Voltage Drop Calcs

[Gloster Boy]

Hi, (new to forum, first thread)

Trying to get my head round voltage drop and using the calcs in practice.

I have radial circuit - 16 amp MCB on a 30mA RCD - 2.5 flat T+E out of household consumer unit, straight through wall and running in plastic conduit along outside of wall and into garage - 12 meter run.

I want to put a shed up top of the garden another twenty meters away. Using junction box can I extend the existing circuit using 4mm SWA from the junction box to the shed?

The circuit would consist of one single 13 amp socket in garage with fused spur for fluorescent strip light, double 13amp socket in shed, with fused spur for another strip light and two 50 watt lamps on the outside of shed.

design current i've calculated = 13.73 amps

So can I split the calculation in two
i.e.

v drop for 2.5mm part of run : 18 x 13.73 x 12m
----------------- = 2.97 v
1000

+ v drop for extension : 11 x 13.73 x 20m
---------------- = 3.02 v
1000


therefore, 2.97 + 3.02 = 5.99 v

5.99 v drop would comply with regs.

Is this a correct way of calculating this circuit voltage drop? And if it is, is this a safe and compliant installation? I'm sure that better practice would be to run the whole thing in 4mm SWA but SWA isnt cheap.

Or am I completely missing the point?

Sorry to those who may be asleep by now, any responses would be greatly appreciated.

Cheers

 

[Gloster Boy]

Sorry the display for the calculations mucked up.

(18 x 13.73 x 12m)/ 1000 = 2.97 v

(11 x 13.73 x 20m)/ 1000 = 3.02 v

 

[Richard Burns]

Welcome to the forum.

If your design current of 13.73A is correct for 3 socket outlets and four light fittings and if this would be the total current taken to the furthest end of the circuit then you have correctly calculated the volt drop.

If you break down the current use to each section this would be more realistic and give better results.
The total current would be used up to the first accessory and then then total current taken by the cable would drop as you get further down the circuit.
The easiest breakdown would be to say current usage in garage and shed would be used for the first section and then current usage in shed only would be used for the second section. Remember to take into account any cables running to the accessories as well.

Can't really comment on the suitable design of circuit that would be for you to determine as the on site electrician.

 

[Gloster Boy]

Thank you for your reply Richard.

Its late now but i'll recalculate tomorrow based on your advice and post what I've come up with to see if I've understood.

Cheers for your time