Gauge wire needed from panels to charge controller

doctorZeus
doctorZeus Registered Users Posts: 24
My debacle at the moment is trying to determine what size/gauge wire I need to run from my panels to my charge controller. I've tried to use a few calculators out there but I'm still pretty confused. They ask if my "system" is 12v, 24v, 48v. Which system, the voltage for an individual panel? The total voltage panels are running (as I was thinking about putting these in series)? The batteries, or what?

I'm starting with two grape solar 250watt panels, a midnite solar classic lite 200 controller, and six trojan t-105s for ~600ah @ 12V. The specs for the panels I have are below if that will help.

I'm looking at needing about 75 to 80 feet, but would like to spec it out to 100 feet in case I run into needing more length to navigate across or around something. What do you guys think?

My solar knowledge has gone from NONE to "slightly informed" over the last few days, but still struggling..so thank you very much in advance for any guidance/suggestions.


Model GS-S-250-Fab5
Electrical Specifications
(STC* = 25 ºC, 1000W/m2
Irradiance, and AM=1.5)
*Standard Test Conditions

Max System Voltage (IEC/UL) 1000V / 600V
Maximum Power Pmax 250 W (-2%, +2%)
Voltage at Maximum Power Point Vmpp 30.7 V
Current at Maximum Power Point Impp 8.15 A
Open Circuit Voltage Voc 37.7 V
Short Circuit Current Isc 8.72 A
Temperature Coefficient of Voc -0.128 V/ºC (-0.34% /ºC)
Temperature Coefficient of Isc 3.49x10-3 A/ºC (0.04% /ºC)
Temperature Coefficient of Pmax -1.20 W/ºC (-0.48% /ºC)

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller

    Welcome to the forum.

    The issue here is what the Voltage of the array will be. This can be variable because you're using an MPPT controller which allows quite a bit of flexibility in array design. System Voltage doesn't enter in to it so long as the array Vmp is high enough to charge the system Voltage. A bit about array configuration: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

    These panels are "GT" type, meaning their Vmp is not standard for use with battery-based systems. As such an MPPT controller must be used, and if the system is 24 Volt the panels need to be two in series and if 48 Volt three in series.

    Now a side note: you appear to be going for a 12 Volt system with three parallel strings of batteries for 600 Amp hours at 12 Volts. Avoid this if you can.

    Another side note: two 250 Watt panels will not be sufficient for fully charging 600 Amp hours @ 12 Volts (or any Watt hour equivalent). You need about twice that.

    Back to the array-to-controller wiring. 75 to 100 feet is a fairly long distance, so it is best to put the Voltage up as high as practical. Trouble is, the greater the difference between array and battery Voltage the less efficient the charge controller will be. It is only a few percent, but it can make a difference. Another reason why going to a 24 Volt system (using a 60 Volt array) would be better.

    Anyway, assuming you run the two panels in series for a 60 Volt array across 100 feet of distance at 8 Amps you could use 10 AWG and keep the V-drop to 3%. Note that if you double the panels the current will double and the V-drop will increase to over 6% so you'd want to increase the wire size to 6 AWG.

    Now let's look at the same thing across 75 feet instead of 100: with 8 Amps & 10 AWG the V-drop decreases to 2.5%. With double the panels you could use 8 AWG instead of 6 AWG.

    You could also up the Voltage more with four panels, putting them all in series for a 120 Volt array. That way the power would be transmitted mostly as Voltage and only 8 Amps so across 100 feet you could use 12 AWG and have a V-drop of just over 2%.
  • Gene.243
    Gene.243 Solar Expert Posts: 35 ✭✭
    Re: Gauge wire needed from panels to charge controller

    I would like to help, If I'm not helping someone say so.

    The system voltage question would be the "nominal" battery voltage.

    If the panels are wired with 12 ga. wire and you wire them in series that will be large enough. (Your max current Isc is 8.72A) This needs to be arranged to properly feed your charge controller. Will your charge controller accept 65.4V (2*Voc)?

