Cable size

[Carrerahill]

On 13/08/2020 at 13:39, Alexphd1 said:

What size of cable between two consumer units in the house. In between sparkies and would like to get cable run in to keep job moving.

@ProDave you dont fancy coming down to the bluetoon for a days work? 

Depends on load and distance.

 

Confirm those and the rest is easy - however, word of advice, always over engineer here. So if it calcs as 6mm go 10 for 20p a meter more!

[Alexphd1]

@Carrerahill I agree with you 100% if in doubt go up a size with cable but the cost difference between 16mm and 25mm earth is £14 (13 meter). Its not going to brake the bank but I am not throwing money away unnecessarily plus its easier to work with.

[Carrerahill]

On 21/08/2020 at 18:09, Alexphd1 said:

@Carrerahill I agree with you 100% if in doubt go up a size with cable but the cost difference between 16mm and 25mm earth is £14 (13 meter). Its not going to brake the bank but I am not throwing money away unnecessarily plus its easier to work with.

I assume you mean £14.00 for the whole job? In which case for £14.00 go up a size - from my supplier the cost difference between 16mm and 25mm is £10.92 on a 13m run.

 

 

 

 

[Alexphd1]

Carrerhill, yes 25mm is ordered for live and neutral (you could argue 16mm would be enough) but I will be running in 16mm for earth as that's what's already in (meter tails) plus its £14 cheaper (on earth cable alone) which is a few extra pints for me when I finally go to the pub. 

You must have a better supplier than me, hes £1 per meter more expensive between 16 &25mm. 

Edited August 25, 2020 by Alexphd1

[Dan F]

We have the same setup; main disbribution board in the garage (along with PV and any future car chargers) and then a conumer unit 12m away in a plant room for the house.  The difference in our case is that both are 3-phase.   We may only use 1-phase in the house in short term (easier to backup with battery) , but it gives us flexibility to rebalance phases later.

 

@ProDave Does the same logic apply, and 16mm 4-core is enough between garage and house as long as an 80A 3-phase breakers is used?

(cable is in duct that goes through the top layer of EPS, partly under heated house and partly under unheated garage)

 

I think the plan was to run SWA 25mm 4-core, but when they saw the 63mm duct, rather than use a 25mm 4-core unamoured, it seems they switched to 16mm 4-core SWA insteasd

 

 

 

 

Edited February 13, 2021 by Dan F

[ProDave]

7 hours ago, Dan F said:

 

@ProDave Does the same logic apply, and 16mm 4-core is enough between garage and house as long as an 80A 3-phase breakers is used?

(cable is in duct that goes through the top layer of EPS, partly under heated house and partly under unheated garage)

 

 

Can you not get the duct under the insulation?  The wring regs are not specific on what counts as "insulation" but I would say being in the middle of a sandwich of EPS  with a heated slab above it and unheated ground under it, would concern me. 

 

Ideally I would want the duct under the bottom layer of EPS.  I would certainly go up to 25mm in this case.

 

What are you using the 4th core for?

 

 

[TonyT]

I wouldn’t supply a sub board via MCB.

 

MCCB or fuse yes, likely there will be no discrimination/selectivity between the mcbs/rcbo in the sub board 

[Dan F]

1 hour ago, ProDave said:

Ideally I would want the duct under the bottom layer of EPS.  I would certainly go up to 25mm in this case

 

Now I think about it it's actually lower down in the EPS.  In the garage it's in the middle layer of 3. In the house there is 400mm EPS and it's in the second layer from the ground.  So 100mm EPS below and 200mm EPS above (aside from a few areas of slab thickening). 

 

1 hour ago, ProDave said:

What are you using the 4th core for?

 

I meant 5-core, sorry: 3 live + neutral and earth.

 

Ideally this is use 25mm armoured, as they initially had said they'd use, but if that won't work (duct is only 50/63mm), what should I do:

 

1) Insist they use 25mm even if it's unarmoured? (any specific type?)

2) Not worry about the 16mm armoured, but just ensure they use 80A (?) fuse/MCCB?

 

80A would limit usage in the house (vs 100A), but as its three phase this shouldn't be an issue I assume, as more than 1 phase can be used if required. 

Edited February 13, 2021 by Dan F

[ProDave]

Table 4E4A   25mm SWA multicore 3 phase, buried in ground or in conduit in ground current carrying capacity 96A  It does not quote a derating for being in insulation.

 

Looking at other tables for non armoured cable you typically derate 20 to 30% for being in insulation.

 

To be safe I would want 25mmSWA fed from a 60A fuse  Will 60A 3 phase do your needs?

[Dan F]

3 hours ago, ProDave said:

Table 4E4A   25mm SWA multicore 3 phase, buried in ground or in conduit in ground current carrying capacity 96A  It does not quote a derating for being in insulation.

 

Looking at other tables for non armoured cable you typically derate 20 to 30% for being in insulation.

 

To be safe I would want 25mmSWA fed from a 60A fuse  Will 60A 3 phase do your needs?

 

Table 4E2A (non-amoured in conduit in insulated wall) seems the closest.  89A (25mm) and 68A (16mm).

 

The electrican didn't ask where the duct went, so he has probably assumed it's in-ground and is thinking he'll be OK with 80A, which isn't good given it's actually wrapped in EPS.  I'll have to talk it over with him Monday, make sure he knows and find out what fuses he was planning to use.

 

60A 3-phase would be enough for our needs yes, but we had planned to try to keep most things (including ASHP) on one phase for battery backup, so it's not really ideal as it means less will be able to go on the backed-up phase.

 

If I do insist on 25mm, but SWA won't work in MBC's 63mm duct, is it possible to get hold of 25mm 5-core non-amoured?   Everything online seems to be only SWA...

