Solar wire sizing problem

[Ray-IN]

On the winnieowners.com website, every thinks just because Winnebago used 10 ga. AWG wire in their solar-prep package, it is the correct size for all solar arrays, a 30A solar panel system is what they are using, I said 10 AWG is too small for a 30A solar system, but that is me just reciting what I've read.
Anyone interested might go visit and state the facts.

 

[Contributor]

10 AWG is rated for 30A, so it's not too small.

 

[Mark_K5LXP]

There's the "safe" ampacity of wire but that's usually in the context of conductors in conduit or a cable. Probably the more important aspect for solar is lost power but even with a less than optimum wire run, it's better than none at all. It's a marketing choice to offer something that works OK for a price people are willing to pay.

Mark B.
Albuquerque, NM

 

[Isaac-1]

Mark, that is a bit of old school solar thinking there, and made sense in the era of $3 per watt solar panels and closely voltage matched solar panels, ie 21VOC panels feeding a PWM solar controller charging a 12V battery. So while there are still voltage losses they are a lot less of an issue when dealing with 40+ VOC panels feeding into an MPPT controller, possibly while wired in series.

 

[Contributor]

Isaac-1 hit the nail on the head. The voltage drop from the panels to the controller will not really affect the controller output. Another factor is that the biggest voltage drop will occur when the panels are at their max current output which happens for a relatively short period (clear sky, no shade and sun directly over the panels). The majority of the time, the 30A limit will not be approached.

 

[Mark_K5LXP]

The voltage drop from the panels to the controller will not really affect the controller output.

"Not really"? Watts is watts, less loss at elevated voltage but generally something to be minimized. Missing from the description is distance, important to know when evaluating system losses.

Mark B.
Albuquerque, NM

 

[Isaac-1]

Mark this is true, but we are now in an era when it is cheaper to buy a slightly higher output solar panel and take the losses vs paying the money for larger diameter wire.

 

[Contributor]

The resistance of 10 AWG is .9989 ohms per 1000 ft. For a typical RV rooftop install distance of 25 ft from panels to controller, you get 50 ft of round trip resistance, or 0.0499 ohm for 50 ft.

So if you have a 30A MPPT controller rated at 95V max input, it's easy to calculate the power loss.

First figure out the max system power, P=VI, so, 95V*30A = 2850W

Now figure out the loss over 50 ft of 10 AWG wire,

Since P=VI, and V=IR, then substitute IR for V in the first figure of P=VI, so you get P=IR*I

Since I=30A and R=0.0499, you get, 30A*0.0499ohm*30A = 44.91W

so the max power loss for a 30 amp flow at 95V is 45W

45W out of 2850 is not worth worrying about.

 

[Isaac-1]

Pedro your math is a bit off, as you should never have 95V panel output in real world conditions, since peak voltage input for an MPPT controller should always be figured as worst case VOC, which is typically figured for temperature adjusted VOC for -20 degrees F. though other temperatures may be used. Which will typically be more like a standard VOC of about 80VDC, often less in warmer real world conditions.

 

[Utclmjmpr]

I just used #6 AWG for the entire run and put it to bed..>>>Dan

 

[Mark_K5LXP]

OK, it's about the scale of the system. I suspect there aren't many 3kW RV solar installations, or ones running near 100V. 4 or 6 panels is much more common, maybe three panels in series at most, generally two. Easy enough to say throw more panels at it, until you run out of roof. Copper isn't cheap but it's one of the dials to turn when factoring system performance. In my experience, the material costs matter but the labor to install is usually a dominant factor, so I wouldn't be shy about wire gauge if I were contracting a system.

Mark B.
Albuquerque, NM

 

[Contributor]

Pedro your math is a bit off, as you should never have 95V panel output in real world conditions, since peak voltage input for an MPPT controller should always be figured as worst case VOC, which is typically figured for temperature adjusted VOC for -20 degrees F. though other temperatures may be used. Which will typically be more like a standard VOC of about 80VDC, often less in warmer real world conditions.

My math was to show a hypothetical situation of max power, not a typical installation. Each installation is custom for wire length, number of panels and configurations, parallels and series. In a real world, 6 panels would be used in two sets of 3 in series. This would give the system a working voltage of about 60V for each leg to go with 5A. Max power for this system comes out to be W = 60*5*2 => 600W. But now, there is only 10A flowing through the 30A, 10AWG wire. That is not a problem.