    I'm not familiar with your charge controller but if it is rated at 200 watts it will be maxed out before one panel is in full sun.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller
    Gene.243 wrote: »
    I would like to help, If I'm not helping someone say so.

    The system voltage question would be the "nominal" battery voltage.

    If the panels are wired with 12 ga. wire and you wire them in series that will be large enough. (Your max current Isc is 8.72A) This needs to be arranged to properly feed your charge controller. Will your charge controller accept 65.4V (2*Voc)?

    I'm not familiar with your charge controller but if it is rated at 200 watts it will be maxed out before one panel is in full sun.

    Uh, no.

    The system Voltage isn't relevant here: we're looking at the Voltage/current handling between the array and charge controller. Since it is an MPPT controller it can operate well above system Voltage, which reduces the V-drop issue considerably.

    The MidNite Classic 200 controller is not 200 Watts; it is 200 Volts maximum array input. It probably is also unnecessary for this application unless he uses four of those panels in series (Voc and temp compensation is what matters for maximum input Voltage to a controller).
  • doctorZeus
    doctorZeus Registered Users Posts: 24
    Re: Gauge wire needed from panels to charge controller

    Thank you so much for the quick reply.. this looks to be incredibly insightful info you've given me to chew on. I will need to take a closer look later this evening and will almost certainly respond with more questions, but you've peaked my interest very much about the batteries. While it's too late to get different batteries at this point, I'm not averse to getting two more and running them in series to 24v pairs if you think that's a better setup for efficiency? (Or even icing two of them and running only four of them in series to get to 24v?)

    To help me understand..if I were running an AC 110v appliance that was a pretty constant 100watts, using a decent inverter, would I get the same or close "hours of usage" from a 500ah battery bank @ 24v as I would a 1,000ah bank @ 12v?

    Assuming I was starting from scratch on the batteries, a better rig would have probably been four 12v batteries paired in series to get 24v and then a parallel string? I'm vested in the 6volt batteries now so I'm stuck with the extra cabling and battery maintenance, but nothing on the load end is keeping me at 12 volts.

    While I need to do some research to fill in a few loose concepts, I followed you 100%. Thanks again..really appreciate the patient and detailed response.
  • Gene.243
    Gene.243 Solar Expert Posts: 35 ✭✭
    Re: Gauge wire needed from panels to charge controller

    No accepted. I'll go back to lurking mode and try to learn something.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller
    doctorZeus wrote: »
    To help me understand..if I were running an AC 110v appliance that was a pretty constant 100watts, using a decent inverter, would I get the same or close "hours of usage" from a 500ah battery bank @ 24v as I would a 1,000ah bank @ 12v?

    This is correct. Have a read through this: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
    Assuming I was starting from scratch on the batteries, a better rig would have probably been four 12v batteries paired in series to get 24v and then a parallel string? I'm vested in the 6volt batteries now so I'm stuck with the extra cabling and battery maintenance, but nothing on the load end is keeping me at 12 volts.

    Whether you lose two batteries or gain two in order to switch to 24 Volt would depend on how much power you need to supply. This in fact is the key element to any off-grid system design. Do you have a Watt hour number? That and maximum Watts at any one time will be essential to having a viable system.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller
    Gene.243 wrote: »
    No accepted. I'll go back to lurking mode and try to learn something.


    It's okay because it's all part of learning and understanding this stuff. For instance you were mislead by the OP's typo of Classic "200w". Along those same lines, most controller's use "model numbers" that relate to their maximum output current not maximum input Voltage (such as Outback's FM60 being 60 Amp and FM80 being 80 Amp output).

