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My Newbie Install.


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The cables are to spec.

 

I have burnt my fingers rather badly, once, trying to disconnect a battery cable rather urgently, testing 2 x 500w 12v UPS inverters on a 102ah battery. I did not see this one coming.

 

The 1st inverter lasted maybe 1 minute then burned out. I thought it was faulty. Cables felt a bit hot but I blamed the inverter. The 2nd inverter lasted longer but when I started to actually smell plastic getting hot, I touched the cables and they felt soft and hotish to touch. I realised something seriously was wrong. I went for the lug on the batt to disconnect the cable, the moment I touched that lug I smelt human flesh burning. Mine. The lug was piping hot.

 

Unbeknownst to me at the time, the 102ah battery was faulty. The batt showed the right voltage but once under load, at near inverter max, the truth set me free. Now understand, I did test the batts volts. They tested 100% full battery. But under load, the volts dropped very fast due to a internal fault, resulting in higher amps.

 

Running a inverter at near or at full power, the lower the volts get, the higher the amps are in the cables.

And if a battery is faulty, it gets pretty bad pretty quick.

 

After that day I started looking at supplier specs. What I have learned is that most suppliers give the bare min specs, assuming a lot, and they never cater for inverter at near or at full power with near flat batteries, or, as in my case, a internal shorting battery. 

 

 

With this in mind, double check with the calculator below, noting the following:

- Cable length is the entire length from batt + via inverter back to batt -, from the furthest point.

- On the value given, it has been suggested to take 2 values higher, to be safer, consider my experience above on just 250w.

- Shorter cables are always ... ALWAYS ... better.

http://www.bulkwire.com/wireresistance.asp

 

If it was my setup, I would make the cables 50% shorter, and thicker.

 

And batteries under a metal shelf, if those shelves and batteries where mine, me being a fearless releaser of smoke, it will be a disaster just waiting to bite me.

 

Just saying.  :)

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I think the cables from the DB to batteries only just to spec. Spec for 1.5m at 48V is 25mm2 @ 1.75m it jumps to 35mm2.

Cables are 35mm2.

These are the exact same cables I have been using for pretty much the last year.

Only difference now is obviously the batteries, and I've shortened all the cables for the new setup.

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I have burnt my fingers rather badly, once, trying to disconnect a battery cable rather urgently, testing 2 x 500w 12v UPS inverters on a 102ah battery. I did not see this one coming.

 

The 1st inverter lasted maybe 1 minute then burned out. I thought it was faulty. Cables felt a bit hot but I blamed the inverter. The 2nd inverter lasted longer but when I started to actually smell plastic getting hot, I touched the cables and they felt soft and hotish to touch. I realised something seriously was wrong. I went for the lug on the batt to disconnect the cable, the moment I touched that lug I smelt human flesh burning. Mine. The lug was piping hot.

 

Unbeknownst to me at the time, the 102ah battery was faulty. The batt showed the right voltage but once under load, at near inverter max, the truth set me free. Now understand, I did test the batts volts. They tested 100% full battery. But under load, the volts dropped very fast due to a internal fault, resulting in higher amps.

 

Running a inverter at near or at full power, the lower the volts get, the higher the amps are in the cables.

And if a battery is faulty, it gets pretty bad pretty quick.

 

After that day I started looking at supplier specs. What I have learned is that most suppliers give the bare min specs, assuming a lot, and they never cater for inverter at near or at full power with near flat batteries, or, as in my case, a internal shorting battery. 

 

 

With this in mind, double check with the calculator below, noting the following:

- Cable length is the entire length from batt + via inverter back to batt -, from the furthest point.

- On the value given, it has been suggested to take 2 values higher, to be safer, consider my experience above on just 250w.

- Shorter cables are always ... ALWAYS ... better.

http://www.bulkwire.com/wireresistance.asp

 

If it was my setup, I would make the cables 50% shorter, and thicker.

 

And batteries under a metal shelf, if those shelves and batteries where mine, me being a fearless releaser of smoke, it will be a disaster just waiting to bite me.

 

Just saying.  :)

I don't know if I agree with that calculator's calculations.

