Sub-main to garage?

[sharpener]

I am planning a new installation of an ESS and EV charger in a garage 15m from the house.

ATM it has the usual garage unit with a main switch and 1 x 16A and 1 x 6A mcbs, fed from a 30mA RCD and dedicated 16A mcb in the house down a 2.5 mm^2 cable cleated to the fence. This will obvs not be adequate.

So I am thinking in terms of a 60A switchfuse off the meter tails and then 6 mm^2 SWA to the garage where a new CU will have

16 or 20A rcbo for a 3.68kW ESS
50A rcbo for an 11kW EV charger
feed to original CU ?via another 16A rcbo, or put new RCD in old CU and swap over the main switch to the new.

Current cable to garage can then be re-purposed as EPS feed back to house for router, alarm, some lights etc.

What do ppl think of this arrangement? I haven't yet done the voltage drop calc but am expecting 6 mm^2 will be enough - rated at 62A according to TLC catalog (not stated which installation method).

Is some degree of diversity allowed? The ESS will charge at night and discharge during the day (no PV). The EV will mostly charge at night. The biggest other loads are very occasional use of vacuum cleaner or electric drill.

[Currently the PME earth is - as approved by the LA inspector - exported to the garage. I might have to change this to local TT to suit the EV charger or global TT for whole establishment but let's not repeat the whole earthing thread here!]

TIA

[Oldgreybeard]

Here's the SWA rating table from the wiring regs:

SWA table from the regs.jpg (62.14 KiB) Viewed 1482 times

and, for completeness, the voltage drop table from the same source:

SWA voltage drop from the regs.jpg (43.05 KiB) Viewed 1477 times

Looks to me as if 6mm² isn't up to the job if you are protecting it with a 60A fuse , so you will need to use 10mm² I think. Even 10mm² looks a bit marginal if you opt to use three core, rather than two core.

[marshman]

Regardless of current ratings go for the biggest CSA you can (10mm2?). 15m is a reasonable run and you can get quite a significant voltage "rise" to the inverter, if the grid volts are at the upper end of what is allowed (253V) then another few volts "rise" on the cable could have the inverter objecting and disconnecting with a report of grid over voltage.

In any case I agree with OGB that for the high loads you are proposing that will run for extended periods overnight even 10mm2 cable will be "running close"

[sharpener]

Thanks both.

10 it will have to be on voltage drop grounds then, I didn't have my On-site Guide handy when I wrote this. I wonder where TLC got their figures from? They seem to agree with those here https://quickbit.co.uk/current-carrying ... le-2-core/, why the discrepancy?


Any thoughts on the rest of the proposed setup?

AFAIR some kind of overcurrent protection is required for the cable as it is more than 2m (?is that the limit) from the meter, and I think I have got a suitable switchfuse in deep storage somewhere. Does it need to be metal clad (I recall it is, have quite a few reclaimed fittings though)?

Fortunately the supply was changed to E7 and at that time along with the new meter BG were persuaded to put in an isolator, which will make the whole job a lot easier. And the LA is helpful so I should be able to do it all myself and then get it inspected by them.

[Oldgreybeard]

Thanks both.

10 it will have to be on voltage drop grounds then, I didn't have my On-site Guide handy when I wrote this. I wonder where TLC got their figures from? They seem to agree with those here https://quickbit.co.uk/current-carrying ... le-2-core/, why the discrepancy?


Any thoughts on the rest of the proposed setup?

AFAIR some kind of overcurrent protection is required for the cable as it is more than 2m (?is that the limit) from the meter, and I think I have got a suitable switchfuse in deep storage somewhere. Does it need to be metal clad (I recall it is, have quite a few reclaimed fittings though)?

Fortunately the supply was changed to E7 and at that time along with the new meter BG were persuaded to put in an isolator, which will make the whole job a lot easier. And the LA is helpful so I should be able to do it all myself and then get it inspected by them.

The disparity between the BS7671 cable ratings and manufacturers ratings has been around for a very long time. I was told, some years ago, that the BS7671 ratings were based on cables not ever exceeding 70°C at the terminations, as although some cables may have a higher temperature rating, virtually all terminals are only rated to work up to 70°C without the risk of deformation or perhaps a reduction in wire clamping force. No idea if this is true or not, but BS7671 consistently rates cables at a lower maximum than many manufacturers.

Yes, over-current protection at source is required for any cable that is over 3m, although from what I remember the 3m limit is only mentioned in the regs with regard to the tails, I think. In effect this sub-main is just a long set of tails, though. Our house is fed from the meter kiosk via a long length of 25mm² three core SWA, so I have an 80A fuse in line with it at the meter kiosk to protect it (80A simply to provide discrimination between that fuse and the 100A company fuse).

