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Basics needed for startup


PaulF007

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As some you know I am on my way to get started with going full off grid. I have a reasonable understanding of how the AC side of things work with DB's breakers ectr. But this will be the first time that I will work with high current DC. From what I have gathered it is not something to take lightly. What I already know is:

You need to Isolate / fuse every group of Panels - Should they be fused by the panels or at the inverter?

Fuse the cable that comes from the panels that goes into the Inverter - Size and Type?

Fuse to the Battery from the charger and to the Inverter - Size and type?

Should I have a fuse between the battery packs as well or just from the batteries to the inverter?

What else have I missed - do you get some type of DC isolators that you could use to Isolate the batteries from the inverter (pulling a fuse seems a bit scary)

regards

 

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As far as I know.

1 Fuse in series with every string of panels (unless it is a very small system where one string is unlikely to overpower the reverse biasing voltage of the other). These often go inside the combiner box (ie not at the inverter).

Positive and negative cable from the combiner box to a double-pole DC-rated isolator, and from there to the charge controller.

A second DC rated isolator between both the positive and negative wires on the charge controller and the battery.

An emergency disconnect between the battery and the inverter. For this I use the Mersen fuse holders you get at Rubicon. It's not a DC rated switch, but in an emergency it will do if you pull it (though it will make a long spark under load). Up to 160 amps comes in one size, after that the fuse size goes up and the fuse holder becomes more expensive too. Another reason to go 48V, a 160A fuse won't get you to 4kva if you don't :-) You can look for better DC switch gear if you want, but it will cost you. You don't have to fuse both lines, but I did.

From the Mersen fuse holder, the cables go to the inverter. Inverter might also have it's own fuse (the blue ones all do :-P).

I don't know what you do with integrated stuff like the Axpert. I would think a second fuse downstairs before the inverter is probably unnecessary, but I'm not an expert in that regard.

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6 hours ago, plonkster said:

As far as I know.

1 Fuse in series with every string of panels (unless it is a very small system where one string is unlikely to overpower the reverse biasing voltage of the other). These often go inside the combiner box (ie not at the inverter). - My installation would be Rooftop so I would have a small distribution/cobiner box at rooftop with all the fuses together in one box? - Is it worth moving it into the Battery room or is the Wire then too long?

Positive and negative cable from the combiner box to a double-pole DC-rated isolator, and from there to the charge controller.

A second DC rated isolator between both the positive and negative wires on the charge controller and the battery.

An emergency disconnect between the battery and the inverter. For this I use the Mersen fuse holders you get at Rubicon. It's not a DC rated switch, but in an emergency it will do if you pull it (though it will make a long spark under load). Up to 160 amps comes in one size, after that the fuse size goes up and the fuse holder becomes more expensive too. Another reason to go 48V, a 160A fuse won't get you to 4kva if you don't :-) You can look for better DC switch gear if you want, but it will cost you. You don't have to fuse both lines, but I did.

From the Mersen fuse holder, the cables go to the inverter. Inverter might also have it's own fuse (the blue ones all do :-P).

I don't know what you do with integrated stuff like the Axpert. I would think a second fuse downstairs before the inverter is probably unnecessary, but I'm not an expert in that regard.

Solar Desighn.png

This the general idea?

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2 hours ago, PaulF007 said:

Solar Desighn.png

This the general idea?

Yes, that is correct. Electrically it would not really make a difference where you install the combiner box. If in the "battery room" you would have to run say 3 pairs of 6mm2 cable between the 3 strings of panels and the combiner box. If you put the combiner box closer to the panels you would have to run thicker cables with the same cross section of copper as the 3 x 6mm2 cables, e.g. 18mm2 cable between the combiner box and the charge controller / inverter. So it is your choice: 3 pairs of 6mm2 cable or one pair of 18mm2 cable. 18mm2 cable is not standard and you might have to use the next size up e.g. 25mm2 - keep in mind that the thicker cable is more difficult to work with e.g. when pulling through conduit. On the other hand if your combiner box is in the "battery room" it is protected from the elements and easy to monitor / replace a blown fuse etc. etc. If I were you I would run 3 pairs of cable to the "battery room" and install the combiner box in the battery room.

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10 hours ago, plonkster said:

 

I agree it would be better to have them by the rest of your "control" panels - Hec I might be inclined to put some analog volt meters over each string just for the fun of it - thats if they dont consume any power :D 

Lucy for my the Battery room will be right below my panels so I can keep the wires shortish. Can one use normal 6mm GP wire or should you use those solar cables that the Solar site wants to sell - We alway had the rule that Conduit is cheap if you install it from the beginning so make sure you put a couple extra in.You will use them in future. 

I remember the first house I build I installed a blank conduit in every room's corner - it just went strait in to the roof and did a very thin skim of plaster over the connector box (After I measured the placing of it carefully and noted it) - few years later the misses wanted to install a standing light in one of the corners that obviously had no power - a swear they have the capacity to smell such things and pick that spot just to see what we will do -  Much to her surprise I took a small hammer knocked the wall (was to lazy to go and lookup the measurements ) until i found the hollow sound that I was looking for. Cleared the skim plaster and pulled the wires trough for the light. So I you ever build a new house and you are DIY inclined or not even - insist that they install some blanks in the wall for future use.It should only be a couple of buks extra but you will use them . 

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51 minutes ago, superdiy said:

3 pairs of 6mm2 cable or one pair of 18mm2 cable. 18mm2 cable is not standard and you might have to use the next size up e.g. 25mm2

@superdiy just a question, the cables, are they not supposed to be decided upon based on amps over length of wires with voltage drops?

