Re-wiring garage supply problem

[robgraham]

An awful lot of years ago - like 25 - when I was seemingly not as knowledgeable about cables as I am now, I put in a buried supply to the garage (some 20m away) using ordinary 2.5mm cable run in a length of ordinary garden house.

Part of the garage is about to become the workshop and the load will go up - 2kW+ circular saw, etc. though the continuous load will only be lighting, the deep freeze and in winter an electric heater, so the load is not all that large. The garage has a CU with RCD.

On the basis that the plastic cable is not 'approved' for such an application, I am considering replacing it with SWA.

My hope (!) is that I will be able to draw the 2.5mm cable out and pull an SWA cable in - I see that 2.5mm SWA cable has a considerably greater capacity than the plastic stuff, but I have the problem that the circular saw has an upgraded motor that takes a start up surge that exceeds the fuse in a 13A socket is therefore hard wired - is there any likelihood that the voltage drop in the cable at start up of this motor is going to give me a problem ?

Thanks Rob

[BigWallop]

2.5mm cable is rated at 25Amps when buried in soil. You would need to take a 4 or 6 mm SWA for that length and load capacity. Personally, if I was using the outhouse as an actual workshop, I'd take a proper sub-mains supply in 16 mm 3 Core SWA and put it on a separate breaker beside the existing house Consumer Unit. With its own CU in the garage to control the local socket and lighting, you'd have a fool proof supply that you can use for most anything.

[Osprey]

I would think hose will have collapsed, and you will be unlikely to draw through the somewhat thicker SWA cable ... worth trying, but probably won't work.

20m should be OK as far as volt drop would be concerned.

BTW ... don't think you were on your won, people do all sorts of things for external supplies .... usual for a garage or shed is

2.5mm2 run along a nearby fence.

Local water authority has 2.5mm2 t&e tied to a barbed wire fence over a 50m run to supply temporary power to a sewer outfall light :-)

[Adrian]

Osprey gurgled happily, sounding much like they were saying:

T&E run through a garden hose - buried in a poured concrete retaining wall to a wooden summer house.

There was a rudimentary ring in the summer house - which terminated in a plug. Which was plugged into a socket. Which was attached to a short length of that shiny varnished two-core lamp flex. Which terminated in a plug. Which was plugged into a three-way "granny burner" socket adapter. Which had the pins drilled and bolted to the T&E from the house.

Now wrap the whole mess in ivy...

Thankfully, it'd long since been disconnected at the house...

[robgraham]

Where can I get these ratings from ?

I appreciate what you're saying about up grading the cable but going to 16mm is seriously unnecessary - what justifies a 100A supply ?

I'm looking for an easy solution here (I'm too old to bother with too many niceties) or the existing cable just stays unless there is a serious drop out in the voltage when the saw start up.

Rob

[js.b1]

You are having a laugh with 2.5mm and 2kW+ circular saw.

4.0mm SWA @ 25m is a good minimum choice. 16A, with say 8m of FTE 2.5mm into house, 5m for garage wiring. Fine for 1 socket, 2 lights, 2kW fan heater, 850W hand tool.

10.0mm SWA is the practical choice.

32A, with say 8m of FTE 6.0mm into house, 5m for garage wiring. Fine for a 2kW+ saw, 3HP, with a big fat induction motor.

Beware seeing SWA current rating and thinking "small is fine". SWA XLPE insulation permits 90oC max conductor temperature, but most wiring accessories are limited to 70oC max conductor temperature. Thus you use the figures for BS6004 PVC insulated copper conductors, so even 4.0mm SWA is substantially less capable than you believe.

If you route SWA thro to the consumer unit, you can avoid having to stick an RCBO at the house end and can put it at the shed end. This is important because a big saw tripping a house RCBO will also trip out your lights! A non-maintained emergency light is =A315 well spent.

Find the wiki re garage supplies. Lots to consider here re your house supply type, the type of garage construction re metal, incoming services, export PME if house PME or make garage a TT supply.

If there is any chance you may have multiple fixed equipment, then the suggestion of 16mm is reasonable. Otherwise you will probably find that

6mm would do, but 6mm & 10mm are rather close in price usually. 2.5mm is out and I suspect 4.0mm is pushing it unless 20A is all you need.

[js.b1]

Where to get the ratings?

Only got the 16th to hand, but that will do.

Table 4D5A 16th BS7671 for PVC BS6004 70oC max conductor temp.

