New to Solar retired and now just reading and learning

[wmgeorge]

Thank you for your concern but I do have a current 2020 Code book handy. Since this a only a back up system my Charge controller could be at the panels and perhaps the batteries and inverter. I can configure as needed.

 

[Hedges]

Charge controller is best near batteries, so its regulated output voltage doesn't have drop to the battery. PV input can tolerate arbitrarily high voltage drop, given sufficient headroom; only lost power, no impact on regulation.

Most people try to follow the couple percent drop used for AC, but I ignore that. To me, it is just a trade-off of copper wire cost vs. silicon PV panels. In my case, nominally 380Vmp, 480Voc PV strings individually wired with 12 awg, up to about 200' runs. My drop isn't much, but for someone with panels far away I suggest long skinny wires.

With 12V battery and 120VAC inverter, less loss if inverter is also at the panels. Unless you need particularly high surge from it.

 

[wmgeorge]

Well in a lucky find yesterday cleaning out my garage I found some 4 conductor 14 gauge THHN/THWN Direct Burial Sunlight resistant cable. I more than likely do 3S2P now and bury the cable. Put the charge controller next to the batteries in the garage. OCV is 24.4 , under load its 20.4 vdc.for each panel.

 

[WYtreasure]

Well in a lucky find yesterday cleaning out my garage I found some 4 conductor 14 gauge THHN/THWN Direct Burial Sunlight resistant cable. I more than likely do 3S2P now and bury the cable. Put the charge controller next to the batteries in the garage. OCV is 24.4 under load 20.4 vdc.for each panel.

You're going to spoil your solar system with nice wire like that.

 

[wmgeorge]

You're going to spoil your solar system with nice wire like that.

After thinking back I know it came from, when I installed my Mini Split, I never paid attention to that wire because I ran it in Sealtite anyway. A person should clean his/her shop at least once in twenty years! I found a lot of other stuff I had forgotten I had or it was under or inside of something else!!

BTW do you have any Prairie Dog hunting out your way?

 

[WYtreasure]

After thinking back I know it came from, when I installed my Mini Split, I never paid attention to that wire because I ran it in Sealtite anyway. A person should clean his/her shop at least once in twenty years! I found a lot of other stuff I had forgotten I had or it was under or inside of something else!!

BTW do you have any Prairie Dog hunting out your way?

Shirley you jest. It's the next best thing to tipping cows.

 

[onokai]

I quit burying cable after a gopher chewed thru one. Now it's conduit always and if if its a long run I use the thinner direct burial conduit the low cost kind bit it cannot see daylight so the schedule 40 or 80 needs to come up from that.Its all gopher proof-maybe even prairie dog proof?

 

[wmgeorge]

No gophers or PDs not that long a run. But in spite of cold and wet Spring we are dry. Not looking forward to digging.

 

[onokai]

No gophers or PDs not that long a run. But in spite of cold and wet Spring we are dry. Not looking forward to digging.

Rest the trencher my friend save your back-or if its short run use the 3 inch trenching shovel They are hard to find but I have a 3 inch a 5 inch and a 6 inch one. The 3 is the ticket

 

[Hedges]

Well in a lucky find yesterday cleaning out my garage I found some 4 conductor 14 gauge THHN/THWN Direct Burial Sunlight resistant cable. I more than likely do 3S2P now and bury the cable. Put the charge controller next to the batteries in the garage. OCV is 24.4 under load 20.4 vdc.for each panel.

Four identical wires, or one bare copper meant for ground?
Will the "2p" occur at far end and all current through one wire pair, or do you plan for each 3s string to have its own two pair?

Your Voc will be 75V under nominal conditions, maybe 85V in extremely cold weather. Not as high as mine, but not something I'd want to contact while standing on wet ground. Grounding the array/frames is for possible breakage or leakage of the panels (some have been known to develop such defects.)

Maybe you've got enough wire for 2 runs.
I used rigid conduit, surface-mount where I could get away with it and in a trench where it crossed open areas.

 

[wmgeorge]

The NEC says I should allow + 22% for my expected cold weather. Yes its a 4 wire insulated cable as I said. The series / parallel will occur at the panels, where else? Its required by Code that frames and metal mount be grounded as I plan to do.

 

[Ranger Rick]

Well since I am putting the panels in parallel the voltage will be the same. Is that your question? Its a MPPT controller 40 amp rated. If I was building a 20 kW system I would chose another way to go. This is only a backup system.

