Electrical supply to garage

[Gaz man]

Hi all, first time on here and was hoping for some sound advice, I am a qualified electrician but took a 2 year gap out to venture in different area's but now I'm back (a little rusty round the edges) , I've been called to a job where an electrical supply sub main cable has been run from the main fuse board in the house to a garage approximately 45 metres away, the submain is a 3 x 2.5mm armoured which is protected with a 20amp mcb and 30ma rcd, it roughly has a working load of 2kw with all appliances and lights on at once!! The cable is partly run in the attic under some insulation and then buried outside for the remainder of the run, the house is a TN-S earthing arrangement. The job has been done quite poorly by the previous owner/ DIY enthusiast, all of the armouring has been cut back at both ends, very poor connections etc the list goes on. As it stands the earth in the garage is being used by one of the 3 cores, my question is am I correct in saying that it should be seperated totally from the house as it is outside of the equipotential zone?? If so This is how I propose to sort the cable out, re terminate both ends of the cable, connect the armouring to earth at the main fuse board (house side), disconnect the earth core so that I will only have phase and neutral connected up to the garage and then drive an earth rod down in the ground at the garage!! Does this HAVE to be done this way?? or is it perfectly ok to keep the third core as an earth, as I say I'm a little rusty round the edges as to why you HAVE to have a separate earth in a garage/outbuilding, TT/earth rod systems aren't always the best as the value can change depending on the weather etc, where as if it were connected as part of the TN-S system in the house it would Shorley be of a more constant value, if that is making sense!! If anyone has the time to read through this and shed some knowledge I would appreciate it many thanks in advance.

 

[Octopus]

Punctuation and paragraphs would make this easier to read... and don't mention sheds!

 

[telectrix]

forget earth rods. it's perfectly acceptable to extend the TN-S to the garage. your only concerns are

1. RCD in house means going back to the house to reset if it trips.

2. bonding of any extraneous metalwork.

 

[Hawk81]

Two points first point is why dump a perfectly good earth to TT this? Second point is that the spelling is surely not shorley

 

[Jonspark]

have a kettle in shed/garage other wise it means going back to house to make a brew.

 

[RogertheBodger]

Also, you mention that the Armour has been cut back at both ends, does this mean it is not connected ?
The Armour needs to be earthed at least at the supply end even if you don't intend to use it as a cpc.

 

[gaulty]

re gland, re terminate and use 3rd core as an earth is fine as long as the Zs is within permitted value. If not it might be easier to make garage a TT system but still have your armour earthed. Rather than running a seperate Earth in. But it's fine to extend TNS from house to garage

 

[Guest 004]

All good

 

[urbanmonkey]

9 amps over 45m on 2.5mm will be just under 3% off the top of my head so the 2.5mm should be fine. Also max load on 2.5mm SWA buried is 29A so 20A MCB is compliant.

 

[GB.Kayak]

9 amps over 45m on 2.5mm will be just under 3% off the top of my head so the 2.5mm should be fine. Also max load on 2.5mm SWA buried is 29A so 20A MCB is compliant.

18 mV / A / m

18 x 9 x 45 / 1000 = 7.3 V

Allowable Volt Drop for Lighting

230 x 0.03 = 6.9 V

Therefore OTT in my book ... 'close but no banana'! The whole thing is therefore dependent upon how approximate the length of run is ... 42.6 m would be 'on the nose'. If you take U0 to be the reduced value in BS 7671 Amd 3, the allowable volt drop becomes:

230 x 0.95 x 0.03 = ~ 6.6 V

I am not certain, have not worked out in my own mind, whether it is necessary to apply this additional reduction factor for volt drop. I do not have my copy of the BYB yet! However, this would reduce the allowable run to a shade under 40.5 m.

 

[Geordie Spark]

Two points first point is why dump a perfectly good earth to TT this? Second point is that the spelling is surely not shorley

"Shorley" is the Geordie pronunciation of the female name "Shirley" - as in "Wor Shorley givis a nosh last neet".

 

[Deleted member 26818]

I won't say what the term 'nosh' means down south.

 

[ian.settle1]

I won't say what the term 'nosh' means down south.

The same in Geordie land no doubt and it is nothing to do with food.

 

[Knobhead]

18 mV / A / m

18 x 9 x 45 / 1000 = 7.3 V

Allowable Volt Drop for Lighting

230 x 0.03 = 6.9 V

Therefore OTT in my book ... 'close but no banana'! The whole thing is therefore dependent upon how approximate the length of run is ... 42.6 m would be 'on the nose'. If you take U0 to be the reduced value in BS 7671 Amd 3, the allowable volt drop becomes:

230 x 0.95 x 0.03 = ~ 6.6 V

I am not certain, have not worked out in my own mind, whether it is necessary to apply this additional reduction factor for volt drop. I do not have my copy of the BYB yet! However, this would reduce the allowable run to a shade under 40.5 m.

Lets have it accurate