- 1.5mm = 20A

- 2.5mm = 27A

- 4.0mm = 36A

- 6.0mm = 46A etc

You can not use SWA quoted figures because they are based on XLPE 90oC max conductor temp. Very few wiring accessories will handle 90oC - consumer unit, junction boxes etc are all 70oC.

You need the wiki FAQ re outdoor supplies.

[tinnews]

Why, what on earth do you think is going to go wrong? The 2kW is presumably the motor power when loaded. 2Kw is somewhere around 9 amps, most of the time therefore it will probably be taking around 5 amps. At start-up it will take more momentarily but so what, it'll start up marginally more slowly, an advantage if anything! :-)

As long as the 2.5mm is correctly protected then I really don't see a big issue. OK, if you're going to redo it then it's worth putting something heavier in there.

I've never really understood the paranoia people seem to have about voltage drop. There's nothing *dangerous* about a bit of voltage drop, it's just a (minor) waste of power. Unless you're really pushing things to the limit a 10% voltage drop (which is *way* more than is 'allowed' normally) won't affect an electric motor all that much.

[Andrew Mawson]

I thought that the major point about voltage drop is that fuses at the far end will take longer to blow and may be outside the safe limits. But then what do I know not being baptised NICEIC !

AWEM

[robgraham]

Many thanks Chris - that was just the point I was going to make. If

2.5mm SWA is rated at 27A in soil (from TLC cable calculator) I can't see why I need to have all this spare capacity. And I'm sorry guys... what is all this bit about 70 oC and 90o C ? With say 10 amps surely the cable is never going to get anywhere near that - these temperatures must be when the cable is running at capacity - 25A plus and if I'm putting a 16A mcb on the line (as the present workshop is) then anything more than 4 mm SWA seems gross overkill.

Rob

[Andy Champ]

Me not baptised either.

But UK mains is 230V +10% -6% so you have to design in a good margin of error.

Andy

[tinnews]

I was assuming that there would be RCD protection for the circuit which will cover most eventualities.

[David Hansen]

On Wed, 10 Dec 2008 00:37:02 -0800 (PST) someone who may be robgraham wrote this:-

"Ordinary" twin and earth is fine, provided it is adequately protected against mechanical damage, weather extremes and so on.

That distance away from the house I would look at the earthing, voltage drop and disconnection times.

[js.b1]

This is why...

1. "Part of the garage is about to become the WORKSHOP"

Workshop implies potential h/d tools...

1. "2kW+ circular saw"

... such as this one - 2000W continuous. Startup 30A?

1. "Lighting"

... so about 100W.

1. "Deep Freeze"

... startup is 30-40A. Continuous 210W.

1. "In winter electric heater"

... 2000W potentially continually.

Ok...

1) Identify Circuit Design Current (Ib)

Continuous load is 4300W. Startup load for freezer & circular at same time 60-70A. That might get interesting if a low In rated Type-B breaker, eg, 20A.

4300W = 18.7A = Ib, Circuit Design Current.

2) Identify OC rating (In) which must > Circuit Design Current (Ib)

Ib = 18.7A, closes OC circuit protective device is 20Amps.

3) Select cable based on Circuit Design Current (Ib)

Tabulated Current Carrying Capacity (It) of 2.5mm SWA is for 90oC due to XLPE insulation, but assume 70oC limit use BS6004.

It at 70oC for 2.5mm SWA is 23Amps, It of 23Amps > In of 20Amps > Ib of 18.7Amps - so all ok.

Cable is 2.5mm SWA.

4) Verify Cable Corrected-Current-Carrying-Capacity (Iz) for Installation Factors > In.

That is, derate cable It figure for installation factors Ambient, Insulation, Grouping & Rewireable fuse (Ca, Ci, Cg, Cr).

Ambient - I assume 30oC. Insulation - I assume cable is not surrounded in insulation. Grouping - I assume cable is not grouped. Rewireable fuses - I assume not used.

So no adjustment required, Iz = It in this instance.

5) Verify Cable Selection & Length satisfies Voltage Drop Limitations.

17th requires lighting to 3%, power to 5% IIRC. That 3% of 230V = 6.9V, 5% of 230V = 11.5V.

Cable route.

- Length A - 10m of FTE 2.5mm from house-CU to SWA on house wall

- Length B - 25m of SWA 2.5mm into garage-CU

- Length C - 10m of FTE 2.5mm into sockets or lights

Voltage drop for Length A @ 20Amps...

- 10m x 20Amps x 18mV/A/m = 3600mV = 3.6V Voltage drop for LengthB @ 20Amps...