    You were also correct about the system nominal Voltage, it's just that it doesn't apply with MPPT controllers where the array Voltage can be anything as long as it's above the charging minimum and below the input maximum. With a PWM controller it's the same nominal array Voltage as system Voltage.
  • doctorZeus
    doctorZeus Registered Users Posts: 24
    Re: Gauge wire needed from panels to charge controller

    I do NOT have those numbers readily available at the moment, but will follow up later this evening when I can review the "additional reading" and direct more of my attention to the subject. This has been more helpful than you probably know - thanks again!
  • doctorZeus
    doctorZeus Registered Users Posts: 24
    Re: Gauge wire needed from panels to charge controller

    So I had a final question re the panel wires that will hopefully put a fork in that (at which point I'd like to continue digressing about my batteries and overall systems specs if that's OK). Based on the feedback here and further reading I feel confident that 10AWG is more than enough for the two panels in series (since 100ft is a max, not expected), and would of course handle four in series as well once I get there. My question pertains to getting the cabling and "mc4" connectors. Are there any pitfalls I should be aware of before I go to home depot and pick up all of the cabling/connectors? Do I need to use the mc4?

    Back on to my "system"..

    I mainly spec'd out the batteries for occasional/emergency use, "minimalist load," targeting the ability to provide necessary power for 48 hours. I'm targeting staying at ~50% dod but it looks like I'm going to need a bit more battery and at least one more panel to do that and/or be more conservative with energy usage. My hope is that I'm able to get close or fully "top off" the batteries every day, worst case scenario being able to top off over the period of 2 days. I also plan to be able to supplement the battery power with an inverter/battery-charger/gas-generator, but I'm thinking that could be its own loaded question. <-- if it's worth tackling here and now, let's go for it, if not I've no problem with starting a new thread.

    Instead of deploying T105s that I have, I was going to start with four of the Sam's GC2 batteries in 12v series pairs and then parallel to get 430ah @ 12V, powered by the two grape solar 250watt panels in series (500 total). Watt usage is mostly 12V DC with very low "maximum watts at any one point," which does I believe point me toward wanting to stick with 12V? My load is looking like mostly DC appliances with the one exception of my cable modem (has built in wifi router).

    Powering fans, the cable modem, two tablets/phones and a laptop at the estimated watts+hours, I'm looking at 1,860 watts per day which is under 50%DoD on the battery bank (according to my calculations, 430a * 12V = 5160 total watts, so 2,580 is my 50% DoD). Total usage/load at any given time is looking to be right around 250 watts.

    So quick summary:
    2 x 250 grape solar panels
    1 x midnite solar lite 200 charge controller
    4 x 215ah Duracell GC2 batteries (430ah @ 12V)
    Average daily load is 1,860 watts with 50% DoD 2,580 watts

    I haven't done much to calculate the amp draw and mostly stuck to watts which is probably a bad idea, but amp draw looks to be very much in the "low" end of what some people run on their rigs. Given this information I'd appreciate any/all guidance or feedback. if there's any key info that would help paint the picture that is my "situation," please ask and I'd be glad to provide it and help you help me.

    P.S. My apologies for the "200w" typo when describing my charge controller in the OP. I've since edited it out.

    Thanks again for your valuable feedback.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller
    doctorZeus wrote: »
    Are there any pitfalls I should be aware of before I go to home depot and pick up all of the cabling/connectors? Do I need to use the mc4?

    You should use the MC4 connectors on the panels. Cutting them off technically voids the warranty. The simplest thing to do is get extension cable(s) http://www.solar-electric.com/incaforsoelp.html and cut them in half to give you a "pigtail" to splice on to standard wire.
    Back on to my "system"..

    I mainly spec'd out the batteries for occasional/emergency use, "minimalist load," targeting the ability to provide necessary power for 48 hours. I'm targeting staying at ~50% dod but it looks like I'm going to need a bit more battery and at least one more panel to do that and/or be more conservative with energy usage. My hope is that I'm able to get close or fully "top off" the batteries every day, worst case scenario being able to top off over the period of 2 days. I also plan to be able to supplement the battery power with an inverter/battery-charger/gas-generator, but I'm thinking that could be its own loaded question. <-- if it's worth tackling here and now, let's go for it, if not I've no problem with starting a new thread.

