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Old 10-10-2012, 09:58   #286
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Re: Simple Solar For A Simpleton

Sorry Vic, but its the combined distance, there and back, calculated for the desired voltage drop.
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Old 10-10-2012, 10:19   #287
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Re: Simple Solar For A Simpleton

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Sorry Vic, but its the combined distance, there and back, calculated for the desired voltage drop.
Thanks... I was afraid that would be the answer.

The panel is a single 320W, Vmpp 54.7, so 5.8 Amp?

On the West Marine catalogue chart, 5.8 amp with a 90ft length makes #12 wire look ok when shooting for a 3% drop, but the wisdom here would be advocating much bigger wire. What am I missing?

Vic
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Old 10-10-2012, 10:28   #288
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Re: Simple Solar For A Simpleton

Vic,

You're not missing anything, this is the advantage of the higher voltage panels, you can run smaller wires for the same power level because the currents are lower.

Since you're feeding into a voltage converter (the MPPT controller) you can also live with a larger voltage drop than a direct feed system. You might not want to because the voltage drop represents power you are giving away (heating your wire instead of feeding your batteries), but everything should work OK. If you lose 1.5V (about 3%) at 5.8A you're losing 8.7W at full power. You could reclaim some of that by running 10AWG. Is it worth it? That's more your call.

[Edit] Just ran the circuit through a voltage drop calculator, got 0.9V drop for your circuit, ~5W at maximum power. If you stepped up to #10 you'd be looking at 0.55V drop and ~3W loss, so a difference of 2W. [/Edit]
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Old 10-10-2012, 11:10   #289
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Re: Simple Solar For A Simpleton

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Vic,

You're not missing anything, this is the advantage of the higher voltage panels, you can run smaller wires for the same power level because the currents are lower.

Since you're feeding into a voltage converter (the MPPT controller) you can also live with a larger voltage drop than a direct feed system. You might not want to because the voltage drop represents power you are giving away (heating your wire instead of feeding your batteries), but everything should work OK. If you lose 1.5V (about 3%) at 5.8A you're losing 8.7W at full power. You could reclaim some of that by running 10AWG. Is it worth it? That's more your call.

[Edit] Just ran the circuit through a voltage drop calculator, got 0.9V drop for your circuit, ~5W at maximum power. If you stepped up to #10 you'd be looking at 0.55V drop and ~3W loss, so a difference of 2W. [/Edit]
Thanks a ton. That's just saved me a difficult job.
I'll run with the #12 that's already in place to the controller, and use heavier gauge from the controller to the batteries.

Now I've just got to figure the distance and wire size for the 2 140W panels on the doghouse.

Vic
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Old 10-10-2012, 11:21   #290
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Re: Simple Solar For A Simpleton

Just 45 ft. What wattage are the panels?
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Old 10-10-2012, 11:31   #291
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Re: Simple Solar For A Simpleton

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Just 45 ft. What wattage are the panels?
320 W, Vmpp 54.7, Amp 5.8

I wasn't sure if the usual calc both ways for a circuit with a load, would be applicable for a solar panel considering it's a power generator and not a 'load'..... maybe?

Vic
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Old 10-10-2012, 11:58   #292
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Re: Simple Solar For A Simpleton

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Originally Posted by Dsanduril View Post

[Edit] Just ran the circuit through a voltage drop calculator, got 0.9V drop for your circuit, ~5W at maximum power. If you stepped up to #10 you'd be looking at 0.55V drop and ~3W loss, so a difference of 2W. [/Edit]
I make it 1.7v (@5.8A) or 9.9w with 90 feet of 12 gauge.
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Old 10-10-2012, 12:10   #293
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Re: Simple Solar For A Simpleton

I used #10 on mine running 35ft. It wasn't too overkill. All of the 3 panels wiring meet at a terminal block where I have a 4 ft. run of # 6 to my MPPT. Then a red and black #4 over to my battery posts.
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Old 10-10-2012, 12:51   #294
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Re: Simple Solar For A Simpleton

The load is at the other end Vic.
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Old 10-10-2012, 14:08   #295
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Re: Simple Solar For A Simpleton

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Originally Posted by VirtualVagabond View Post
Thanks a ton. That's just saved me a difficult job.
I'll run with the #12 that's already in place to the controller, and use heavier gauge from the controller to the batteries.

Now I've just got to figure the distance and wire size for the 2 140W panels on the doghouse.

Vic
how old is that 12# wire that is in place?
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Old 10-10-2012, 15:03   #296
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Re: Simple Solar For A Simpleton

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how old is that 12# wire that is in place?
It was put in place 3 years ago, none of it exposed to UV because it just goes to the base of the davits and is all internal.
It looks like new.

Vic
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Old 10-10-2012, 15:58   #297
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Re: Simple Solar For A Simpleton

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It was put in place 3 years ago, none of it exposed to UV because it just goes to the base of the davits and is all internal.
It looks like new.

Vic
lucky you! the previous owners must have done it with panels in mind!
that will save a lot of expense and effort
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Old 10-10-2012, 22:53   #298
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Re: Simple Solar For A Simpleton

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Originally Posted by VirtualVagabond View Post
Now I've just got to figure the distance and wire size for the 2 140W panels on the doghouse.
If I recall your proposed system you're talking about going essentially direct with the 140's. That means you'll be running about 10A from each panel. You'll lose more than half a Volt/about 5% with #12 even on just a 20' (one-way) run. I suspect you'll be looking at #10 or #8 if you want to avoid the losses. If you run both panels on a single pair you're looking at 20A and might even end up at #6.

And that shows the difference between the high voltage panels and the lower voltage one.
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Old 14-10-2012, 15:49   #299
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Re: Simple Solar For A Simpleton

Don't forget the future, Vic. You might want to add some more panels one day, so with the two 140s going direct, you should go with at least #6. It is easier to set up for future higher capacity now. (Our three 230 W panels go direct and we use marine #4 AWG to run them straight to the batteries.) Lower V in parallel means higher A which means bigger cables.
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Old 14-10-2012, 16:34   #300
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Re: Simple Solar For A Simpleton

I don't want to read 285 posts on the off chance that I would learn what wattage you have settled on. Some basic advice - for longer runs of wiring use higher voltage. I think you've settled on an MPPT controller. If you can run your panels in series and run a higher voltage between the panels and the controller then you can get away with smaller gauge wire - much smaller if you go high enough on the voltage. You can run multiple smaller gauge wires to accomplish the same effect as a single larger gauge wire. Its the cross sectional area of the wire that matters so you have to do a bit of math but you can figure it out.

There's any number of voltage drop calculators on the internet but many of them aren't clear about whether you need to input the roundtrip wire length or the single distance length. I like American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire strength That page also has the wire cross section information that you will need to determine what you can do if you choose to use multiple smaller conductors. Personally I'd want to keep my voltage drop well under 10% but YMMV.
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