Looking at this setup, he has 520Ah battery bank over 48V. The calculator suggests a 21.15mm2 wire is sufficient. I would have run a 35mm2 cable to each battery bank, at least. 50mm would be better IMO

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@ Viceroy: The cables from the batt positive via the inverters back to the batt negative, is in my opinion, a wee bit long.

 

 

I don't know if I agree with that calculator's calculations.

Looking at this setup, he has 520Ah battery bank over 48V. The calculator suggests a 21.15mm2 wire is sufficient. I would have run a 35mm2 cable to each battery bank, at least. 50mm would be better IMO

 

The AH of the batt bank makes no difference. Larger bank will just supply the load for longer. 

However, depending on the systems specs, it is always the highest amps inverter can draw from batteries at the lowest possible volts over the length of the cable, that is where the problem may rise.

 

Jip, 21.15mm would upsize to be 25mm2 here in SA, due to cable size and lug availability. 4mm2, 6mm2, 10mm2, 16mm2, 25mm2, 35mm2 and 50mm2 are the most commonly found cables and lugs in my experience.

 

Calculator was given to me years ago by off-grid guys in the US of A.

That is also where I got this from: On the value given, it has been suggested to take 2 values higher, to be safer

 

As they would say, 50mm2 would be nice but then there are the complications of getting connectors crimped on properly and the chances are they will not fit inside the inverter / controllers contacts so if you start snipping away threads to make the cables fit, it defies the purpose of having 50mm2. And then the cost of the fuses and lugs for the cables.

 

It is a fine balance between costs versus safety.

 

The rule of thumb is: The shorter the right cable, the better.

In this case, I would probably have ended up with shortest possible 25mm2 - 35mm2 cables.

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Nice install Viceroy!!!

 

Before you do anything else, please make sure about your battery cabling length too.

The ideal is to have the cabling to each bank the same length.

Any difference in length can affect the voltage and charging of the banks.

As your batteries is your biggest asset, protect them!!

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Nice install Viceroy!!!

 

Before you do anything else, please make sure about your battery cabling length too.

The ideal is to have the cabling to each bank the same length.

Any difference in length can affect the voltage and charging of the banks.

As your batteries is your biggest asset, protect them!!

That's exactly what I had to do after the batteries were put in place and wired together.

I don't know what it is with these installers, but both times now they've wired the batteries like so.

48v%203%20parallel%20solution%201_zps6vh

Does my head in why they do this!!!

The red cable on either side of the bank are the same length, and connected together. before connecting to the inverters through the fuse box.

I will be replacing these with proper bussbars soon.

I will be shortening cables further once I have the conduits in place too.

 

 

Here is another calculator: http://bdbatteries.com/wirerules.php

 

And the recommended fuses required for recommended cables thickness: http://www.bdfuses.com/fusesnwires.php

So basically, from the batteries to the fusebox, 35mm2 is the min required.

1x 200A fuse or 2x 100A fuses are required.

And then 25mm2 from the fuse box to the inverters.

 

I already have 50mm2 interconnecting the batteries, and will upgrade the cable from the batteries to the fuses to 50mm2 as well, but the 35mm2 from the fuses to the inverters is way above required, so that can stay.

I have 2x 100Amp fuses in the box as well as 2x 100amp breakers, which is sufficient too I believe.

 

Then this morning I disconnected the batteries and removed them from the frame. I have made it clear to the installer that despite their claims the stand will hold, that they must either provide a stand which will not bend under the weight of the batteries or they must return my 120ah stand (I did a straight swap of the smaller stand for the larger) and I'll repurpose it somewhere else in the garage, and I'll have a properly engineered stand made up for me.

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Does my head in why they do this!!!

The red cable on either side of the bank are the same length, and connected together. before connecting to the inverters through the fuse box.

I will be replacing these with proper bussbars soon.

I will be shortening cables further once I have the conduits in place too.

Does my head in too!

Until your bussbars are in place move one of the terminals positive or negative from string 1 to string 3 it will help even out battery string resistance but as you know bussbars are the way to go.

 

Edit: I had a look at you photos again and you only have two strings so move a terminal from string 1 to string 2 rather than string 3 as mentioned above.

 

Then this morning I disconnected the batteries and removed them from the frame. I have made it clear to the installer that despite their claims the stand will hold, that they must either provide a stand which will not bend under the weight of the batteries or they must return my 120ah stand (I did a straight swap of the smaller stand for the larger) and I'll repurpose it somewhere else in the garage, and I'll have a properly engineered stand made up for me.

