Hi Guys,

Please see the attached image, i spent quite a bit of time putting this together so it is clear for everyone to answer on.

My install has the left configuration. I have been told it needs to be done in the right configuration and left is wrong.

Are either of these methods correct, and if either is not correct, why is it bad?

Please help me get to an answer, i feel end users need to know what is right and what is wrong, so we can spot a bad install.

Lastly i assume that it doesnt matter what hardware used, the method should be the same?

Left one looks like the Fusebreaker is set up as a "shared" Busbar,which afaik isn't "wrong" - your batteries are shared,the inverters are shared,and the batteries are sharing with eachother

Right side seems closer to a rack-busbar type config which i'd probably go for if you start stacking many batteries - your Fusebreaker "busbar" would run out of space eventually for more wires to be connected

23 minutes ago, PsyWulf said:

Left one looks like the Fusebreaker is set up as a "shared" Busbar,which afaik isn't "wrong" - your batteries are shared,the inverters are shared,and the batteries are sharing with eachother

Right side seems closer to a rack-busbar type config which i'd probably go for if you start stacking many batteries - your Fusebreaker "busbar" would run out of space eventually for more wires to be connected

I agree on your "wrong" statement. We find a lot of times purely because we can measure 1 system and proclaim it is the better way with 1millivolt better voltage or 1W does not mean other methods are wrong and need to be changed. 

Look at the left diagram.  The red battery cable from Fuse to Master is shorter than the red cable from Fuse to Slave.  That is a problem.  They should be equal length.  Now I know this is only a diagram, your real world setup could use equal lengths or it could be worse than what is shown in the diagram.  Ditto for the black battery cables.

Also, the cables from the Master Inverter to the Fuse is longer than those from the Slave inverter to the Fuse.  Here it is not quite as important but still sub-optimal.  You want both the master and the slave to read the same battery voltage.  With different length cables, the longer cables will produce more voltage drop.  In the left diagram, the Master will read a lower voltage than the slave.

The better way is to connect it as in the right picture.  However, even there you should watch out for cable lengths.  For instance, make sure that the red and black battery link-cables are of the same length.

I would stick my neck out and say that a system with perfectly matching cables in the left, will be BETTER than the right hand diagram if the right hand side is using vastly differing cable lengths.

Imagine the current running from the red cable to the battery, through the battery and then back via the black cable.  Every cm of length will have a fixed amount of resistance that will drop some voltage.  Rule 1 is - have about the same loop-length between similar devices.  Rule 2, keep the wires as short as possible.  In that order.  Only if you have infinite small distances, then it doesn't matter. 😂

 

For brevity I assumed that the installer would have used equal lengths of cabling for each split path for the left image - painting in MSPaint isn't a mathematical science,but yes,cable lengths would affect the balancing of loads

A very brave statement to make that if cables are not of equal length it is wrong. But a debate for another day. 

6 hours ago, roadkill said:

My install has the left configuration. I have been told it needs to be done in the right configuration and left is wrong.

Since the current for both inverters pass through a single fuse on the left configuration you'll need fuses rated for twice the current of the right configuration. This also means all the battery cables for the left-hand configuration have to be twice as thick as they have to be for the right-hand configuration.

As long as the cables match the fuses then there's nothing wrong with the left-hand configuration, but I suspect that is not the case and that may be the actual reason why the installer wants to change it to the right-hand configuration.

Thanks for the info so far, yes the cable lengths in the diagram don't represent the real life cabling, it is just so it is easier to see the wiring paths on the diagram. One of the things I have thought about is if 1 of the invertors is faulty and needs to be disconnected from the system, then with 1 disconnect the system will need to be taken down and the cables removed, instead of just pulling 1 disconnect like on the second diagram. There was an argument about every invertor needing its own disconnect as a legal requirement but I don't know if this is true or false. 

The cables are very short on the left battery about 10cm into the disconnect, the right battery cables are about 40-50cm in length, it might not make a huge difference on short lengths but I want to fix that just incase. It was done like this because the batteries are wall mounted side by side, instead of the cage type stack, so the patch leads from one to other are too short. Before I had these 2 invertors I had a single growatt and the batteries were daisy chained with the latch leads, positive into master, negative return from slave. 

For further info, I have 2 X pylontech us3000c and the fuses on the disconnect are rated at 160A each

 

Edited May 27, 20233 yr by roadkill

55 minutes ago, roadkill said:

For further info, I have 2 X pylontech us3000c and the fuses on the disconnect are rated at 160A each

The Pylontech cables are AWG4 (about 21mm^2), which is generously rated at 120A max by Pylontech. If I am correct in assuming that you're using the standard Pylontech cables, then your fuses may not be larger than 120A.

Edited May 27, 20233 yr by PierreJ

Yes that's correct using the pylontech cables, this disconnect was put in by the first installer and recent Installer reused it. Must it still be rated at max 120A now that there is 2 cables into the disconnect as per left diagram, or does it double the 120a to 240a so 160@ should be fine?

