UK Underground Wiring

[Khuz360]

Hi All

I'm looking to run a 10/16mm SWA cable from the main house to the Garage. Currently the power supplied to the garage is with a twin and earth cable in some plastic pipe. The plastic pipe is buried under the conservatory and concrete in the garden areas.


The plastic pipe is of sufficient size that a 10mm cable could in theory be pulled through. Note I did say in theory.

My question is two parts.

1. Is my theory correct in that it is possible to attach the 10mm SWA cable to the existing cable and pull it through.

2. If I were to dig up the concrete garden areas, is it feasible for a domestic sparky to drill through the base of the conservatory roughly 6m long because tearing down the conservatory is an absolute no go.

Thanks

K

 

[pc1966]

Option 2 is your cheapest option, but the spark would have to be satisfied the system was safe, and at the very least determine they cable type and condition before deciding if it can be safely reused, and if so what the load limit should be for it. However it might not be sufficient for your goals.

Option 1 with 6mm SWA certainly would meet the safety aspect, the issue is the total load limit you can get away with. To compare with what is already in, and based on 11.5V and 50m cable drop your limits are:

​	​	Vdrop​	11.5​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	R (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I (A)​
1.5​	11.8​	31​	1.55​	7.4​	22​	7.4​
2.5​	13.2​	19​	0.95​	12.1​	29​	12.1​
4​	14​	12​	0.6​	19.2​	37​	19.2​
6​	14.4​	7.9​	0.395​	29.1​	46​	29.1​
10​	18​	4.7​	0.235​	48.9​	60​	48.9​
16​	19​	2.9​	0.145​	79.3​	78​	78.0​

Diameter is for 2 core SWA to BS5467 from Prysmian data sheet. The SWA resistance might also limit the current on breaker trip but you could get round that at modest expense with an up-front 100mA S-type RCD, or look at using 3-core SWA (slightly larger diameter) so you can disconnect in under 5s on a short.

Realistically 6mm is the max that you could expect to pull through at 20mm ID pipe, so 29A is your limit unless you play with adding a joint to 10mm or 16mm, etc.

Option 0 is using split concentric provided you can be satisfied it is sufficiently protected. If all of the buried duct section is under concrete already then it would be OK, otherwise as davesparks points out, it needs protection against damage if in the ground and anywhere it could be dug up in the future. It may be practical to put in duct to BS EN 61386-24 for those areas, but that would need to be factored in to the overall cost and the cable is not cheap, at £6.36/m for 16mm (first on-line figure I found), but is has more usable CSA per diameter than other cables (and is far better suited to outdoor use than T&E!)

 

[Spoon]

@pc1966 Just a reminder mate: The OP wants lighting in the garage, so the max volt drop is 3%, not 5%

 

[pc1966]

@pc1966 Just a reminder mate: The OP wants lighting in the garage, so the max volt drop is 3%, not 5%

Ah, OK that is another thing to consider. It is a surprise it is still the 3% given most lights are now LED and not the voltage-sensitive filament lamps, but there you go!
[automerge]1589910604[/automerge]
And by the miracle of a spread sheet:

​	​	Vdrop​	6.9​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	Res (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I​
1.5​	11.8​	31​	1.55​	4.5​	22​	4.5​
2.5​	13.2​	19​	0.95​	7.3​	29​	7.3​
4​	14​	12​	0.6​	11.5​	37​	11.5​
6​	14.4​	7.9​	0.395​	17.5​	46​	17.5​
10​	18​	4.7​	0.235​	29.4​	60​	29.4​
16​	19​	2.9​	0.145​	47.6​	78​	47.6​

 

[JD6400]

What sort of distance does the mole need to be , and could you post up some pictures of the run ?
That price was outright vicious !?
You can send me a PM if you like .

 

[davesparks]

Option 0 is using split concentric provided you can be satisfied it is sufficiently protected.

The electrician doing the job also needs to be satisfied that the installation method is suitable.
I'm sure a lot of us would have different opinions on what would, or would not be suitable arrangements when using split concentric.

 

[pc1966]

The electrician doing the job also needs to be satisfied that the installation method is suitable.
I'm sure a lot of us would have different opinions on what would, or would not be suitable arrangements when using split concentric.

I meant "you" in the broad sense of addressing the customer (OP) and the electrician they select who ultimately signs off on any design.

None of us have seen the site, or known more than a few descriptions posted here, so until they get someone in to look at the situation and options available all we can do is make suggestions of what might be an acceptable solution for various possible situations.

