Sub main help

[t0m87]

been asked to supply a site containter which is going to be used as an office. the run is 125 metres long. 2 fluorescents, a few sockets and a 3k heater in the office. sockets are going to be used for computer only. the swa is going to be run on the perimeter fence clipped direct. I'm thinking 10mm on a 32amp breaker. any opninions?

 

[IQ Electrical]

been asked to supply a site containter which is going to be used as an office. the run is 125 metres long. 2 fluorescents, a few sockets and a 3k heater in the office. sockets are going to be used for computer only. the swa is going to be run on the perimeter fence clipped direct. I'm thinking 10mm on a 32amp breaker. any opninions?

Have you done volt drop calculations?

 

[t0m87]

i did but i wasnt exactly sure id done them right :S can you show me a quick example, be highly greatful.

 

[IQ Electrical]

I'm just about to catch a train but just use the usual mV/A/m with values from table 4D4B with a design current (Ib) of say 20 Amps and that'll give you an idea.

 

[IQ Electrical]

Also remember that if your Ib value is significantly less than the current carrying capacity of the cable, the calculation will be artificially high as it is based on max conductor operating temperature (full load).

 

[t0m87]

ok so u mean i should work it out using ib as whatever size breaker im going to use ? apologies dont do these calcs often

 

[t0m87]

these are the different calcs
4.4 x 32 x 125/1000 = 17.6 v
4.4 x 20 x 125/1000 = 11v

 

[Welchyboy]

remember its the lighting that needs to be considered only 6.9VD max

just done a job in a stables, small stables was 70m from origin to carry 20A max load, i used 16mm SWA this gave me a VD of 3.9v to my sub main CU under full load conditions, then around 2v drop for my biggest/longest lighting circuit 5.9vd alltogether

so there is a small window for additions on the install but not much

If you go by the book even 16mm is gonna be over 7v to end of sub main(in the real world this would probably be fine, but if its not..........)

Looks like its 125m of 25mm SWA.....ouch!!!

 

[Rockingit]

OK, so here's something that's always bugged me. How come it is that I have to install (hypothetically) a run of expensive 25mm to carry 50 A for 60m (or whatever the calc would be) but the DNO can lay in a 16mm concentric to deliver me 80A from a spur 100yds up the road.

What magic do they have in their screwdrivers that is missing from mine?

 

[Welchyboy]

OK, so here's something that's always bugged me. How come it is that I have to install (hypothetically) a run of expensive 25mm to carry 50 A for 60m (or whatever the calc would be) but the DNO can lay in a 16mm concentric to deliver me 80A from a spur 100yds up the road.

What magic do they have in their screwdrivers that is missing from mine?

^^I have allways wondered exactly the same

Allthough, i actually carried out a test on my summer house in my garden with a 6mm SWA run, loaded it up with 2x 3kw heaters, calculated the VD then measured it and it wasnt that far out at all

 

[Guest111]

OK, so here's something that's always bugged me. How come it is that I have to install (hypothetically) a run of expensive 25mm to carry 50 A for 60m (or whatever the calc would be) but the DNO can lay in a 16mm concentric to deliver me 80A from a spur 100yds up the road.

What magic do they have in their screwdrivers that is missing from mine?

wondered that myself, come on Tony put us out of our misery