oversized wiring to charge controller

Cougarnewbie wrote:

Open ended plan to expand as as we can afford.

Panels are cheap now. It is easier to do things right the 1st time than changing/upgrading later.

Go by the check-list in 12thgenusa post.

Start from your energy audit - how many watt-hours or amp-hours you need daily.
Then estimate total panel wattage that you think you need now, and maximum wattage that you can squeeze in on that roof. Panels should be mounted at least 18" away from A/C and 6"-10" from vents, to avoid shading. On most rigs there won't be space for more than 800-900W.

You might need #4 or #12 wire, there is no way to tell until you go through that check-list. 900W array comprised of 24V panels in series will require a thinner wire than 500W array of 12V panels in parallel.

When you will know panels wattage, voltage, series or parallel, and panels location, - only then you will be able to estimate what wire you need.

In this order.

Here is a good voltage drop calculator if you don't have one. Voltage Drop Calculator

While you don't want to undersize there are things to consider beyond what is the biggest wire that can be used. There are good reasons why most controllers don't accept wire bigger than #6. Here are some things to consider.

How much real estate do you have for panels?
What is the max wattage you plan for the future?
Will it be series, parallel or S/P?
What is the wire length from combiner box to controller?
What type of controller will you use (PWM or MPPT)?
What is the max current/voltage the controller can accept?
What is the max controller output without clipping?
Will you use one large controller or two smaller ones?
How many connections will there be in the circuit?
Difficulty in routing #4 wire as well as increased cost.

At some point the law of diminishing returns takes over. When other resistance points (connectors, splices, fuses, switches) in the circuit contribute as much or more than the wire, it makes little sense to continue to lower the resistance of the wire. Is a .25V drop worthy of pursuing over a .5V drop?

My real estate is maxed out with two 24V panels, 185W each. Actual voltage in warm temps is about 33V. I used #8 wire which was overkill as I could double the amount of panels if I had the room and still be close to 1% drop.

Almot wrote:
Still remains to be established whether they NEED #4, or even #6 :)

Open ended plan to expand as as we can afford.

rjxj... When V drop is under 1% already with cable #10, there isn't much reason for going higher. People may want to go higher, yes. They sometimes (or often?) want something that they don't need.

I have a feeling that we don't know what the OP really needs. SMK have found a controller that accepts wire as big as #2, it's a good news, now they can run #2 :)

Almot wrote:

rjxj wrote:

While buying parts from the solar dealer in Yuma he asked if we wanted him to calculate it for minimum volt drop. I said absolutely, you are the solar man. He said 4 gauge. He said he always recommends shooting for the lowest v drop and best practices in connections.

Dealer is never a "solar man", he is a "seller man" :D

"shooting for the lowest V drop" - yeah, sure. The lowest drop would be 0.000000000...1%, doesn't mean that everybody has to install a cable as thick as his hand.

Example. 2*24V panels in series, 500W total, 30ft cable to controller. V drop in cable #12 = 1.5%, in cable #10 = 0.9%. This is at peak current 8.5A. As RoyB noted, you don't always get max current stated in the specs (if ever), so the daily difference in solar harvest btw #12 and #10 would be 0.3 AH or less. Conclusion: #12 is enough, #10 is plenty and leaves some margin to add one more panel in future. There is no reason to run #6 or #4, in this case. For future expansion - add another 24V panel in series, with close Vmp and Imp, and go 24V bank if the controller is maxed up. Things should be planned for.

The OP is at the planning stage. Panels will be purchased at the same time - at least, for initial setup. Panels number, wattage and voltage will determine the wiring and the controller model. Now, WHAT panels and HOW MUCH wattage - I asked this and got no answer. There isn't much to suggest at this point.

No, he's a pretty good solar dealer and just going by a chart. It seems common for people to ask about going to the higher side on wiring mainly just not settling for the common 3 % loss. Never worked with anything over 4/0 or seen anyone solar system with hand sized wire. Must be something new. Thanks

Morningstar has #2 connector :B

http://www.morningstarcorp.com/wp-content/uploads/2014/02/TriStar-Datasheet-English.pdf

rjxj wrote:

While buying parts from the solar dealer in Yuma he asked if we wanted him to calculate it for minimum volt drop. I said absolutely, you are the solar man. He said 4 gauge. He said he always recommends shooting for the lowest v drop and best practices in connections.

Dealer is never a "solar man", he is a "seller man" :D

"shooting for the lowest V drop" - yeah, sure. The lowest drop would be 0.000000000...1%, doesn't mean that everybody has to install a cable as thick as his hand.

Example. 2*24V panels in series, 500W total, 30ft cable to controller. V drop in cable #12 = 1.5%, in cable #10 = 0.9%. This is at peak current 8.5A. As RoyB noted, you don't always get max current stated in the specs (if ever), so the daily difference in solar harvest btw #12 and #10 would be 0.3 AH or less. Conclusion: #12 is enough, #10 is plenty and leaves some margin to add one more panel in future. There is no reason to run #6 or #4, in this case. For future expansion - add another 24V panel in series, with close Vmp and Imp, and go 24V bank if the controller is maxed up. Things should be planned for.

The OP is at the planning stage. Panels will be purchased at the same time - at least, for initial setup. Panels number, wattage and voltage will determine the wiring and the controller model. Now, WHAT panels and HOW MUCH wattage - I asked this and got no answer. There isn't much to suggest at this point.

Almot wrote:
Still remains to be established whether they NEED #4, or even #6 :)

I rewired a friends rig that's running an Blue Sky mppt 30 amp 3024I. I figured 4 gauge because it was 20 ish feet. While buying parts from the solar dealer in Yuma he asked if we wanted him to calculate it for minimum volt drop. I said absolutely, you are the solar man. He said 4 gauge. He said he always recommends shooting for the lowest v drop and best practices in connections.

Yes his mppt is wired in parallel. He has mismatched panels so he cant run series. Well, he could but he would only be able to run his two 140's in series so as to stay under 46 volts. He also has a third 190 watt panel so all three are wired in parallel so he doesn't go over max voltage in series. He's still getting more by running it as pwm vs mppt on only 280 w. If he wanted to buy a 4th panel it would be even more of a mess as he couldn't series parallel them. From what I can tell in reading various sources it seems even more important to choose ALL the panels on day one when running mppt. I would also go with a high voltage mppt if using mppt. His system grew over time with poor guidance in the beginning. My friend was never shy with spending money but the original dealer installed 3 agm batteries (300ah), Magnum 2800 watt inverter with charger and ONE 140 watt panel and told him he'd be good. :)

Then someone told him to add a panel and they did another 140 in parallel. It was still wimpy so he added an 190 and the blue sky. Instead of the 190 they should have added 2 more 140's in series par for two strings of 280 at 40ish volts per string. While all that was going on it was wired with 6 g and several splices and a nice 14 g wire fuse holder that could also serve as a mini hand warmer. So he skimped and avoided his coffee maker etc.

Resistance= Distance, conductor size, and temperature of conductor, temp being the least of our worries.

Still remains to be established whether they NEED #4, or even #6 :)

I used #4 AWG that was a multi-strand welding wire. At the controller to get it to fit, I just cut a few strands out, twisted it, and stuck it in. Works fine.

Mike