1 hour ago, Scorp007 said:

A very brave statement to make that if cables are not of equal length it is wrong. But a debate for another day. 

You can connect it whichever way you want and it will work.  I even said that the right way might be more problematic than the left diagram, depending on less-than-ideal cable lengths.

I also realise that theory is one thing and practise might be a very, very different thing.  Meaning, it is not always practical, and sometimes not even possible, to make the setup as dictated by science.  I really think that a few millivolt difference between batteries are nothing to worry about.  It is good to understand the basic science of it and try to implement it as close to ideal as is practically and in a way that also doesn't break the bank.  These cables are not exactly cheap.  If I have a less-ideal system and someone told me to change it and spend R1000 for a few mV, I would tell him to take a hike.

BTW, I know very little about parallel inverter operation.  "Nothing" would perhaps be the more accurate word.  I think that connecting PV to multiple inverters' MPPTs in parallel would be a terrible idea.  Two separate MPPTs would likely fight with each other.  However, paralleled systems "know" of each other so it would be feasible to sort it out in firmware and thus keep the peace.  So the question is, can parallel inverters also have their MPPTs paralleled?  I am asking this because if the answer is yes, would it not then perhaps be better to give each inverter it's own battery?  I realise this would have a drawback on redundancy and maintenance.
 

The DC resistance of 16mm diameter copper cable is 1,21 ohms per 1000 metres, so a few cm of cable is not going to make any measurable difference at the current (amperage) levels used in domestic inverters 

An imperfect join to a lug or an imperfect connection will make a larger difference

50 minutes ago, Modina said:

So the question is, can parallel inverters also have their MPPTs paralleled

That's a good question . The answer is not directly , MPPT would indeed fight. The standard topology for mainstream inverters is all inverters share a common battery bus. It's actually very simple the way it works. Let's say we have  2 inverters in parallel , one (A) has 10kw of panels, other one (B) has none. 

Consider that the battery itself is fully charged. Now we apply a  10kw load . Because the inverters shares equally, each produces 5 kW on AC. Inverter A has no panels, so it draws 5kW from the battery bus. Inverter B supplies 5kw from panels , with another 5 kW of pv doing nothing.

However because A is drawing 5 kW from battery , the battery bus voltage drops accordingly.  B sees this and thinks the battery now needs a charge , and delivers 5 kW charge power to battery bus . But in reality , A is consuming this charge power to deliver its share of 5kW to load , because the battery is full.in this manner is PV power shared but indirectly thru battery bus.

So the battery bus has consumers and producers multidropped onto it. And in reality you can have currents flowing thru the battery bus cable with no current flowing thru the battery itself.

In terms of inverters having seperate batteries : for reason above they are all pooled together.and secondly , it's very ineffective to have 2x 5kw battery ,each connected seperately to an inverter. One inverter draws its battery near flat , while other one is near full. Bad utilization.

Edited May 27, 20233 yr by BritishRacingGreen

10 minutes ago, BritishRacingGreen said:

That's a good question . The answer is not directly , MPPT would indeed fight. The standard topology for mainstream inverters is all inverters share a common battery bus. It's actually very simple the way it works. Let's say we have  2 inverters in parallel , one (A) has 10kw of panels, other one (B) has none. 

Consider that the battery itself is fully charged. Now we apply a  10kw load . Because the inverters shares equally, each produces 5 kW on AC. Inverter A has no panels, so it draws 5kW from the battery bus. Inverter B supplies 5kw from panels , with another 5 kW of pv doing nothing.

However because A is drawing 5 kW from battery , the battery bus voltage drops accordingly.  B sees this and thinks the battery now needs a charge , and delivers 5 kW charge power to battery bus . But in reality , A is consuming this charge power to deliver its share of 5kW to load , because the battery is full.in this manner is PV power shared but indirectly thru battery bus.

So the battery bus has consumers and producers multidropped onto it. And in reality you can have currents flowing thru the battery bus cable with no current flowing thru the battery itself.

In terms of inverters having seperate batteries : for reason above they are all pooled together.and secondly , it's very ineffective to have 2x 5kw battery ,each connected seperately to an inverter. One inverter draws its battery near flat , while other one is near full. Bad utilization.

And here we have the answer why not the right circuit. Eavh battery can have it's own fuse/disconnect but every manual I have seen have the inverter battery input connected to each other. 

As is indicated the MPPTs to panels are split. 

Thanks for sharing @BritishRacingGreen

47 minutes ago, BritishRacingGreen said:

The answer is not directly , MPPT would indeed fight

 

That's hogwash that I have stated there. Yes MPPTs will fight if sharing panel  input directly, but will not fight if the outputs of the MPPTs are wired together.  In fact , the low voltage MPPTs of older Axperts sits directly on the 48v battery bus , so in a parallel configuration they are effectively connected as two producers directly on the battery bus. 

The MPPTs of high voltage inverters are on the HV DC bus , so pv sharing amongst inverters via the battery bus means double dc-dc conversion effectively  . But hey , the sun pays for the amount of loss!

