Solar panels - sizing wiring for voltage drop

[noelex 77]

Originally Posted by Don L

you guys have some side bet on who can make this the most complex?

I hope you are paying attention Don, there is going to be exam at the end if this.

[sailorboy1]

Originally Posted by noelex 77

I hope you are paying attention Don, there is going to be exam at the end if this.

What happens if I pass or fail so I can schedule the correct study time?

[Bambolera]

I have a similar question to the OP, but in my case it's a new installation.

Purchased 4 of these panels:

Amazon.com: DM 158w Monocrystalline Solar Panel (2 Pack): Patio, Lawn & Garden

Will run each panel 16' (one way) in Parallel to PWM 60. The panels are rated at Vmp 18.9 and 8.5 amps.

At first, I considered #8 wire to be sure, but after checking the voltage drop tables, I became convinced that #10 would be fine.

Now, after reading this thread, I'm no longer sure. There are many things that I didn't consider. This seems far more complex than I had imagined.

John

[hellosailor]

Guys, not that ambient temperature is never a factor, but really? If the wiring is adequate at standard temperature and pressure (20C, 68F) it will be substantially adequate at any ambient cabin or deck temperature. And if it isn't adequate, a little more heat won't make it any better. So...

Long wiring run, expensive wire...zero loss would be nice but in this case I think the question is simply "What will it cost me to reduce loss?" versus "what will it cost me to add a panel?" assuming the only consideration isn't "I need all the power I can get" in which case you go for near-zero loss and rob a 7-11 to pay for the rest.

If the MPPT controller is doing its job, the panels "should" be operating at Vmp and putting out full rated amperage and the worst-case condition for line loss will be at Vmp and carrying the full rated power. Why make it more complicated than that, when the wallet is likely to have the final vote at that point?

[noelex 77]

Originally Posted by Bambolera

I have a similar question to the OP, but in my case it's a new installation.

Purchased 4 of these panels:

Amazon.com: DM 158w Monocrystalline Solar Panel (2 Pack): Patio, Lawn & Garden

Will run each panel 16' (one way) in Parallel to PWM 60. The panels are rated at Vmp 18.9 and 8.5 amps.

At first, I considered #8 wire to be sure, but after checking the voltage drop tables, I became convinced that #10 would be fine.

Now, after reading this thread, I'm no longer sure. There are many things that I didn't consider. This seems far more complex than I had imagined.

John

Sorry John it should not be complex the first thing is to calculate the voltage loss at maximium power. These are: (note I work mainly with metric wiring and I think the table I use may be slightly out for AWG)

AWG 10 2.3v
AWG 8 1.3v
AWG 6. 0.7v
AWG 3 0.3v


Larger wire will always yield more power, but it is more expensive and difficult to install.
If there are great difficulties such as difficulties fitting appropriate wire inside SS tubbing it is sometimes worthwhile accepting compromises involving less power, but if these factors do not apply I would recomend between AWG 6 and AWG 3.
Given the high Vmp of your panels AWG 6 is OK.

[noelex 77]

Originally Posted by hellosailor

Guys, not that ambient temperature is never a factor, but really? If the wiring is adequate at standard temperature and pressure (20C, 68F) it will be substantially adequate at any ambient cabin or deck temperature. And if it isn't adequate, a little more heat won't make it any better. So...

The STC question arose not in terms of wiring where the effect is generally not relevant, but in terms of solar panels where it can be a major factor.
Specifically STC specification for Vmp are a couple of volts above the real world results (or NOCT results if you want some standardisation). This is very significant, particuarly when trying to asses the effect of voltage loss in the wiring.
It is a peculiarity of solar panels that the results under STC are very unlikely to realised the real world. The only redeeming factor is that all the panel manufactures quote under the same conditions.

[galacticair]

Originally Posted by foggysail

Sure........... your quoted post above has high merit. But you overlook what it was I attempted to explain which specifically is "too much importance is placed on wire sizes!" I am not sitting at my keyboard recommending that solar users wire their panels with #18 or even #12 wire. My intent was to show that even with #12 wire with a total length of 110' could perform well in most cases. And I also want to point out that in most cases wire run lengths are much shorter. Even Dave (Goboatingnow) put emphasis on reducing the 110' wire run length for the OP's application.

So on shorter run lengths #12 could possibly meet all performance expectations in many, probably most applications. Personally, I recommend at least #10 minimum because I usually over kill stuff like this. I am also not in the camp of recommending 00 wire as I remember one once recommended. I also believe you are placing too much emphasis on connection voltage drops.

From my my earlier post #15 in this thread where my data came from Kyocera's KC130TM specifications:

"Now notice that this is for Mpp and for battery charging who cares what the wiring drop is if it is charged with a current source? This is true as long as the battery terminal voltage plus all the voltage drops do not EXCEED Vmpp at which point the panel's current avalanches downwards."

As your attached performance curves indicate.

