Outdoor electric circuit

[rustynails]

Hi,

I'd like to install electric to an outside deck I have, supplying three brick lights and a weatherproof socket. It's about 13m to the point where a weather proof junction box is required under the deck, then another 1.5/2m to each fitting. The total wattage of all the fittings is 3240, given that a 13amp outdoor socket is 3120 watts and the lights are E27 lampholder max 40 watts each. I used an SWA calculator and It came back saying that I can use 2.5mm 3 Core SWA? Which means I'll need to install a 13m length of 2.5mm 3 Core SWA, buried 600mm deep, from an outside junction box on the external house wall to a bigger outisde junction box underneath the deck where I can spur off to the 3 lights and the socket, in a radial. Can I use 1.5mm to the lights from the deck junction box? Obviously I need to run 2.5mm to the socket. Does this all sound like good practice?

THanks

 

[rustynails]

* BUMP *

Anyone got anything to say...

 

[rustynails]

According to Table 4D4A p338 in the 17th I could use 1.5mm SWA everywhere because the design current load is 14amps (3240w/230=14.08A) which would make 1.5mm 3 core swa ok as it's max is 18A. Even the 2m drop from the CU could be in standard 1.5mm because according to p334 1.5mm t & e max load is 19.5 clipped direc. I just love talking to myself...

 

[Richard Burns]

You also need to work out your volt drop for the load, which is probably why the calculator said 2.5mm2 SWA.
You can split the volt drop so that you can see if just the lights can be run on a lower csa cable.
For standard light fittings your calculation is based on 100W per fitting.
The lighting should be fused down to say 5A with a SFCU.
The nominal voltage used is 230V not 240V.
The buried cable shoudl also have marker tape above it to prevent damage
You will also need to ensure that you have earthed the SWA armour as an exposed conductive part.
This work is notifiable to building control under part p of the Building regulations.

 

[pencilpusher]

I have a job coming up soon installing outside lighting and power. I have completed all the calcs and am happy 2.5mm SWA is fine BUT when the builders did the customers patio, they installed 2.5mm SWA under the patio and left it coiled up at each end of the run. I will be checking it is 2.5mm cable throughout it's unseen length by measuring the resistance and doing the calcs. However, there are two things I do not know. One is how deep it is buried and the other is if the cable had marking tape above it. As I will be signing the certs and notifying, can I carry on just noting the fact that I cannot check this without ripping up the patio?

Cheers

PP

 

[Guest55]

Dig up patio lol ?
Just make a comment on your EIC that ground works were completed by a third party.

 

[alarm man]

* BUMP *

Anyone got anything to say...

aye,you will answer your own question soon..as per the norm..

 

[pencilpusher]

Dig up patio lol ?
Just make a comment on your EIC that ground works were completed by a third party.

I did not suggest I dug it up! I suggested noting the cert and you have confirmed that. Thanks. PP :8:

 

[rustynails]

You also need to work out your volt drop for the load, which is probably why the calculator said 2.5mm2 SWA.

Using 1.5mm 3 Core SWA PVC

Volt Drop (mV) is 8.8V (mv = 25 (cable voltage drop per amp per metre, table 4D4B) x 14.08 (Cable design current lb) x 25 (Length of run in m)

Using 2.5mm 3 Core SWA PVC

Volt Drop (mV) is 5.28V (mv = 15 (cable voltage drop per amp per metre, table 4D4B) x 14.08 (Cable design current lb) x 25 (Length of run in m)

You can split the volt drop so that you can see if just the lights can be run on a lower csa cable.

Not entirely sure what you mean by splitting the volt drop?

For standard light fittings your calculation is based on 100W per fitting.

I have 3 brick lights, each are E27 lamholder 40w bulb max.

The lighting should be fused down to say 5A with a SFCU.

I think I understand this, but it's outside under a deck. Why would I need a switched fused connection unit. I have an outdoor enclosure box which is where the spurring occurs. I'd need to fuse down inside that ideally because of the glanding. I presume?

 

[HandySparks]

How are you intending to protect this circuit? Fuse/MCB/RCD? Ratings?

 

[HandySparks]

I have a job coming up soon installing outside lighting and power. I have completed all the calcs and am happy 2.5mm SWA is fine BUT when the builders did the customers patio, they installed 2.5mm SWA under the patio and left it coiled up at each end of the run. I will be checking it is 2.5mm cable throughout it's unseen length by measuring the resistance and doing the calcs. However, there are two things I do not know. One is how deep it is buried and the other is if the cable had marking tape above it. As I will be signing the certs and notifying, can I carry on just noting the fact that I cannot check this without ripping up the patio?

