got a trench dug, and I would like to know what cable i need
russ18,
Mar 27, 3:02pm
Depends on electrical load and lenght of cable run.
ghafmond,
Mar 27, 3:05pm
16.5 metres, I am looking at 2 + E cable 2.5 mm2
budgel,
Mar 27, 3:40pm
I was advised to go up in size so that welders and other stuff can be run together. Your sparkie that will do the connection will advise you.
nzjay,
Mar 27, 3:44pm
Describe how you are going to lay this cable in the ground.
raven71,
Mar 27, 3:53pm
If you're having a sub main board installed in your garage, I believe that you have to use 4.0 mm2 cable.
johotech,
Mar 27, 4:00pm
Why do you believe that?
ghafmond,
Mar 27, 4:43pm
At the moment I have a 16.5 metre trench at 600mm with plastic conduit running along the bottom. what I was going to do was use a heavy duty extension cord in there, but I am rapidly going against that idea as i would have to upgrade the cable anyway. I am going to set the garage up as an off ice, so computers, lights etc. I am following NZECP 51:2004 as a guideline
andrew1954,
Mar 27, 6:12pm
Soon or later in this project Mr Sparkie is going to be involved, the open trench before you have purchased the cable stage is the ideal time to get him involved !
nzjay,
Mar 27, 6:21pm
I absolutely agree.
nzjay,
Mar 27, 6:22pm
So the total cable run isn't 16.5m, just the trench?
johotech,
Mar 27, 6:54pm
As he is doing the work himself under ECP51 it will have to be an inspector, not a sparky.
ghafmond,
Mar 27, 7:05pm
total cable run is around 20m, I am doing this job in stages as funds allow
cleggyboy,
Mar 27, 7:18pm
When I run an underground cable to my workshop, it had to be 3ft deep with H4 tanilised timber on top of the cable. Back then it was Waitemata Power Board inspector had to do the inspection before it was filled in.That was back in the late 70's.
ngawakard,
Mar 28, 1:29am
You need to get a sparky involved. reading your comments I believe you may not understand the rules and regs involved in this job. Please contact a registered sparky or inspector to get advice.
t_naki,
Mar 28, 1:54am
I agree with jonotech on this one, if you are going to be doing the work yourself then you need to have an inspector involved. I would get him involved now so that he can advise you on what he requires in order to sign off the whole job. While he is doing that he can let you know what sized cable to use. To start with avoid the extension lead idea, its not a starter.
treecave,
Mar 28, 2:07am
comes back to do it once and do it right
oliver6,
Mar 28, 3:28am
While the trench is open, run ph lines, fibre optic, and a 100ml pipe to run anything else through later.
easygoer,
Mar 28, 5:43am
You can't use flex as permanent wiring, you need to use TPS as a minimum, I would use no less than 4.00mm but as suggested get a sparky involved now otherwise you may not get the work signed off if you bury the cable first
russ18,
Mar 28, 10:36am
What rule says you can't use flex as permanent wiring?
t_naki,
Mar 28, 1:49pm
A question for others but from my interpretation the switchboard shown above would need two RCD's?
Most often, I now do not fit a board in a garage and just wire it all on 2.5mm cable and then it becomes a circuit from the main switchboard instead of a sub main. Its fine for most garages unless someone is wanting a workshop, welder or other sort of high draw appliances.
johotech,
Mar 28, 3:18pm
If the trench is 16.5m, I can't see how the total cable run could possibly be 20m. That length "calculation", suggests to me that you intend to take it to a power point in the house, close to the house end of the trench. If that's the case, then your total run will be much more than 20m when you include the distance from that power point to the switchboard.
The maximum length of run for 2.5mm2 cable on a 16A circuit breaker is around 21-23m. You really should be using 4mm2 cable in the trench to the first power point in the garage, then change to 2.5mm2 cable to wire the rest of the lights and power points in 2.5mm2 as T_Naki said. You don't need a switchboard in the garage for just a few power points and lights to use as an office.
Then there is the requirement for an RCD for any new power points installed. It would be best if you ran the garage cable all the way to the main switchboard in the house and have an RCD and circuit breaker at the main switchboard. But if you really only want to run to a power point in the house you would need to confirm that that power circuit is wired in 2.5mm2 first. A lot of power wiring was done in 1.5mm2 because of different rules from about 30 years ago up to around 10 years ago.
If you confirm that power point is wired in 2.5mm2, then that power point could be changed to an RCD type and protect everything down the line from it (i.e. the garage). The circuit breaker for that power circuit should still be changed to a 16A.
As others have said, you really should have an electrician involved before you run the cable. Or if you are determined to do it all yourself, you will need an electrical inspector to do the testing and connection, so you should be getting them involved for advice now, rather than after you have run the cable. It would usually be cheaper to use an electrician though.
t_naki,
Mar 28, 3:46pm
Do you have a reference for that cable length, it seems very short. By that reckoning most power circuits in a house would need to be 4mm as the cable run is over 20m. I am looking a table B1 and it says that a 2.5mm is on a 16A MCB is 85 meters, 68m on a 20A MCB. I realise that there will be other factors involved to reduce it, just wondering where you got your figure from.
johotech,
Mar 28, 4:04pm
My figures are from GenCalc.
Table B1 is only for EFLI for protective devices. It's useful for checking protection on long runs at low current but it doesn't take into consideration voltage drop at maximum current & conductor temp considering the type of enclosure for installation.
Considering all cabling must now be considered as partially surrounded by insulation (which is similar rating to enclosed underground), all previous "rules of thumb" should be thrown out.
andrac63,
Mar 28, 4:17pm
don't forget phone line,It slipped my mind because house phones are cordless but of course still need phone line to base unit
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