Voltage drop

Charles in Aus.

Well-known Member
Could someone please help me out with a projected voltage drop calculation ?

I need to lay cable from the solar panels at my barn to the machine room that houses the battery bank .
The distance is 40 metres / 130 feet . I have two[ one positive one negative ] 25mm2 cables with cores of about 20 copper strands each and am hoping that these will be heavy enough to use over this span without compromising the charge current too much .
Output from the panels is aproximately 18.5 volts at full capacity , regulator charges at 13.5 v .
 
(quoted from post at 19:02:36 10/09/17) Could someone please help me out with a projected voltage drop calculation ?

I need to lay cable from the solar panels at my barn to the machine room that houses the battery bank .
The distance is 40 metres / 130 feet . I have two[ one positive one negative ] 25mm2 cables with cores of about 20 copper strands each and am hoping that these will be heavy enough to use over this span without compromising the charge current too much .
Output from the panels is aproximately 18.5 volts at full capacity , regulator charges at 13.5 v .
on't care much about voltage, but need to know current. Also, is the 'copper' 25mm2?
 
(quoted from post at 16:02:36 10/09/17) Could someone please help me out with a projected voltage drop calculation ?

I need to lay cable from the solar panels at my barn to the machine room that houses the battery bank .
The distance is 40 metres / 130 feet . I have two[ one positive one negative ] 25mm2 cables with cores of about 20 copper strands each and am hoping that these will be heavy enough to use over this span without compromising the charge current too much .
Output from the panels is aproximately 18.5 volts at full capacity , regulator charges at 13.5 v .
25mm2 is about 3awg gage wire.
 
(quoted from post at 18:02:36 10/09/17) Could someone please help me out with a projected voltage drop calculation ?

I need to lay cable from the solar panels at my barn to the machine room that houses the battery bank .
The distance is 40 metres / 130 feet . I have two[ one positive one negative ] 25mm2 cables with cores of about 20 copper strands each and am hoping that these will be heavy enough to use over this span without compromising the charge current too much .
Output from the panels is aproximately 18.5 volts at full capacity , regulator charges at 13.5 v .
18.5 volts at 10 amps over a distance of 130 feet will have 17.99 volts at end under ideal conditions. 0.51v or 2.76% drop.
 
I posted a little over a week ago about my scrap yard 12ga. extension
cords. They are very heavey and work great. Here is something that
suprised even me. I had my handy dandy harbor freight heat gun for the
shrink tubing. If I plugged the gun into the outlet it makes a sound
at this frequency. A certian pitch. You plug in the 100 footer and
then plug the gun back in to the end of that cord and it makes a
slightly lower pitch. Not even a half a note but it is different. I
was very suprised. You would never think a 100 foot 12ga. cable would
have any drop. That is why I wired my generator house input with 10 /
3 ga. Wire. I would say to go as big as you can afford. Also ask at
the electrical supply place and go one size bigger than they say. Just
my opinion. Since china slowed down two years ago the price of copper
really tanked. Almost 1/2 the price. Go get it.
 
You lost me with your wire-size designation. What is the total solar array wattage?

With a 130 foot run from the array to the controller at 18 volts . .

100 watts needs #4 AWG copper, .37" diameter
200 watts needs #2 AWG copper, .43" diameter
400 watts needs 2/0 AWG copper, .54" diameter
800 watts needs 250MCM copper, .79" diameter
1000 watts needs 300MCM copper, .84" diameter
 
> You would never think a 100 foot 12ga. cable would have any drop.

All copper wire has resistance and voltage drop. There are voltage drop calculators available on-line. The HF 1500W heat gun draws 12.1A at full power. Assuming a 120V source, a 100FT 12Ga extension cord feeding 12.1A would have a voltage drop of 3.84V delivering 116.16V to the heat gun.

For comparison a 16Ga extension cord would have a 9.71 volt drop with the same load.
 

Thanks all for the fast replies .
Two panels at 200 watts each planed for the moment with a possible third or fourth in the future depending on what I may power with it .
As far as I can tell 25 mm squared cable [ that refers to the copper core without insulation ] is a little larger than a no 4. AWG and smaller than a no. 3 AWG.
 
