I put together a portable kit of 2 x 50w panels. They are 18.0v, and have junction boxes rather than the MC4 connectors. I'm not going to be using MC4's, either. I want to use 25' of wire. I want it to be flexible more or less like an extension cord. I've been wanting 10-2 something.I see 10-2 solar cable, sometimes called tray cable. The wire is stranded but the jacket looks pretty stiff. Is that correct, or is it flexible?I also see 10-2 low voltage landscape cable. It appears to be like big lamp cord? So it should be flexible?What should I be looking at? Also, do I get much benefit from 10 versus 12 going into a PWM controller? (18.0v, 5.5a, 25 feet)I've run the loss calcs but I don't know if the difference is significant going into the controller. Loss with #10: 1.7% or 0.3v. Loss with #12: 2.7% or 0.5v. So even with #12, that's 17.5v at the controller.

Snowman9000 wrote:I've run the loss calcs but I don't know if the difference is significant going into the controller. Loss with #10: 1.7% or 0.3v. Loss with #12: 2.7% or 0.5v. So even with #12, that's 17.5v at the controller.Aren't you forgetting that 25ft of tray cable means 50ft total? This is 3.3% loss with #10 cable, or 5.5% with #12.

I've used both tray cable, and 10/2 SOW for portable installs. SOW is the heavy black rubber extension cord type wire. It's really flexible, tray cable is stiffer, but not too unwieldy. I like tray cable for installs where the owners are a little more aware of the cable being "slightly" more vulnerable.

Hi,On twenty five feet with PWM you get zero benefit from going to #10 vs #12.MPPT it would matter.The PWM ignores voltage that is higher than the battery voltage.

With 5.5% loss on 25ft tray #12, the voltage will be 16, and the controller will taper it to 14.8. This is - at 70F. When it gets hotter, the voltage drops, and you won't have 14.8.

Hi Almot,They are 18 volt panels. Panel ratings are done at 25 C (77 f)

Wire type and size for portable solar panel kit

13 Replies

Snowman9000
Explorer
Aug 14, 2013
OK, yes, Voc = 22.3. Vmpp = 18.0.
BFL13
Explorer II
Aug 14, 2013
With PWM the mp figures are meaningless, so what is an 18v panel? It is either a 21v "12v" panel or a higher voltage panel (pick a number)

I will ASSume these are a couple of "12v" 50w panels that will get about 6a and a bit in full sun as a 100w combo in parallel. This will get 6.3a in full sun but need to allow 20% over that at times, so call "max expected max amps" with 20% 6.3 plus 0.63 = 6.93a.

You need a 7a controller or so. Cost is about $20 for a 7a PWM controller or so.

Wiring was the actual question. #12 is fine. In fact our neighbour just got a whole bunch of "well pump wire" from Home Depot which is #12 and it is fine for his 12a Isc panels.
Almot
Explorer III
Aug 14, 2013
Snowman9000 wrote:
Maybe the Southwire calculator is already accounting for the pair, too?

I'm not sure if I am talking about the same thing as you WRT temperature, but my controller is temperature compensating.

I don't see "pair" mentioned anywhere in Southwire calcualtor, here. It only says in the comments under the results "The total combined load and length of the circuit must be used". I used Direct Overhead rather than Conduit, since this is similar to loose cables in your case.

Temperature compensation means that controller will increase the setpoint voltage with temperature increasing, BUT - it will only increase the voltage when there is some margin left after tapering. The panel outputs 17.5V at 77F. After 2.5% cable loss (if it is 2.5%) you have 17V. Say, the battery needs 14.8V, so the controller tapers off 2.2V. When temperature increases, the voltage of the panel drops and at some point the panel outputs 15V and you have 15-2.5%=14.6V at the battery. No volts to taper, and the battery is undercharged.
Snowman9000
Explorer
Aug 13, 2013
Almot wrote:
Snowman9000 wrote:

The calculator I used accounted for both directions in the one way number.

http://nooutage.com/vdrop.htm

Southwire calculator shows 5.5% for 50ft #12. Assuming the Southwire is wrong, you have 2.5% voltage drop - this is what I got with your calculator for 25ft of "one way single set of conductors", whatever it means :)

Temperature drop is still a threat. But then, if it's more than 15%, you don't get 14.8V even with #10 wire.

Maybe the Southwire calculator is already accounting for the pair, too?

I'm not sure if I am talking about the same thing as you WRT temperature, but my controller is temperature compensating.
time2roll
Nomad
Aug 13, 2013
I would be tempted to go 14 but #12 should be fine to at least 50'

Since polarity is critical I would get some marked wire such as:

http://www.genuinedealz.com/
rexlion
Explorer
Aug 13, 2013
Go 12, you'll be fine. I rigged mine with common extension cord plugs between the panel and controller. That way I can plug together directly, or insert a 12 ga. extension cord of 25' or 50' (or both, but I can't imagine needing both). Worked fine for me this summer.
Almot
Explorer III
Aug 13, 2013
Snowman9000 wrote:

The calculator I used accounted for both directions in the one way number.

http://nooutage.com/vdrop.htm

Southwire calculator shows 5.5% for 50ft #12. Assuming the Southwire is wrong, you have 2.5% voltage drop - this is what I got with your calculator for 25ft of "one way single set of conductors", whatever it means :)

Temperature drop is still a threat. But then, if it's more than 15%, you don't get 14.8V even with #10 wire.
Snowman9000
Explorer
Aug 13, 2013
Almot wrote:
Snowman9000 wrote:
I've run the loss calcs but I don't know if the difference is significant going into the controller. Loss with #10: 1.7% or 0.3v. Loss with #12: 2.7% or 0.5v. So even with #12, that's 17.5v at the controller.

Aren't you forgetting that 25ft of tray cable means 50ft total? This is 3.3% loss with #10 cable, or 5.5% with #12.

The calculator I used accounted for both directions in the one way number.

http://nooutage.com/vdrop.htm
pianotuna
Nomad III
Aug 13, 2013
Hi Almot,

They are 18 volt panels. Panel ratings are done at 25 C (77 f)
Almot
Explorer III
Aug 13, 2013
With 5.5% loss on 25ft tray #12, the voltage will be 16, and the controller will taper it to 14.8. This is - at 70F. When it gets hotter, the voltage drops, and you won't have 14.8.