EDIT : this loss again is somewhat made up for when pv is supplying the AC load directly .in this case there is no dc-dc conversion because the pv output already sits on the HV bus.

Edited May 27, 20233 yr by BritishRacingGreen

2 hours ago, roadkill said:

Must it still be rated at max 120A now that there is 2 cables into the disconnect as per left diagram, or does it double the 120a to 240a so 160@ should be fine?

The fuses are there to protect the cables. With the current configuration there are several fault conditions that can cause the 120A current limit in any of the cables to be exceeded, so if the electrician is paying attention then a 160A fuse should fail the CoC inspection.

If this setup was to code then it would also be legal to connect all the plug circuits in your DB board directly to a single 60A breaker, with the argument that the total current carrying capacity of all the circuits exceed 60A. This is a ridiculous example, but it illustrates the point: Every individual conductor must be protected against overcurrent.

Edited May 27, 20233 yr by PierreJ

I don't want to get into the debate of left vs right here , but right addresses all the problems you can sit with per left, albeit at the cost of two breakers/fuses.  

Each inverter has its own battery fuse in order to look after itself.but having a seperate fuse per inverter , slightly lower in rating will allow that fuse to blow before the inverter internal one . When the latter blows it's a product failure , you need to get serviced.

Secondly it allows for the individual isolation of battery power to an inverter.inverter becomes fault for some reason and you must isolate it . Other inverter can still limp in single mode .

On the subject of cable protection , you cannot really use a single  400A fuse to protect two 200A cables in parallel that has unequal lengths or where the  far end of of each cable is wired to a different circuit , example one cable goes from fuse to one inverter , another one from fuse to another inverter.one can have short , other one not , now the 400A fuse will not blow at 200A for the sake of  the one cable being shorted.

EDIT: rule of thumb : if you have parallel cables or cables that stars out to different endpoints  , you must derate your breaker or fuse to the weakest cable. Otherwise it's a mismatch. Unless you have 2 perfectly equal lengths of the same cable , connected together at both ends.

 

Edited May 27, 20233 yr by BritishRacingGreen

Ok so from the comments I gather for peace of mind it would be better for me to get another battery disconnect, change out the 160a fuses with 120a and wire it like the right diagram,so will need to get some extra pylon cable too. 

I could continue to use the left diagram but with 120a fused in a single disconnect, but it is not the best way to do it. Would left pass COC that way?

Edited May 27, 20233 yr by roadkill
Edit

4 minutes ago, roadkill said:

hat mentioned, all of those cost wise together is still cheaper than potential blown components on an invertor or a fire in a worse case scenario, and not being able to replace under insurance.

For sure , that's planning for good product cost-of-ownership during its life cycle.

Thanks for all the information guys, it has been helpful

10 minutes ago, roadkill said:

I could continue to use the left diagram but with 120a fused in a single disconnect, but it is not the best way to do it. Would left pass COC that way?

Yes, that would pass CoC. Given that you've only got two US3000C batteries their BMS protection should kick in and disconnect long before you blow the 120A fuse.

Below are characteristic curves for size NH00 fuses. These are for the ETI brand, but it's fairly typical. As you can see the 125A fuse will carry 200A for almost an hour before it blows:

To throw the cat amongst the pigeons , when it's comes to cable protection on a battery bus ,  centralised fusing is technically incomplete.reason , the bus is bidirectional .when discharging any short  in cable between fuse and battery cannot be protected by the main fuse .conversely , when charging , any short in cable  between inverter and main fuse cannot be protected. But fear not , both individual inverter and individual battery has an endpoint fuse built in to cater for this.

So technically , if you want to add external fuses , there must be individual fuses as close the the battery terminals and as close to the inverter terminals as possible possible.  But that's pedantic overcooking.

 

So in a scenario like my dad's install, he has 4 X pylon up5000 connected to a deye 12kw, on a single battery disconnect.  I see on my us3000c nominal is 37A and max is 90A, but what I don't understand is in the manual it states the pylon cables hold a max of 120A constant. Does this mean in his case, his cable and short patch leads should also be thicker due to his amperage capability being much higher or his batteries split into 2 pairs to a bus bar, or am I not understanding this correctly. The cable from his disconnect to the invertor is much thicker, but the cable from the disconnect to the batteries is the standard pylon cable. 

5 minutes ago, roadkill said:

So in a scenario like my dad's install, he has 4 X pylon up5000 connected to a deye 12kw, on a single battery disconnect.  I see on my us3000c nominal is 37A and max is 90A, but what I don't understand is in the manual it states the pylon cables hold a max of 120A constant. Does this mean in his case, his cable and short patch leads should also be thicker due to his amperage capability being much higher or his batteries split into 2 pairs to a bus bar, or am I not understanding this correctly. The cable from his disconnect to the invertor is much thicker, but the cable from the disconnect to the batteries is the standard pylon cable. 

No , you can merely complete the circuit with a ring by adding one more thin cable from the last battery back to the disconnect .now you have affectively double the capacity of the cables.

Thanks @BritishRacingGreen he has some spare leads so going to do that.