How much can I practically/legally DIY, and other newb questions

[Stinsy]

If your inverter is rated at 3kW, then you may find that 4mm­² cable is too big to fit the glands and terminals that easily. You will have to use 4mm² if using a 32A RCBO, as that's too high a current rating to protect 2.5mm² cable. Largest size RCBO that can be used to protect 2.5mm² cable is 20A.

Inverter is 3.6kW. Good point on the cable - 2.5mm² should be good for 32A in free air but it's a bit borderline (going on info from http://www.cable-ratings.co.uk/).

Luckily, cable is cheap, and I only need a couple of metres, so if the 4mm² cable won't go into the inverter, I'll have to go for 2.5 and get a 20A RCBO. It would be an expensive mistake though to pick an RCBO with too low a rating, and find it trips out whenever the inverter ramps up. Although, being curve C makes that unlikely, you would hope.

The cable rating table you need to use is the one in the wiring regs, that rates 2.5mm² flex cable at an absolute maximum of 25A for single phase, 4mm² flex is rated at 32A maximum:


Flex cable rating from the regs.jpg

Our Sofar is connected using a 20A RCBO, which is fine, as the maximum current normally (for 3kW) is slightly over 13A. A 3.6kW inverter could draw or supply about 15.6A, so still fine on a 20A RCBO.

You need to worry about more than just the current carrying capacity of the cable! You need to account for Voltage rise and your local supply voltage.

Eg, my 3.6kW inverter could’ve been wired in 1.5mm² but I went 2 sizes bigger to 4mm² so that the inverter didn’t cut out on sunny days due to over Voltage condition.

[Oldgreybeard]

You need to worry about more than just the current carrying capacity of the cable! You need to account for Voltage rise and your local supply voltage.

Eg, my 3.6kW inverter could’ve been wired in 1.5mm² but I went 2 sizes bigger to 4mm² so that the inverter didn’t cut out on sunny days due to over Voltage condition.

Very true, here's the other part of that table from the regs that shows the voltage drop with cable length:

Voltage drop.jpg (41.64 KiB) Viewed 1502 times

For our installation, the cable is about 8m long, so the voltage drop at the maximum of 13A is 0.152V. I can live with that, as it's only a loss of under 2W, and then only when the inverter is running flat out.

[dangermouse]

I'm mounting the inverter right next to the CU, with a cable length of about 1m so I was not going to worry about voltage drop

[dangermouse]

After several days of "offering up" and various fiddling I'm nearly there!

Just need fuses for the battery isolator, and crimps for the battery cables to connect to it.

My only small concern is that the trunking I've used for the battery cables might be a bit too small and they might get a bit warm, once I am ready to switch it all on I'll monitor the temperature - if they seem too hot I can always run another length of trunking under the existing one and move one of the DC cables into the new one.

Many thanks to everyone who offered very helpful advice.

[ducabi]

After several days of "offering up" and various fiddling I'm nearly there!

Just need fuses for the battery isolator, and crimps for the battery cables to connect to it.

Have you bought the cables between inverter and fuses or diy? What cables are these?

[dangermouse]

Have you bought the cables between inverter and fuses or diy? What cables are these?

Those are the Pylontech battery cables, cut so that there's a short length at the inverter end.

I didn't realise that the Mersen fuse holders have an M8 bolt connection rather than a clamp terminal, so I'm waiting for some crimps to go on the bare cable ends to be able to connect them up.

[ducabi]

Those are the Pylontech battery cables, cut so that there's a short length at the inverter end.

I didn't realise that the Mersen fuse holders have an M8 bolt connection rather than a clamp terminal, so I'm waiting for some crimps to go on the bare cable ends to be able to connect them up.

Thanks. I thought I would utilize the spare battery2battery cables but they are too short. I don't want to cut the long cables so will probably just buy some 4 awg cables and crimps.

[Oldgreybeard]

Those are the Pylontech battery cables, cut so that there's a short length at the inverter end.

I didn't realise that the Mersen fuse holders have an M8 bolt connection rather than a clamp terminal, so I'm waiting for some crimps to go on the bare cable ends to be able to connect them up.

Thanks. I thought I would utilize the spare battery2battery cables but they are too short. I don't want to cut the long cables so will probably just buy some 4 awg cables and crimps.

The cable size needed is 25mm², which is a bit bigger than 4 AWG and generally easier to get hold of here than American cable, I think. IIRC, the terminals in the Sofar inverter are M6, those in the Mersen switch fuse are M8, and although M8 terminals will fit OK on M6 studs I opted to fit the right sizes of terminal at each end. Crimping these on needs a hefty tool. I have a hydraulic crimp tool that will do this easily, but the non-hydraulic crimp tools for 25mm² cable work OK, just need a bit more effort.

[dangermouse]

Just to round off this thread, and say (again) thanks for all the advice, my DIY installation just got signed off for part P approval

In the end it was somewhat anticlimactic, the electrician tested the RCBO, did some earth impedance tests (I think), stuck a couple of stickers on my CU and that was it.

[Oldgreybeard]

Good result!

The whole Part P thing seems to be a bit of a scam to me, if I'm honest. I've seen plenty of really poor quality work from supposedly qualified electricians that have Part P certification and suspect there is little oversight of their workmanship by the certification body they belong to. I had an electrician install our temporary building supply here, when I started building the house, and his workmanship was not only very untidy, but also downright dangerous. Rather than chase him up to do a proper job I chose to just rip all his dodgy work out and do the job myself. My thinking was that if I got him back to do the job properly there was every chance he'd still screw it up. I had a vested interest in making sure the TBS was safe and properly installed, as most of the time it was going to be me using it for the time spent building the house.