Best implementation for long cable runs?

Re: Best implementation for long cable runs?

Sounds like you have a 12 volt system. But there are some questions - -
1) What batteries do you have?
2) what do you have for PV? Depending on what you have, you might be able to use a MPPT controller, run higher voltage DC to the controller at the batteries, which will down convert to 12 volts for charging.
3)What are your loads on the battery?

EDIT: Sorry Mike, I'm too slow, didn't see your post until after I hit send.

Re: Best implementation for long cable runs?

the wire efficiency increases with the increase in voltage, but the controller efficiency goes down with higher voltage. now you haven't said what you intend to run with these batteries and what their draw is, but i'll approach this from the angle of good charge %. the range we recommend for charging is between 5% and 13% with 10% a good overall point for most batteries. one good point is your agm batteries are more efficient and if you wanted to you could go above the 13% charge maxima we usually recommend without any problems.

anyway, at 5% you need 11a and at 10% you need 22a to charge the batteries. pvs usually don't deliver this with stc ratings so a derating to about 77% is usually done here. first putting the current into watts we see 11a x 12v = 132w and 22a x 12v = 264w. now derating we have 132w/.77=171.43w and 264w/.77=342.86w.

the voltage drop percentage can vary depending on the pv voltage and the current passed of the pvs you purchase. now this you can figure out with a v drop calculator like the one in my signature line. if you have a specific pv in mind tell us how many you intend to get and how you intend to wire them and we could give you a rough idea as to what gauge wire you'd want to run, however, you indicated too that you wish to expand this in the future so you will not get away with using small wire to do so as you will see in the calculator.

Re: Best implementation for long cable runs?

higher voltage pvs work just fine if you use a mppt type controller to be able to downconvert to your 12v battery voltage.

i did a bit of a run on a few calculations for rough illustrative purposes with various cc input voltages showing a good wire gauge to use around 2% v drop. note that for a 12v output at 11a this is going by the wattage here as the mppt controller will up the current nearly proportionally as it downconverts the voltage to give 22a at 12v.

24v pv at 11a---- #2 copper wire at 1.83% v drop.
36v pv at 7.34a---#6 copper wire at 2.05% v drop.
48v pv at 5.5a----#8 copper wire at 1.84% v drop.
60v pv at 4.4a---#10 copper wire at 1.87% v drop.
72v pv at 3.67a--#10 copper wire at 1.56% v drop.
84v pv at 3.14a--#12 copper wire at 2.12% v drop.
96v pv at 2.75a--#12 copper wire at 1.86% v drop.

so what happens if say you went with the 1st one at 24v and later decided to upgrade the system by another 24v 11a in pvs? this brings it to a total of 24v and 22a if you were to parallel them with the same wire. running the calculations it now shows it as,
24v pv at 22a----#2 copper wire at 3.65% v drop. this is unacceptable.
if you put another 24v in pvs at 11a in series with the other at 24v and 11a you get,
48v pv at 11a----#2 copper wire at .91% v drop. this is very acceptable and is 25% of what it would be in parallel because cutting the current in half cuts the v drop % in half, but doubling the voltage will also cut the v drop % in half. this could be expanded to 48v at 22a total when paralleling another 48v pv at 11a to the original 48v pv at 11a and would be back to 1.83% and is still very acceptable.

now #2 is a bit large so you may wish to go to a 48v pv input to start allowing the use of #8 copper wire. when you use higher voltages like 48v this means the pv expansions can go with single identical pvs in series until one reaches the danger zone for the controller's max input voltage. colder weather can push pv voltages even higher yet so one needs to take great care when venturing over 100v. also do the v drop calculations to be sure of your system losses for my calculations kept the same wattage and when adding pvs this ups the wattage at the same current.

also keep in mind that the downconverting mppt controllers will limit the output current so one has to watch what they input to the cc so as to not impede the ability to gain through mppt by going with too many watts for the specified voltage output. for instance many mmpt ccs have a 60a limit. at 12v this is 720w as 12v x 60a = 720w. at 24v this is 24v x 60a = 1440w. and the last common output voltage of 48v is 60a x 48v = 2880w. remember we need more stc watts to get to the delivered watts due to that 77% derating and one can't be 100% certain that derating will be accurate as it may be more or less. one last point is the nec limits the controllers to 80% of their current output capability. now i and many others will agree how stupid this ruling is, but on the plus side if you do this you allow for 20% of mppt to take place too. there are many variables involved in a system as you can see and ultimately it could boil down to your decision on what you purchase now and what you propose for the future. now i know #8 is by far cheaper wire, but if you can swing say #6 that added current capacity could give you much more flexibility in the future and at the very least more efficiency with your initial system. i went with about 2% v drop as the goal and some say under 3% total while others say under 2% total and this would include the v drop % from the cc to the battery. that one should be as low as you can get it and definitely below 1% for that section of wire. that section will be at 12v and higher current so it will take a considerably larger wire and need to be as short as possible.

well, i didn't expect to get long winded here and it was either overboard for you or just opened up more questions or even confused you so i'll stop now.