I'd say that if it's covered by paving slabs over the whole length then it doesn't matter how deep it is. No one's going to be putting a spade through it.

 

[Richard Burns]

Using 1.5mm 3 Core SWA PVC

Volt Drop (mV) is 8.8V (mv = 25 (cable voltage drop per amp per metre, table 4D4B) x 14.08 (Cable design current lb) x 25 (Length of run in m)

Using 2.5mm 3 Core SWA PVC

Volt Drop (mV) is 5.28V (mv = 15 (cable voltage drop per amp per metre, table 4D4B) x 14.08 (Cable design current lb) x 25 (Length of run in m)

Not entirely sure what you mean by splitting the volt drop?

I have 3 brick lights, each are E27 lamholder 40w bulb max.

I think I understand this, but it's outside under a deck. Why would I need a switched fused connection unit. I have an outdoor enclosure box which is where the spurring occurs. I'd need to fuse down inside that ideally because of the glanding. I presume?

Maximum volt drop for lighting is 6.9V; for sockets 11.5V; the 1.5mm2 does not make it for lighting.
If you calculate the volt drop for each section then the lights may comply using 1.5mm2 for the lighting.
See the diagram below.

Appdx 1 in the OSG suggests using 100W minimum for each light fitting.
Maximum OCPD for lighting with ES lamp holders is 16A (559.6.16) also the fittings may not be rated for taking the current that maybe expected from a socket. Does not need a switch but does need a fuse.

 

[rustynails]

Ok, so your saying my lb = 15 Amps (rounded up)

13amp socket is 2990 watts +
100w(0.5A) for 3 light fittings 300 watts = 3290 watts / 230 = 14.3 Amps

Following the diagram, Volt Drops would be:

17th p339 2.5mm 3 core swa voltage drop per amp per metre = 15
17th p339 1.5mm 3 core swa voltage drop per amp per metre = 25

VdB 2.93 = (15 x 13m x 15A (Total load))/1000
VdC 0.2 = (15 x 1m x 13A (Socket))/1000
VdD 0.02 = (25 x 1.2m x 0.5A (Light))/1000
VdE 0.02 = (25 x 1.5m x 0.5A (Light))/1000
VdF 0.03 = (25 x 2m x 0.5A (Light))/1000

Socket Vd = 3.13V
Light D Vd = 2.85V
Light E Vd = 2.85V
Light F Vd = 2.86V

Lights are way under 6.9V as required and power way under 11.5V so that would be acceptable. Just out of interest I did these calculations using 1.5mm throughout and added on the forgotten 3m drop from the CU in standard 1.5mm two core and earth. Am I right in saying that I could do it in 1.5mm straight from the CU, that can't be right can it. Something wrong with my calculator possibly...?

VdA 1.31 = (29 (p335 17th) x 3 x 15)/1000
so then that would make the following vd when using 1.5mm:

Socket Vd = 4.44V
Light D Vd = 4.16V
Light E Vd = 4.16V
Light F Vd = 4.17V

and using 2.5mm two core and earth for CU drop gives:
VdA in 2.5mm 0.81 = (18 x 3 x 15)/1000

Socket Vd = 3.98V
Light D Vd = 3.66V
Light E Vd = 3.66V
Light F Vd = 3.67V

As for overcurrent protective device I am required to use a maximum 16A as said which Is fine because I have a new board with a spare Type B 16A 30mA RCBO 61009-1.

Only thing is I know that the regs page 426 and table H2.1 p174 OSG say A3 Radial 20A OCPD 2.5mm Minimum live conductor.

Calculations allow acceptable use of 1.5mm 3 core swa throughout the circuit, however like I said my calcs could be wrong and regs state 2.5mm throughout...but picture shows socket radial serving 20m.

If you calculate the volt drop for each section then the lights may comply using 1.5mm2 for the lighting.

It looks that I could.

 

[sedgy34]

sorry to put a spanner in your works but your not a registered electrician and should not be doing these works.

 

[Guest55]

sorry to put a spanner in your works but your not a registered electrician and should not be doing these works.

How have you come to that conclusion ?
And you're incorrect anyway.
Anyone can carry out electrical work in their own home so long as they notify the local council , pay the appropriate fees , and have the work inspected and tested by a competent person.
Simples.