So Earth cable size for the DC side of a grid tied installation for multiplus 2 8000kVa 48V, two MPPT 250/85, and a 16kWh battery.
I recall reading that the Earth cable size needs to be the same as the pos and neg cable size from the battery, which in my case are 2 x 50mm2 each.
So can anybody confirm that I need 2 x 50mm2 Earth cable for all the DC earthing requirements (Inverter to Lynx and MPPT (via an Earth busbar).
At the moment I just have 16mm2 Earth cable.
Edit - I just found this in the wiring documents, says Earth cable needs to carry fault current at least equal to the DC fuse rating, how can I work out the DC fault current and apply that to the cable size ?, lets assume the fuse is 200A
are you aware of the Victron Toolkit App?
You could use that to determine the needed conductor size (mm²) for a given length and voltage drop.
Manual calculation would be (german mathematical terms):
A=(L x I)/(k x Ua)
A is conductor size in mm²
L is lenght of cable
k is conductivity of cable material (usually copper, which is 57,14 S/m)
Ua is accepted voltage drop in % (in Germany this is usually set to 2%, Victron app states that anything below 3% is safe)
So in your case as an example for 200 Amp fuse, 48 Volt, 2% drop and a lenght of let’s say 3 meters:
A=(3m x 200A)/(57,14S/m x 2% x 48V) = 10.93 mm²
Other way round: If you would accept 3% of voltage drop on earthing cable (which is in my opinion very well acceptable), you can go for a little more than 5 meters from your busbar to every unit to be earthed with your 16 mm² (assuming 200A and 48 Volt).
Have a nice one,
Alex
Edit:
If you use the Victron Toolkit it is assuming DC cabling for supply, not earthing.
Means: they are calculating you cable length twice the length of your input.
For earthing you have to input half the cable lenght into the Toolkit app.
@kevdav100
Hmm. I was just wondering how the DC-side earthing was supposed to work and found your post. On the AC side the UK BS 7671 requirement is for earthing cables to be half the section of the live/neutral conductors (once bigger then 16mm2). That’s half the section of each cable, not both added together! From table 54.7 in the regs, reproduced here: Earthing and bonding requirements when relocating the gas meter - Installer Online
So that might lead to a 25mm2 earth lead.
But that table is actually the result of using the adiabatic equation: Adiabatic Calculation - Wiring and the Regulations BS 7671 - IET EngX - IET EngX
which is a calculation to check that the cable can carry enough current to trip an RCBO, RCD, MCB or fuse quickly enough without getting dangerously hot.
The Smartsolar manual says the device should be grounded, but the PV array cables shouldn’t be (except frames). Suggesting that it is the same ground the batteries use. But I don’t think batteries need to be earthed as they are ELV (<50V) and thus not providing a dangerous voltage that needs earth protection.
What does need grounding is the SPD on the PV DC side as it can’t operate unless there is a ground. That needs a cable <0.5m to the device it is protectiong (the MPPT device in this case). But those are quite low currents, or at least so transient that they can be assumed to be effectively low (the installation requirements are for cable 2.5mm2-6mm2). If you don’t have an SPD on the PV you can ignore this bit.
The current Regs section 712 covers PV systems and is discussed here: Delving into BS 7671: Section 712 and Solar Panel Systems — RenewSolar
That says nothing in particular is specified for DC-side earthing, but the requirement is as above to protect from metal things being at dangerous voltages.
So in fact just bonding is needed, not earthing. They are not the same thing.
So I conclude from all this that using standard 10mm2 bonding-size earth cable to the earth point on the side of the Smartsolar MPPT (i.e the metal heat-sink) is what’s required and appropriate (protecting from the high DC PV voltage). A short cable from there to the SPD of any size (2.5-10mm2) should go in if there is an SPD. And the 10mm2 cable should be taken on to any other metal boxes (e.g. my PV DC isolated is in a 4-way metal DIN box). You could also connect to the battery case, but there is no need to as it’s only 48V. My battery doesn’t even have an external earth point.
Happy to hear from people who have reached other conclusions. I’m just wiring mine up right now!
Bottom line I’m fairly sure you don’t need 100mm2 of earth cable! The 16mm2 you have is fine. 10mm2 would be fine too.
Thanks for reply, sorry for delayed reply, not been here for a while, yes I already decided to use 10mm earth cable, did the batteries too as there is an earthing bolt at the back. I have my electrician coming next week hopefully, so will see how he gets on with his tests and if he advises anything different.
