Cable size inverter to meter

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Cable size inverter to meter

Postby Phil_C » Mon Jul 14, 2014 8:34 pm

Hello All
I currently have a gross feed solar array that is 70m from the meter to the inverter (80m cable length). The inverter feeds the meter via two underground 70mm2 cables. I am looking at adding a secondary net metered system that will be the same distance from the net meter to the inverter.
The new inverter will need to be 3 phase so I need to pull out the two 70mm2 cable and pull back through the conduit the same cables with four additional cables for the 3 phase system. I know now that the 70mm2 cables are massively oversized for the intended purpose and as such I have been looking at a cheaper sized cable for the three phase system.
Some sparkies say the cable size does not need to be any bigger than 4mm2 others say 6mm2 so I was thinking along the lines of 10mm2 simply because it gives less voltage drop. According to the calculator at http://www.calculator.net/voltage-drop-calculator.html and given the distance of 80m, 415volts, 3-phase, single set of conductors and 20amps current draw, 10mm2 cable gives a voltage loss of 0.63%. Is that an acceptable level? The 20 amps are based on peak output of the panels with a 60% margin.

Also I was thinking about replacing the 70mm2 cables with smaller cables as well to reduce the space taken up in the conduit but again I am not sure what size cable should be used.

Basically what I need to know is what is an acceptable level of voltage drop in a PV system.
Regards

Phil
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Re: Cable size inverter to meter

Postby davidg » Mon Jul 14, 2014 9:40 pm

Phil_C wrote:Basically what I need to know is what is an acceptable level of voltage drop in a PV system

How big is the system you have now on the gross FIT?

It's not voltage drop, it's voltage rise you need to compensate for, In NSW the regs are you are not allowed to have more than a 2% voltage rise with solar connected. Your cable may not be massively over-sized, it depends on a number of other factors, including maximum demand also has to calculated as well it not as simple as you are making it out to be, there needs to be more detailed study done and it needs to done properly.

I have 25mm2 for a slightly shorter distance approx 60m in Vic, that keeps it to under a 2% rise and I didn't have to comply with a reg like NSW's.

By the way if the cable is really so oversized then one of the 70mm2 cables might be able to be used as a common neutral for all of them saving one cable, there is provision for this in AS3000, would have to be verified though.
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Re: Cable size inverter to meter

Postby Phil_C » Mon Jul 14, 2014 9:54 pm

davidg wrote:
Phil_C wrote:Basically what I need to know is what is an acceptable level of voltage drop in a PV system

How big is the system you have now on the gross FIT?

2.5kw
It's not voltage drop, it's voltage rise you need to compensate for, In NSW the regs are you are not allowed to have more than a 2% voltage rise with solar connected. Your cable may not be massively over-sized, it depends on a number of other factors, including maximum demand also has to calculated as well it not as simple as you are making it out to be, there needs to be more detailed study done and it needs to done properly.

Wouldn't the voltage be regulated by the inverter? It thought that was part of it's function. If the cable is too small it cannot give a voltage rise. If the cable is too large it will not cause any issues except for cost. 90% of the power produced by the net system will be used in house and not exported to the grid.
I have 25mm2 for a slightly shorter distance approx 60m in Vic, that keeps it to under a 2% rise and I didn't have to comply with a reg like NSW's.

Is that a three phase system?

By the way if the cable is really so oversized then one of the 70mm2 cables might be able to be used as a common neutral for all of them saving one cable, there is provision for this in AS3000, would have to be verified though.

Thanks for that. I was not sure if a common neutral could be used for two separate systems such as this.

Regards

Phil
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Re: Cable size inverter to meter

Postby davidg » Mon Jul 14, 2014 10:04 pm

Phil_C wrote:2.5kw

Gees, (A toy) a little one, yes don't need that size cable for it.

Phil_C wrote:Wouldn't the voltage be regulated by the inverter?

No. an inverter follows grid voltage and will push the voltage by an x volts needed to produce and export power to the grid.

Phil_C wrote:It thought that was part of it's function. If the cable is too small it cannot give a voltage rise.

No. if its too small the voltage must rise more to overcome the internal resistance of the wire.

Phil_C wrote:If the cable is too large it will not cause any issues except for cost.

you would windup with almost no rise due to much lower internal wire resistance.

Phil_C wrote:Is that a three phase system?

No. see my signature :D
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Re: Cable size inverter to meter

Postby Phil_C » Mon Jul 14, 2014 10:37 pm

davidg wrote:
Phil_C wrote:2.5kw

Gees, (A toy) a little one, yes don't need that size cable for it.

