) you would only be hitting just over 8V but that would not include any applicable diversity which could lower it below 7V.

So yep I reckon you are good to go with your 1.5mm cable .......

 

[markc17]

Thanks all,
I appreciate your time in looking this over for me. Must admit i was scratching my head a little. All LED so no 1.8 factor multiply thank fully. The picture is accurate.. and i tried to be over generous with my lengths (as one does!) to compensate for the few metres in switch drops.
Thanks again
Mark

 

[DPG]

Seems a lot of power for LEDs? I may have read it wrong though. Daz