The requirement for fire resistant enclosures isn't 100% clear, my take on it is that they are needed for indoor consumer units or consumer units in attached buildings like a garage, but not for locations outside the house or in detached, non-habitable, buildings. I might be wrong, though, I'd need to dig out my old copy of the regs and try and find what it says exactly.

[John_S]

It is not clear from your request if you are aware that 11kW EV chargers are three phase and that you supply is three phase.

Can you clarify?

Personally I would not touch three phase electrics unless I were a fully qualified electrician, which I am not.

When I had my 7.2kW EV charger installed with a similar cable run, I insisted on using 10mm2 SWA. Even if not absolutely required, it reduces the voltage drop and the power lost to the resistance in the cable.

[Oldgreybeard]

Good spot about the three phase charger. AFAIK the largest single phase ones are only 32A, and the three phase ones are either 16A or 32A per phase, so very roughly 11kW or 22kW, although I don't think many EVs can charge at 22kW (ours can, but it is one of the very few that has high power AC charging).

[sharpener]

Thanks @John_S, I had missed that.

Forgot to ask, which ESS inverters have a CT which can be 15m away?

I had been under the impression that the largest single phase EV chargers were 11kW, if not then I will have to stick with 7kW. Then the calcs become a lot more favourable and I don't think there will be a problem with the 6 mm^2 cable.

Cetainly not worth upgrading to a 3-phase supply, ironically a previous house (sold 1991) had been a smallholding with 3ph supply but I had no use for it. I am not frightened by the thought (have been Technical Director of wind turbine firm) but it would be a big upheaval, my best man has just had a big Mitsubishi ASHP installed and the 3ph alone cost £ks. Then he has had his AGA converted to electricity ?!?

The ESS will be generating in the daytime when the EV is not charging, so the voltage rise should not be an issue. If at worst we need to top-up the EV during the day then the two currents will subtract and only the net draw will load the cable. In both cases 16A is safely within the capabilities of the 6 mm^2 whichever table you look at! But it is still needed for the max night-time current.

[Oldgreybeard]

Thanks @John_S, I had missed that.

Forgot to ask, which ESS inverters have a CT which can be 15m away?

I had been under the impression that the largest single phase EV chargers were 11kW, if not then I will have to stick with 7kW. Then the calcs become a lot more favourable and I don't think there will be a problem with the 6 mm^2 cable.

Cetainly not worth upgrading to a 3-phase supply, ironically a previous house (sold 1991) had been a smallholding with 3ph supply but I had no use for it. I am not frightened by the thought (have been Technical Director of wind turbine firm) but it would be a big upheaval, my best man has just had a big Mitsubishi ASHP installed and the 3ph alone cost £ks. Then he has had his AGA converted to electricity ?!?

The ESS will be generating in the daytime when the EV is not charging, so the voltage rise should not be an issue. If at worst we need to top-up the EV during the day then the two currents will subtract and only the net draw will load the cable. In both cases 16A is safely within the capabilities of the 6 mm^2 whichever table you look at! But it is still needed for the max night-time current.

I don't think there's a problem with extending any CT cable out to maybe 20m to 25m or so. I didn't give it a thought, but just ran a length of outdoor Ethernet cable (the stuff with the tough sheath and the gel filling) from our meter kiosk to the building that houses our inverter and batteries, that is probably around 20m away. It's worked fine, so I can say for sure that the Sofar inverters work with around a 20m run of cable to the CT. I did wire the Ethernet cable so all the solid colours were in parallel and connected to one wire of the CT and all the striped wires were connected to the other wire, but this may well have been overkill, as the current from a CT is pretty low (typically 1/1000th of the measured current).

[Marcus]

There are two common types of SWA; PVC insulated 60c (70?) And XLPE insulated 90c. The 62A rating is assuming XLPE, allowing it to run up to 90c mounted in free air (ambient temps up to 30c) and not accessible for people or animals to touch, with suitable terminations for the temperature of course.

If you're going with 10mm2 i would fuse at 45A / 50a bsen 60898 breaker personally. If that's not enough I'd advise 16mm2

As for amd3 (metal) c.u.; if it's an attached outbuilding; metal. Detached; as a rule of thumb, >3m away from house can be plastic, but otherwise metal. A bit of common sense needs applying of couse - if there's little to catch fire and enough distance from the dwelling then plastic is fine, if it's a timber barn filled with dry hay then metal even if it's 20m from the house.