Wire size, which parameters
- Thread starter neoflyer
- Start date
OffGridInTheCity
Solar Wizard
This web page does an OK job defining terms - https://www.electronicsforu.com/mar...-nominal-voltage-voc-vmp-isc-imp-solar-panels
Vmp = Voltage at maximum power - this is more representative of when the panels are producing power.
Imp - Current at maximum power - this is more representative of when the panels are producing power.
The VoC is used so you don't exceed the maximum voltage the Charge Controller can handle - this can go up in very cold weather when there's no active power (yet) - so you want to keep it in mind when designing a system.
Wire for PV power transmission .... I wouldn't overthink it unless you are doing over 200ft lenghts. For 'regular lengths' such as you would find in home wiring from the main panel to circuits around the house - e.g. up to 150ft... regular amps / awg are fine in my experience. You'll notice that no one does 'special wiring' for a regular home worrying about 2% loss
The basic for awg / amps for home wiring (e.g. up to 150ft of PV transmission):
14awg = 15a max
12awg = 20a max
10awg = 30a max * Panels have 10awg pigtails (30amps) but typically you don't want to go right up to 30amps ... say 25a max.
8 awg = 40a max
6 awg = 50a max
4 awg = 80a max
etc... standard wire sizes per amps will do you OK.
However, if you want to try for just a bit more efficiency, there's nothing wrong with oversizing wire - its just expensive for small gain.
The #1 (by far) loss of efficiency in my system is the DC -> AC inverters. The wiring losses (up to 150ft) is so small I can't detect it, even the charge/discharge loss to the battery looks 1% max? BUT the inverters average 15% loss and if I don't keep power consumption % up higher on the inverters, they can sink down to 20% loss. To illustrate - I produce 18,000kwh/year but loose 3000kwh/year to inverter losses. If you want to spend money - do it on an efficient inverters is my best advice
Vmp = Voltage at maximum power - this is more representative of when the panels are producing power.
Imp - Current at maximum power - this is more representative of when the panels are producing power.
The VoC is used so you don't exceed the maximum voltage the Charge Controller can handle - this can go up in very cold weather when there's no active power (yet) - so you want to keep it in mind when designing a system.
Wire for PV power transmission .... I wouldn't overthink it unless you are doing over 200ft lenghts. For 'regular lengths' such as you would find in home wiring from the main panel to circuits around the house - e.g. up to 150ft... regular amps / awg are fine in my experience. You'll notice that no one does 'special wiring' for a regular home worrying about 2% loss
The basic for awg / amps for home wiring (e.g. up to 150ft of PV transmission):
14awg = 15a max
12awg = 20a max
10awg = 30a max * Panels have 10awg pigtails (30amps) but typically you don't want to go right up to 30amps ... say 25a max.
8 awg = 40a max
6 awg = 50a max
4 awg = 80a max
etc... standard wire sizes per amps will do you OK.
However, if you want to try for just a bit more efficiency, there's nothing wrong with oversizing wire - its just expensive for small gain.
The #1 (by far) loss of efficiency in my system is the DC -> AC inverters. The wiring losses (up to 150ft) is so small I can't detect it, even the charge/discharge loss to the battery looks 1% max? BUT the inverters average 15% loss and if I don't keep power consumption % up higher on the inverters, they can sink down to 20% loss. To illustrate - I produce 18,000kwh/year but loose 3000kwh/year to inverter losses. If you want to spend money - do it on an efficient inverters is my best advice
Last edited:
Thanks. Great information. My installation is on a motorhome. The distance from the combiner box to the charge controller is no more than 25’ so 50’ total cable.This web page does an OK job defining terms - https://www.electronicsforu.com/mar...-nominal-voltage-voc-vmp-isc-imp-solar-panels
Vmp = Voltage at maximum power - this is more representative of when the panels are producing power.
Imp - Current at maximum power - this is more representative of when the panels are producing power.
The VoC is used so you don't exceed the maximum voltage the Charge Controller can handle - this can go up in very cold weather when there's no active power (yet) - so you want to keep it in mind when designing a system.
Wire for PV power transmission .... I wouldn't overthink it unless you are doing over 200ft lenghts. For 'regular lengths' such as you would find in home wiring from the main panel - e.g. up to 150ft... regular amps / awg are fine in my experience. You'll notice that no one does 'special wiring' for a regular home worrying about 2% loss
The rundown is:
14awg = 15a max
12awg = 20a max
10awg = 30a max * Panels have 10awg pigtails (30amps) but typically you don't want to go right up to 30amps ... say 25a max.
8 awg = 40a max
6 awg = 50a max
4 awg = 80a max
etc... standard wire sizes per amps will do you OK.
However, if you want to try for just a bit more efficiency, there's nothing wrong with that / just expensive - maybe double the standard wire size?
Note: The #1 (by far) loss of efficiency in my system is the DC -> AC inverters. The wiring (up to 150ft) is so small I can't even detect it - but the inverters average 15% loss and if I don't keep power % up higher on them, they can go up to 20% loss. To illustrate - I product 18,000kwh/year but loose 3000kwh/year to inverter losses. If you want to spend money - do it on an efficient inverters is my best advice
An online wire calculator just asks for voltage and current so your answer helps a lot.