Originally I had planned to quadruple the size of the system until I found out you could not do so without losing the 60c feed in tarrif so I shelved the plans and later scrapped them.

No. an inverter follows grid voltage and will push the voltage by an x volts needed to produce and export power to the grid.
No. if its too small the voltage must rise more to overcome the internal resistance of the wire.


Exactly what I was thinking in that the inverter controls the voltage. To me if the wire size was too small the inverter would increase the voltage to compensate but at the meter due to voltage loss through the cable you would still be within the limits.

you would windup with almost no rise due to much lower internal wire resistance.

Wouldn't the inverter regulate the voltage to give the rise?


Phil_C wrote:Is that a three phase system?
No. see my signature :D


I have a question about your power output. Your sig says you have a 7.2kw system but your stats say your maximum output is about 20-22 kw on a good day. My 2.5kw system puts out around 12-14kw on a good day which would equal around 37kw if I had the same size system. I am located on the NSW side of the Queensland border does the difference in Lattitude make that big a difference in solar output or is there another reason for the low figures?

Regards

Phil
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Re: Cable size inverter to meter

Postby davidg » Tue Jul 15, 2014 12:26 am

Phil_C wrote:Your sig says you have a 7.2kw system but your stats say your maximum output is about 20-22 kw on a good day. My 2.5kw system puts out around 12-14kw on a good day which would equal around 37kw if I had the same size system. I am located on the NSW side of the Queensland border does the difference in Lattitude make that big a difference in solar output or is there another reason for the low figures?


At this time of the year it's down quite a bit. We have had a lot poor weather for some weeks, also did you notice it says it's facing 20 degrees east of true north...... which at this time of year also has a substantial impact. True north would be a lot better. Had I, a choice in how to place them on the roof and still fit them all. I didn't and still get that amount of PV on that roof, so it is what it is.

And yes difference in location to the north does have an impact as well.

During the summer its a lots more output

The average per day 24.80kWh for the lifetime of the system to date, actually slightly low PVbeancounter is dropping a small amount most days, adds up though.
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Re: Cable size inverter to meter

Postby Phil_C » Tue Jul 15, 2014 6:22 am

Thanks for the explanation about your solar.

Looking around the web it appears the acceptable voltage drop is 1% for solar and by voltage drop I mean from the inverter to the mains.

Given the 1% voltage drop tolerance the 10mm2 cable with it's 0.63% voltage drop is more than adequate for the new system. It is also suitable for the 240v system but I think I will leave it as is and common up the neutral for both systems.

Can anyone see any problems with that setup?

Regards

Phil
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Re: Cable size inverter to meter

Postby sparkycoffs » Tue Jul 22, 2014 5:43 pm

I am trying to get my head around some of your responses. You have a gross system installed with an FiT of 60 cents correct? Why are you running more cable to the metering position if you want an additional Nett System?
You also mention 3 phase, do you have 3 phase connected to the house?

If you are planning a Nett system connected to single phase just hook it into the sub board at the house. You would be best having an accredited electrician design it all as application has to be made to the supply authority and voltage rise calcs need to be completed for every leg of the installation including your submains to the house
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Re: Cable size inverter to meter

Postby Phil_C » Tue Jul 22, 2014 6:02 pm

The cable is for the new 10kw NET system which is three phase. Yes the house has three phase connected (not the house but the shed near the house). The meter box has a three phase meter capable of metering both consumption and generation and next to it is the gross meter for the existing solar system. The panels and inverter are 80m from the meters.

I am well aware of what is required for the system to be accredited. The issue here is whether I put the system on the roof or on a stand alone frame alongside the existing Gross system which is 80m from the house. If I put it on the house it will be cheaper but will not generate as much power as it is partly shaded and not facing north.

Regards

Phil
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Re: Cable size inverter to meter

Postby sparkycoffs » Tue Jul 22, 2014 6:56 pm

for a 10kW 3 phase system you will have to use 16mm 4C+E or building wire which still need the earth for EP bond. This would need 40mm up to 63mm conduit

You may change the 70mm you already have to 25mm. 16mm wont make the 80 metre run

If you run all of this in the one conduit you will have to be aware of the heating effects and conduit size. You would need 63 or 80mm.

You may be better off running a new conduit, even though it is more money etc, as it will be a difficult pull

Hope this helps

All sizing above is per AS3000:2007 and amendments
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