For 25mm2 is for a "ma moerse" lot of amps.

Plonkster, I love that story! Wish you told it 4 years ago, for I would have done the blanks all over the houses.

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On 7/22/2016 at 11:47 AM, The Terrible Triplett said:

@superdiy just a question, the cables, are they not supposed to be decided upon based on amps over length of wires with voltage drops?

For 25mm2 is for a "ma moerse" lot of amps.

That was not an official calculation, but depending on the length of cable used you can run either 4mm2 or 6mm2 per string of panels producing in the range of 8-9 amps - again not calculated exactly. 3 strings in parallel will require 3 x the cable cross-section of the cable used per string and you always go a size up instead of down when you use cable, because the thicker the cable the lower the resistance, losses, heat etc. etc.

If you are using 4mm2 per string, for 3 strings you would use 12mm2 or the next size up which would be 16mm2. (or 3 x 4mm2 cables in parallel - your choice)

If you are using 6mm2 per string, for 3 strings you would use 18mm2 or the next size up which would be 25mm2. (or 3 x 6mm2 cables in parallel - your choice)

If 25mm2 is the next size up then be it. In these low PV voltage configurations you will have a lot of current flowing from the panel array and especially if using long cable lengths, you rather want to use thicker cable.

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On 7/25/2016 at 11:49 AM, superdiy said:

That was not an official calculation, but depending on the length of cable used you can run either 4mm2 or 6mm2 per string of panels producing in the range of 8-9 amps - again not calculated exactly. 3 strings in parallel will require 3 x the cable cross-section of the cable used per string and you always go a size up instead of down when you use cable, because the thicker the cable the lower the resistance, losses, heat etc. etc.

If you are using 4mm2 per string, for 3 strings you would use 12mm2 or the next size up which would be 16mm2. (or 3 x 4mm2 cables in parallel - your choice)

If you are using 6mm2 per string, for 3 strings you would use 18mm2 or the next size up which would be 25mm2. (or 3 x 6mm2 cables in parallel - your choice)

If 25mm2 is the next size up then be it. In these low PV voltage configurations you will have a lot of current flowing from the panel array and especially if using long cable lengths, you rather want to use thicker cable.

This is where most people under estimate DC power. Don't skimp on the cables. Go one size bigger as far as possible. 

To the OP:

 

1. Fuse either at the panel, or in below - wherever is easiest / convenient for you to replace fuses. Simple rule of thumb:

2. Use a fuse at least 25% bigger than your expect current rating. For a 250W-300W panel, I have found that 10A fuses tend to blow often in summer. 12A seems fine. Fuses protect the cable, not the equipment. "10x38" barrel fuses are common for Solar and they cost about R35 each. A fuse hold cost about R60

3. Yes! See what the manufacturer suggest and go up by 25%. The Axpert 5Kva (common inverter) for example suggest a 100A fuse. Their typical Amperage is 87A, according to the manulDo the math: 4Kw / 48V = 83A. 5Kw / 48V = 104A. You don't get a 83A or 104A so use a 120A fuse. Blade fuses, as used in sound installations work well and are quite cheap - about R300 for a holder + 2 fuses. Or the most expensive fuse isolators are preferred but are expensive - about R800-1200 for a 160A fuse + holder. 

4. DC isolators are SUPER expensive and quite scarce. The fuse isolator will work fine, but over time will develop welding on the contacts. You could easily pay R2K for a 160A DC isolator. It is definitely better but expensive. 

 

 

 

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  • 4 weeks later...
On 2016/07/29 at 4:35 PM, SilverNodashi said:

to deliver the same Watt (Joules?)

To answer the bit in paranthesis... you're close :-) A watt is a joule per second, so 1wh == 3600 joules, and 1kwh == 12960000 joules. Watt is a measure of power (the ability to turn energy into work), while Joule is a measure of energy (just like it's cousin kwh).

Re the original story about more amps as the volts drop: Depends on your loads and the quality of the inverter. Some very cheap MSW inverters have a fixed boost stage and as the voltage on the input drops you simply get a similar drop on the other end. Other MSW inverters will compensate (for the fixed boost stage) by modulating the width of the square bit in the modified sine wave to keep the energy level the same. The best inverters have an adjustable boost stage and as the voltage drops, the inverter itself compensates by drawing more amps.

For a UPS, your load is half-likely a computer power supply, and such SMPS units will compensate volts for amps anyway, so even on the cheap MSW where the voltage might sag to well below 200V, you'll still get the same effect: amps go up as the volts go down. Only place this will not happen is with a purely resistive load (eg Incandescent light, kettle) on a very cheap inverter. Then the amps will actually come down, but so will the output power, so your lamp will go dimmer and it will take longer to boil the water.

:-)

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Side note on similar place where power vs energy always comes up: Car engine. Diesel has a higher carbon content per liter (so it's actually no surprise that they get better mileage, right?), but it burns slower. More energy (aka torque), but less power (it takes more time to turn that into work). So in layman's terms it means the power is proportional to the torque multiplied by the rpms (but Diesel's don't rev high).

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On 25/08/2016 at 0:26 PM, plonkster said:

Side note on similar place where power vs energy always comes up: Car engine. Diesel has a higher carbon content per liter (so it's actually no surprise that they get better mileage, right?), but it burns slower. More energy (aka torque), but less power (it takes more time to turn that into work). So in layman's terms it means the power is proportional to the torque multiplied by the rpms (but Diesel's don't rev high).

But, when you drive a turbo charged V8, you don't care too much about fuel consumption ;)

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