LSZH SWA cable is cheaper than PVC...am I missing something here?

[Thorfun]

I need approx 30m of 4 core 35mm SWA cable for the connection from the cabinet to the CU in the plant room (damn that stuff is expensive!). I've been doing my research and have found on superlecdirect.com their LSZH SWA cable is £1.01+VAT cheaper per meter (https://www.superlecdirect.com/cables/steel-wire-armoured-cables-bs6724-lszh/bs6724-35mm-4core-xlpe-swa-lszh-cable-harmonised-black/) compared to the PVC version (https://www.superlecdirect.com/cables/steel-wire-armoured-cables-bs5467-pvc/6944x-35mm-4core-bs5467-xlpe-swa-pvc-cable-harmonised-black/).

 

My electrician said that he's previously used LSF cable and it's a pig to strip and as my SWA is going in a duct in the ground he said just get the PVC stuff.

But LSF is different to LSZH, right? so is the LSZH cable also a pig to strip? And if the LSZH cable is cheaper than the PVC why wouldn't you buy it? it's better, right? am I missing something here?

 

I have a LOT of cable to buy and will be agonising over the whole LSZH/LSOH vs PVC argument lots in the near future. can someone also please tell me if it's worth using the LSZH stuff even if it's more expensive than PVC? I would've thought that in a domestic environment by the time the cable catches fire, i.e. has gone through the plasterboard, I've got bigger issues than the PVC burning, right? is LSZH more for commercial buildings when you have to evacuate a multi-storey office block or high-rise buildings like apartment blocks etc

 

Can someone please help to clear up this whole issue for me? 🙂 

[PeterW]

8 minutes ago, Thorfun said:

need approx 30m of 4 core 35mm SWA cable for the connection from the cabinet to the CU in the plant room (damn that stuff is expensive!

Who has sized that ..?? Has the DNO Installed a 3x80 head or is it a 3x100 head ..? You would be pulling 55kW with 3x100 which is industrial power levels not house levels.

 

And how are you dealing with earthing ..? Separate conductor back to the DNO earth point ..?

 

 

[Thorfun]

14 minutes ago, PeterW said:

Who has sized that ..?? Has the DNO Installed a 3x80 head or is it a 3x100 head ..? You would be pulling 55kW with 3x100 which is industrial power levels not house levels.

 

And how are you dealing with earthing ..? Separate conductor back to the DNO earth point ..?

 

 

DNO installed 3 x 100A head. our electrician has sized 35mm 4-core SWA with a separate 35mm earth. our system is a TT earthing system so I will be hammering an earthing rod in to the ground in the not too distant future!

 

 

we're planning on running the house off a single phase but I am planning on utilising 3-phase in the future for EV charging, mostly. so am running the cable for future proofing. I also need a TP&N switch fused isolator which isn't cheap.

[PeterW]

I’d be looking at that differently - what’s the heating system too..?

 

I’d run a single 25 to the house CU, one to the garage (assuming not attached) and leave the 3rd for the car charger. You’ve got more flexibility and you can then terminate each with single fused isolators. Unless you’ve got a big meter housing then you won’t get a TP&N fused isolator in a normal box  
 

 

[Thorfun]

9 minutes ago, PeterW said:

I’d be looking at that differently - what’s the heating system too..?

 

I’d run a single 25 to the house CU, one to the garage (assuming not attached) and leave the 3rd for the car charger. You’ve got more flexibility and you can then terminate each with single fused isolators. Unless you’ve got a big meter housing then you won’t get a TP&N fused isolator in a normal box  
 

 

we do have a big cabinet (https://ccf-fibreglass.co.uk/products/electric-kiosk-housing-meter-box-cabinets-enclosure-grp) so the size of the TP&N isn't an issue. the reason for running everything off a single phase is our Solar PV inverter is single phase and batteries will be on the single phase and so I didn't want to complicate matters by having different phases in the house/garage. we have a room above the garage as well so that is part of the house really. and the garage is attached to the house.

 

our electrician said, as our cabinet is about 25m from the CU we're on the cusp of 25mm and suggested going up to 35mm which I'm fine with, just in case. 

 

we have an 11kW ASHP (Mitsubishi Ecodan) which, again, I will want to power from our single phase PV array. 

 

in theory, I don't yet need to run cables/isolators etc for 3-phase but I'd rather do it now than running for a single phase and then needing/wanting to utilise the 3-phase later down the line and having to run more cables/install more isolators then.

 

hope that makes sense! it does in my mind. 😉 

Edited August 21, 2022 by Thorfun
forgot to mention that the garage is attached

[PeterW]

30 minutes ago, Thorfun said:

we have an 11kW ASHP (Mitsubishi Ecodan) which, again, I will want to power from our single phase PV array. 

