diy solar

diy solar

Wire sizing question

J

JPetronek

New Member
Joined
Dec 2, 2019
Messages
31
Afternoon all-

I am wanting to double check and ask a question on wire sizing and amperage.

currently I have 1-160 w panel on my roof with a 30 amp pwm controller 10guage wire coming in from panels, and going out from controller.
I am adding 1 more 160 w panel in series w my existing, also adding 2-180 w panels in series that I will parallel in with the 2 - 160 panels. I would like to avoid another roof penetration, can I do all the S&P on the roof and be safe with the 10 wire? I plan to remove the controller and junction the wires together and locate new 40 amp mppt in different location.
If my thought is correct I should be at roughly 18-20 amps from panels if I do it as stated above since I’m doubling voltage amperage won’t double?
also what can be expected as output from controller max in this scenario? Does that stay at the total wattage /12?
 
You'll have 320w @ about 36ish V from the first string = about 9 amps, parallelled with 360w @ about 36Vish = about 10 amps ...so your math is correct...20 amps is a safe bet and your 10 gauge should be fine, as long as it's not a long distance.
 
JPetronek said:
also what can be expected as output from controller max in this scenario? Does that stay at the total wattage /12?
Click to expand...


Yup, more like /14 though, but yup. You can have a 200w controller but it can only output what your panels produce. 680/12 = 56a, so size those wires accordingly. Not 10g of course....

That 56a @12v would be above your 40a mppt, so consider going to a 24v system to knock it down to ~28 amps @ 24v so you "get your money's worth"...and then could use the 10g...but i'd still upsize that.
 
SolarRat said:
Yup, more like /14 though, but yup. You can have a 200w controller but it can only output what your panels produce. 680/12 = 56a, so size those wires accordingly. Not 10g of course....

That 56a @12v would be above your 40a mppt, so consider going to a 24v system to knock it down to ~28 amps @ 24v so you "get your money's worth"...and then could use the 10g...but i'd still upsize that.
Click to expand...
So the 40 amps is the output of the controller and not the max input? Would I be better off having 2 parallel panels to my 30 pwm and a set of series to my new one to use all of my available output of the panels?
part of me is thinking since they are flat mounted in a rv I’ll never see 56 amps and to stick with original plan?
 
Put simply PWM controllers operate by connecting the battery directly to the panels while monitoring the battery voltage. When the battery voltage rises up to the charging target it disconnects the panels from the battery while again monitoring the battery voltage and when it hits a low voltage point repeats the process. This can happen 25 or more times a second. Instantaneous amps in is the same as instantaneous amps out. This is why PWM controllers should have PV input close to the battery voltage. Higher voltage from the panels effectively means an increasing amount of the panel's output is discarded.

MPPT controllers take the power from the panels and convert it (typically down) to the voltage your battery requires to be charged at. If you have 500 watts of power (let's say 5 x 100 watt panels in any arrangement) from the panels and a notionally 12V battery the amps in may only be 15 ( 500 / Vmp (32v or what ever)), but the amps out of the controller to the battery will be 41 (500 watts / 12V). MPPT controllers are the better way to go for most circumstances.

Typically the amp rating of an MPPT controller applies to both the input and output, regardless of the voltage. Assuming a typical MPPT charger rated at 50 amps, If your panels supply more than 50 amps a good controller will only draw 50 amps (to protect itself). If when that input is converted down to battery voltage the current available exceeds 50 amps the good controller will again limit the current to protect itself. If we connect 1000 watts of panels to the input of a 50 amp controller, but the controller is connected to a 12v battery at least half of the capacity of the panel's peak production will go unused because the charge controller will limit the current to 50 amps (600 watts).
 
Last edited:
JPetronek said:
So the 40 amps is the output of the controller and not the max input? Would I be better off having 2 parallel panels to my 30 pwm and a set of series to my new one to use all of my available output of the panels?
part of me is thinking since they are flat mounted in a rv I’ll never see 56 amps and to stick with original plan?
Click to expand...

Most good CC's can throttle back some excess wattage...some cheapies can't. With flat mounted panels you are correct, you may never go much above the 40a so you'll be ok with a good CC if you live up north. Closer to the equator you'll surely waste some watts, but should stay within safe "max input" of a good CC. 40amps is the max output (charge current). Usually they list max input wattage separately. No, don't use the old PWM.
 
