Cable size and how to earth this system?

HappyHippyDad · Jun 15, 2019

Morning....

I have a friend of a friend who is just looking for some advice regarding his 12V solar set up.

It's a metal shed approx 5m x 3m.

My 2 questions are:

1. Does the cable joining each of the 5 batteries have to be 50mm as well as the cable from battery to 3kW inverter? I'm sure it does but just wanted to be certain.

2. How to earth this system? I have drawn an earth rod coming from the 230V consumer unit but I realise this is no good by itself. I'm just not sure how this gets joined to the neutral (or which neutral) in order for it to become a working earthing system that would allow the RCD's to trip with earth leakage? (See picture below)

I realise I could have an earth free system (as I have in my wooden shed) but with this being a metal shed that didn't seem like a good idea.

The picture below only shows 1 solar panel, please ignore this as there will be more.

[ElectriciansForums.net] Cable size and how to earth this system?

HappyHippyDad · Jun 15, 2019

telectrix · Jun 15, 2019

50mm is optimistic @ 250A. maybe use tri-rated cable. rod looks good and bond the metal structure. the RCDs would get the required earth leakage to trip by means of L-person-earth. that's what they're designed for.also note you'll need 14-15V for battery charging.

snowhead · Jun 15, 2019

Cables from Charge controller to first battery need to match the max output and fuse from the charge controller.

Cables from first battery and between others and to the Inverter need to match the max input and fuse.

As Tel says, 50mm may be under.
You'd need to use Vehicle battery cable for that part.

This link shows 50mm at 12v 345amp, but that'll be short duration for starter motors.
They do 70mm.

Oceanflex Extra Flexible Tinned Copper PVC Battery Cable - 50mm² 345A

Oceanflex 345A 50mm² extra flexible, tinned copper PVC marine grade battery cable. Particularly suitable for corrosive environments.

www.12voltplanet.co.uk

Strima · Jun 15, 2019

Have a look a the inverter specs, it might need the chassis of the inverter referencing to earth but it could well be done on the neutral.

HappyHippyDad · Jun 15, 2019

snowhead said:
Cables from Charge controller to first battery need to match the max output and fuse from the charge controller.

Cables from first battery and between others and to the Inverter need to match the max input and fuse.

As Tel says, 50mm may be under.
You'd need to use Vehicle battery cable for that part.

This link shows 50mm at 12v 345amp, but that'll be short duration for starter motors.
They do 70mm.

Oceanflex Extra Flexible Tinned Copper PVC Battery Cable - 50mm² 345A

Oceanflex 345A 50mm² extra flexible, tinned copper PVC marine grade battery cable. Particularly suitable for corrosive environments.

www.12voltplanet.co.uk

50mm is rated at 246A - 275A (Table 4E1A) ref method F so I would have thought ok, especially as the max demand will not be at 250A (12V) for long periods. I'll definitely suggest battery cable though.

telectrix said:
50mm is optimistic @ 250A. maybe use tri-rated cable. rod looks good and bond the metal structure. the RCDs would get the required earth leakage to trip by means of L-person-earth. that's what they're designed for.also note you'll need 14-15V for battery charging.

I thought the earth rod had to have some link back to the supply neutral? In a home which is TT the neutral is linked to earth somewhere which i thought meant any earth fault going through the TT rod has a pathway back to the supply neutral and will therefore be a effective earthing system. otherwise its just a copper rod in the ground.

Lucien and Davesparks tried explaining this to me sometime ago with my shed but I didn't quite get it!

telectrix · Jun 15, 2019

which beggars the question....... would it not be cheaper to run a 40A supply from the house?

HappyHippyDad · Jun 15, 2019

telectrix said:
which beggars the question....... would it not be cheaper to run a 40A supply from the house?

It's not by a house. It's out in the country at a plant nursery. There are Mains voltage electrics in a giant greenhouse fairly nearby but he wants to have a play with solar.

davesparks · Jun 15, 2019

telectrix said:
50mm is optimistic @ 250A. maybe use tri-rated cable. rod looks good and bond the metal structure. the RCDs would get the required earth leakage to trip by means of L-person-earth. that's what they're designed for.also note you'll need 14-15V for battery charging.