 

[Khuz360]

You say that the existing cable is wired back to your consumer unit.
Does it wire back to it's own breaker? (I'm presuming it is but it's not good to presume everything) If not then what circuit does it wire back to?
What size is the breaker?
Why add a consumer unit in the garage now?
I'm presuming the 'fuse box' which is in the garage, is a fused spur? (See.... presuming things again)
Why not stick with the fused spur until you upgrade the cable to the garage?

So the existing cable runs to a an old style consumer unit probably known as a fusebox with wired round fuses, that you would need to wrap new fuse wire round if it ever blew the black type casing. I don't know the size of the fuse breaker.

The current unit in the garage is a fuse box of some description and again I don't know the size. The reason for replacing it is one I don't believe it is safe and two I the circuits within the garage to be separated properly. I've attached images for reference.

Sticking with the current system, can lead to problems if I upgrade the lighting and start to use more equipment even if using the low power variety.

Option 2 is your cheapest option, but the spark would have to be satisfied the system was safe, and at the very least determine they cable type and condition before deciding if it can be safely reused, and if so what the load limit should be for it. However it might not be sufficient for your goals.

Option 1 with 6mm SWA certainly would meet the safety aspect, the issue is the total load limit you can get away with. To compare with what is already in, and based on 11.5V and 50m cable drop your limits are:

​	​	Vdrop​	11.5​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	R (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I (A)​
1.5​	11.8​	31​	1.55​	7.4​	22​	7.4​
2.5​	13.2​	19​	0.95​	12.1​	29​	12.1​
4​	14​	12​	0.6​	19.2​	37​	19.2​
6​	14.4​	7.9​	0.395​	29.1​	46​	29.1​
10​	18​	4.7​	0.235​	48.9​	60​	48.9​
16​	19​	2.9​	0.145​	79.3​	78​	78.0​

Diameter is for 2 core SWA to BS5467 from Prysmian data sheet. The SWA resistance might also limit the current on breaker trip but you could get round that at modest expense with an up-front 100mA S-type RCD, or look at using 3-core SWA (slightly larger diameter) so you can disconnect in under 5s on a short.

Realistically 6mm is the max that you could expect to pull through at 20mm ID pipe, so 29A is your limit unless you play with adding a joint to 10mm or 16mm, etc.

Option 0 is using split concentric provided you can be satisfied it is sufficiently protected. If all of the buried duct section is under concrete already then it would be OK, otherwise as davesparks points out, it needs protection against damage if in the ground and anywhere it could be dug up in the future. It may be practical to put in duct to BS EN 61386-24 for those areas, but that would need to be factored in to the overall cost and the cable is not cheap, at £6.36/m for 16mm (first on-line figure I found), but is has more usable CSA per diameter than other cables (and is far better suited to outdoor use than T&E!)

@pc1966 Just a reminder mate: The OP wants lighting in the garage, so the max volt drop is 3%, not 5%

Ah, OK that is another thing to consider. It is a surprise it is still the 3% given most lights are now LED and not the voltage-sensitive filament lamps, but there you go!
[automerge]1589910604[/automerge]
And by the miracle of a spread sheet:

​	​	Vdrop​	6.9​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	Res (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I​
1.5​	11.8​	31​	1.55​	4.5​	22​	4.5​
2.5​	13.2​	19​	0.95​	7.3​	29​	7.3​
4​	14​	12​	0.6​	11.5​	37​	11.5​
6​	14.4​	7.9​	0.395​	17.5​	46​	17.5​
10​	18​	4.7​	0.235​	29.4​	60​	29.4​
16​	19​	2.9​	0.145​	47.6​	78​	47.6​

Yeah I imagine option 2 is certainly the cheapest just incredibly limiting. Even the original plan of using 10mm was a compromise as I incorrectly believed it could be pulled through the existing pipe. At the very least the 6mm will be safer overall even if I have to compromise on the what I can use in the garage.

Those tables are very handy. Thanks.

You alluded to the VD for lighting at 3% being a surprise becaue of LED lights. I don't think I mentioned this earlier, however the lights I'm installing are going to be LED. Will that change anything?

What sort of distance does the mole need to be , and could you post up some pictures of the run ?
That price was outright vicious !?
You can send me a PM if you like .

I'm based in the North East, so it may be a trek. The company are the only ones in the area who do the work. I understand the pricing and as eye watering as it is, I can't fault them.

The mole needs to be no more than 6m long which will be under a conservatory. The area in front and behind would be clear and excavated.

I meant "you" in the broad sense of addressing the customer (OP) and the electrician they select who ultimately signs off on any design.

None of us have seen the site, or known more than a few descriptions posted here, so until they get someone in to look at the situation and options available all we can do is make suggestions of what might be an acceptable solution for various possible situations.

