Navitron Renewable Energy and Sustainability Forum

SOLAR PHOTOVOLTAIC => Solar Photovoltaic Systems => Topic started by: antsals on March 31, 2020, 09:04:20 AM



Title: Inverter Position 30m Away
Post by: antsals on March 31, 2020, 09:04:20 AM
Good Morning,
I want to install a second solar array; but at the back of the garden.  The current array is on the house with the inverter in garage.  Preference would be to have the two inverters next to each other.  Can I run the DC current from the panels 30m (ish) to the garage without any problems? 

System is approx 1.5kw array, string installation.

Cheers,
Ant


Title: Re: Inverter Position 30m Away
Post by: kdmnx on March 31, 2020, 09:41:02 AM
Yes you can!

Just do the sums on voltage drop. You might need/want to use 6mm cable instead of 4mm for the long run.


Title: Re: Inverter Position 30m Away
Post by: antsals on March 31, 2020, 10:04:37 AM
The panels have 4mm cable MC4 connectors?  If they are a string would that no mean all the panels need cabling in 6mm?

Or

Can I cable together then to a jb and take an armored cable through to the garage as a 6mm?

Details below but I'm not sure how to calc correctly I am working on:-

40.3v x 5 panels = 201.5v
9.51a = 9.51a

Do the numbers below look good?

 
(https://i.postimg.cc/svj5Mr4f/Presentation1.jpg) (https://postimg.cc/svj5Mr4f)



(https://i.postimg.cc/BL3drgdv/Presentation2.jpg) (https://postimg.cc/BL3drgdv)

Cheers,
Ant


Title: Re: Inverter Position 30m Away
Post by: JohnS on March 31, 2020, 10:16:13 AM
You have put the open circuit voltage in the calculator but should use the maximum power voltage, or may be a bit less.

Using 150 volts gives a result of 6mm cable.



Title: Re: Inverter Position 30m Away
Post by: kdmnx on March 31, 2020, 10:27:46 AM
The panels have 4mm cable MC4 connectors?  If they are a string would that no mean all the panels need cabling in 6mm?

Or

Can I cable together then to a jb and take an armored cable through to the garage as a 6mm?

Details below but I'm not sure how to calc correctly I am working on:-

40.3v x 5 panels = 201.5v
9.51a = 9.51a

Do the numbers below look good?

 
(https://i.postimg.cc/svj5Mr4f/Presentation1.jpg) (https://postimg.cc/svj5Mr4f)



(https://i.postimg.cc/BL3drgdv/Presentation2.jpg) (https://postimg.cc/BL3drgdv)

Cheers,
Ant


Thanks for the additional info.

It is only the long run that you need to be concerned about, the rest can stay as it is. No JB required, buy 6mm DC cable, crimp on the MC4 connectors, hook that up to each end of the array and run that to your garage.

Use the "max power voltage" in your calculations (not open circuit voltage).

But with the additional info you're fine with 4mm˛, I'd still lay 6mm˛ because: the cost difference is tiny in the scheme of things, those few extra V make all the difference on a cloudy day, and it give flexibility to upgrade the array in future. Up to you though...


Title: Re: Inverter Position 30m Away
Post by: antsals on March 31, 2020, 10:35:33 AM
Thanks very much!

The "Amp" rating of the 6mm cable seemed very high....seemed like I was over killing it.

Much appreciated!


Title: Re: Inverter Position 30m Away
Post by: antsals on March 31, 2020, 10:39:42 AM
Just re-run the numbers with the 163v and 9.51a.......seems to kill it on 4mm or 6mm, its the acceptable loss that makes the difference.

What are we supposed to calculate the loss at 3%?

Also this might be a random question; but I see a lot of articles about 48v solar panels etc. but none of them seem to reference 48v.....whats all this about?

Thanks again,
Ant


Title: Re: Inverter Position 30m Away
Post by: kdmnx on March 31, 2020, 11:30:45 AM
Just re-run the numbers with the 163v and 9.51a.......seems to kill it on 4mm or 6mm, its the acceptable loss that makes the difference.

What are we supposed to calculate the loss at 3%?

Also this might be a random question; but I see a lot of articles about 48v solar panels etc. but none of them seem to reference 48v.....whats all this about?

Thanks again,
Ant

The "rule" is 3% for the "system" which many people position this as 2% for DC side, 1% for AC side. However if your consumer unit is in the garage with your inverter then you could call the AC voltage drop negligible. On the other hand where voltage drop is concerned, less is better.

Almost all panels are c. 30V, there is a fancy 400W "bi-facial" LG panel that is 42V, and some Panasonic panels are 60V, there may be others.




Title: Re: Inverter Position 30m Away
Post by: brackwell on March 31, 2020, 11:35:26 AM
I always thought cable size was about amps not volts.


Title: Re: Inverter Position 30m Away
Post by: Ted on March 31, 2020, 11:50:45 AM
It's about both. But usually, on shortish runs, if you take care of the amps then the volt drop will automatically be OK too. It is only on the longer cable runs that it can become an issue.


Title: Re: Inverter Position 30m Away
Post by: kdmnx on March 31, 2020, 11:54:33 AM
I always thought cable size was about amps not volts.

It is all about "voltage drop"! That is the most important figure to calculate. Obviously it is all intertwined, amps, volts, cable length...

Some people think that because a cable is "rated" for 6A (or whatever) that is what you can push through it. However that isn't how it works...


Title: Re: Inverter Position 30m Away
Post by: pj on March 31, 2020, 12:36:02 PM
I beg to differ. Acceptable voltage drop is an engineering decision or compromise. What is suitable in one scenario may not suit another.
However, amps is different. As you say, you can push as many amps as you like through any cable, but if you exceed a certain value in a certain environment, then the cable will melt or catch fire.
Amp ratings of cables include implicitly the environment they are used in and the type of temperature tolerance they have. So, for example,  'House Wiring' amp ratings would be different to 'Vehicle' amp ratings.


Title: Re: Inverter Position 30m Away
Post by: kdmnx on March 31, 2020, 01:08:16 PM
I beg to differ. Acceptable voltage drop is an engineering decision or compromise. What is suitable in one scenario may not suit another.
However, amps is different. As you say, you can push as many amps as you like through any cable, but if you exceed a certain value in a certain environment, then the cable will melt or catch fire.
Amp ratings of cables include implicitly the environment they are used in and the type of temperature tolerance they have. So, for example,  'House Wiring' amp ratings would be different to 'Vehicle' amp ratings.

This is a common mistake. The "voltage dropped" tells you how much power is being used per m of cable (ie converted into heat), it is this figure that is therefore the most important when preventing your house from burning down.

For very short runs you can get away with just having a "current rating" for a particular cable. However this a dumbing down of the reality that can in some circumstances be very dangerous. It ignores the basic principle that: you need a thicker cable for long runs even if the current is the same. The fact that some cables have a "maximum current" printed on the side can be misinterpreted by people who shouldn't be messing with wires!