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BFL13's avatar
BFL13
Explorer II
Oct 04, 2013

Wire Gauge Required Question

For some reason I just can't understand the various wire gauge tables, and I have tried to, so I am hoping somebody will take pity and do all the work! :)

One table that I can follow says that for a 12v system, and a 16 amp solar controller, you need (to be within 1/2 volt drop) for a solar array connection to controller of 30 ft (round trip)-- #12, or for 40ft-- #10

I take that to mean either 15 ft away or 20 ft away.

This same source says to double the voltages for 24v in their example for calculating voltage drop, so does that mean with 24v, 1.0v drop is ok on that line? NOTE this is in a manual for a PWM controller. ISTR with MPPT you want a tighter control on voltage drop.

Anyway I am lost here. My question is, "What wire gauge of stranded copper would you need for 16 amps with a 24v system 30ft away (60 ft round trip) with an MPPT controller?"

EDIT--also, please with a 24v panel and MPPT, do you use the 12v figures for the controller battery, but the 24v figures for the controller array wires?

Thanks.

207 Replies

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  • BFL13's avatar
    BFL13
    Explorer II
    Oct 20, 2013
    I can now see that it is not really correct, but the net amps in -out approach gets the right answer so I like it.

    I also like when they call the ends of a compass needle N and S Don't want to know about solenoids. Almost as bad as Watts.
  • ktmrfs's avatar
    ktmrfs
    Explorer II
    Oct 20, 2013
    BFL wrote:
    Of interest perhaps, is that the instructions for the solar controller say do not run loads from the load terminals of the controller without the battery connected to the controller. Why not, if the battery cannot charge and discharge at the same time?

    A couple of thoughts.

    First, if the solar panel can't supply ALL the load needs, the battery can pickup the extra; E.G. a cloud roals in.

    Second a CYA in the case for some reason the panel can't supply ALL the current needed, and the load for some reason then drops out.

    Basically I think they are assuming that the panel can't be relied on to ALWAYS supply the necessary load current, that all, but then that just my opinion for what little it is worth.!!!
  • 12thgenusa's avatar
    12thgenusa
    Explorer
    Oct 19, 2013
    BFL13 wrote:
    I think the issue here is that the Trimetric does indeed show net amps across its shunt, but there is an alternative way to look at whether all those amps are going through the battery to supply the loads or some just go across the battery pos terminal common connection and the shunt's neg common connection without going down inside the battery. The controller, converter and batteries are all parallel power sources. Think about it.

    -Doing a test for what amps the converter can do on its path from DC panel to battery. Batteries are jammed full at 100% SOC

    -Turn on kettle via inverter. Trimetric shows -90 amps draw. Turn on converter. Tri shows -40 amps. This shows that the converter is supplying 50a. and the battery is supplying 40a as shown by the Trimetric.

    Of interest perhaps, is that the instructions for the solar controller say do not run loads from the load terminals of the controller without the battery connected to the controller. Why not, if the battery cannot charge and discharge at the same time? Many controllers will not operate or are damaged if the panels are connected without the batteries on line.

    Perhaps what happens is that the battery "sees" different voltages as one net voltage wrt either loads or chargers and responds plus or minus to that net voltage diff with amps heading one way or the other.True in that current only flows where there is a difference of potential. Both the converter and solar controller are at higher potential than the battery or it would not charge. That difference causes them to power an external load while simultaneously charging the battery. However, the current does not flow through the battery on its way to the load.
  • BFL13's avatar
    BFL13
    Explorer II
    Oct 19, 2013
    I think the issue here is that the Trimetric does indeed show net amps across its shunt, but there is an alternative way to look at whether all those amps are going through the battery to supply the loads or some just go across the battery pos terminal common connection and the shunt's neg common connection without going down inside the battery.

    -Doing a test for what amps the converter can do on its path from DC panel to battery. Batteries are jammed full at 100% SOC

    -Turn on kettle via inverter. Trimetric shows -90 amps draw. Turn on converter. Tri shows -40 amps. This shows that the converter is supplying 50a.