    Safest method is to plan on 25% DOD average for one day. This gives you a second day's worth before you hit 50% DOD. You need more battery this way and more panel to recharge it. For emergency use generators tend to be far more economical.
    Instead of deploying T105s that I have, I was going to start with four of the Sam's GC2 batteries in 12v series pairs and then parallel to get 430ah @ 12V, powered by the two grape solar 250watt panels in series (500 total). Watt usage is mostly 12V DC with very low "maximum watts at any one point," which does I believe point me toward wanting to stick with 12V? My load is looking like mostly DC appliances with the one exception of my cable modem (has built in wifi router).

    Normally I recommend against 12 Volt systems unless there's a specific need for that Voltage (mobile application). But sometimes for emergency systems it works fine, providing the loads aren't major. Here you are looking at four 6 Volt batteries, so you can put them in series for 24 Volt and have the same amount of power available. But sometimes the 12 Volt inverters are cheaper and easier to find.
    Powering fans, the cable modem, two tablets/phones and a laptop at the estimated watts+hours, I'm looking at 1,860 watts per day which is under 50%DoD on the battery bank (according to my calculations, 430a * 12V = 5160 total watts, so 2,580 is my 50% DoD). Total usage/load at any given time is looking to be right around 250 watts.

    It's amazing how fast the small stuff adds up. :D Personally my emergency load would be the refrigerator & freezer, to avoid having to throw away a lot of expensive food.

    You're looking at 1860 Watt hours per day. It's important to keep the Watts and Watt hours straight as they are two different things. On 12 Volts that's 155 Amp hours, so the minimum battery bank size would be 310 - which doesn't included powering the inverter and losses in conversion. As such going with 440 would be a good idea.

    Although that means you need at least 685 Watts of array to fully recharge it. So adding one more of those Grape panels will put your array size at 750 which would work well.

    You can check this with the Icarus formula:

    750 Watts * hours of good sun (4 minimum usually) * 0.52 over-all efficiency rating = 1560 Watt hours AC. That is low for your estimated use.

    If this were an off-grid system the first thing you'd do is round the usage up to 2kW hours per day. The second thing you'd do is calculate the batteries based on that: 336 Amp hours @ 24 Volts (based on 25% DOD average). Then you'd round that up to an available battery size, such as 390 Amp hour Crown 6 Volts. Then you'd calculate the array size based on that and get 1215 Watts of array which would again be rounded up to nearest available size. All that rounding up is to install plenty of margin for those days when usage is a bit high or sun is a bit low.

    But all you really have to do is ask yourself "what can I shut off if the power is running low?" and "what happens if it runs out?" :D
  • doctorZeus
    doctorZeus Registered Users Posts: 24
    Re: Gauge wire needed from panels to charge controller

    Thank you so much, your feedback has been incredibly insightful (and helpful). Had one more question regarding the use of cat 5. I have well over 1,000 feet of cat5 which is 8 lines of twisted 24 gauge wire. According to this calculator, if I combined all 8 wires from 3 runs of cat 5 I would get a wire roughly equal to 10 awg. http://home.hiwaay.net/~rgs/awgcalculator.html

    Cat5 = 8 twisted strands of 24awg, I would get 15 awg if I stripped all 8 and combined them, I would get 12 awg if I stripped and combined 16 of them (two runs of cat5), and 10 awg if I stripped and combined 24 of them (three runs of cat5).

    Has use combined runs of cat5 ever come up before? Specifically, I'm looking to use it for the run from my panels to my charge controller, so heat and weather and all of that should definitely be considered. Also, not sure if the "twisted" nature of the wires is helpful, harmful, or indifferent for what I'm trying to do. Any thoughts good bad or otherwise on using cat5? I have so much of it I would love to be able to put it to actual use.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller

    It is not a good idea to use several runs of smaller wire to carry the current normally put on a larger one. The reason being if one of those wires breaks or has a poor connection its share of the current gets "off loaded" to the remaining wires. And so on: cascade failure is common when the current is in the higher range. The only way to do this safely is to fuse each and every strand to handle its maximum current rating, and the more strands you have to do that with the messier it gets.