I'm starting increasingly to believe Foggysail's more relaxed view... Took a few more real-world measurements today:
- First I actually spent time measuring the cable length: it's more like 45-50ft than 50-55ft. So total cable run of 90-100ft return.

- Again I saw max output of ~5-5.5A for a 125W panel in midday sun. Not great, but I am at 45 deg north and the sun wasn't very directly overhead, so all-in I think the losses vs. 7.15A rated Imp (17.5V Vmp) is more due to temperature and PWM regulation bringing down the effective voltage to battery's 13.0-14.0V range (measured at battery via Steca charge controller + Xantrex Pro battery montor) measured with some loads applied. Unless I'm misunderstanding completely what should be the impact of PWM regulation....

- During cloudy moments, the panel was still outputting substantial Amps to the charge controller (~1.5-2A depending on type of cloud), rather than the 0amps I would expect if panel's voltage (less voltage loss) was dropping below the battery voltage... So no signs of falling off a cliff due to the voltage losses.

Overall, I'm not yet seeing a significant loss that I would attribute to the voltage drop rather than PWM... My surprised conclusion is that 11 AWG for a 45-50ft one-way cable run with effective 5-6A current does appear to work in a PWM/AGM combo system, even though it still loses ~25% vs theoretical (lab) output at Vmp/Imp (which might however be more reachable with MPPT + thick cables)... My current hunch would be I should only worry about the voltage drop if I were to upgrade the PWM controller to MPPT since that's when the voltage losses will actually matter to ultimate amps output...

Again, I'm not an electrical expert so would love to understand if I'm mis-understanding this...

P.S. We actually have 2 such 125W panels wired individually direct to a single charge controller in parallel (one panel on each outboard hull of a trimaran). I've been focusing on the single panel narrative since I don't think that having the 2nd in parallel changes the outcome beyond just doubling the amps. Someone do tell me if I'm wrong to assume parallel setup doesn't need to be accounted for when calculating the per panel output with separate wire runs.

[svmariane]

I've found this calculator and the explanations / discussions to be of interest: Wire sizing calculator for Solar Panel Arrays

Then I just bought 8 AWG 'cause that's what I could afford

[Bambolera]

Originally Posted by noelex 77

I would recomend between AWG 6 and AWG 3.
Given the high Vmp of your panels AWG 6 is OK.

That's funny Noelex.

To wire my 4 panels in parallel, I would need to run eight #6 wires? I'll need a hole the size of a port light to run that bundle. How would I adapt the lugs inside the charge controller to accommodate such a mass?

Do people really do such things?

According to the wiring calculator, the #10 AWG wire should have less than a 3% voltage drop for the 16' run I'm contemplating, but I should have known that was too easy!

John

[noelex 77]

Originally Posted by Bambolera

That's funny Noelex.

To wire my 4 panels in parallel, I would need to run eight #6 wires? I'll need a hole the size of a port light to run that bundle. How would I adapt the lugs inside the charge controller to accommodate such a mass?

Do people really do such things?

No I was referring to the total output. So a single AWG 6 wire carrying the current from the panels to the controller.

Normal practice is to combine the wires at the panels and then use a single wire to carry this to the controller.

This is usually the easiest, neatest and most efficient way to do the wiring.

If you do want to use 8 separate wires, two from each of the panels, (presumably because the panels are all in very different locations) instead of combining the output, then you can of course use smaller wires.

[noelex 77]

Originally Posted by galacticair

- Again I saw max output of ~5-5.5A for a 125W panel in midday sun. Not great, but I am at 45 deg north and the sun wasn't very directly overhead, so all-in I think the losses vs. 7.15A rated Imp (17.5V Vmp) is more due to temperature and PWM regulation bringing down the effective voltage to battery's 13.0-14.0V range

The sun angle has a big impact on he solar panel output and I think your poor performance is primarily due to you latitude, but a couple of points may help you understand your solar systems better.
7.15 A is the maximum rated output a 17.5v. If we allow for normal operating temperature effects this translates to about 7.1A at 16v (the temperature has a big impact on voltage very little on current). This is measured at 1000W/m2 which is very, very bright conditions the sort of conditions that we will only see around midday near equatorial latitudes, on a clear day, or when there is direct sun and reflected sun from cloud at the same time.
In these very bright conditions we should be seeing from a PWM regulator about 7.2-7.3A ( current should go up slightly) at battery voltage.
You will only see these currents if conditions are suitable and in locations where 1000W/m2, or more, is possible.

Originally Posted by galacticair

so all-in I think the losses vs. 7.15A rated Imp (17.5V Vmp) is more due to temperature and PWM regulation bringing down the effective voltage to battery's 13.0-14.0V range (

When the PWM regulator brings the panel voltage down to the battery voltage we should see a slight increase, not decrease in current.

Originally Posted by galacticair

- During cloudy moments, the panel was still outputting substantial Amps to the charge controller (~1.5-2A depending on type of cloud), rather than the 0amps I would expect if panel's voltage (less voltage loss) was dropping below the battery voltage... So no signs of falling off a cliff due to the voltage losses.
.