 It’s irrelevant - it’s all the consumer side of the meter so doesn’t matter what phase it’s on..? 
 

 

[Thorfun]

9 hours ago, PeterW said:

 It’s irrelevant - it’s all the consumer side of the meter so doesn’t matter what phase it’s on..? 
 

 

Hi Peter. I fail to see how it's irrelevant. my PV inverter is on a single phase and so feeds back to my CU on that single phase. the electricity generated by the PV can only be utilised on that phase so if our ASHP is on another phase then how can the PV generated electricity power it? afaik the only way is to ensure that net metering is in place so that any power drawn by the ASHP on a second phase will be cancelled out by the exported electricity on the first phase.

 

unless I'm completely misunderstanding this whole thing!

[PeterW]

So your 3P meter should net meter but that will depend on whether you are looking at maximising self use and not pushing back to the grid - it also limits your options on expanding your solar as you can put a lot more on a 3P install than you can on 1P without DNO approval  

 

My concern would be that you’ll end up in the situation on a sunny day that you want to charge the car, the battery is full but you’re not properly set up at metering - at that point you would be much better having full 3P at the house and a 3P inverter (they are ~£100 more than single) and do a full 3P install from the outset. Has the electrician not suggested this ..??

[Carrerahill]

15 hours ago, Thorfun said:

DNO installed 3 x 100A head. our electrician has sized 35mm 4-core SWA with a separate 35mm earth. our system is a TT earthing system so I will be hammering an earthing rod in to the ground in the not too distant future!

 

 

 

we're planning on running the house off a single phase but I am planning on utilising 3-phase in the future for EV charging, mostly. so am running the cable for future proofing. I also need a TP&N switch fused isolator which isn't cheap.

What is the cable route?

Edited August 22, 2022 by Carrerahill

[Thorfun]

1 hour ago, PeterW said:

So your 3P meter should net meter but that will depend on whether you are looking at maximising self use and not pushing back to the grid - it also limits your options on expanding your solar as you can put a lot more on a 3P install than you can on 1P without DNO approval  

 

My concern would be that you’ll end up in the situation on a sunny day that you want to charge the car, the battery is full but you’re not properly set up at metering - at that point you would be much better having full 3P at the house and a 3P inverter (they are ~£100 more than single) and do a full 3P install from the outset. Has the electrician not suggested this ..??

ahh....(firstly I must apologise for not giving all the facts)....even though we have a 3-phase supply trying to get a 3-phase meter is next to impossible so I haven't bothered yet. again, as the whole house is being run off a single phase we are having a single phase smart meter installed in a couple of weeks (hence getting the cabinet and CU in ready for that to happen) and then, in the future, we can upgrade to a 3P meter when they become available again and if we deem it necessary. but I want the SWA cable in place from the cabinet to the CU to be able to handle 3P if/when it happens. 

 

yes, we could potentially fit more PV but I've pretty much run out of roof space! so the 10.5kWp is enough for us for now I think. and we already have a single phase inverter here and paid for as I've always only ever wanted to run the house off a single phase. I wanted to avoid the complication of running a house across 3 phases, having a battery that spans 3 phases or 3 battery storage systems (one for each phase) and wanted to avoid any risk of arcing across the phases no matter how small that risk might be. 

[Thorfun]

1 hour ago, Carrerahill said:

What is the cable route?

underground in a 110mm twin-wall duct from the cabinet to plant room in the basement.

[andy]

4 hours ago, Thorfun said:

afaik the only way is to ensure that net metering is in place

Net metering is standard and means you don't need to worry about which phase is importing or exporting.  I have 3P to house, 3P PV and Tesla Powerwall on phase 1.  Phase 1 has load that I want battery back up for power outage reasons not that we've had any power cuts yet!  I'd go full 3P from the outset.

[Thorfun]

3 minutes ago, andy said:

Net metering is standard and means you don't need to worry about which phase is importing or exporting.  I have 3P to house, 3P PV and Tesla Powerwall on phase 1.  Phase 1 has load that I want battery back up for power outage reasons not that we've had any power cuts yet!  I'd go full 3P from the outset.

I'm unable and unwilling to have to choose the loads I want for battery back up. I want the whole house to have battery backup should I require it. hence running the whole house on a single phase. not to mention that running a house on 3-phase, afaik, needs to split each phase by at least per floor for safety reasons due to the arcing potential. and so all battery backup loads would need to be on a single floor.

 

(note. I am not an electrician and am only going on what I remember reading regarding 3-phase and arcing. but what I remember reading filled me with worry about potential dangers and so I want to avoid it)

[Carrerahill]

51 minutes ago, Thorfun said:

underground in a 110mm twin-wall duct from the cabinet to plant room in the basement.