- 25m x 20Amps x 19mV/A/m = 9500mV = 9.5V Voltage drop for LengthC @ 20Amps...

- 10m x 20Amps x 18mV/A/m = 3600mV = 3.6V

i) Your total voltage drop on 25m of 2.5mm SWA alone is 9.5V.

17th regs limit lighting to 6.9V drop, power to 11.5V.

ii) Voltage drop on startup is going to be significantly worse. However a circular saw does NOT start in a stalled state.

6) Calculate EFLI, Fault Current, Shock Voltage & Disconnect Time

EFLI Zs = Supply-Resistance Ze + (Phase R1 & CPC R2 resistance).

Length A R1+R2 & Length C R1+R2...

- R1 (Phase) = 10m * 7.41mohm at 20oC or 8.89mohm at 70oC

---- for length of 10m = 88.90mohm

- R2 (CPC) = 10m * 12.10mohm at 20oC or 14.52mohm at 70oC

---- for length of 10m = 145.20mohm

- R1+R2 = 88.90+145.20 = 234.10mohm

Length B R1+R2

- R1 (Phase) = 25m * 7.41mohm at 20oC or 8.89mohm at 70oC

---- for length of 25m = 222.35mohm

- R2 (CPC) = 25m * 7.41mohm at 20oC or 8.89mohm at 70oC

---- for length of 25m = 222.35mohm

- R1+R2 = 222.35+222.35 = 444.70mohm

Length A-B-C R1+R2 = 234.1+444.7+234.1 = 912.9mohm - 0.91ohm at 70oC.

70oC is used because in a fault condition high currents flow, heating, which increases resistance.

i) Assuming your supply is TN-C-S, Ze < 0.35ohm.

- EFLI Zs = 0.35 + 0.91 = 1.26ohm

ii) Assuming your supply is TN-S, Ze < 0.80ohm.

- EFLI Zs = 0.80 + 0.91 = 1.71ohm

Fault current under i) at 230V = 182A. Fault current under ii) at 230V = 134A.

A 20A Type-B MCB trips at 5*In or 100A. Under both i) & ii) your fault current is >100A so ok.

Fault disconnect time will be 0.1sec at 5*In.

Prospective shock voltage for Length A-B-C R2 is...

- Pros Shock Voltage = Fault Current * A-B-C CPC R2

- Pros Shock Voltage = 182 * (145.20+222.35+145.20/1000)

- Pros Shock Voltage = 182 * (512.75/1000) = 93.32V

For a shock voltage of 93.32V you need a fast disconnect time. However we know a Type-B MCB trips at 5*In or 100A in 0.1sec. Ok.

Actual shock voltage for is thus 100 * (512.75/1000) = 51.28V. The wrong side of 50V for a touch of an appliance casing.

So most people would prefer a larger CPC. That could be achieved by combining SWA armour (steel) with a core (I'm assuming 3 core SWA BTW).

7) Verifying CPC CSA for Disconnect Time

We want to verify via the adiabatic calculation that the CPC will not overheat.

Assuming copper CPC throughout, k = 115 at 70oC (IEE 16th Table 54C).

- CPC min-size = SQRT (Fault-Current^2 * disconnect-time) / 115

- CPC min-size = SQRT (182^2 * 0.4) / 115 = 1mm CPC.

The FTE 2.5mm has a 1.5mm CPC > CPC min-size so ok.

Basic problem with 2.5mm SWA at 20A is voltage drop.

- For 2.5mm SWA *alone* Vdrop @ 20Amps = 9.5V.

- For 4.0mm SWA *alone* Vdrop @ 20Amps = 5.5V.

- 17th requires lighting to 3%, power to 5% IIRC.

- That 3% of 230V = 6.9V, 5% of 230V = 11.5V.

Even 4.0mm SWA does not leave much room.

- Any House-end cable run before the 25m of SWA

- Any Garage-end cable run (within the shed)

Next problem may be your fuse of 20A may not be enough.

- May not be enough for the circuit - 4300W is 18.7A.

- May not be enough if saw & freezer startup together.

Remember a Type-B 20A MCB will trip

[robgraham]

JS - very many thanks indeed - that is a tour-de-force and should be added to the DIY Wiki as an example of how these calculations can/ should be done. I will certainly paste this lot out and file it for reference, and study it in detail this evening.

The only variation is that I'm TT earth, and will put a new earth point near the outbuilding. If I remember rightly I don't isolate this new earth point from the existing house one ?

Many thanks again

Rob