Teach me please

[axel132132]

Hi

Can someone please teach me how to do a proper cable calculation, step by step would be really really appreciated so i can learn and know for future.

It will be a standard house TN-C-S system
The run is going to be 30meters
It will be clipped along a wall and tied to a cantenary wire then into a garage

I need to work out what size cable i will need
What type of cable
What size breaker i will need

Kind Regards
Dave

 

[GaryM]

How about working through this template that Lenny created - http://www.electriciansforums.net/e...ons/47059-cable-calc-template.html#post413994

I take it you have access to BS7671:2008 (2011)?

 

[JUD]

What are the loads going to be??

 

[axel132132]

Using his calculation for my board outside with 1 double socket and 2 fluorescent lights of a 360 pir is roughly 4000 Watt (about 3800)

TN-C-S
30meters

Step 1
4000 watts / 230 volts = 17.39

Step 2
20 Amp breaker

Step 3
Cg = No grouping single cable
Ci = No insulation clipped direct outside
Ca = 30 degree (outside) = 1
Cf = not a bs3036 its a bs60898 so = 1

so 20 / 2 = 10 ??


Dont get
STEP 4. Select cable size fromBS 7671 App 4. (Iz/t) > (In) > (Ib).
(Iz/t) = ________amps
From table __________ Reference Method __________ Cable size = _______mm##

I ONLY HAVE AN ON SITE GUIDE (ANY IDEA WHAT PAGE TO FIND THE ABOVE)but it looks like 1.5mm2 from a table i did find Clipped direct (20a)

Step 5
30meters x 17.39 x 29 / 1000 = 15.12 volts max is 9.6 so 1.5 not good

2.5 is 30 x 17.39 x 18 / 1000 = 9.39 volts which is below the max 9.6

So if i was to INSTALL this cable chosen 4mm rather than 2.5mm in the chance he wants more sockets or an auto garage door etc

4mm 3 Core NYY-J Cable Black
4mm 3 Core NYY-J Cable Black

That would be good enough to run from his mains-board inside out along a wall and wire into garage to a mainsboard?? from a 20A breaker (Upgrade to 32A breaker incase of upgrade)

Cheers

 

[JUD]

Using his calculation for my board outside with 1 double socket and 2 fluorescent lights of a 360 pir is roughly 4000 Watt (about 3800)

TN-C-S
30meters

Step 1
4000 watts / 230 volts = 17.39

Step 2
20 Amp breaker OK

Step 3
Cg = No grouping single cable = 1
Ci = No insulation clipped direct outside = 1
Ca = 30 degree (outside) = 1
Cf = not a bs3036 its a bs60898 so = 1

so 20 / 2 = 10 ?? Wrong....20 ÷ (1 x 1 x 1 x 1) = 20


Dont get
STEP 4. Select cable size fromBS 7671 App 4. (Iz/t) > (In) > (Ib).
(Iz/t) = ________amps
From table __________ Reference Method __________ Cable size = _______mm##

Choose a cable that can carry at least 20A

I ONLY HAVE AN ON SITE GUIDE (ANY IDEA WHAT PAGE TO FIND THE ABOVE)but it looks like 1.5mm2 from a table i did find Clipped direct (20a)

Step 5
30meters x 17.39 x 29 / 1000 = 15.12 volts max is 9.6 so 1.5 not good

2.5 is 30 x 17.39 x 18 / 1000 = 9.39 volts which is below the max 9.6 Wrong....max is 6.9V(including the VD of your final circuit) as you are feeding lights

So if i was to INSTALL this cable chosen 4mm rather than 2.5mm in the chance he wants more sockets or an auto garage door etc

4mm² would give you a VD of 5.77V on the distribution circuit which leaves you with 1.13V to play with on the final lighting circuit

4mm 3 Core NYY-J Cable Black
4mm 3 Core NYY-J Cable Black

4mm² armoured is cheaper
4.0mm 6943X 3 core SWA Steel Armoured Cable

That would be good enough to run from his mains-board inside out along a wall and wire into garage to a mainsboard?? from a 20A breaker (Upgrade to 32A breaker incase of upgrade)

Cheers

My Red & ​Green

 

[Guitarist]

If you are an electrician you should have the BGB as well as the on-site guide as a minimum. Several other books I wouldn't be without.

 

[axel132132]

Can you suspend the armoured though on the catenary wire, i cant dig it or anything like that has to be suspended

 

[JUD]

Can you suspend the armoured though on the catenary wire, i cant dig it or anything like that has to be suspended

Can't think of a reason why not

 

[axel132132]

Thanks for your guidance and help really appreciate it

 

[JUD]

One thing to remember though is there are minimum heights that over head cables need to be suspended at and maximum support intervals.

Tables D1 and D2 of Appendix D in the On-site Guide. Minimum height is 3.5m and the maximum support interval is 350mm