A system with a PWM regulator and wiring that is too small will perform normally (as good as the system with thicker wiring) in cloudy conditions. The reduced current output means the voltage drop in the wiring is reduced to acceptable levels. The voltage output of solar panels goes down very little in cloudy conditions, it can sometimes even go up due to lower cell temperatures.

The time when wiring that is too thin with a PWM regulator will have its biggest impact is when
Battery voltage is high
Conditions are bright
Temperatures are high

Local shadowing in the form of a shadow that only falls on some cells from say a shroud can also have a much bigger impact than it would have in a system with adequate wiring.

Originally Posted by galacticair

P.S. We actually have 2 such 125W panels wired individually direct to a single charge controller in parallel (one panel on each outboard hull of a trimaran). I've been focusing on the single panel narrative since I don't think that having the 2nd in parallel changes the outcome beyond just doubling the amps. Someone do tell me if I'm wrong to assume parallel setup doesn't need to be accounted for when calculating the per panel output with separate wire runs.

If you are running separate wires to the controller from each panel it does not effect the calculations, but you will need ensure the wires from he controller to batteries are of adequate size as they are carrying twice the current. It is the total voltage drop from the panels to battery that is important with a PWM regulator.

In conclusion:
The low maximum output from your system 5.5A verses the expected 7.2A under ideal conditions is caused by your latitude (I suspect this is the dominant factor) and the wiring. It is difficult to separate the relative contributions of these two factors.
The easiest way would be to temporally use some addition wiring and note the improvement in average and peak output.

[ashmun]

Wow, a lot great conversation and calculations. I’d like to add a few items to the conversation. First is adding a couple wire calculators to the conversation such as http://www.boat-project.com/tutorial...ulator.htm#wsc on the web, Wire Sizer 3.0 for Windows (Wiresizer 3.o) and DC Wire Sizer for iOS (supports any voltage, ABYC E-11 or ISO 10133, meters or feet, AWG or mm2). Since the initial conversation started in mm2 (vs AWG), can I assume this boat is Canadian which would fall under ABYC/Canadian Marine Electrical standards (aka 3% voltage drop) vs European ISO which requires 1% voltage drop. And last, the length of the wire is pretty long. We ran our two 200 watt solar panels in series so that we could run a single properly sized duplex cable at 72volts for 30 feet that two 36 volt panels (aka we save a bit on the cable but spent more on a higher-end controller).

[foggysail]

Be careful with wire calculators!!! Stranded wire resistance is higher and is a function of the number of wires in the strand and their sizes. Its not copper's physics change when stranded its that the build factor for stranded results in less copper for a particular wire size as multiple strands bundle.

There is info on this but you need to really look. Dave (Goboatingnow) gave an eariler example in this thread. My take is an easy rule of thumb is to add 10% to the resistance of stranded sizes or just go down one wire size for calculations to be on teh safe side. Its close but far from exact.

[ashmun]

I concur with Foggysail that one needs to be careful of the wire calculators they use, especially those that are based on NEC 310 Table for house/land based wiring (which is usually solid cooper vs stranded). Thank you for pointing that out Foggy.

ABYC E-11 or ISO 10133 tables and formulas are based on stranded cooper since most, if not all, marine standards do not allow solid copper (which would break too easy from vibration). There are a number of marine DC wire sizing calculators/applications that use ABYC (and even ISO) formulas and tables. Some of the applications even use mm2, any entered volt and derating tables (insulation rating, bundles, etc.) and provide wire size results with voltage drop percentages and/or, as Dave (Goboatingnow) pointed out, ampacity (safe current carrying capacity of the wire). I also concur with Dave that using higher voltage panel(s) for that long a run would make quality marine cable of the correct size more affordable. And with the solar technology improving in watts while decreasing in price each year, even we are considering upgrading our panels (200 watts/each) to the next generating which are currently 300 Watts/each this year but might be 350 or 380 watts/each by the time we replace them in 2 years. We’ll re-use the old panels on an electric golf cart or something.

[ashmun]

TYPO CORRECTION: ISO requires 10% not 1% as indicated at the time in the INTERNATIONAL STANDARD ISO/DIS 10133:2007:

"4.6 The length and cross sectional area of conductors in each circuit shall be such that the calculated voltage drop shall not exceed 1 % of the nominal battery voltage at any appliance when every appliance in the circuit is switched on at full load.”

The revision/correction is in the 2000-12-01 version of 10133 (the 1% error is now 10%):

"4.6 The length and cross-sectional area of conductors in each circuit shall be such that the calculated voltage drop shall not exceed 10 % of the nominal battery voltage for any appliance, when every appliance in the circuit is switched on at full load."

and BS EN ISO 10133:2017:

"4.7 The length and cross-sectional area of conductors in each circuit shall be such that the calculated voltage drop does not exceed 10 % of the nominal voltage."