 

 

[TonyT]

25mm 4core. All you need is 25mm in diameter 

 

25mm 5 core is 29mm diameter. 
no reason why that won’t go

through a 63mm duct

 

 

 

 

[Dan F]

7 minutes ago, TonyT said:

25mm 4core. All you need is 25mm in diameter 

 

Sorry for the ignorance, but don't you need 3-live + nuetral + earth?  I think that's what the 16mm SWA they've put in is. I'm pretty sure the supply has a nuetral.. but I'll check.

 

7 minutes ago, TonyT said:

25mm 5 core is 29mm diameter. 
no reason why that won’t go

through a 63mm duct

 

Might have been an issue with the bends, but I wasn't there when they tried so not sure.  Might have to insist that they try again and show me.  Is a non-amoured equivilant available though?

 

 

[TonyT]

The armour is the earth.

needs to be armoured unless you are Protecting by RCD.

 

do you have a schematic/ sketch of what’s in, going in?

 

can I ask why you are installing 3 phase when the photos of page 1 show single phase?

 

 

[Dan F]

2 hours ago, TonyT said:

can I ask why you are installing 3 phase when the photos of page 1 show single phase?

 

My bad, I hijacked an old post as topic was very similair (just different number of phases).  This is what we have:

 

- 3 phase (3x100A) to a site kiosk.

- 5-core 25mm armoured cable from site kiosk to 3-phase distribution board in garage.

- Garage distribution board will serve i) garage ii) house iii) garden room iv) front gate v) garden vi) car charger(s).  3-phase inverter is also wired up to this board.

- Plant room consumer unit (serves house) is 12m from garage board, and connected via 50mm ID duct through the EPS layer of the passive slab foundation.

- Intially the plan was to use one phase in the house, but then we decided it was best to bring 3-phase to plant rooom, even if we only used 1-phase initially, because if we ever have 2 three-phase car charges there wouldn't be much left for house on just a single phase.  Reason to try to use 1 phase for most things initially, is to ony put battery on a single phase but still backup almost everything.

 

Our electirian was going to put 5-core 25mm from garage to plant room, but has instead used 16mm.  They were unaware duct is insulated. 

 

2 hours ago, TonyT said:

The armour is the earth.

 

Our electrician is using a core for earth, not the armour.   Using a 4-core 25mm and earthing the amour might be an option thoughm if they can't get 5-core through duct.

 

2 hours ago, TonyT said:

needs to be armoured unless you are Protecting by RCD.

 

Even if already mechanically protected by 100mm of reinforced concrete?   Anyway, might still be best to use 25mm unamoured and RCD vs. 16mm armoured, right? 

 

 

[ProDave]

22 minutes ago, Dan F said:

 

Our electirian was going to put 5-core 25mm from garage to plant room, but has instead used 16mm.  They were unaware duct is insulated. 

 

 

Our electrician is using a core for earth, not the armour.   Using a 4-core 25mm and earthing the amour might be an option thoughm if they can't get 5-core through duct.

 

 

If you only actually need single phase in the house how about 2 cores for L, 2 cores for N 1 core plus SWA for earth.

That should take care of it.

[Dan F]

20 minutes ago, ProDave said:

If you only actually need single phase in the house how about 2 cores for L, 2 cores for N 1 core plus SWA for earth.

 

Noone has done full loading calcs, but we assumed 100A was enough (as ASHP is only 7kW).  But then I realised that 1 or 2 future 3-phase car-charges could pull up to 32A per phase which would only leave 68A for the house.  This is definitely too tight, especially with no gas.

 

So idea was to:

- run 3-phase cable to plant room

- minimize use of phases now (to backup more things on single pwerwall)

- when we have car chargers installed and/or more batteries then balance phases.

 

I guess, if I can't get them to use 25mm cable,  we could potentially use your suggestion for 100A single-phase and then repurpose the same cable and get 3x60A if/when required.

 

 

[TonyT]

If you don’t have any 3 phase equipment why run 3 phase to the house?

 

can you not live with charging the cars in the garage where the 3 phase lives anyway?

 

Happy paying for 3 phase chargers?

 

the duct may be under the ground and eps but has to come up through the concrete and cable pass out of duct at either end.

need an RCD if it’s not swa.

no RCD if swa.

 

fitting a 3 phase RCD into a panel board will be expensive unless it’s going to be silly separate enclosures for each RCD?

 

29mm diameter cable through a duct is easy.

 

the electrician will be signing this off, so it’s his design, best see if they can come up with something as they have seen the building.

 

 

[Dan F]

1 hour ago, TonyT said:

the duct may be under the ground and eps but has to come up through the concrete and cable pass out of duct at either end.

need an RCD if it’s not swa.

no RCD if swa.

 

fitting a 3 phase RCD into a panel board will be expensive unless it’s going to be silly separate enclosures for each RCD?

 

29mm diameter cable through a duct is easy.

 

the electrician will be signing this off, so it’s his design, best see if they can come up with something as they have seen the building.

 

Good points, let's see what I can sort out.

 

3-phase to house isn't because there is 3-phase equipment, but simply because once you deduct 16-32A (on each phase) to future proof for car charger(s) and another 16-20A (house phase) for the ASHP,  that doesn't leave any/much headroom at all, especially when house has no gas supply.

 

[Nickfromwales]

Check with the sparky about exporting the earths. What will be the distances between the MET and the final CPC connections at all CU’s?

PME?

[TonyT]

Exporting the earths is really only a consideration for the EV charger, depending on type of charger some require a local earth rod, some have fancy electronics to allow for dc leakage.

 

yoinneed to watch for type a RCD for EV which not all manufacturers do, and you can’t just fit anybody else’s RCD into anyone else’s enclosure- not type tested for it.