garage power help from the pros

[chen77]

Hi

I am studying to be an electrican and I quite often set myself some challenges to see if i'm learning well.

My mate is wanting to run a new supply to his garage. So I thought I would do some calculations to see if I can work out the cable required correctly. Could someone tell me if I am correct with my answer.Also there are a couple of questions at the end as well.

He has a TNCS 230v supply

His usage will be about 8.48KW (he has a lot of power tools and three ponds for koi)

It we be run from a 17th edition consumer unitin the house. So I thought use a 40amp MCB.

From consumer unit in house to consumer unit in garage is about 25 meters.

The cable would be a swa thermosetting cable clipped directly to the wall.

Ive worked out that he should use a 4mm cable. Would you agree?

Also ive seen on previous threads not to connect the earth to the earth on the house, to create a new earth path from the garage using an earth rod in the ground. Is this correct? and if so what is the reason behind it.

And also would you use a 2 core cable? if so would you connect the armoured cable to the earth on the garage side and not the house side?

If I have missed anything please let me know.

behind

 

[Andy™]

not done any calc, but 4mm on 40a over 25 doesnt sound right

as for the armour earth, this MUST always be earthed at the supply end. if TTing the other end, then isolate the armour earth at the load end

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TLC volt drop calc shows 10mm

 

[sellers]

A few things you need to consider,

Diversity (you may have already done that)

Current Carrying Capacity

Volt Drop

Giving the answers would be easy

 

[ccfc]

Hello

Is there any water or gas in garage. Is it a metal framed constrution.

If so id TT it if not imho no problem exporting earth from PME supply if none of above. If there is above need bonding if so need to extend equipotental zone from house with a 10mm earth.

 

[chen77]

I sat down with my copy of BS7671 and worked out the current carrying capacity and volt drop. And I came up with 4mm2 on both of them.

I just wanted to make sure I had worked it out correctly.

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CCFC

No there is no water or gas and it is brick built.

If there were any of the above why would you use TT and not the PME supply?

 

[sellers]

You could show your workings?

Have you allowed for diversity? For example, the customer isn't going to be using all power tools all at once.

 

[chen77]

[quote name='Andy

 

[Andy™]

i only get 4mm on TLC volt drop calc if i use non-lighting circuit

even using non-lighting circuit, 4mm gives a 4.8% volt drop. which means all your final circuits (sockets) will have to achiece 0.2% volt drop. unless the sockets are right next to the CU, then you aint going to achieve this. and if you add a light, then you have no chance of complying

use 10mm and you have a VD of 1.9%. giving 3.1% to play with for sockets, and 1.1% for lighting

 

[Electroglow]

10mm SWA with exported earth .. Run off a 40a mcb into a garage cu

 

[chen77]

You could show your workings?Have you allowed for diversity? For example, the customer isn't going to be using all power tools all at once.

With ref to the diversity I worked it out to worse case senaryo all the pond pumps, heaters and filters running, lights on in the garage radio on, outside lights and heater and using the chop saw (since this is the tool which use,s the most watts and he can only use one tool at a time)

40 diveded 1.03 (not sure if I was to use this for ambient temp of 25deg since the cable would be outside) =38.83 amps.

Whic on table 4E4A for a two core cable would be 4mm.

Then for volt drop

12 mV/A/m x 36.96 25m diveded by 1000 =11.088 (11.1 volts which is JUST under the 11.5 5% drop) so 4mm would be ok

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[quote name='Andy

 

[steptoe]

He has a TNCS 230v supply

the PME supply

so, is it PME or simply TNCS?

yes, it makes a massive difference.

Electroglow:10mm SWA with exported earth

yoiu feel happy about saying that without knowing the actual logistics, site layout, usage, or even the correct earthing system? never mind a load of other stuff to be taken into account.

on your cable calcs chen, are you using PVC or XLPE?

my own choice would be NO LESS THAN 10mm on a 40A MCB over that distance.

 

[SLIPSHOD & SLAPDASH]

I would not connect through any Rcd in the house, but only to a Mcb with an Rcd at the garage end.

Very confusing this '17th edition board' terminology as any board fitted today is one, isn't it ? Its how the circuits are wired to it that makes it compliant.

 

[chen77]

He has a TNCS 230v supply

the PME supply

so, is it PME or simply TNCS?

yes, it makes a massive difference.

Electroglow:10mm SWA with exported earth

yoiu feel happy about saying that without knowing the actual logistics, site layout, usage, or even the correct earthing system? never mind a load of other stuff to be taken into account.

on your cable calcs chen, are you using PVC or XLPE?

my own choice would be NO LESS THAN 10mm on a 40A MCB over that distance.

I was using XLPE

 

[Andy™]

I was using XLPE

and i assume your using appropriate terminals, i.e those designed for 90

 

[alpha spark]

If there were any of the above why would you use TT and not the PME supply?

 

[Zeespark]

If you lose the means of earthing on a TNCS, you also lose the neutral, not so with TT.

AndyGuinness

 

[alpha spark]

Where in 7671 or osg does it say this really didnt know this and I've seen lads of garages done with out tt is it a new reg

Thanks though Andy news to me

 

[Zeespark]

TNCS is supplied in concentric cable is it not? and exporting earth on a TNCS system is a no no according to the DNO's.

AndyGuinness

 

[kme]

Look back through the previous forum threads; using the "search" facility, for "exported earth". There are a few, including "the" big `un! They should help to explain the issues surrounding this.

4mm XLPE isn`t going to be sufficient for your requirements, IMHO; and, as was previously touched on, the rating for XLPE is allowing for a 90 deg C conductor temp - "normal" termination methods aren`t rated above 70 deg.

It is probably also worth noting the RCD requirements; especially in regard to avoidance of nuisance tripping......just something else for you to think on.........

KME

 

[Zeespark]

Alpha, treat BS 7671/OSG as a starting point for wiring regulations, not the be all and end all, there is far too many regulations in existence to fill them inside one publication, and BS 7671/OSG is not enforceable by statute law in of themselves.