Solar wire sizing

Slidegate · 02-29-12, 08:45 PM

Okay I have a conundrum. If my panels are producing 18 Volts and 6 panels at 3.8 amps each which gives me approx 23 amps to my system. I have a 20foot run from my combiner box to my charge controller. Do I calculate wire sizing useing 12V or 18V in the formula?

Ryland · 02-29-12, 11:00 PM

Your wire size should be based off the amps, not the voltage, but your voltage under load should drop down to 13 to 14v, depending on your battery bank state of charge.
You can also get a Maximum Power Point Tracker Charge Controller (MPPT) that can handle a higher input voltage and adjust the output voltage to give you the highest wattage possible, often higher then you could otherwise get with a cheaper charge controller, also if you get a MPPT you can wire up your panels to a higher voltage so you can either run a longer run of wire and have your panels in an even more ideal location or you can use a smaller gauge wire.

Slidegate · 03-01-12, 04:44 PM

Thanks Ryland. I have an MPPT controller. I was just checking to see if my wire calcs were correct. I have a 20 ft run to my breaker box and with the 6 panels I have at a combined 23 amps so # 4 copper should be fine

Xringer · 03-01-12, 07:15 PM

What is the max voltage your MPPT controller will take from PV??

If it's high enough, you can put panels in series, (instead of parallel).
Higher voltage means lower current, and cheaper wire.

My four 18v (125w)panels are connected in series, giving my controller about 72v input.
Use that to keep my 48V battery bank changed up..

Since the current is only about 7 amps, I used about 100 feet of old RF coax
cable to bring the juice indoors to the controller and 48v bank..

Didn't need to break the bank on copper..

The bad thing about using 72 volts, is it could hurt or even kill you.
It's hard to get shocked by 18V.

rhino 660 · 03-01-12, 11:16 PM

when installing systems ive been shocked by 360VDC and it just feals really funny its the amps that kill. higher voltage allows a smaller wire because when you series the volts double and the amps stay the same lotts of amps need a big wire if you parallel volts stay the same and amps add together

Xringer · 03-02-12, 08:02 AM

Using two strings of 3 panels in Series-Parallel, you would be running 8 amps,
which is way better than 23 amps..

Since the line loss, is the current Squared, times the resistance..

8^2 = 64, but 23^2 = 529

So, 0.1 ohms of wire resistance would be 6.4 watts of loss,
but using 23 amps w/ 0.1 ohms is 53 watts of loss..

As you can see, it's not a linear thing.

Xringer · 03-02-12, 08:30 AM

Quote:

Originally Posted by rhino 660

when installing systems ive been shocked by 360VDC and it just feals really funny its the amps that kill. higher voltage allows a smaller wire because when you series the volts double and the amps stay the same lotts of amps need a big wire if you parallel volts stay the same and amps add together

It feels funny?? At 360 volts, even a tiny bit of current can stop your heart.

I've found that anything over 45 volts across my fingers can stings like crazy,
and I would hate to have that zap going across my chest (left-hand to right hand).

MN Renovator · 03-02-12, 12:27 PM

It all depends on where the current is passing. I've been zapped by enough power from a small photo capacitor(rated 350v) holding 280 volts that it burned two black marks where the current passed through the tip of my finger and the base of my finger. Source was 2 AA batteries passing through a transformer and then stored in a capacitor the size of a tic-tac.

If the voltage is there and it has any ability to create power and it goes through your heart, you are done. The amperage kills you think doesn't matter so much in the case of very low voltage because 12v won't pass through skin. It takes voltage to push the amperage through the impedance between your skin, your heart, and back through skin again.

A 360v array can kill you if you passes through, say both hands. If it passes through a finger or one part of your right hand through to ground you are lucky. Just hope it doesn't pass through your heart or get you trapped between the conductor and earth through the means of gravity. As always, use only your right hand when handling high voltage unless you are wearing HV resistant gloves and absolutely know it won't touch any other area of your body.

Slidegate · 03-03-12, 02:41 PM

I want to thank you guys for your replies. I have 3 homemade panels on a tracker. Each panel is 18V-65W-3.7A. I am going to expand the framing to add 3 more panels. Thus my question on the wire sizing. I want to thank Xringer because he got me to research a few things. After reading my MPPT manual I found that it will accept up tp 100VDC from the panels to charge my 12V battery system. So if I series 3 panels I get 54V-195W-3.7A. Then if I parralel both 3 panel series I will have 54V-390W-7.4A. So I won't have to run any new wire. Just change some wiring to my combiner box. If anybody see's anything wrong with this please reply

Xringer · 03-04-12, 03:56 PM

Just remember to use heavy wire between the charger and battery..

But if you keep the runs short, #10 wire will be fine.

Another nice thing about running low current and high voltage PV,
is when you want to charge up a 24, 36 or 48 volt battery bank,
you just need to re-config your charge Controller. The PV is ready to go..