There won't be a path via the person to earth if one pole of the supply isn't connected to earth.

davesparks · Jun 15, 2019

Depending on the layout and method of actually connecting to the battery you could use a solid copper or brass bar across the top which bolts to each one (assuming it's some sort of bolted connection)
When I put twin batteries in my landrover I used offcuts of lightning conductor to link the batteries.

Or just thinking out loud you could maybe have two copper bars mounted either side of the row or batteries used as busbars, then short cable to each battery terminal. You'd probably be able to use smaller cables then and make it easier to swap out batteries.

HappyHippyDad · Jun 15, 2019

davesparks said:
Depending on the layout and method of actually connecting to the battery you could use a solid copper or brass bar across the top which bolts to each one (assuming it's some sort of bolted connection)
When I put twin batteries in my landrover I used offcuts of lightning conductor to link the batteries.

Or just thinking out loud you could maybe have two copper bars mounted either side of the row or batteries used as busbars, then short cable to each battery terminal. You'd probably be able to use smaller cables then and make it easier to swap out batteries.

Thanks for the suggestion Dave.
Could I ask your opinion on the earthing side of it?
Is it as simple as putting in another rod going from the neutral on the 12V side of the inverter? This is a guess as I just don't understand the principle properly.

I cant understand why the RCD has to have an earth to trip. If I was to touch the live cable (line) and there was no earth, as long as >30mA of current travelled through me (to the general mass of the earth) then the RCD would detect an imbalance and trip, wouldn't it? On a lighting system in a house without an earth the RCD would still trip if you touched the line ( assuming >30mA again).

buzzlightyear · Jun 15, 2019

it would be better to get a genny in or his a tree hugger .

Deleted member 26818 · Jun 15, 2019

happyhippydad said:
Thanks for the suggestion Dave.
Could I ask your opinion on the earthing side of it?
Is it as simple as putting in another rod going from the neutral on the 12V side of the inverter? This is a guess as I just don't understand the principle properly.

I cant understand why the RCD has to have an earth to trip. If I was to touch the live cable (line) and there was no earth, as long as >30mA of current travelled through me (to the general mass of the earth) then the RCD would detect an imbalance and trip, wouldn't it? On a lighting system in a house without an earth the RCD would still trip if you touched the line ( assuming >30mA again).

No.
You are earthing the 230V, so the return is to the 230V side of the inverter.
You would need the neutral output from the inverter, referenced to earth, which it probably already is.

HappyHippyDad · Jun 15, 2019

spinlondon said:
No.
You are earthing the 230V, so the return is to the 230V side of the inverter.
You would need the neutral output from the inverter, referenced to earth, which it probably already is.

When you say referenced to earth could you explain that a little more? 'Referenced' can't mean the same as 'connected' otherwise that would mean just connecting the same earth rod to the 'neutral in' terminal on top of the main switch in the CU.
How do I achieve this 'reference' to earth? How can the inverter already be referenced to earth if it is not in contact with earth in any form?

davesparks · Jun 15, 2019

happyhippydad said:
.

I cant understand why the RCD has to have an earth to trip. If I was to touch the live cable (line) and there was no earth, as long as >30mA of current travelled through me (to the general mass of the earth)

Where is the 30mA current going to flow to? For current to fow there must be a complete circuit, if the supply is not connected to the general mass of earth then the current cannot flow to via the general mass of earth.

If you don't have the N - E link then if you make contact with the L whilst in contact with the earth no current will flow and you will not receive a shock, this is the principle of how isolated supplies, shaver sockets etc work.
This is all fine for something simple like a shaver socket, or a single piece of equipment fed from an isolating transformer. But it can be a problem if you try to use this for an installation, the first time a fault occurs between any conductor and exposed metal (known as the first fault) will not present a danger but will be undetected.
If another fault occurs it becomes very dangerous

Cable size and how to earth this system?

HappyHippyDad

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HappyHippyDad

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telectrix

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snowhead

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Oceanflex Extra Flexible Tinned Copper PVC Battery Cable - 50mm² 345A

Strima

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HappyHippyDad

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Oceanflex Extra Flexible Tinned Copper PVC Battery Cable - 50mm² 345A

telectrix

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HappyHippyDad

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davesparks

davesparks

HappyHippyDad

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buzzlightyear

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Deleted member 26818

HappyHippyDad

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davesparks

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