 

While you are about it just euthanase the lot. Your installer has wired the battery bank the same way and now expects different results. I've got people like that that work for me and it takes all my self-will not to choke the living $#@t out of them some days. Here we have an installler paid good money to stuff things up. I reckon he needs to do an apprenticeship with Mike. For those of you who like a neat install have a look at Mike's FB page (https://www.facebook.com/groups/ThorneSolar/) Hope this does not embarrass him it not the intention.

 

Perhaps use wood for the shelves of your custom battery stand. The non conductive properties of wood may make the difference one day.

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Man, I WISH I had all of you when I did my original design!  :D

 

I have a question, teach me:

Why, from the batteries to the fuse box, is it 35mm2 and from the fuse box to inverter, 25mm2?

Reason I ask is matching 2 different cable mm2, must the fuse now be specced for the 35mm2 wire of the 25mm2 wire?

 

I am under the impression that from batts to inverter, wires must be same size, the fuse to cater for the wires?

 

As per Chris, see the negative wire:

post-122-0-18996200-1454927001_thumb.jpg

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Man, I WISH I had all of you when I did my original design!  :D

 

I have a question, teach me:

Why, from the batteries to the fuse box, is it 35mm2 and from the fuse box to inverter, 25mm2?

Reason I ask is matching 2 different cable mm2, must the fuse now be specced for the 35mm2 wire of the 25mm2 wire?

 

I am under the impression that from batts to inverter, wires must be same size, the fuse to cater for the wires?

 

As per Chris, see the negative wire:

attachicon.gif48v.jpg

 

The same amount of current will flow between the batteries and fuse box and fuse box and inverter. The only reason I can think of why someone would want to use different cable sizes, is if they use thinner cable for shorter distances and thicker cable for longer distances to try and have the same resistance and voltage drop. Maybe the installer have read something about the voltage drops in parallel battery strings, where you want to keep the resistance / voltage drop per string equal and misunderstood the explanation and tried to keep resistance and voltage drops similar in other parts of the battery cables as well.

 

Since you want the lowest overall resistance and lowest possible voltage drop at high currents between the battery bank and inverter, you should try to use the thickest cable in all instances.

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Man, I WISH I had all of you when I did my original design!  :D

 

I have a question, teach me:

Why, from the batteries to the fuse box, is it 35mm2 and from the fuse box to inverter, 25mm2?

Reason I ask is matching 2 different cable mm2, must the fuse now be specced for the 35mm2 wire of the 25mm2 wire?

 

I am under the impression that from batts to inverter, wires must be same size, the fuse to cater for the wires?

 

As per Chris, see the negative wire:

attachicon.gif48v.jpg

 

I based my fuse information from the Axpert install manual.

Basically what it says is:

Recommended breaker specification of battery for each inverter:

*If you want to use only one breaker at the battery side for the whole system, the rating of the breaker should be X times current of 1 unit. "X" indicates the number of inverters connected in parallel.

 

Model

1 unit*

4KVA

80A/60VDC

5KVA

100A/60VDC

So my understanding from the above is for each inverter, you need a 100A breaker, alternatively, you could also use a single 200A breaker before splitting the cables off to each inverter.

 

Since 25mm2 cable can safely handle a current of about 125A, it should be safe to use with 100A breakers/fuses.

 

So theoretically, using 25mm2 from the breakers to the inverters should be safe as the breakers will go before the cable?

 

Personally, I would actually be pretty worried if my system was pulling more than 100A from the batteries at any one time.

 

I did note Chris's negative wire, but I'll rather wait till I have the bussbar till I reconnect everything

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Check my logic:

On max of 83amps Dc for inverter max, 44 lowest volts from batteries, 3m (?) cable length: AWG12 or 3.3mm2

Take 2 levels up, it is then AWG 8, we do not get easily 8-9mm2, so 10mm2 cable size is the required cable.

For simplicity, AWG 6-8 cable need 50-80 amp fuses.

 

So Viceroy having 25mm2 (for the 35mm2 does not count) wires with 100amps fuses, is safe for the inverter should never get there, unless there is a problem in the batteries.

 

 

Personally, I would actually be pretty worried if my system was pulling more than 100A from the batteries at any one time.