03-01-12, 07:15 PM	#4
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	What is the max voltage your MPPT controller will take from PV?? If it's high enough, you can put panels in series, (instead of parallel). Higher voltage means lower current, and cheaper wire. My four 18v (125w)panels are connected in series, giving my controller about 72v input. Use that to keep my 48V battery bank changed up.. Since the current is only about 7 amps, I used about 100 feet of old RF coax cable to bring the juice indoors to the controller and 48v bank.. Didn't need to break the bank on copper.. The bad thing about using 72 volts, is it could hurt or even kill you. It's hard to get shocked by 18V. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..

03-02-12, 08:02 AM	#6
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	P = (IR)(R) or P = I^2(R) Using two strings of 3 panels in Series-Parallel, you would be running 8 amps, which is way better than 23 amps.. Since the line loss, is the current Squared, times the resistance.. 8^2 = 64, but 23^2 = 529 So, 0.1 ohms of wire resistance would be 6.4 watts of loss, but using 23 amps w/ 0.1 ohms is 53 watts of loss.. As you can see, it's not a linear thing. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less.. Last edited by Xringer; 03-02-12 at 08:19 AM.. Reason: it's not a linear

03-03-12, 02:41 PM	#9
Slidegate Lurking Renovator Join Date: Oct 2011 Location: Mercer, Pa. Posts: 19 Thanks: 1 Thanked 4 Times in 3 Posts	Wire size I want to thank you guys for your replies. I have 3 homemade panels on a tracker. Each panel is 18V-65W-3.7A. I am going to expand the framing to add 3 more panels. Thus my question on the wire sizing. I want to thank Xringer because he got me to research a few things. After reading my MPPT manual I found that it will accept up tp 100VDC from the panels to charge my 12V battery system. So if I series 3 panels I get 54V-195W-3.7A. Then if I parralel both 3 panel series I will have 54V-390W-7.4A. So I won't have to run any new wire. Just change some wiring to my combiner box. If anybody see's anything wrong with this please reply Attached Thumbnails

02-29-12, 08:45 PM	#1
Slidegate Lurking Renovator Join Date: Oct 2011 Location: Mercer, Pa. Posts: 19 Thanks: 1 Thanked 4 Times in 3 Posts	Solar wire sizing Okay I have a conundrum. If my panels are producing 18 Volts and 6 panels at 3.8 amps each which gives me approx 23 amps to my system. I have a 20foot run from my combiner box to my charge controller. Do I calculate wire sizing useing 12V or 18V in the formula?

02-29-12, 11:00 PM	#2
Ryland Master EcoRenovator Join Date: Mar 2009 Location: Western Wisconsin. Posts: 913 Thanks: 127 Thanked 82 Times in 71 Posts	Your wire size should be based off the amps, not the voltage, but your voltage under load should drop down to 13 to 14v, depending on your battery bank state of charge. You can also get a Maximum Power Point Tracker Charge Controller (MPPT) that can handle a higher input voltage and adjust the output voltage to give you the highest wattage possible, often higher then you could otherwise get with a cheaper charge controller, also if you get a MPPT you can wire up your panels to a higher voltage so you can either run a longer run of wire and have your panels in an even more ideal location or you can use a smaller gauge wire.

03-01-12, 04:44 PM	#3
Slidegate Lurking Renovator Join Date: Oct 2011 Location: Mercer, Pa. Posts: 19 Thanks: 1 Thanked 4 Times in 3 Posts	Thanks Ryland. I have an MPPT controller. I was just checking to see if my wire calcs were correct. I have a 20 ft run to my breaker box and with the 6 panels I have at a combined 23 amps so # 4 copper should be fine

03-01-12, 11:16 PM	#5
rhino 660 Apprentice EcoRenovator Join Date: Oct 2010 Posts: 105 Thanks: 0 Thanked 10 Times in 8 Posts	when installing systems ive been shocked by 360VDC and it just feals really funny its the amps that kill. higher voltage allows a smaller wire because when you series the volts double and the amps stay the same lotts of amps need a big wire if you parallel volts stay the same and amps add together

03-02-12, 12:27 PM	#8
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	It all depends on where the current is passing. I've been zapped by enough power from a small photo capacitor(rated 350v) holding 280 volts that it burned two black marks where the current passed through the tip of my finger and the base of my finger. Source was 2 AA batteries passing through a transformer and then stored in a capacitor the size of a tic-tac. If the voltage is there and it has any ability to create power and it goes through your heart, you are done. The amperage kills you think doesn't matter so much in the case of very low voltage because 12v won't pass through skin. It takes voltage to push the amperage through the impedance between your skin, your heart, and back through skin again. A 360v array can kill you if you passes through, say both hands. If it passes through a finger or one part of your right hand through to ground you are lucky. Just hope it doesn't pass through your heart or get you trapped between the conductor and earth through the means of gravity. As always, use only your right hand when handling high voltage unless you are wearing HV resistant gloves and absolutely know it won't touch any other area of your body.

03-04-12, 03:56 PM	#10
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Just remember to use heavy wire between the charger and battery.. But if you keep the runs short, #10 wire will be fine. Another nice thing about running low current and high voltage PV, is when you want to charge up a 24, 36 or 48 volt battery bank, you just need to re-config your charge Controller. The PV is ready to go.. __________________ My hobby is installing & trying to repair mini-splits EPA 608 Type 1 Technician Certification ~ 5 lbs or less..