The reason for my previous post hypothetical situation, was to show that for the very worst case where the 30A controller limit was reached and max voltage, it would require 6 parallel panel legs of 5A each to get to 30A. But each leg would only be one panel at 20V and 5A. My hypothetical, used 95V to make a point that it would be possible, not probable, that someone could build that system.

The controller DC to DC efficiency and sun position are bigger factors than the 10AWG wire.

 

[Contributor]

Another thing to consider in the 30A system using 10AWG wire, is the controller output limit of 30A. If you feed a 30A controller with 6 parallel 100W panels of approx 20V and 5A output each, in order to reach 30A on the wire, then you are using 600W of panels. The thing is that the 30A controller can only output about 444W (14.8V*30A). So one is using 600W to get 444W to the battery bank. The 10AWG wire loss is much smaller than the inefficiency of the system needed to reach the 30A on the wire.

 

[zulu]

On the winnieowners.com website, every thinks just because Winnebago used 10 ga. AWG wire in their solar-prep package, it is the correct size for all solar arrays, a 30A solar panel system is what they are using, I said 10 AWG is too small for a 30A solar system, but that is me just reciting what I've read.
Anyone interested might go visit and state the facts.

This may be a case of comparing Apples with Oranges.
That 10 AWG wire Winnebago is using might be just intended for one PV panel or several in a series.
That 30A may be the Charge Controller output as opposed to the PV array output.
However, the combined output of PV panels wired in parallel can quickly add up . . .

 

[solarman]

On the winnieowners.com website, every thinks just because Winnebago used 10 ga. AWG wire in their solar-prep package, it is the correct size for all solar arrays, a 30A solar panel system is what they are using, I said 10 AWG is too small for a 30A solar system, but that is me just reciting what I've read.
Anyone interested might go visit and state the facts.

ok, well the first red flag here is 30 Amps..
solar systems are NEVER specified in amps or volts but ALWAYS in Watts.
Amps are DIY talk, and if Winnebago is promulgating this then their engineer ( if one exists ) needs a kick in the ass as it's wrong, period.

I don't know their reasoning for specifying 10 AWG, but its probably what they consider correct for the anticipated maximum wattage that can be placed on a winne roof.

for 10 AWG and a 30 A rating, panel wattage varies considerably, if they specify "12 V" panels and a PWM controller this could yield a max of approx 360 Watts. if using a MPPT controller and 50V panels then it could yield upwards of 1500 Watts.

secondly, Pedro's math is not off.. what it demonstrates is the power loss due to cable resistance, that is a design choice based on acceptable losses.. typically one should design for better than 3%

 

[solarman]

Pedro your math is a bit off, as you should never have 95V panel output in real world conditions, since peak voltage input for an MPPT controller should always be figured as worst case VOC, which is typically figured for temperature adjusted VOC for -20 degrees F. though other temperatures may be used. Which will typically be more like a standard VOC of about 80VDC, often less in warmer real world conditions.

"as you should never have 95V panel output in real world conditions"

really.. well you had better tell the solar industry that as we clearly have been doing it wrong for years..

we usually use a 1.25 x VOC max as a working point..

 

[Isaac-1]

All I was saying was that in the real world you would never see a 95VDC rated MPPT controller running at its voltage limit, that any good installation would leave a fairly good safety margin for things like lower than expected operating temperatures.

 

[Ex-Calif]

I would posit that a "factory" install of solar wiring is likely to be, as solarman said, either sized for anticipated capacity of roof panels on the Winnie but more likely a conservative "battery recharging" array of probably no more than 200-400w. This sort of system allows the extension of boondocking to reach 2-3 days, which I would guess is the manufacturers goal. Remember, not many manufacturers provision for more than 2 house batteries, which without solar would likely be consumed in about 1/2 a weekend without real energy conservation techniques.

However a lot of temptation is out there now to add craploads of solar in the hopes of being able to run all manner of load "off-grid"

As a rule of thumb (kiss theory) I would say, "If your RV is prewired for solar you are very likely to be fine up to 400w and a 30amp controller."

Above that is time to start getting an education about solar systems design and installation.