Cable to garage

[johnb2713]

Hi,
I am a sparkie by trade, now retired but have always done heavy electrical work i.e. rolling mills, arc furnaces etc.

I need to run a cable to my garage, it already has one but I'm to install a battery and inverter in their which will need a bit more capacity. I intend to run the cable under my block paved drive and really need to feed it from the Henley blocks / meter tails in the external meter cabinet. The cable run will be around 10 metres end to end and I can squeeze into my meter cab and isolator and mcb to protect the cable. Looking at my incoming 100A supply, it seems way too small, but it's there and has a 100amp fuse in the termination unit.

Could I please have your thoughts on cable size, I'm thinking SWA 10mm+ ? 50 amp breaker between this cable and Henleys, or should run a cable duct and meter tails? I have to run conduit / duct anyway for the CT's. Is it OK to use the armour as earth or is that now frowned upong and therefore 3 core cable needed?

Yes, i do know the DNO's can frown on equipment added to meter cabinets but I do have room in mine !

Any help / advice gratefully received.

 

[Barx]

Stick with SWA John, then there is no need for RCD protection at the supply end. You can still use the Armour as CPC on that size cable, but most tend to just install 3c now a days. Personally, I would run 3c and banjo the armour both sides too. Best of both worlds.
Also, the swa can be direct burried, but if in a duct, it makes it replaceable if needs be.

 

[Fleeting]

Stick with SWA John, then there is no need for RCD protection at the supply end. You can still use the Armour as CPC on that size cable, but most tend to just install 3c now a days. Personally, I would run 3c and banjo the armour both sides too. Best of both worlds.
Also, the swa can be direct burried, but if in a duct, it makes it replaceable if needs be.

Why do you assume there is no need for rcd protection.

 

[Barx]

Why do you assume there is no need for rcd protection.

Distribution circuit, run in SWA (so mechanically protected). Why would you RCD protect it?
You owuld run it to a Consumer Unit within the garage and that is where your RCD protection goes, as required for the local circuits.

 

[Fleeting]

Distribution circuit, run in SWA (so mechanically protected). Why would you RCD protect it?
You owuld run it to a Consumer Unit within the garage and that is where your RCD protection goes, as required for the local circuits.

You assume fault protection is satisfied without an rcd?
We would need more information before we make a judgement,

 

[johnb2713]

Why do you assume there is no need for rcd protection.

That would be covered in the garage consumer unit, little point in having 2 of them in series?

 

[johnb2713]

Stick with SWA John, then there is no need for RCD protection at the supply end. You can still use the Armour as CPC on that size cable, but most tend to just install 3c now a days. Personally, I would run 3c and banjo the armour both sides too. Best of both worlds.
Also, the swa can be direct burried, but if in a duct, it makes it replaceable if needs be.

Thanks for the advice, I will banjo each end as you say.

 

[johnb2713]

You assume fault protection is satisfied without an rcd?
We would need more information before we make a judgement,

What other information is needed? I propose at the meter end to have a 50A MCB and a two pole isolator switch, in the garage a standard metal clad disboard with RCD protection.

 

[Fleeting]

That would be covered in the garage consumer unit, little point in having 2 of them in series?

What about the swa supply cable will your 50A protective device afford fault protection.

 

[Fleeting]

What other information is needed? I propose at the meter end to have a 50A MCB and a two pole isolator switch, in the garage a standard metal clad disboard with RCD protection.

Earthing system, Ze, type of protective device not just 50A, your calculated Zs.

 

[Fleeting]

It also requires notification under Part P of the Building Regulations.

 

[johnb2713]

What about the swa supply cable will your 50A protective device afford fault protection.

Care to enlighten me?

The way I have viewed it is it's different to the supply cable to my house, in fact it's better than that because my proposed cable is bigger and the MCB lower rating than the fuse on my incoming supply.

In practical terms I'm dont really see the difference from a safety perspective between what I have now (garage fed from CU in house) and what I'm proposing, all I have done is remove a couple of devices in the circuit and used a bigger cable.

Thanks for your help.

 

[johnb2713]

It also requires notification under Part P of the Building Regulations.

May I ask why it would require notification and who I need to notify?

 

[Fleeting]

The circuit to the garage must comply with ADS (Automatic Disconnection of Supply). It is possible your 50A device may satisfy the requirement but without more info as in #10 we cannot say.

 

[Fleeting]

May I ask why it would require notification and who I need to notify?

Building Control it is a legal requirement for what you intend to do.

 

[johnb2713]

Building Control it is a legal requirement for what you intend to do.

Replacing an existing cable needs notification?

I've just had a quick read at 2.7 which says "Regulation 12(6a) sets out electrical installation work that is notifiable. All other electrical installation work is not notifiable namely - additions and alterations to existing installations outside special locations and replacements, repairs and maintenance anywhere"

Looking at 12(6a)
I'm not installing a new circuit or consumer unit, I'm replacing an existing cable?

I suppose it may be on a thin line feeding from the Henley blocks rather than the CU.

How difficult (and expensive) is it to make the notification?

J

 

[binky]

Replacing an existing cable needs notification?

I've just had a quick read at 2.7 which says "Regulation 12(6a) sets out electrical installation work that is notifiable. All other electrical installation work is not notifiable namely - additions and alterations to existing installations outside special locations and replacements, repairs and maintenance anywhere"

Looking at 12(6a)
I'm not installing a new circuit or consumer unit, I'm replacing an existing cable?

I suppose it may be on a thin line feeding from the Henley blocks rather than the CU.

How difficult (and expensive) is it to make the notification?

J

that is basically a new cct, plus it's a 'special location' ie outdoors.

 

[Fleeting]

You are installing a new circuit however you look at it along with some kind of distribution board.

 

[johnb2713]

You are installing a new circuit however you look at it along with some kind of distribution board.

Thanks guys, I'll talk to building control tomorrow, I see I can do it by phone here.

 

[Barx]

Care to enlighten me?

The way I have viewed it is it's different to the supply cable to my house, in fact it's better than that because my proposed cable is bigger and the MCB lower rating than the fuse on my incoming supply.

In practical terms I'm dont really see the difference from a safety perspective between what I have now (garage fed from CU in house) and what I'm proposing, all I have done is remove a couple of devices in the circuit and used a bigger cable.

Thanks for your help.

What @Fleeting is getting at is that you need to make sure your current Ze, plus your projected r1+r2 of the cable (which gives you a anticipated Zs value for the circuit) is within the limits of the Max allowable Zs of the 50A MCB you intend to use. The Maz Zs value being different if the type of MCB is a B, C or D type.