    Of interest perhaps, is that the instructions for the solar controller say do not run loads from the load terminals of the controller without the battery connected to the controller. Why not, if the battery cannot charge and discharge at the same time?

    Perhaps what happens is that the battery "sees" different voltages as one net voltage wrt either loads or chargers and responds plus or minus to that net voltage diff with amps heading one way or the other?
  • 12thgenusa's avatar
    12thgenusa
    Explorer
    Oct 19, 2013
    BFL13 wrote:
    The way I look at it the loads are still run from the battery but the solar recharges that. The Trimetric shows net amps of whatever is on its shunt.

    Was:
    Draw = 0
    Charge= +3
    Net on Tri= +3

    Now:
    Draw= -6
    Charge= +9
    Net on Tri= +3

    Still have 3 to go before anything changes on the Tri.

    Not to belabor the point, but you can't charge and discharge the battery at the same time. Power beyond what is accepted by the battery goes directly to the external load and not through the shunt (or battery). The trimetric measures actual amps not net amps. The +3 will not change until the external load plus battery acceptance exceeds the panel output.
  • NinerBikes's avatar
    NinerBikes
    Explorer
    Oct 18, 2013
    ktmrfs wrote:


    all very good questions and good analysis, especially if starting from scratch. In my case, since I use portable panels, size/weight for carrying, and storage space are much more important to me than in a fixed roof mount. Also, wanting to be able to go up to 100ft away from the trailer gets into power loss issues much more so than in a more typical use case. And I already had the two 80W panels anyway and was just looking to maximize output at a long distance.

    At least there are lots of alternatives to pick and choose from to do what each individual thinks best fits their application.

    choice between MPPT and PWM is kinda like the choice between 6V GC batteries and 12V batteries. First understand carefully your needs and what's important, then determine which choice best fits your needs and budget.


    From what I am reading, folks are pretty lucky to get more than a year out of a 75 ah 20hr rate $70 group 24DC that has a lifetime of 150 50% charge cycles, where as a GC-2 from Costco or Sams Club puts out 210 to 225 ah at 20hr amp rate, and folks seem to get 6 years out of them that use them a few times a year... no brainer there. Now if you full time reside in your RV and cycle daily, then the Trojan brand GC-2 T-105 is the gold standard, and you pony up and get it instead for the lowest cost per recharge, lifetime.

    As usual... the answer always is... it depends...
  • NinerBikes's avatar
    NinerBikes
    Explorer
    Oct 18, 2013
    BFL13 wrote:
    Yes, thanks. I was checking just for changing actual wire sizes.

    I think it is starting to make sense (to me---others got there long ago!) how to use the linked calculator in real life to pick the size of wire on either side of the controller for how much change that might mean in charging amps for a particular array wattage size.

    Kind of an eye-opener, especially learning that 12v applies to one side and 24v to the other side of the controller. This is not easy, like 12v PWM.


    Disregard the voltage, voltage in water terms is like water pressure, the higher the voltage, the higher the pressure.

    Amperage is like volume flow, like a dams release measured in cubic feet per second.

    It is easy. We do it all the time wiring residental. You are interested in I where I = Amperage.
    If the run is 30 feet or less...

    15 amps or less, use 14 gauge
    20 amps or less, use 12 gauge
    30 amps or less, use 10 gauge


    Wire gauge size made easy.

    Think of amperage as flow of water, and wire gauge as hose diameter, where gauge is number of balls to the pound. 10 gauge is 10 balls to the pound, so it is larger in diameter than 12 gauge which is 12 balls to the pound. Like a shot gun.

    Now, if you exceed 30 feet, go up a size in wire from above, get a fatter, lower gauge number wire. More than 60 or 70 feet, go up a size in wire again. But know that copper is an expensive component when you go up a gauge is wire, and is priced accordingly. At some point, adding watts to the panel and allowing for losses in wiring can become more cost effective, depending on all your parameters.

    In general, low voltage = 12v = requires fat wire. If you have 180 watts per panel and 18V Vmax, you'll have 10 amps. If you have 180 watts and you double the Vmax to 36V, since you doubled the voltage (pressure), you halved (ie the inverse) the amperage... so you can use skinnier wire on the higher voltage size. Battery side of charger controller is fat wire if with MPPT, it's the same size (fat) on both sides of the controller with PWM.

