Hi,
I have a Victron Multiplus-II 5kVA inverter/charger with Pylontech US5000 batteries installed in my house along with a 6.8kWp PV array and associated charge controllers. This installation has full DNO G99 approval and was installed to BS7671 and has been running great for about 18 months, initially with batteries only and then the PV array was installed in July last year. The inverter supports island mode where it can continue to supply the house in the event of a grid outage and for this it has its own earth rod as required by the regs. We have also split our original consumer unit into two "maintained" and "unmaintained" consumer units where the former is fed by the inverter's output and is maintained in a grid outage whereas the latter such as electric shower, range cooker and the garage feed is fed from the primary consumer unit and is not maintained in a grid outage.
However, the inverter is quite noisy and I'm looking into the feasibility of relocating it into my detached garage. We currently have two SWA cables underground between the house and garage.
The first is a 10mm 3 core XLPE SWA cable which is used as the garage sub-main feed and is running off a 40A MCB currently. It uses two cores for line and neutral and the third core is used as an earth bond to export the TN-C-S earth from the house. This feed runs a 7.4kW EV charger plus the sockets and lights for the garage. The cable is buried for about 4m of its 12m total length with the other 8m a mix of clipped direct and free air.
The second cable is a 10mm 4 core XLPE SWA cable which is currently used to bring over the two solar PV DC strings from the garage, where the PV array is mounted, to the house where the Victron charge controllers are located next to the inverter. This cable takes the same route as the 3 core cable and is the same approximate length of 12m.
My question is whether I can repurpose the 4 core 10mm cable to act as two pairs of line conductors to carry the AC In and AC Out to and from the inverter. This would allow me to relocate the Victron and Pylontech gear to the garage where it would be closer to the PV array and would run much cooler than it currently does under our stairs. It also removes all that gear from under the stairs which is beneficial for fire safety and noise. The batteries are LiFePO4 chemistry so pretty safe but still...
I've got the 18th edition regs and OSG and have found table 4E4A which gives current ratings for 2 core 1PH or 3/4 core 3PH but doesn't specifically cover using the 4 cores as two independent 2 core pairs of line conductors. For my application the worst (albeit unlikely) case would be to have the inverter fed with 50A via its 50A MCB and then boost this to 70A to return back to the house. This is highly unlikely in practice and I have 1 minute resolution data to show that the highest input current seen in the last month was 28.4A and the max return current was 27A. The maximum instantaneous sum of these currents seen was 47.3A. Ref method D (buried) for 3/4 core 3PH 10mm is 58A and for ref method C it's 73A. So I think using the 4 core 10mm cable is probably fine but I would probably install an MCB on the return line to limit return current to 50A since I've never seen more than 27A in practice and can't imagine ever needing the fully boosted 70A. I realise that table 4D4A might be more appropriate because you are only supposed to use 90C temperature rating of XLPE if all the other switchgear is 90C rated. But in my case the cables are only buried in the middle 4m section of the 12m cable to will be much cooler as they approach both ends of the switchgear.
Sorry for the long ramble, hence the addition of the TL;DR.
Thanks in advance.
I have a Victron Multiplus-II 5kVA inverter/charger with Pylontech US5000 batteries installed in my house along with a 6.8kWp PV array and associated charge controllers. This installation has full DNO G99 approval and was installed to BS7671 and has been running great for about 18 months, initially with batteries only and then the PV array was installed in July last year. The inverter supports island mode where it can continue to supply the house in the event of a grid outage and for this it has its own earth rod as required by the regs. We have also split our original consumer unit into two "maintained" and "unmaintained" consumer units where the former is fed by the inverter's output and is maintained in a grid outage whereas the latter such as electric shower, range cooker and the garage feed is fed from the primary consumer unit and is not maintained in a grid outage.
However, the inverter is quite noisy and I'm looking into the feasibility of relocating it into my detached garage. We currently have two SWA cables underground between the house and garage.
The first is a 10mm 3 core XLPE SWA cable which is used as the garage sub-main feed and is running off a 40A MCB currently. It uses two cores for line and neutral and the third core is used as an earth bond to export the TN-C-S earth from the house. This feed runs a 7.4kW EV charger plus the sockets and lights for the garage. The cable is buried for about 4m of its 12m total length with the other 8m a mix of clipped direct and free air.
The second cable is a 10mm 4 core XLPE SWA cable which is currently used to bring over the two solar PV DC strings from the garage, where the PV array is mounted, to the house where the Victron charge controllers are located next to the inverter. This cable takes the same route as the 3 core cable and is the same approximate length of 12m.
My question is whether I can repurpose the 4 core 10mm cable to act as two pairs of line conductors to carry the AC In and AC Out to and from the inverter. This would allow me to relocate the Victron and Pylontech gear to the garage where it would be closer to the PV array and would run much cooler than it currently does under our stairs. It also removes all that gear from under the stairs which is beneficial for fire safety and noise. The batteries are LiFePO4 chemistry so pretty safe but still...
I've got the 18th edition regs and OSG and have found table 4E4A which gives current ratings for 2 core 1PH or 3/4 core 3PH but doesn't specifically cover using the 4 cores as two independent 2 core pairs of line conductors. For my application the worst (albeit unlikely) case would be to have the inverter fed with 50A via its 50A MCB and then boost this to 70A to return back to the house. This is highly unlikely in practice and I have 1 minute resolution data to show that the highest input current seen in the last month was 28.4A and the max return current was 27A. The maximum instantaneous sum of these currents seen was 47.3A. Ref method D (buried) for 3/4 core 3PH 10mm is 58A and for ref method C it's 73A. So I think using the 4 core 10mm cable is probably fine but I would probably install an MCB on the return line to limit return current to 50A since I've never seen more than 27A in practice and can't imagine ever needing the fully boosted 70A. I realise that table 4D4A might be more appropriate because you are only supposed to use 90C temperature rating of XLPE if all the other switchgear is 90C rated. But in my case the cables are only buried in the middle 4m section of the 12m cable to will be much cooler as they approach both ends of the switchgear.
Sorry for the long ramble, hence the addition of the TL;DR.
Thanks in advance.
- TL;DR
- Can a 4 core 10mm XLPE SWA cable safely handle 50A flowing in all 4 conductors simultaneously (single phase, 2 pairs of L+N). Cable length is 12m with 4m ref method D and 8m ref methods C & E. Table 4D4A and 4E4A in regs don't cover this scenario.