    Also NEC does not allow parallel conductors under a certain size (I think it is 1/0), so such use would be technically illegal as well.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Gauge wire needed from panels to charge controller
    doctorZeus wrote: »
    Has use combined runs of cat5 ever come up before? Specifically, I'm looking to use it for the run from my panels to my charge controller, so heat and weather and all of that should definitely be considered. Also, not sure if the "twisted" nature of the wires is helpful, harmful, or indifferent for what I'm trying to do. Any thoughts good bad or otherwise on using cat5? I have so much of it I would love to be able to put it to actual use.

    Perhaps you can sell it, and then buy #10 wire. You can make profit as well.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Gauge wire needed from panels to charge controller

    also keep in mind that as long as you are putting the pvs in series that you will be ok with the #10 wire, but if you put in parallel any strings that this ups the current and also ups the requirements for the wiring. you could keep putting into series up to the recommendations set forth in the midnite pv string calculator.

    sidenote-you can parallel up to 2 strings of pvs without the need for fusing. beyond that you have to have fusing.

    keep in mind that if you go with a 24v battery bank then the inverter must also be able to work at 24v. the 24v arrangement would be better to avoid too many parallel strings of batteries.
  • doctorZeus
    doctorZeus Registered Users Posts: 24
    Re: Gauge wire needed from panels to charge controller

    I understand.. It's a bit of a "game" trying to spec out a scalable solar system. Thank you for everyone's feedback!

    Assuming batteries are within a few feet of charge controller and inverter, will #4 AWG be OK to wire 6V batteries into 24V series and then to the inverter and CC? Max load is looking like ~250 watts at any given time, spiking could be as high as 500 or 600, but nothing major. (Is there a loose formula for how far, and how many watts or amps would incline me to to use bigger cables?)

    My charge controller is a midnight solar classic lite 200. Since it has aux1 and aux2 connections, is it possible/practical to charge two different banks? (I'm just thinking out loud here). I'm looking at eight 6V batteries and rather than run them in two parallel strings of 24v series I was considering if it would be better to run them as two separate "banks" of 24V. It seems like that could have its own problems with how to draw power from the different banks, etc., but curious if that sounds lesser compared to running a parallel string on the batteries. (I'm starting to get scared off from the parallel connections.)
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Gauge wire needed from panels to charge controller
    doctorZeus wrote: »
    I understand.. It's a bit of a "game" trying to spec out a scalable solar system. Thank you for everyone's feedback!

    Well I would have said "nightmare" ... :D
    Assuming batteries are within a few feet of charge controller and inverter, will #4 AWG be OK to wire 6V batteries into 24V series and then to the inverter and CC? Max load is looking like ~250 watts at any given time, spiking could be as high as 500 or 600, but nothing major. (Is there a loose formula for how far, and how many watts or amps would incline me to to use bigger cables?)

    No quick-and-easy way to do it: know the maximum expected current and size the wire, then check against a V-drop calculator over the distance to see how much you suffer. In this case it shouldn't be much of a problem as the power demand is low. 4 AWG will handle 90 Amps, and at 24 Volts that's more than 2000 Watts. You would have less than 50 Amps on it, so it could be 20 feet long (not recommended) before the V-drop was above 2%. Change any of those factors and the results change.
    My charge controller is a midnight solar classic lite 200. Since it has aux1 and aux2 connections, is it possible/practical to charge two different banks? (I'm just thinking out loud here). I'm looking at eight 6V batteries and rather than run them in two parallel strings of 24v series I was considering if it would be better to run them as two separate "banks" of 24V. It seems like that could have its own problems with how to draw power from the different banks, etc., but curious if that sounds lesser compared to running a parallel string on the batteries. (I'm starting to get scared off from the parallel connections.)

    Extra wiring and no advantage.