    As the new kid on the block, I would like to thank everyone with their patience the past 4 days as I read, re read, and digested some very heavy material for me, a non EE backrounded guy, with all the myriad of variables that engineers must take into consideration when building and designing power supplies for our RV toys.

    I've learned a ton of practical knowledge, I won't feel so dumb or overwhelmed around the the campfire next time with beers in hand when the motorhome guys start up with their electrical charging systems. I'm now beyond the Eu1000 /2000 /3000 or yamaha 2400 hurdles, and going more green each day.
  • ktmrfs's avatar
    ktmrfs
    Explorer II
    Oct 18, 2013
    NinerBikes wrote:
    ktmrfs wrote:
    smkettner wrote:
    I would love for BFL13 to do an empirical test to quantify that 14 vs 10 loss.


    For BFL's case run =30ft Imp=7.7A

    for #10 cable power loss = 3.6W = about 0.27A at the output under max power. (0.06ohms x 7.7x7.7)

    for #14 same conditions, power loss = 9 watts, about .75A under max conditions. (0.15ohms x 7.7x7.7) Note that #10 to #14 more than doubles the resistance. each increase/decrease by 3 wire sizes doubles or halves the resistance.

    so depends on if 3/4 A is signficant to you.

    How does this "theoretical" calculation compare to real world. Using the same method I calculated the power loss in output amps for my 160W panels connected in parallel vs. series for a 100ft run to the portable panels. Wire is #10 on one side #7 on the other side. (how?? well it is 30A Rv cord with one side being 2 #10's the other side being 1 #10 for calculations I used 150ft of #10 for total wire length)

    doing the calculations power loss in a parallel connection was 12W about 1A. In a series connection power loss was calculated to be 3W or about 1/4 amp.

    so.... last time I was out batteries near 50% SOC, using the trimetric in bright sun had them in parallel and looked at battery charging current, then switched to series and looked at battery charging current. Guess what, in series the current went up by real close to 3/4A. controller is a sunsaver 15 MPPT controller

    So again how important is another 3/4A to you? Only you can decide.

    For me, I don't have enough solar to ever get beyond bulk charging mode. 500AH of battery (4 T-125's) , only 160 W of solar. In fact I used the SW for the sunsaver to modify the charge profile to only use bulk and equalize mode and don't let bulk mode time out. So the "extra" 3/4 A or so most of the day is an extra 3/4A. It all helps.

    In mid summer with sun all day I can recover 80-90% of a days power use. In the fall or under less sunny conditions (most common in our part of oregon) it's less. My goal is to maximize the days between firing up the generator.


    Having 160W of solar and cost of your MPPT versus having 200 watts of 12v solar panel, and a 20 amp PWM by Morningstar, and 10 gauge wire. What is the gain or loss in Amps, and what is the cost difference?

    This is assuming space is not at the limit currently, and you aren't running 10 ga over a 30 foot run from panel to controller, to batteries.

    Can we assume $1.00 a watt per panel? One can spend a little more on watts and 12V panels that are larger, and will it offset the price of a Morningstar MPPT -15L at $230 plus shipping, vs a Morningstar PWM Sunsaver SSL 20L at $77.... well unless roof space is at a premium, the slightly more watt and larger panels, in a 2 panel application with PWM will be more cost effective, and produce more amps, will it not?


    all very good questions and good analysis, especially if starting from scratch. In my case, since I use portable panels, size/weight for carrying, and storage space are much more important to me than in a fixed roof mount. Also, wanting to be able to go up to 100ft away from the trailer gets into power loss issues much more so than in a more typical use case. And I already had the two 80W panels anyway and was just looking to maximize output at a long distance.

    At least there are lots of alternatives to pick and choose from to do what each individual thinks best fits their application.

    choice between MPPT and PWM is kinda like the choice between 6V GC batteries and 12V batteries. First understand carefully your needs and what's important, then determine which choice best fits your needs and budget.
  • NinerBikes's avatar
    NinerBikes
    Explorer
    Oct 18, 2013
    ktmrfs wrote:
    smkettner wrote:
    I would love for BFL13 to do an empirical test to quantify that 14 vs 10 loss.