I appreciate everything being said here is purely hypothetical and do not take anything said here as gospel, unless it's a fact like pulling a 19mm cable through dubious 20mm pipe might be impossible. I understand the necessity to obtain professional advice from a registered sparky who has visited the site and made an assessment. I am simply arming myself with knowledge.

 

[pc1966]

So the existing cable runs to a an old style consumer unit probably known as a fusebox with wired round fuses, that you would need to wrap new fuse wire round if it ever blew the black type casing. I don't know the size of the fuse breaker.

The current unit in the garage is a fuse box of some description and again I don't know the size. The reason for replacing it is one I don't believe it is safe and two I the circuits within the garage to be separated properly. I've attached images for reference.

To say they are unsafe if in good condition is not quite right, they met the wiring regulations of many years ago.

But they are far less safe than a new CU would be for a couple of reasons and upgrading both is probably the single best thing you can do since a new CU offers:

• RCD protection - a major reduction in shock risk (and to a smaller degree fire).
• MCB trip on a lower overload and let through a lot less fault energy (at least up to the 1kA or so fault current) than fuses.
• MCB can be reset easily by an unskilled (electrically speaking) person, and no risk of wrong fuse wire being used.
• MCB can be used to isolate/restore a circuit without powering everything off (never ever change a rewirable fuse with the power on!).
• It is also going to result in you house wiring being inspected/tested (done properly), either as an EICR before the CU change or as part of the deal. That can find faults that are not noticed in normal (no-fault) operations.

Also if you need any new circuits having a CU with spare slots allows better separation so faults are contained to less impact.

You alluded to the VD for lighting at 3% being a surprise because of LED lights. I don't think I mentioned this earlier, however the lights I'm installing are going to be LED. Will that change anything?

The reason for the wiring regulations having a maximum voltage drop of 3% on lighting circuits (compared to 5% on other) is to reduce the impact of load changes on the voltage, and resulting change in brightness.

Old filament lights are very sensitive to voltage, so a few percent has a noticeable effect, hence this concern. However LED lights have nowhere near the same change in brightness with voltage so they probably don't need the 3% rule.

But the regulations as they stand still say 3% for lights, and still advise on assuming 100W per light fitting when computing power use!

I appreciate everything being said here is purely hypothetical and do not take anything said here as gospel, unless it's a fact like pulling a 19mm cable through dubious 20mm pipe might be impossible. I understand the necessity to obtain professional advice from a registered sparky who has visited the site and made an assessment. I am simply arming myself with knowledge.

Some folk might think "arming yourself" sounds like it is a showdown with the electrician as many have had problems before with customers who think they know a lot more than they really do. But if you treat some of this discussion as points to go over with the sparky for ideas on how some of the issues could be solved it should be fine, and we don't know the details of your setup so none of us can do the job remotely. Whoever you do get will have to go over all of the calculations, etc, any way.

Good luck!

 

[JD6400]

No that is a vicious price .
Depending on the size of bore , ground type , if the pits have been prepared for us and distance to travel , we start at £250 plus vat upwards.
If you were local to Buckinghamshire your one would be £250 or £350 if we dig the pit .

I actually live near Skegness and commute each day at the moment to our yard in Buckinghamshire , my day tomorrow consists of a 7:30 am meeting in Milton Keynes then Thame then Southampton then Winchester then Lechlade then Lambourn , back to Oxford and finally home again to Lincolnshire , so you are not that far away to be honest if you wish to pm me some more Pacific information I could take a look and maybe price it for you .

 

[pc1966]

If you can get a large enough duct put in then all your problems go away, you can put in whatever size of SWA power you want and a few networks cables to finish the job!

Well, maybe not total price...

 

[Khuz360]

To say they are unsafe if in good condition is not quite right, they met the wiring regulations of many years ago.

But they are far less safe than a new CU would be for a couple of reasons and upgrading both is probably the single best thing you can do since a new CU offers:

• RCD protection - a major reduction in shock risk (and to a smaller degree fire).
• MCB trip on a lower overload and let through a lot less fault energy (at least up to the 1kA or so fault current) than fuses.
• MCB can be reset easily by an unskilled (electrically speaking) person, and no risk of wrong fuse wire being used.
• MCB can be used to isolate/restore a circuit without powering everything off (never ever change a rewirable fuse with the power on!).
• It is also going to result in you house wiring being inspected/tested (done properly), either as an EICR before the CU change or as part of the deal. That can find faults that are not noticed in normal (no-fault) operations.

Also if you need any new circuits having a CU with spare slots allows better separation so faults are contained to less impact.