SolarRat said:
Most good CC's can throttle back some excess wattage...some cheapies can't. With flat mounted panels you are correct, you may never go much above the 40a so you'll be ok with a good CC if you live up north. Closer to the equator you'll surely waste some watts, but should stay within safe "max input" of a good CC. 40amps is the max output (charge current). Usually they list max input wattage separately. No, don't use the old PWM.
Click to expand...
Thanks for the info. We are in Oregon, all of our camping is from Southern Oregon to Northern Idaho. Most of the time our exposure has some trees and shading. I am installing the epever 40 amp. I believe it will be safe for my application. Excited to get it installed and working.
 
Hopefully they are 200ah worth at the very least...if FLA's that should be ok, sorta, not quite but.... Lead acids don't like much more than C/10, C/6 is about max.

Wives can get mad...at freakin anything :).
 
SolarRat said:
Hopefully they are 200ah worth at the very least...if FLA's that should be ok, sorta, not quite but.... Lead acids don't like much more than C/10, C/6 is about max.

Wives can get mad...at freakin anything :).
Click to expand...
I have 3-group 27 deep cycles. 100 ah each.
 
gnubie said:
Put simply PWM controllers operate by connecting the battery directly to the panels while monitoring the battery voltage. When the battery voltage rises up to the charging target it disconnects the panels from the battery while again monitoring the battery voltage and when it hits a low voltage point repeats the process. This can happen 25 or more times a second. Instantaneous amps in is the same as instantaneous amps out. This is why PWM controllers should have PV input close to the battery voltage. Higher voltage from the panels effectively means an increasing amount of the panel's output is discarded.

MPPT controllers take the power from the panels and convert it (typically down) to the voltage your battery requires to be charged at. If you have 500 watts of power (let's say 5 x 100 watt panels in any arrangement) from the panels and a notionally 12V battery the amps in may only be 15 ( 500 / Vmp (32v or what ever)), but the amps out of the controller to the battery will be 41 (500 watts / 12V). MPPT controllers are the better way to go for most circumstances.

Typically the amp rating of an MPPT controller applies to both the input and output, regardless of the voltage. Assuming a typical MPPT charger rated at 50 amps, If your panels supply more than 50 amps a good controller will only draw 50 amps (to protect itself). If when that input is converted down to battery voltage the current available exceeds 50 amps the good controller will again limit the current to protect itself. If we connect 1000 watts of panels to the input of a 50 amp controller, but the controller is connected to a 12v battery at least half of the capacity of the panel's peak production will go unused because the charge controller will limit the current to 50 amps (600 watts).
Click to expand...
What can I expect to lose efficiency wise if I go to 24 v output and use a converter for my trailer loads?
 
The 40a charge is perfect for your 12v 300ah battery. As long as your trailer loads are under 1kw, stay with the 12v. If you had higher loads or were running a larger inverter however, a bump to 24v would be worthwhile.
 
JPetronek said:
What can I expect to lose efficiency wise if I go to 24 v output and use a converter for my trailer loads?
Click to expand...
If you were to change to a 24v battery and then used a converter to reduce that to 12v for the trailer loads you would typically see in the order of 80-85% conversion efficiency, ie for every 100 watts into the DC-DC converter you'd get about 80 watts out (the loss goes out as mostly heat). Actual efficiency is a function of the load and the design of the converter.
 
gnubie said:
If you were to change to a 24v battery and then used a converter to reduce that to 12v for the trailer loads you would typically see in the order of 80-85% conversion efficiency, ie for every 100 watts into the DC-DC converter you'd get about 80 watts out (the loss goes out as mostly heat). Actual efficiency is a function of the load and the design of the converter.
Click to expand...
Thanks All. It sounds like my setup will fit my needs for now. Appreciate the info!
 
You must log in or register to reply here.

Similar threads

T
Dokio 220w panel problem?
Replies
6
Views
197
TucnaWildcat
T
C
Working on string configuration adding more panels
Replies
1
Views
213
MaikaiLife
MaikaiLife
nwsolar
Solar PV wire sizing
Replies
19
Views
678
dopeassjackson
D
JBoffgrid2022
Off Grid Wire Sizing / Best Ground Methods
Replies
17
Views
270
garybryan33
G
M
Mpp solar lv6548 solar array
Replies
9
Views
297
Matthew judah
M

diy solar

diy solar
Back
Top