I am putting in SWA cable to the garage, total length max 30 metres, load just a few sockets for low powered tools, and lights, connected to 32A RCBO in the house CU. Should I use 4mm or 6mm SWA ?
Cheers, Simon.
It's a garage.
One day you may want to charge an electric car...
Andy
So are we assuming the design load is 32A?
Current capacity for either are fine (the current carrying capacity "clipped direct" for PVC clad [1] SWA would be 38A/49A (4mm^2/6mm^2))
So we can check voltage drop. Since you have lighting, that ought to be a 3% or 6.9V. At 11mV/A/m 32A load would give ~ 10.5V, so 4mm^2 cable will not meet the spec.
Going to 6mm^2, that would be 7.3mV/A/m or ~7V
So you could easily argue that 6mm^2 will get you close enough - especially as it does not sound like the circuit will likely have anything near that load on it normally. (you could also argue that the effects of small voltage drops with modern low energy lighting are overstated anyway).
You have not said what the earthing system is at the head end, but we can make a crude disconnection time check: If you are using 2 core 6mm^2 SWA, the core resistance will be about 3 mOhms/m, and the armour
7mOhms/m for a total of about 10 mOhms/m round trip. That gives us 0.3 Ohms. If it were a TN-S head end you could add on 0.8 for Ze, so 1.1 Ohm total. Low enough to meet then 1.37 Ohms min for a B32 MCB [3].Prospective fault current = 230/1.1 = 253A, enough to be in the magnetic part of the trip response.
Adiabatic check, sqrt( 253^2 x 0.1 ) / 115 [4] = 0.7mm^2 minimum CSA wire required. The copper equivalent 7mm^2 of armour being >3mm^2 if we use the 2.255 conversion factor. So plenty good enough.
[1] Much SWA will be the 90 deg C XLPE style sheathed stuff anyway, which has higher limits) [2]6 or 10 for me
I suspected 4mm would be iffy. Load will be lowish (def. no electric cars !) Thanks for John's calcs. I will use 6mm. Most of my neighbours have a string of 2.5mm nailed to the fence.
Simon.
6mm minimum.
No electric cars ATM:-) What would be the extra cost to put 10mm cable in?
That's probably okay in winter.
Owain
Well last July I bought 20 m of 10mm2 three phase SWA cable for 70 quid from denmans so I would expect 30 m of 10mm2 to be around 105 quid....
My SWA cable is awaiting connection at both ends with suitable coloured sleeving to mark it has single phase, neutral and earth.
one is at the garage end and the other is for the wood shed and greenhouse
Realistically, while not correct, how often do most people use enough power in a shed or garage to actually trouble it unless they use electric heating there?
SteveW
Depends what you do in there.. but if its used for anything of any duration in winter, then some form of heating seems likely.
First hit for 10mm 3 core on line finds 30m at £104 inc delivery.
When I did the voltage drop calculations on 20m SWA assuming a max load of 32A, I found that the voltage drop exceeded 2% for 6mm2 so that was why I ended up with 10mm2 SWA in a trench across the lawn along with 25mm MDPE pipe.
For now it's for a wood shed and a greenhouse and garden lighting and garden sockets.
There is also now spare capacity for a hot tub on the patio at the bottom of the garden if SWMBO or the rugrat so desires!
Also we may one day convert house integral garage to living accommodation so we would build a replacement garage at bottom of the garden and as water and power is already there... I currently appreciate the hot water I have on tap on the garage sink!
This is where the tightening up of lighting circuits to a 3% voltage drop in the regs may need to change.
The 3% voltage drop for lighting became a reg before the use of LED lighting became mainstream.
The use of a deviation from the regs is a useful tool. If the OP fits LED lighting that works from 80 to 250V (most of it does) then you can ignore the 3% voltage drop and calculate for a 5% drop.
There are other reasons not to design the circuit to the absolute minimum allowed.
The three most obvious ones are
Point 3 is very important for garages used to park daily cars in.
Eventually
Point 3 is important particularly for detached garages used for storing daily use vehicles overnight.
Eventually we will all be driving electric vehicles which will need charging overnight....
I have been informed that my 80a cutout is only sufficient for charging 1 vehicle. If I want to charge two vehicles overnight, I have to have the cutout upgraded to 100a and possibly the service Cable to the cutout.....
So a detached garage having a 3 core 25mm2 SWA cable does not now seem excessive to put in while the trench is still open instead of that 6mm2 or 10mm2 SWA cable.....
Believe it or not the most logical thing to put in the trench is a duct with a draw cord next to your SWA.
As for upgrading your cut out to 100A. If you have to do this then so would most the people on your street. The infra stucture of the grid cannot not cope with this.
Electric only vehicles may be fine many people but not ATM for me, no matter how fast they charge or far they can travel.
They are probably suitable for my gf with her fixed mileage to work (9 miles round trip) and a couple of shopping trips a week. A 13A supply could keep her car topped up. I seldom do less than 100 miles a day.
I have 63A out here (the SWA cable predates the building!) but even if I wanted I couldn't go all the way to 100A. That's the house main fuse limit.
Andy
Regarding your point about the infrastructure being unable to cope....
I reckon that electric car owners will sign up to economy 7 electric which means their cars will be charged between midnight and seven am where the majority of people will be in bed.
The infrastructure is clearly able to cope with peak load during the morning get up, shower and have beakfast run and again for when people get home, shower cook and watch tav, surf internet and then go to bed.
So instead of two peak time periods every day we could see three peak periods every day.
When you consider all the kettles, electric showers, toasters, electric cookers etc during day time Vs battery Chargers during the night....
Having said that, houses with night storage heating systema may struggle to charge cars without having a 250 amp cutout supply!
When I moved in (1993) I re-wired and also replaced the CU. The next time the meter reader came, he noted the new CU and told me that they'd send someone to upgrade the main fuse from 63A to 100A.
On the day they came, they took the fuse out, smashed the cast-iron box to pieces, while wearing thick rubber gloves, replaced the box with a modern moulded one, connected up, filled the cable entry with what looked like an epoxy resin, put the new fuse in and departed.
SteveW
OK thanks all. I've ordered 30m of 10mm 3-core SWA even though its way over for current usage. If I ever needed to make use of its capacity I'd have to upgrade some internal wiring, but hey ho as they say. And it will certainly sort out the earthing requirements.
Simon.
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