The reference data book I use for line (voltage) drop calculations is printed by Sams Co.......the guys that used to do the Sams Photo Fax
(or something like that) TV schematics we used in the old days (1960s) for TV repairing. He uses 2% for line (voltage) drop when sizing
wire for a given current at a given distance. Considering the fact that our COOP runs our power lines at 124v we have a lot of lee-way in
things like marginal gauge extension cords for power tools.
 

A 48V PV system has less voltage drop concerns and 48V inverters handle more power for less $$$ than 12V equipmemt.
Is the plan to provide power for an home, farm, ranch , livestock watering pump or what? 200W of panels will be hard pressed to supply a few LED lights, a microwave oven and a TV set.
Calculate your batteries at 25% use, not 100% discharge . Otherwise the batteries will be ruined in a few months .
Using deep cycle golf cart or fork lift batteries ? Ordinary vehicle or tractor starting batteries are not a match to solar applications .
 
RE . . your comment . . " Calculate your batteries at 25% use, not 100% discharge . Otherwise the batteries will be ruined in
a few months ."

Not true. Trojans can be discharged to 100% and survive 750 cycles. If done to 50% they survive 1600 cycles. If at 25% they
survive 3500 cycles. NO down side to a full discharge.

I will further note that in the solar off-grid world, Trojan T-105s are not considered true HD deep-cycle batteries. They are
a good compromise between cost and longevity. Some of the most durable batteries made for solar come from Canada (Rolls
Surette).
a174565.jpg
 
Not really doable with 18 volts unless you want to invest a fortune in 300 MCM copper cable nearly 3/4" diameter. I'd go with
all the panels in series, for higher voltage. There are charge-controllers that can take up to 200 volts at the input and
output a lower 12, 24, or 48 volt charge for the battery bank. I have a 60 foot run from my 5400 watts of solar, to the
controller. I have the panels wired to make 100 volts and only need #6 copper cable. My battery bank is 48 volts with
Canadian batteries. When I did the install, USA batteries were not allowed due to lack of warranty. In retrospect, I wish I
had done the bank in 12 volts.
 
I still have a few of thr AR books for car radios. Great help bqck in the day when I used to do repairs.
 
(quoted from post at 19:02:36 10/09/17) Could someone please help me out with a projected voltage drop calculation ?

I need to lay cable from the solar panels at my barn to the machine room that houses the battery bank .
The distance is 40 metres / 130 feet . I have two[ one positive one negative ] 25mm2 cables with cores of about 20 copper strands each and am hoping that these will be heavy enough to use over this span without compromising the charge current too much .
Output from the panels is aproximately 18.5 volts at full capacity , regulator charges at 13.5 v .
etting past all the "other info" you got here, you asked about voltage drop & got one answer close to what you asked & at that point he didn't know about your 400W number, so basically double his drop number. ANS.: is 1.1 volt drop. Can you tolerate that? JDEM wants you to have 0.437 volt drop. That is nice, but do you need that small a drop? Only you can answer that, knowing you own equipment.
 
I figure any of my wiring with loss at around 3%. Certainly not uncommon. 1/2 volt in an 18 volt system is substantial loss.
Loss is just what it sounds like. Energy not being used for the intended purpose. Usually just takes a couple of extra bucks
to keep it to a minimum. All the figures I gave were with losses 2.2% to 3.1%.
 
(quoted from post at 11:21:41 10/10/17) I figure any of my wiring with loss at around 3%. Certainly not uncommon. 1/2 volt in an 18 volt system is substantial loss.
Loss is just what it sounds like. Energy not being used for the intended purpose. Usually just takes a couple of extra bucks
to keep it to a minimum. All the figures I gave were with losses 2.2% to 3.1%.
0.5v/18v=2.8%
 
I am not sure what your point is. Mine is simple. 1/2 volt loss at 18 volts is plenty and I would not intentionally exceed it
by much.
 
(quoted from post at 11:51:38 10/10/17) I am not sure what your point is. Mine is simple. 1/2 volt loss at 18 volts is plenty and I would not intentionally exceed it
by much.
Just quantifying your "substantial loss".

JDEM, "1/2 volt in an 18 volt system is substantial loss. "

Me: "0.5v/18v=2.8%"
 
(quoted from post at 12:46:10 10/10/17) VD is bad in all situations. Size matters.
rue sport, but there is always a cost trade off. Could really minimize it with super cooled conductors! :wink:
 
Hello Charles in Aus,

I would think all is well with what you have. I'm thinking that the only think a voltage drop will do, is take longer for the battery system to be charged. I know you will post later, I will be watching...........