The reason for the wiring regulations having a maximum voltage drop of 3% on lighting circuits (compared to 5% on other) is to reduce the impact of load changes on the voltage, and resulting change in brightness.

Old filament lights are very sensitive to voltage, so a few percent has a noticeable effect, hence this concern. However LED lights have nowhere near the same change in brightness with voltage so they probably don't need the 3% rule.

But the regulations as they stand still say 3% for lights, and still advise on assuming 100W per light fitting when computing power use!


Some folk might think "arming yourself" sounds like it is a showdown with the electrician as many have had problems before with customers who think they know a lot more than they really do. But if you treat some of this discussion as points to go over with the sparky for ideas on how some of the issues could be solved it should be fine, and we don't know the details of your setup so none of us can do the job remotely. Whoever you do get will have to go over all of the calculations, etc, any way.

Good luck!

Ok so it's a case of regulations not catching up with current technology, which is understandable given that there'll be plenty of people who still use the old tech.

Poor choice of words lol. What I should say is that with the knowledge I have I can filter out the cowboys who'll promise the everything and not deliver on anything, I'm sure you can agree there are plenty of them around who make finding a quality electrician much more difficult.

No that is a vicious price .
Depending on the size of bore , ground type , if the pits have been prepared for us and distance to travel , we start at £250 plus vat upwards.
If you were local to Buckinghamshire your one would be £250 or £350 if we dig the pit .

I actually live near Skegness and commute each day at the moment to our yard in Buckinghamshire , my day tomorrow consists of a 7:30 am meeting in Milton Keynes then Thame then Southampton then Winchester then Lechlade then Lambourn , back to Oxford and finally home again to Lincolnshire , so you are not that far away to be honest if you wish to pm me some more Pacific information I could take a look and maybe price it for you .

I think it might be a bit too much of a drive. I'll try to find other prices. Thanks for the offer though, I will keep it in mind.

If you can get a large enough duct put in then all your problems go away, you can put in whatever size of SWA power you want and a few networks cables to finish the job!

Well, maybe not total price...

If I can afford to do it and get a bigger duct installed then, I'll probably just go crazy and throw a 35mm cable in so it can handle 100A lol.

 

[Spoon]

If I can afford to do it and get a bigger duct installed then, I'll probably just go crazy and throw a 35mm cable in so it can handle 100A lol.

I know you are joking above, but you can't have 100A to your garage, unless you get a dedicated supply. The supply to your garage has to be lower than your main fuse rating.

 

[JD6400]

No worries , but your first price was way off .
I've just got to the Oxford job so off home soon , should be back by 10pm. ?

 

[pc1966]

Already said by @Spoon is to allow for your supply limit

If you really want to be sure that a worst-case garage fault can't take out your main fuse then you would generally want it fused at less than 60% the DNO rating (checking selectivity would need a bit more care than just that) so you would look at 50A/63A max on a 100A supply fuse, etc.

But out of curiosity you can look at the following for other cables sizes and your 50m run below (based on 2-core cable using Prysmian data sheet):

Drop % =​	3​	Vdrop​	6.9​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	Res (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I (A)​
1.5​	11.8​	31​	1.55​	4.5​	22​	4.5​
2.5​	13.2​	19​	0.95​	7.3​	29​	7.3​
4​	14​	12​	0.6​	11.5​	37​	11.5​
6​	14.4​	7.9​	0.395​	17.5​	46​	17.5​
10​	18​	4.7​	0.235​	29.4​	60​	29.4​
16​	19​	2.9​	0.145​	47.6​	78​	47.6​
25​	22​	1.9​	0.095​	72.6​	99​	72.6​
35​	24​	1.35​	0.0675​	102.2​	119​	102.2​

Drop % =​	5​	Vdrop​	11.5​	V​	​	​
​	​	Length​	50​	m​	​	​
CSA (mm^2)​	Dia (mm)​	Res (mOhm/m)​	R (Ohm)​	Max on VD​	Max 4D4A (70C)​	Max I (A)​
1.5​	11.8​	31​	1.55​	7.4​	22​	7.4​
2.5​	13.2​	19​	0.95​	12.1​	29​	12.1​
4​	14​	12​	0.6​	19.2​	37​	19.2​
6​	14.4​	7.9​	0.395​	29.1​	46​	29.1​
10​	18​	4.7​	0.235​	48.9​	60​	48.9​
16​	19​	2.9​	0.145​	79.3​	78​	78.0​
25​	22​	1.9​	0.095​	121.1​	99​	99.0​
35​	24​	1.35​	0.0675​	170.4​	119​	119.0​