Copper vs Aluminum cable

kaipo_boy

Sorry if this is old news, but I looked on the 'net and couldn't find the answer.  The cost of a longish 25ft run of 4/0 copper cable is pretty serious, so I'm thinking of using aluminum cable instead. How much would I have to increase the alum cable diameter by to get roughly the same loss rates as the 4/0 copper?

vtmaps

Usually aluminum needs to be the next thickest size... I don't know if you can get anything thicker than 4/0, but if you can get it, it will be very difficult to bend.  In order to avoid fire, you will need special (expensive) adapters to go from copper to aluminum, and from aluminum back to copper. 

--vtMaps

Raj174

Hi kaipo_boy,

Try this voltage drop calculator.. http://www.calculator.net/voltage-drop-calculator.html
Just plug in your numbers and choose copper or aluminum.
Can I ask what you are connecting with this run?

Edit....My bad, this calculator doesn't list cable bigger than 4/0 AWG.  

mike95490

It's fine to use the aluminum wire for the AC runs (I've got a 980' run of #6) but for DC, all the battery terminal lugs are only rated for copper.
The Rule of Thumb to replace copper with aluminum, is 2 gauges larger.  If you use #6 copper, aluminum requires #4 wire.

Fro 25' you need to re-design the layout of things to be closer together.  There has been some success using flattened copper water pipe as a bus bar.  4/O copper would translate to needing 6/O aluminum.  (note it is 4/ capitol O not a zero)

BB.

Is there anyway you can change the setup (i.e., shorten cable run, run higher voltage/lower current, etc.)?

-Bill

kaipo_boy

Unfortunately, I don't think so.  An obvious way to shorten the run would be to mount the panels on my roof, but the problem with my roof is, it is around 18 years old but is a type of composite shingle (supposedly 30 year life when they sold it to me) and the maker has since gone defunct. I didn't know they went under until I asked around for spare shingles recently (was thinking of mounting the panels up there and wanted spare shingles in case I cracked a few) and found out that their other roofs were cracking and some lost shingles in the wind or something; or water swelling problems, I'm not too sure which; there were a lot of lawsuits and he went under.  I only found out a little while ago... but, long story short, the roof shingles are not likely to survive if I go up there and walk on them to do a PV install. So I have been clearing weeds and making ground panel mounts .   I suppose I COULD mount it closer, but that would be good arable land that I wanted to start growing food crops in... I've gone vegan in my old age, I'm afraid (not due to animal rights awareness... I love animals, but I also love hunting). THAT's a whole 'nother story

BB.

The standard suggestion would be to wire Vmp to a higher voltage and get a (not cheap) MPPT solar charge controller. Then you can use much thinner wiring.

That is usually the best solution. But if cost/roof damage is a major issue, you have to make your choices.

-Bill

kaipo_boy

Ah, so that is why people select the Midnite Classic 200 instead.... i'll play with that sizing tool a bit to see if I can run more than 3 in a series string...

kaipo_boy

Well, I currently have 3 of these same panels (trina 285w, Vmp is supposed to be 35.6v each, but with 3 in series I sometimes see, in bright sun, 112v DC going into my Kid controller).  So, if I have 9 panels, 3 strings of 3 in series, I should see at max 112v and roughly 82A DC input into the Classic. If betting on close to a 25ft run, what would be a good wire size in copper and in alum for that DC run from the panels to the Classic?

vtmaps

kaipo_boy said:

Ah, so that is why people select the Midnite Classic 200 instead.... i'll play with that sizing tool a bit to see if I can run more than 3 in a series string...

As I mentioned to you in another thread, the Classic becomes less efficient (makes more heat) as you raise the input voltage.
That also means that the capacity (both amps and watts) of the Classic is reduced at higher input voltage.

The Midnite calculator will allow you to raise the voltage of the string to the limits... but it is not good optimal design to operate at the limits all the time.

For example (on a 24 volt system) a Classic 150 with an input voltage of 90 volts can put out 94 amps (2700 watts), but a Classic 200 with 140 volt input can only put out 72 amps (2066 watts).   Either Classic will run hot when operated at its limits for sustained periods.

--vtMaps

mike95490

kaipo_boy said:

Well, I currently have 3 of these same panels (trina 285w, Vmp is supposed to be 35.6v each, but with 3 in series I sometimes see, in bright sun, 112v DC going into my Kid controller).  So, if I have 9 panels, 3 strings of 3 in series, I should see at max 112v and roughly 82A DC input into the Classic. If betting on close to a 25ft run, what would be a good wire size in copper and in alum for that DC run from the panels to the Classic?

285W @ 35v = 8.1A   x 3 strings = 25A max for your high voltage DC run from PV array to the charge controller,  The big amps come OUT of the controller and toward the battery, not at the PV side.

kaipo_boy

mike95490 said:

285W @ 35v = 8.1A   x 3 strings = 25A max for your high voltage DC run from PV array to the charge controller,  The big amps come OUT of the controller and toward the battery, not at the PV side.

Argh! Sorry, yes you're right of course, Mike. Thanks for catching that.  27A is the number that stuck in my head because that's the number i'm always watching from my current single string from Kid into the batteries. Thanks! But its amazing how little I see that, so the other point brought up by VTmaps, of operating too often near the limits doesn't bother me much.... too many clouds here. Of the 10 months or so I've been running this sytem, I've probably seen that 27A limit a half dozen times, and then not for very long.
So I should be asking for wire diameter for a 25ft long run, 112v @ 25A... (not 82A)

mike95490

>  So I should be asking for wire diameter for a 25ft long run, 112v @ 25A... (not 82A)
And that is going to be easily affordable.  10 ga is safe to 30A, and 8 ga will have a little bit of loss, but still be affordable copper..  Only consider alu wire for standard 120VAC wiring, because lots of connectors, outlets, breaker terminals and such are rated Cu/Al   Very little stuff on the DC side of things carries Al rating

kaipo_boy

Thanks Mike! I think I'll go up one more to be safe, so shooting for 6awg!

mike95490

#6 will be overkill, and much harder to pull and route (quite stiff) but check them both out at the store.   You will SELDOM be pumping the full 25A through the cable (only on a foggy day if it clears right at noon)

The run from the controller to the battery is MUCH more critical, and effort there will have the most payoff in tuning your system

Don't forget the 3 proper sized overcurrent protection at your combiner box (I prefer DC rated breakers for ease of troubleshooting)
and a breaker between the controller and the battery (close to the battery)