Outbuilding

[Traineeboy]

Hi guys I’m new to this forum I hope you can help. I am an apprentice and while on lockdown I am continuing to study my electrical course. When we come out of lockdown there is job that I will be on with my boss that he has asked me to kind of oversea to help me gain experience and confidence. He has asked me while on lockdown to study the scenario as if it was my own job. 

The main incoming fuse is 100A and the Earthing system is a TN-S and it is split via Henley block to which meter tails go to the house and one set of cable goes to an outbuilding. In the out building it goes to an single RCD board. From this board it powers a few circuits in the out building and an outside DB which powers a pond pump and some outside lights. Another distribution board is connected to the outbuilding that goes to a large shed. This is just used to power a few sockets and lighting. 

The customer is wanting to run another distribution circuit from the outbuilding to a new building which will be a gym/games room. The Length of cable needed would be 400ft. This new building would need to have 2 rings at 32A and lighting 6A and a some heating 32A. 

In the current outbuilding I would change the current RCD board to an RCBO board to prevent unwanted tripping to various DB circuits and make it easier to find a fault in future.

A few questions I just want to clarify .........

As it is a long run of cable ( 400ft) using 16mm2 I would use a 2 core armoured cable ( using an earthing nut on the supply side to earth the steel wire ) to the new building. I would then not earth nut at the new building end but install an earth rod. Would this be the correct thing to do ? 

Hope this makes sense. If you need me to give more detail in order for you to help me please let me know. Thanks 

 

[binky]

That's one way of doing it. Now look up exporting earths, and equivalent CPC values of SWA armour.

I wouldn't RCBO sub main - no need.

 

[ProDave]

So what protects the cable from the supply to the first outbuilding?  I just see   "split via Henley block to which meter tails go to the house and one set of cable goes to an outbuilding. In the out building it goes to an single RCD board."  No mention of even a fuse to protect that first cable?

Then look up voltage drop and work out what size cable you will need for a 400ft run to keep the voltage drop to an acceptable value.  you might e quite shocked at how big it has to be.

 

[Murdoch]

what is the cable size to the existing outbuilding?

What is the distance from the origin to the first outbuilding?

Without these details you can't start a calculation that would give a meaningful solution

 

[Traineeboy]

So what protects the cable from the supply to the first outbuilding?  I just see   "split via Henley block to which meter tails go to the house and one set of cable goes to an outbuilding. In the out building it goes to an single RCD board."  No mention of even a fuse to protect that first cable?

Then look up voltage drop and work out what size cable you will need for a 400ft run to keep the voltage drop to an acceptable value.  you might e quite shocked at how big it has to be.

Hi Pro Dave thank you that’s helpful. When I say Single RCD board in the outbuilding I mean a CU with a 100a double pole switch with an RCD not just a garage unit with RCD as main switch. I presume that’s what you mean when you ask what fuse the cable from the Outbuilding to the supply, it would be the 100A fused switch. 
 

Sorry it was my wording. Yes I hear you about volt drop and i presume I also need to obtain the distance from the current outbuilding to the supply also. Hope that clearer if no please ask as I appreciate your help. Thanks 

 

[Traineeboy]

what is the cable size to the existing outbuilding?

What is the distance from the origin to the first outbuilding?

Without these details you can't start a calculation that would give a meaningful solution

Thank you I will try and find that information out. If say it was 100ft then I presume I would work out the volt drop based on 400ft of cable even through there would be a CU joining the cable ? That obviously there would be some voltdrop already between the Origin and the outbuilding. 

 

[Traineeboy]

That's one way of doing it. Now look up exporting earths, and equivalent CPC values of SWA armour.

I wouldn't RCBO sub main - no need.

My thought behind putting an RCBO board in the outbuilding is currently the garage is fed of the outbuilding RCD so if the garage trips then it trips the outbuilding. A new board is needed anyway in the outbuilding. Otherwise I guess I could install a duel RCD board which would be cheaper but run the garage CU, Pond DB and the new building off separate MCB off the non RCD protected side. I could do this as all 3 of the 

distribution circuits would be mechanically protected. But either way and I right in thinking both ways are correct but obviously one is cheaper ? 

 

[Murdoch]

Thank you I will try and find that information out. If say it was 100ft then I presume I would work out the volt drop based on 400ft of cable even through there would be a CU joining the cable ? That obviously there would be some voltdrop already between the Origin and the outbuilding. 

Volt drop needs to take into account all the length from the service head so you need 100 + 400 = 500 foot

ouch

but first you need to clarify the ccc for the cable to the first building and the design load .... as you may not be able to add the 2 nd building at all

 

[Traineeboy]

Volt drop needs to take into account all the length from the service head so you need 100 + 400 = 500 foot

ouch

but first you need to clarify the ccc for the cable to the first building and the design load .... as you may not be able to add the 2 nd building at all

great thanks if say we can’t add to the existing outbuilding then because the tails are already spilt into two ( one the house and one to the outbuilding) then can a new Henley block be put in place to spilt the tails 3 ways ?  Then that way it can run straight from the Henley block to the new outbuilding ? What do you think ? 