Guido.
 
I assume anyone installing a solar panel that only works for 1/4 of the day(and often less), wants all power available. Your comment about loss is kind of perplexing, at least to me. Granted if someone always make more power then can be used , every day, it matters less. Such a case is rare. Loss is loss. Energy made that you cannot get back.
 
Thank you all again .

My property is really a retreat used one week in a month at the most . The power demands are not great , lighting mainly , but it would be nice to run the television and music without having to operate the generator .
An electric pump to transfer water up to the header tank was one future thought , again trying to reduce the number of small petrol engines or things dependent on the generator .
Batteries are quite large , I have four German Sonnenschien six volt batteries of 900 amp hours each wired as a 12 volt supply .
As there are three weeks in a month where the batteries can recuperate a slow rate of charge is acceptable , thanks for that Guido.
Good points by all , series wiring of the panels and a different regulator would be a good insurance policy and easy to rejig if necessary .
 

If I had he choice between spending $$$ on upsizing a cable loosing 1/2 a volt or adding more PV panels. There would be more PV panels.
 
Yeah, and if you added even more panels with the same cable, you'd have even more voltage drop. That is, unless the voltage
gets raised.
 
How much are solar panels costing where you are? When I installed my first solar array 40 years ago, it was around $10 per
watt. That is like maybe $40 per watt today? I know I used to buy lunch at Mcdonalds for less then a buck back then and a new
car could be had for $3000. 15 years ago, I paid around $5 per watt. Last week I bought a 800 watt kit with two controllers,
all the wires and roof mounts for $1200. It is amazing how low panels have gotten. Also controllers from China. On the other
hand, inverters and batteries keep going up and up.
 
Hello JDEM

I agree with you on power loss, but a simple set up like him? like splitting hairs> A large industrial array, that is another story!

Guido.
 
Hello Charles in AUS.

You are welcome, you can post back for the non believers, as I know you will do just fine,

CIOA................Guido.
 


Grazie Guido, molto apprezzato.

And a big thank you to everyone else who gave thought and time .

I was fortunate to get the batteries I have very cheaply , they came from an emergency power supply at a Public building and were part of a routine change over . Yes batteries like this are astronomical in cost . It is because I have these that I am upgrading the system from a simple single 80 watt panel charging four golf cart batteries for lighting to something that might power small drain appliances and give us a little more convenience and less dependency on the generator . I have had the 25mm squared cable for years so I am eager to use it , the same cable for the distance is well over eight hundred dollars here , factor in conduit and fittings and it adds up quickly .
Higher voltage panels are cheaper than they used to be here as well but for some reason 12 volt ones are still expensive , I think that is because they are used by recreational campers and so prices are tailored for want rather than need .
36 and 48 volt panels from grid feed systems are cheaper and reasonably available , I may take advice from a previous post and use this type with an upgraded regulator instead of the 12 volt panels I was going to use .
Inverters are not so bad , 3000 watt models about $250 Aus , not the most reputable brand though .
I have enough to go on I think , as I sat here I realised that I could just lay the cable out , connect to the battery bank and run a multimeter over it :roll: Dumb aren't I ?

One last question , do you measure voltage drop under load or statically ?
 
One last question , do you measure voltage drop under load or statically ?

If there's NO current flow, there's NO V = I x R Voltage Drop...........Open circuit unloaded voltage isn't the same as when a current drawing load is applied after which time you DO HAVE A V = I X R VOLTAGE DROP. The Voltage Drop across a load/resistor through which NO current is flowing is V = 0 x R WHICH IS ALSO ZERO

John T
 
Voltage drop when choosing wire size is calculated as a worse-case under highest possible load. So . . if a 12 volt solar
panel is rated 17 volts and 120 watts - drop is figured with 7 amps. 3% drop is usually the max desired in a low voltage
system.

120 watt panel at 120 degrees F, with 130 foot run of plastic covered copper wire:

7 amps @ 18 volts: 10 gauge - 11.5% drop, 6 gauge - 4.2% drop, 4 gauge -2.9% drop

4 amps @ 18 volts: 10 gauge - 6.6% drop, 6 gauge - 2.6% drop, 4 gauge -1.1% drop

2 amps @ 18 volts: 10 gauge - 3.3% drop, 6 gauge -1.3% drop, 4 gauge -.8% drop


In cold areas that also get lots of snow, you add 10% to the panel rating since they can overproduce in those conditions.
 