Ed
12VoltInstalls
life passes by too quickly to not live in freedom
I overcable things. I’m maybe the wrong guy to advise
I’m only 20’ and 10ga calcs with headroom. So I bought a spool of 8ga and on the rare crystal clear days when I was running pwm w/ 200W a couple years ago it got me to 11.x amps whereas with 10ga I only saw 9.8 or 10.1A and though it’s possible I just never saw it higher by changing to fat oversized multi strand wire my current peaks apparently went up.
Now that’s not a guarantee but in my (then poor) location I saw an improvement that actually surprised me a lot.
My point is - if that 1/2% calculates to a significant number of watts, then maybe fatter wire is in order. If the wire has the headroom to add panels for the same or close dollars, add panels instead.
What is the combined VOC and Amps?
I’m only 20’ and 10ga calcs with headroom. So I bought a spool of 8ga and on the rare crystal clear days when I was running pwm w/ 200W a couple years ago it got me to 11.x amps whereas with 10ga I only saw 9.8 or 10.1A and though it’s possible I just never saw it higher by changing to fat oversized multi strand wire my current peaks apparently went up.
Now that’s not a guarantee but in my (then poor) location I saw an improvement that actually surprised me a lot.
My point is - if that 1/2% calculates to a significant number of watts, then maybe fatter wire is in order. If the wire has the headroom to add panels for the same or close dollars, add panels instead.
The critical factor here is the DC input voltage of your inverter. A 0.5 Volt drop on a 48 volt system means you are losing about 1% of your stored Kwh. That same 0.5 Volt drop on a 12 volt system means your are losing almost 4% !! Over size wiring on 12 volt systems is usually a good idea because of the much high currents. But don't forget that badly cramped connectors, corrosion and loose nuts will usually be a greater problem.
diysolarforum.com
Voltage Drop Calculator
Useful voltage drop calculator with a given wire gauge and current. This, in combination with a current carrying chart is essential for low voltage i.e. 12 volt builds which usually have very high currents. Choosing wire size based only on current carrying limits will often result in large...
diysolarforum.com
chrisski
Solar Boondocker
- Joined
- Aug 14, 2020
- Messages
- 5,326
I go with the lower voltage Vmp and higher Amps IOC.
That gives a more conservative measurement that allows for thicker wire. I do this for a couple of reasons. When temp goes up, the heat causes the panels to put out less volts, but about the same amps, meaning wire gauge would be thicker. THese thicker Wires are also less susceptible to voltage drops on the high wattage loads and will not heat up as much for the medium size loads that are on for a long time.
This often gets me wires 2 AWG thicker than others will use. In my mind makes me less likely to be part of the “Up in Smoke” section of the forum.
There’s some limits on this though when paneling. THis can get me to thicker than 10 AWG pretty quickly and MC4s are limited to 10 AWG. There is a AMphenol H4 connector that can go to 8 AWG. So for paneling, I’ve started to go to a voltage that will keep me 10 AWG or thinner. And at a 3% or less voltage loss.
That gives a more conservative measurement that allows for thicker wire. I do this for a couple of reasons. When temp goes up, the heat causes the panels to put out less volts, but about the same amps, meaning wire gauge would be thicker. THese thicker Wires are also less susceptible to voltage drops on the high wattage loads and will not heat up as much for the medium size loads that are on for a long time.
This often gets me wires 2 AWG thicker than others will use. In my mind makes me less likely to be part of the “Up in Smoke” section of the forum.
There’s some limits on this though when paneling. THis can get me to thicker than 10 AWG pretty quickly and MC4s are limited to 10 AWG. There is a AMphenol H4 connector that can go to 8 AWG. So for paneling, I’ve started to go to a voltage that will keep me 10 AWG or thinner. And at a 3% or less voltage loss.
It depends what you are calculating for. With voltage drop (mostly an efficiency concern), you can just use Imp and Vmp. With ampacity or when calculating the max voltage, you are better off using more conservative specs (Isc and Voc*temp coefficient)
Probably not (and this comes from someone who is a little over obsessive when it comes to efficiency).
But its up to you. More a personal preference and cost/benefit decision than a technical decision.
Think of it in dollars if that is more intuitive. The difference between 1% and 1.5% voltage drop, is equivalent to being 'paid' $100 but receiving only either $99 or $98.5. Or whatever other analogy might help make those %'s more intuitive to you.
At the power level of most solar, voltage specs are only used to select the insulation grade. The maximum expected amperes are used to select wire gauge by calculating voltage drop. With the charts, I use 1% to allow for drop across fuses and connections. 1.5% is reasonable.
12VoltInstalls
life passes by too quickly to not live in freedom
You have my vote.
(I have a very low tolerance for smoke…just as low a tolerance for “acceptable” when the same work and a few dollars more basically eliminates any gambles)