What is the anticipated board load? TPN board, each a potential of 100A, but what is the demand actually going to be? Reason I ask is 25mm could fly quite easily if the loads were OK.

 

At 25mm² you might save £5 a metre!

 

Also, 35mm² additional earth is a bit excessive, on 25/35mm² SWA the armour will be fine, I have never had a cable calc fail on a 25/35 armour earth on a SWA, if worried tack a 16mm² on in tandem. Put it this way, practically every cable schedule we do, for SWA's up to about 70mm² will go out with armour as the earth. 

 

Next time you are in a building with exposed services, look for all the sub-mains, most of them will be SWA on their own.

 

What is being proposed is not wrong I hasten to add, just a lot of overkill and maybe some over egging on some figures which you are paying for. 

[Thorfun]

1 hour ago, Carrerahill said:

What is the anticipated board load? TPN board, each a potential of 100A, but what is the demand actually going to be? Reason I ask is 25mm could fly quite easily if the loads were OK.

 

At 25mm² you might save £5 a metre!

 

Also, 35mm² additional earth is a bit excessive, on 25/35mm² SWA the armour will be fine, I have never had a cable calc fail on a 25/35 armour earth on a SWA, if worried tack a 16mm² on in tandem. Put it this way, practically every cable schedule we do, for SWA's up to about 70mm² will go out with armour as the earth. 

 

Next time you are in a building with exposed services, look for all the sub-mains, most of them will be SWA on their own.

 

What is being proposed is not wrong I hasten to add, just a lot of overkill and maybe some over egging on some figures which you are paying for. 

sorry, I can't answer the anticipated board load! I'm not that clever. But I have decided to go with this CU (again, bit of overkill but plenty of space for anything I want to throw at it and not much more than a smaller board) which is single phase. our electrician did think that we could be ok with 25mm but thought for the extra the 35mm will give us a lot of headroom should our loads ever require it in the future. I think I'd rather pay an extra £100 now than have to run a new cable further down the line! but, I guess that now I have prices on the cable I could ask him if we can go back to the 25mm cable.

 

I understand what you're saying about the earth but it's what our sparky has suggested to do and as he will be installing it I feel I need to run the separate earth cable as he's requested. hope you understand. although, I, again, could simply have a chat with him about it! 🙂 

 

I'm not very good with electrics and need to trust our electrician on all of these things. I've read pretty much every thread in the CU subforum on here so I've a bit of knowledge, but, as they say, a little knowledge is a dangerous thing.

[Thorfun]

22 hours ago, Carrerahill said:

What is the anticipated board load? TPN board, each a potential of 100A, but what is the demand actually going to be? Reason I ask is 25mm could fly quite easily if the loads were OK.

 

At 25mm² you might save £5 a metre!

 

Also, 35mm² additional earth is a bit excessive, on 25/35mm² SWA the armour will be fine, I have never had a cable calc fail on a 25/35 armour earth on a SWA, if worried tack a 16mm² on in tandem. Put it this way, practically every cable schedule we do, for SWA's up to about 70mm² will go out with armour as the earth. 

 

Next time you are in a building with exposed services, look for all the sub-mains, most of them will be SWA on their own.

 

What is being proposed is not wrong I hasten to add, just a lot of overkill and maybe some over egging on some figures which you are paying for. 

I spoke to our electrician and he has done the voltage drop calculations and concluded that 25mm SWA will suffice which is great. he also said that although we officially only need a 16mm earth he’d just go 25mm because of the poor earth loop at source we have. he's suggesting 25mm 5-core SWA now so that I don't have to run a separate earth.

 

still doesn't answer my question about LSZH vs PVC though! 😉 

[Thorfun]

1 minute ago, Thorfun said:

still doesn't answer my question about LSZH vs PVC though! 😉 

and interestingly for the 5-core SWA the PVC is cheaper than the LSZH. it just makes no sense to me. 😭

[Carrerahill]

9 minutes ago, Thorfun said:

I spoke to our electrician and he has done the voltage drop calculations and concluded that 25mm SWA will suffice which is great. he also said that although we officially only need a 16mm earth he’d just go 25mm because of the poor earth loop at source we have. he's suggesting 25mm 5-core SWA now so that I don't have to run a separate earth.

 

still doesn't answer my question about LSZH vs PVC though! 😉 

Basically low smoke, zero halogen and various variants are used in every commercial building and indeed most new residential developments it is not in BS7671 but is spec'ed as a standard now and oddly seems to be one of the things that everyone just accepts and rarely value engineer out (also because I think contractors buy so much is is cheap enough anyway), they now sell more of the low smoke variants than normal PVC, I also think the time will come grey T&E will go and we will just have low smoke variants as it will become more of a hindrance to make the normal PVC version. 