 

Ja, I also came around to that kind of thinking after I burnt the living dinges out of my fingers. :-) 

Fuse ideally should go LONG before it gets problematic, which is way before it becomes dangerous.

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Check my logic:

On max of 83amps Dc for inverter max, 44 lowest volts from batteries, 3m (?) cable length: AVG12 or 3.3mm2

Take 2 levels up, it is then AVG 8, we do not get easily 8-9mm2, so 10mm2 cable size is the required cable.

For simplicity, AVG 6-8 cable need 50-80 amp fuses.

 

Using your figures this calculator comes up with 25mm2 or AWG3 (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html) which is similar to this coastguard chart.

 

post-822-0-44063100-1454934566_thumb.jpg

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Man, I WISH I had all of you when I did my original design!  :D

 

I have learnt so much from all of you on this forum, you too TTT you just don't give yourself credit where credit is due.

 

Edit: If you don't believe me go and look at my searching questions in the aptly named Solar beginners section.

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Thanks Chris.  :)

Using your figures this calculator comes up with 25mm2 or AWG3 (http://www.solar-wind.co.uk/cable-sizing-DC-cables.html)

 

Using your calculator, it all depends on % voltage drop over the length of the cable. 

On 3% drop, using 48v, 83amps and 3m total cable, I get 10mm2, AWG 8 - not acceptable.

On 1% drop, using 48v, 83amps and 3m total cable, I get same as you, 25mm2 - much better!

 

The crunch:

On 1% drop, using 48v, 83amps and 1m total cable, I get 10mm2, AWG 8 again - now we are talking!

(Cable one 500mm from batt pos to inverter positive, and cable two 500mm from batt neg to inverter negative)

 

Goes right back to: Less is more = shorter cables.

For the shorter the cable run, not only is it cheaper ito actual cost per meter, fuses are also cheaper and it is as safe as thicker cables over longer distances ... and thinner cables handle better.  :)

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Lots of good reading on this.

 

I'm going to leave my 35mm2 cable between the fuse box and inverters, but I do still want to up the cable size to 50mm2 from fuse box to battery to match the battery interconnects.

And obviously, going to shorten cables where I can once the new battery shelf is finalised. I think when I spec it, I'll raise it off the floor by about 300mm. That way I can shorten the battery cables.

 

Speaking of the battery shelf. I have left messages for the installer to call me urgently regarding this. Flipping annoying that he has not gotten back to me, but I guess he won't get paid the final amount until he sorts this out.

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Viceroy, between the batteries, shortest possible cables, go as big as you can.

At one stage, I used flexible Busbars similar to these, mine was rated at 250amps: http://www.cubic.eu/en-US/Products/Cu-flex-Flexible-copper-busbars

Overkill by a large factor = wasted monies.

 

From batts to inverters, no need to oversize, just follow the maths. It is a fine line.

 

The bigger the cable, more difficult to use, less likely you are to pick up a problem before it turns dangerous i.e. extreme perspective: At one point you can have such large cables, with associating fuses, that the inverter burns out before the fuses go.  :D

 

It is all maths, with a few caveats, based on equipment used.

Go lean, go mean. 35mm2 is perfectly fine for 48v Axperts in my opinion, if the cable length is short and sweet.

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I have a crimping tool with dies from 6mm2 up to 70mm2 so thats not a problem.

I would have a problem cutting 50mm2 cable. with my cable cutter it's pretty much on its limit at 35mm2.

 

The interconnects I have already on the batteries are already 50mm2, so I figure, at the very least from each string to the bussbars should be 50mm2. From there to the fuse box could remain 35mm2 or if I raise the shelves as I'm planning on doing with the new battery shelf, the distance from bussbars to fuses will be half what it is now.

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To cut a big cable if you do not have the correct tools...

Take good nitto tape and wrap the section you would like to cut. About 5cm wide should do it. Make sure you have at least 5 layers wrapped tight.

Now use a small hacksaw, or a big one, but with a dull blade reversed, and saw through the section without too much pressure.

With welding cable the thin strands tend to pull, so be aware.

Once through, take a carpet knife and remove as much of the wayward strands as possable to give you a clean "cut".

 

In the past I have also used a "biltong" cutter, but it was not mine :)

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