    For BFL's case run =30ft Imp=7.7A

    for #10 cable power loss = 3.6W = about 0.27A at the output under max power. (0.06ohms x 7.7x7.7)

    for #14 same conditions, power loss = 9 watts, about .75A under max conditions. (0.15ohms x 7.7x7.7) Note that #10 to #14 more than doubles the resistance. each increase/decrease by 3 wire sizes doubles or halves the resistance.

    so depends on if 3/4 A is signficant to you.

    How does this "theoretical" calculation compare to real world. Using the same method I calculated the power loss in output amps for my 160W panels connected in parallel vs. series for a 100ft run to the portable panels. Wire is #10 on one side #7 on the other side. (how?? well it is 30A Rv cord with one side being 2 #10's the other side being 1 #10 for calculations I used 150ft of #10 for total wire length)

    doing the calculations power loss in a parallel connection was 12W about 1A. In a series connection power loss was calculated to be 3W or about 1/4 amp.

    so.... last time I was out batteries near 50% SOC, using the trimetric in bright sun had them in parallel and looked at battery charging current, then switched to series and looked at battery charging current. Guess what, in series the current went up by real close to 3/4A. controller is a sunsaver 15 MPPT controller

    So again how important is another 3/4A to you? Only you can decide.

    For me, I don't have enough solar to ever get beyond bulk charging mode. 500AH of battery (4 T-125's) , only 160 W of solar. In fact I used the SW for the sunsaver to modify the charge profile to only use bulk and equalize mode and don't let bulk mode time out. So the "extra" 3/4 A or so most of the day is an extra 3/4A. It all helps.

    In mid summer with sun all day I can recover 80-90% of a days power use. In the fall or under less sunny conditions (most common in our part of oregon) it's less. My goal is to maximize the days between firing up the generator.


    Having 160W of solar and cost of your MPPT versus having 200 watts of 12v solar panel, and a 20 amp PWM by Morningstar, and 10 gauge wire. What is the gain or loss in Amps, and what is the cost difference?

    This is assuming space is not at the limit currently, and you aren't running 10 ga over a 30 foot run from panel to controller, to batteries.

    Can we assume $1.00 a watt per panel? One can spend a little more on watts and 12V panels that are larger, and will it offset the price of a Morningstar MPPT -15L at $230 plus shipping, vs a Morningstar PWM Sunsaver SSL 20L at $77.... well unless roof space is at a premium, the slightly more watt and larger panels, in a 2 panel application with PWM will be more cost effective, and produce more amps, will it not?

    In this case, with the bigger panels, you will at the start end up with 2.4 or more Amps than with your 160W panels, that you can take the .75 Amp loss from. Or am I missing something?

    Seems like panels, to me, are like engines, can't replace displacement, need more power go up in size.

    Understand, I am dealing with small potatoes real estate with a travel trailer compared to a motor home crowd type of set up. Guess I just prefer the KISS. Take my losses with PWM and upsize a bit with more watts per 12V panel, and call it good. Or find a PWM that is tweekable adjustable also via computer that you can program the bulk charge feature on it also, if you are under on amp hours to the size of your usage on the battery system and sit in bulk for what you can also, instead of timed out.
  • BFL13's avatar
    BFL13
    Explorer II
    Oct 15, 2013
    Point taken. The idea of running the loads was so whatever the solar was doing would show on the Trimetric even if the batts were too full to accept all the solar.

    The way I look at it the loads are still run from the battery but the solar recharges that. The Trimetric shows net amps of whatever is on its shunt.

    In the afternoon while on solar, where the solar can get 12a, you see plus 3a on the Tri from battery charging. You turn on your laptop which takes 6a.

    Nothing happens on the Tri. The solar is doing 3 of its 12 already on the batt but has 9a to spare. The laptop takes 6 of those 9.

    Was:
    Draw = 0
    Charge= +3
    Net on Tri= +3

    Now:
    Draw= -6
    Charge= +9
    Net on Tri= +3

    Still have 3 to go before anything changes on the Tri.

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