Running of SWA, sub main

[frank3323]

Hi guys,

I am just trying to price a job up, which I am a bit unsure of. It is at a commercial premises, the supply I have got is 300a three phase, with ruffley 150a - 200a per phase already being used. The customer has asked me to run a supply, 300m (buried) for a 50kw load. It will supply staff living quarters, like a three storey house, and also possible expention in the future.
I have calculated the cable to 70mm2 SWA, 4core, with a 50mm2 earth. Do you think I should still put a earth rod in at the load side, house side?
Also a bit in unsure on protection, I calculated 125a 60947-2?
And then I was thinking of splitting the phases for each floor at the house, so ground floor L1, 1st floor L2 + 2nd floor L3?
Any help will me much appreciated

Whats the earth rod for?? is it a TT installation? and if you ran a new submain in the new DB would be TNS...... you need to work out the load on the new Db, amount of sockets, lights etc, the diversity, to design a building you work back from the power required, that determines the size of board needed, which determines the size of cable feeding it, once you have that, then you will look at spare capacity in the existing board, and 3 phase installations are not designed by simply throwing one phase per floor!! you will need to balance the phases, which means looking at which parts will be used the most, and if you have say 2 socket circuits within touching distance they need to be on the same phase, and lights, if you have 2 way lighting for example with 2 circuits in the switch, again, same phase, all the while thinking about the same load being on each phase, protection - if it was me the new submain would be 100mA protected, then drop to 30mA for sockets etc... housebashing it aint.... regs will give you the cable sizes needed, dont forget grouping factor, (if needed) derating factor for ambient temp, (if needed), and volt drop, if you are using SWA - eland cables website do great easy to follow data sheets,

like i said you will have to work back, if the guy wants a price then a true price would involve 1- 2 days design on the new building just to work out the power requirement.....

 

[Guitarist]

Though nothing to add to the thread, I just want to know where you have been G, I never had a note from ya mum excusing you

Sorry I didn't ask permission malc. I'm sure everyone has coped just fine without me naggin em

 

[Robert Turnbull]

Think expansion on your part is needed here

downstairs lights L1
Upstairs Power L2

all in the same floor space

 

[lauriehurman]

Thanks Robert, more succinct than I could have been. As I say, I'm a beginner but it really didn't sound right to me.
Laurie

downstairs lights L1
Upstairs Power L2

all in the same floor space

 

[malcolmsanford]

To be honest guys I can't really see why we would be so concerned with a lighting circuit on one phase in a commerical environment being run next to a RFC on another phase.

I could see in a domestic situation you try your best to have perhaps one area on one phase and not have the chance of 400 volts in close proximity, but even in the best designed and run installation, there is going to be somewhere in it that phases are close together, the main intake for one and sub mains feeding boards for another

 

[Engineer54]

Whats the earth rod for?? is it a TT installation? and if you ran a new sub main in the new DB would be TNS......

No it wouldn't!!

Well if the original supply is PME/TNC-S, it's always prudent to provide a local earth electrode to reinforce the multiple N-E connections. But don't waste your time, if your thinking along the lines of your typical 200 ohms!!... lol!!

if it was me the new sub main would be 100mA protected, then drop to 30mA for sockets etc...

It would need to be 100mA S type RCD to ensure full discrimination with downstream 30mA RCDs. Most active faults will be in excess of 100mA, so unlikely that a straight 100mA RCD will discriminate, both units would likely trip...