Inverters are a crap-shoot. Most so-called "sine-wave" or "true sine-wave" inverters do NOT make a real sine-wave. I have
had more compatibility issues with "true sine-wave" inverters then cheaper modified-wave inverters. Note I am talking about
portable off-grid inverters. NOT certified grid-tie inverters that can cost thousands of bucks.

On the issue of known name-brand versus unknown Chinese? Again, a crap-shoot. I am still using a 2000/4000 watt mod-wave
inverter I bought new for $89 ten years ago and it still works fine. I also have an Xantrex "sine-wave" and an Outback
"certified sine-wave" that both went bad when less then two years old and very little use. At least Outback sent me the
parts to fix it.

I bought a new AIMs 3000/6000 watt "true sine-wave" last year and used it for 20 days before it started turning itself on
and off. I called the company and they cannot even come up with a schematic for it.

The funny part of this is - I have yet to have an el-cheapo no-name Chinese inverter crap out on me. I guess I have been
lucky. I also have yet to have a "true sine-wave" inverter make a real 60 Hertz sine-wave. Imperfect wave really shows up
when running a smart phone, lap-top computer, batter charger, motor starting, etc.
 
I see it the other way around. I have a cabin in the Michigan UP, in the middle of heavy forest with a "port-hole" in the tree-tops for sun. 800 watts of solar that only works for a few hours a day. I want NO loss. Makes no sense to have it. Difference of a few dollars to prevent loss with big enough wire or high enough voltage.
 
I recently installed a solar pump and shopped for 2 100 watt panels to run in series. Prices were about $1 per watt.
My wife is having a system installed using 12 280 watt panels. She paid significantly less, I think about 65 cents per watt. She says prices are due to rise dramatically due to one of the trade deals going on.
 
per dollar spent the extra PV capacity would still charge the batteries faster than saving a few watts of I2R losses on the cable .
Going to 48V would reduce most of the loss concerns to minimal.
 
CIOA? Shame on you Guido. If you're going to use the one Italian word the whole world knows (not counting the names of food, anyway), you should at least spell it right. After all, you're Italian.

Stan
 
Hello JDEM,

In is case, he stated that he has three weeks for charging time and one week of use time, with minimal low amp usage. It should work for him just fine,

Guido.
 
Hello Stan,

How did I do that? I only had 12 years of schooling in Italy, Back to school I guess. I was 17 years old when I crossed the big pond. As you recall we served about the same time. Think I got in a couple of years before you. Keep a sharp eye, I will do the same.........

Guido.
 
We have a small solar system at our hunting cabin. It just
does lights and a tv to watch the Packers. Our only problem
is that the German charge control is temperature compensated
that means less charge the colder it gets. Our next step is
to make a separate room for the batteries and pipe some of
our wood heat there.
 
OT

Speaking of super cooled, the focusing magnets for the Superconducting Super Collider Ion beam were designed for 4^K. Design called
for a Cryo unit every 5 miles of the 52 mile circumference. Thank Sam Donaldson, 60 minutes for crashing that. Cern in Switzerland did
the same thing on a smaller scale and lots less money. He saved the taxpayer $200M per year in operating expenses alone, not including
the cost of construction. Purpose was to prove the equations of the international scientists there about their Top Quarks and such when
collided at the speed of light....big train wreck.
 

Is the charge controller just a zenor diode shorting the pos and neg incoming line together when ever the voltage is too high? Or a power transiter dropping voltage ?
 

Everyone is talking about voltage drop, but if the controller is for a 12 volt system and dropping it to 13.8 volts anyway...............................................................

a couple of volts drop on an 18 volt feed.... will not affect the system.

Now in reality, do the panels keep the voltage at 18 at full current rating... if so... no problem.

but if that's the open circuit voltage,,, they will drop voltage, pending on load as current increases, it may be a whole different ball game.
 
Hello sotxbill,

Now that makes sense! I Didi not mention any voltage drop. I
said he will be OK. He only needs 14 - 15 Volts Right?
Doubt it he will drop that much. HVing the batteries full
when needed it's all about power and time,

Guido.
 

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