 

[ProDave]

great thanks if say we can’t add to the existing outbuilding then because the tails are already spilt into two ( one the house and one to the outbuilding) then can a new Henley block be put in place to spilt the tails 3 ways ?  Then that way it can run straight from the Henley block to the new outbuilding ? What do you think ? 

You are still not explaining what you have very well,  It still sounds like Henley block direct to cable feeding outbuilding.  The issue there is you cannot rely on the suppliers 100A fuse to protect your cable.  The feed to that outbuilding needs to be fed by at least a switch fuse with a fuse appropriate to the cable and certainly less than 100A

Normal Henley blocks have 5 connections so you can split to 4 destinations with one pair of Henleys.

 

[Traineeboy]

You are still not explaining what you have very well,  It still sounds like Henley block direct to cable feeding outbuilding.  The issue there is you cannot rely on the suppliers 100A fuse to protect your cable.  The feed to that outbuilding needs to be fed by at least a switch fuse with a fuse appropriate to the cable and certainly less than 100A

Normal Henley blocks have 5 connections so you can split to 4 destinations with one pair of Henleys.

great thanks there is a double pole switch In between the Henley block and the service fuse but I understand what you are saying now. 
 

I guess the safer option would be to probably run a cable direct from the new building straight into a Henley block but having a switched fuse appropriate to that cable. 

 

[Murdoch]

I guess the safer option would be to probably run a cable direct from the new building straight into a Henley block but having a switched fuse appropriate to that cable. 

so now you can do your calcs based on 400 foot and your maximum design load ..........

 

[binky]

OK, with TN-S you can export the earth, so no need ot TT any part of the ysstem, although some on here would rather do it that way . Sub mains don't need RCD protection, but abvioulsy RCBO every other cct to compky with todays regs. 

I'm a bit of a fan of 2 core swa and usuing armour for earthing, however, 3 core cable tends to be cheaper these days as there is little demand for 2 core.

 

[Murdoch]

whats the installation method for the cable going to be?

 

[Traineeboy]

whats the installation method for the cable going to be?

Underground in depth of 600mm I know it’s only got to be 500mm but just in case of double digging it will be 600mm. 

 

[Traineeboy]

OK, with TN-S you can export the earth, so no need ot TT any part of the ysstem, although some on here would rather do it that way . Sub mains don't need RCD protection, but abvioulsy RCBO every other cct to compky with todays regs. 

I'm a bit of a fan of 2 core swa and usuing armour for earthing, however, 3 core cable tends to be cheaper these days as there is little demand for 2 core.

OK, with TN-S you can export the earth, so no need ot TT any part of the ysstem, although some on here would rather do it that way . Sub mains don't need RCD protection, but abvioulsy RCBO every other cct to compky with todays regs. 

I'm a bit of a fan of 2 core swa and usuing armour for earthing, however, 3 core cable tends to be cheaper these days as there is little demand for 2 core.

Thanks again. In relation to earthing the building I’m told will have steel construction. I know any metal work that comes into contact with earth needs to be bonded. If the steel construction is continuous then one bonding cable will do but if say there is a break between and it’s not continuous then would I bond each part ? Hope that makes sense ?

 

[Murdoch]

Underground in depth of 600mm I know it’s only got to be 500mm but just in case of double digging it will be 600mm. 

What makes you think it should “only” be 500 mm?

 

[Traineeboy]

What makes you think it should “only” be 500 mm?

unless I’ve mis read my Electricians guide to the building regulations  book it states 500mm. But is says if double digging is to occur then 600mm. Please correct me if I’m wrong. P82 5.7.2 Fixed cables. 

 

[Traineeboy]

unless I’ve mis read my Electricians guide to the building regulations  book it states 500mm. But is says if double digging is to occur then 600mm. Please correct me if I’m wrong. P82 5.7.2 Fixed cables. 

I know it says preferably deeper but at least 500mm so that’s why it will probably be 600mm. Yes you can go deeper but at long as it is at least 500mm deep or 600 mm it double digging is to occur then it complies to regulations. But again sorry if I’m wrong I want to learn so don’t mind being corrected. 

 

[Murdoch]

I know it says preferably deeper but at least 500mm so that’s why it will probably be 600mm. Yes you can go deeper but at long as it is at least 500mm deep or 600 mm it double digging is to occur then it complies to regulations. But again sorry if I’m wrong I want to learn so don’t mind being corrected. 

For the record there is no depth stipulated in BS 7671 .... AFAIK