Whether a system is a 600w simple backup system or 20,000w system, panel efficiency and a solar charge controller's ratings must be followed. A 6P arrangement has its problems with fusing and more. IMO, a 3S2p is the best combination of your PVs.

 

[wmgeorge]

Whether a system is a 600w simple backup system or 20,000w system, panel efficiency and a solar charge controller's ratings must be followed. A 6P arrangement has its problems with fusing and more. IMO, a 3S2p is the best combination of your PVs.

And Ranger Rick that's what I am doing....

 

[Hedges]

The NEC says I should allow + 22% for my expected cold weather. Yes its a 4 wire insulated cable as I said. The series / parallel will occur at the panels, where else? Its required by Code that frames and metal mount be grounded as I plan to do.

My panels are in series at the array, of course. 24x 12V panels in series, 12x 24V in series, 8x 36V in series. Each series string has a pair of 12 awg for home run to inverters or combiner box.
When I first installed it, my inverters were 2500W so each got a single string, about 3000W STC of panels.
Since I swapped for higher wattage inverters, multiple strings are paralleled either in a combiner box after fuses, or at an attached disconnect switch/fuse box on the inverter.

Having the separate runs let me rearrange easily. In particular, although manufacturers used to say all panels feeding a single MPPT had to be same orientation, it is fine if multiple strings are different. 3000W || 3000W || 3000W STC rated panels is a good fit for a 5000W to 6000W PV inverter (or SCC) if the strings are oriented differently.

+ 22% is conservative. For my panels with -0.4%/degree C temperature coefficient, -15 degree record cold vs. +25 degree nominal would mean +16% rise in Voc. I don't remember a specific percentage in the NEC but I only look at parts of it. We either use a conservative percentage or calculate more precisely for a given location and set of hardware it it is close.

What the code does call for is sizing wires and breakers 1.56x Isc. That's the usual 1.25x to avoid nuisance tripping, times 1.25x for cloud effect. Full sun on panels plus additional light reflecting off clouds and falling on panels. If 14 awg has 15A ampacity (or rather allowed limit of current and breaker size), an array 3s2p feeding through that wire should have panels with Isc no greater than 4.81A. Of course, 14 awg (with 90 degree insulation and operating at 30 degree ambient) actually has 25A ampacity so works fine. Code just has a more stringent limit.

Ground also is to be same gauge, because it has to carry current continuously in case of a fault with no breaker tripping. In the case of small gauge like 14 awg, I think ground is same as the others so you're good there.

I normally buy 12 awg and 250' rolls, but prices were up so much that recently I bought 50' 14/2 + ground UF to hook up a yard light. Wire still cost more than the light fixture.

In the case of my PV system, conduit fill is so high the 30A ampacity of 12 awg derates to about 12A.

 

[Hedges]

And Ranger Rick that's what I am doing....

I missed that was his actual user name, thought you were just being snarky. (maybe "also" not "just")?

 

[Ranger Rick]

And Ranger Rick that's what I am doing....

A wise choice and a very wise change from 6P.

 

[wmgeorge]

You folks are missing this is 90C wire and direct burial allows even more amps. Got it covered...

 

[Hedges]

I'm familiar with the bundled wire tables and single wire ampacity in free air, also ambient temperature adjustment going up to 1.04 for cooler ambient. I've found a couple references to direct burial that are for higher voltages.

Can you point me at a link that addresses our sort of application?
Not that I can't imagine cool thermally conductive earth helping with heat dissipation, but I'm not sufficiently versed in NEC.

Thanks.

 

[LywWyr]

Table 310.16 covers not more than 3 current carrying conductors in a raceway, cable or earth. (Direct buried).

 

[wmgeorge]

Table 310.16 covers not more than 3 current carrying conductors in a raceway, cable or earth. (Direct buried).

I am only using 2 current carrying, and one more for equipment ground.

 

[LywWyr]

I'm familiar with the bundled wire tables and single wire ampacity in free air, also ambient temperature adjustment going up to 1.04 for cooler ambient. I've found a couple references to direct burial that are for higher voltages.

Can you point me at a link that addresses our sort of application?
Not that I can't imagine cool thermally conductive earth helping with heat dissipation, but I'm not sufficiently versed in NEC.

Thanks.

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