 

They always kept the low smoke variants more expensive because it was a premium product and it was on every spec up and down the country, but a shift may see this change soon. 

 

So on your costings, the price difference will be driven by the market.

 

I have terminated both types of SWA cable and in all honestly I cannot see the big difference, knife to ring the sheath, slice it down the length, that pulls off, normal hacksaw or armour tool for the armours, internal bedding is fine too. Commercial boys have no choice really anymore and they get on with it. 

 

 

[Thorfun]

3 hours ago, Carrerahill said:

Basically low smoke, zero halogen and various variants are used in every commercial building and indeed most new residential developments it is not in BS7671 but is spec'ed as a standard now and oddly seems to be one of the things that everyone just accepts and rarely value engineer out (also because I think contractors buy so much is is cheap enough anyway), they now sell more of the low smoke variants than normal PVC, I also think the time will come grey T&E will go and we will just have low smoke variants as it will become more of a hindrance to make the normal PVC version. 

 

They always kept the low smoke variants more expensive because it was a premium product and it was on every spec up and down the country, but a shift may see this change soon. 

 

So on your costings, the price difference will be driven by the market.

 

I have terminated both types of SWA cable and in all honestly I cannot see the big difference, knife to ring the sheath, slice it down the length, that pulls off, normal hacksaw or armour tool for the armours, internal bedding is fine too. Commercial boys have no choice really anymore and they get on with it. 

 

 

if it was your place would you spec LSZH over PVC regardless of the cost?

[Carrerahill]

3 minutes ago, Thorfun said:

if it was your place would you spec LSZH over PVC regardless of the cost?

When it was about double the cost, if I am honest, probably not, but I see the difference is £13 for 2.5mm² PVC vs. LSF, so yes at that price difference I would. Even if I only did all my 1.5mm and 2.5mm and say had 1 x 6-10mm PVC cable, it would be better in a fire having most of my cable LSF.

 

If your house is on fire and the cable is burning, you would hope in a modern house you would be long out of it before it got bad enough, however, if something happened, and I did not get out of it, me and my family, pets too, would have a much higher chance of survival. So even if I needed a good few drums, I would probably use T&E LSF. 

 

As for your SWA, how much of it is in the house/outbuilding, if 95% of it is in the ground then, maybe I'd take that risk.

 

 

[Thorfun]

5 minutes ago, Carrerahill said:

When it was about double the cost, if I am honest, probably not, but I see the difference is £13 for 2.5mm² PVC vs. LSF, so yes at that price difference I would. Even if I only did all my 1.5mm and 2.5mm and say had 1 x 6-10mm PVC cable, it would be better in a fire having most of my cable LSF.

 

If your house is on fire and the cable is burning, you would hope in a modern house you would be long out of it before it got bad enough, however, if something happened, and I did not get out of it, me and my family, pets too, would have a much higher chance of survival. So even if I needed a good few drums, I would probably use T&E LSF. 

 

As for your SWA, how much of it is in the house/outbuilding, if 95% of it is in the ground then, maybe I'd take that risk.

 

 

that all makes a lot of sense, thank you. yeah, our SWA is basically around 90% - 95% in the ground. 1m above ground in the cabinet and, at most, 2m in the plant room, then around 24m - 25m under ground.

[Carrerahill]

Just now, Thorfun said:

that all makes a lot of sense, thank you. yeah, our SWA is basically around 90% - 95% in the ground. 1m above ground in the cabinet and, at most, 2m in the plant room, then around 24m - 25m under ground.

Buy whatever is the most economical for your SWA then, especially if it is in relative isolation unlikely to be burning, such as in a ceiling/floor/wall space. Unless of course you want to use the LSZH stuff. 

[Thorfun]

2 minutes ago, Carrerahill said:

Buy whatever is the most economical for your SWA then, especially if it is in relative isolation unlikely to be burning, such as in a ceiling/floor/wall space. Unless of course you want to use the LSZH stuff. 

yeah. cheers. plant room is in the basement so it's surrounded by concrete. I think I'll be ok with the cheapest option there. 

 

thanks for your help on this thread. it's really appreciated.

[Thorfun]

On 23/08/2022 at 12:07, Thorfun said:

I spoke to our electrician and he has done the voltage drop calculations and concluded that 25mm SWA will suffice which is great. he also said that although we officially only need a 16mm earth he’d just go 25mm because of the poor earth loop at source we have. he's suggesting 25mm 5-core SWA now so that I don't have to run a separate earth.

just finished pulling through 35m of 25mm 5-core SWA. that stuff is super blooming heavy! but it's in now ready for the sparky on Thursday.