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bdb2047

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Solar is new to me also new to fourm. I have done power distribution and a/c wiring for my past career 40 years. DC has me reading a lot. I am planning to put 4 120w panels on roof I am thinking to series them 2 together and parallel the 2 sets. Low voltage panels. I plan to run wires from panels to inside,5' #10, and parallel them run larger wire,#4 15', to MPPT controller. #2 to batteries,315AH AGM 4'.Charge controler is 60A, inverter charger 2000w pure sine. Does this seam like a reasonable approach

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You read your AWG charts... :D You're hitting your minimums for the distances you need to run. My only recommendation would be to go a bit heavier on your wiring. Especially on your 15' and battery runs. I tend to encourage folks to size their wiring for the maximum capacity of their controller rather than what they currently have sitting up top. In this type of 24v system I would tend to go more with say.. #2 up top, and 2/0 for the battery connections. You didn't mention how far of a run you are looking at for your inverter/charger.

 

Jack Mayer has some solid information on wiring.

 

I know that heavier gauge can get spendy, but it really does pay for itself. You'll have increased efficiency, less heat, and the ability to expand your system.

 

Everything else looks really solid. Not a single suggestion on that front. You're not playing around. :lol:

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BDB, look at the math. 480 watts at 24 volts (two sets of 12 volt panels in series) computes to 20 amps even though charging voltage is higher which would reduce those amps and the actual charging is likely much less due to the sun and angle etc etc. Therefore if you use 30 amp rated wire and conduct 20 amps max over say even 20 feet I dont envision there being any significant voltage drop. HOWEVER SURE BIGGER WIRE REDUCES VOLTAGE DROP PLUS ALLOWS FOR FUTURE EXPANSION. All Im saying is if you run 20 amps MAX of current in 30 amp rated wire over 20 feet the voltage drop is small BIT I DIDNT LOOK IT UP SO NO WARRANTY. This is for wiring at 24 volts from panels down to charge controller ONLY. Sure bigger is better so look at what the Solar manufacturers recommend NOT my calculations as voltage drop may be wayyyyyyyyyy more important meaning you need much bigger wire then 30 amp rated even if 20 is you max amperage and you run 20 feet.

 

For a 2000 watt inverter at 12 volts, that computes to 167 amps HOWEVER figuring less then 100% efficiency and the 125% rule, from the batteries to the inverter I would use wire rated for 225 to 250 amps depending on the length.

 

NOTE Im long retired from electrical engineering practice and more familiar with AC then DC distribution so voltage drop may be more important then my calculations above in which case MORE then 30 amp rated wire may be necessary for conducting 20 amps over 20 feet (panels to charge controller) BIGGER IS BETTER AND REDUCES VOLTAGE DROP although 30 amp rated wire can sure handle 20 amps and dissipate the heat and not yield any great voltage drop over 20 feet.

 

John T Tooooooooooo darn long retired EE and rusty as an old nail so there lol

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Very good start. In fact it may be the "finish".... :)

 

Just run the numbers through the voltage calculators to predict voltage drop. On a high voltage system you can definitely run smaller wires. The "big" wires come into play on low voltage systems. I also recommend wiring for your equipment capacity, not just what you have now.

 

The numbers don't lie....just run them through the calculators. Make sure you use the RIGHT numbers, though. I have some samples on using the calculators on my website.

 

Also, remember if you want to be NEC compliant you need to calculate using 125% of your rated power.

 

Keep your inverter-to-battery run as short as practical. No more than 10' MAX. And follow the manuf. wiring directions and you will be fine.

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OKAY, I just arrived at SKP in Wauchula Florida, have smoking hot internet and time to kill, so I will do as Jack suggested and crunch some numbers for you.

 

1) Two 120 watt panels in parallel = 240 watts at 12 volts = 20 amps max ALTHOUGH the charging voltage will be somewhat higher and the full wattage will likely never be realized BUT LETS USE THAT ANYWAY TO BE CONSERVATIVE.

 

2) Once you complete the series/parallel wiring and have 480 watts at 24 volts you again have only 20 amps max current.

 

3) Five feet of 10 AWG Wire conducting 20 amps drops 0.20 volts (one set of 12 volt panels to junction)

Fifteen feet of 4 AWG wire conducting 20 amps drops 0.15 volts (24 volt junction to MPPT Controller)

Ten feet of 2 AWG wire conducting 20 amps drops 0.06 volts (MPPT to batteries) I guessed at the 10 feet

TOTAL VOLTAGE DROP IF YOU ACTUALLY CONDUCTED 20 AMPS 0.41 Volts

 

4) Engineering comments: Since the five foot run using 10 AWG wire creates the largest of the 3 drops, you might consider increasing that wire size!!!!!!!!!!!!!!!!!!!!!!!!

 

5) NOTE you are dealing with a few DIFFERENT PARAMETERS HERE. A) The wires Ampacity tells you how many amps the wire can conduct and dissipate the heat generated so as to NOT degrade or damage the wire. B) Voltage drop tells you how many volts are dropped across the wire run. C) When you select the wire size you should size it 125% larger then the maximum continuous load HOWEVER even using 30 amp rated wire that can still handle 24 amps and since you're at 20 max, theres no problem.

 

6) I Already addressed the 2000 watt inverter cable and suggested using wire having an ampacity of 225 amps subject to length and actual loads.

 

DISCLAIMER I did this pretty quick using an on line voltage drop calculator SO NO FREAKING WARRANTY lol

 

BOTTOM LINE if you actually had a 0.41 voltage drop (but that's a worse case scenario and you will be less) at say 28.4 beginning charging volts, that still amounts to 28 at the charge controller so your suggested wire sizes WILL STILL SUFFICE QUITE WELL although SURE bigger is better and if you increase wire size you reduce voltage drop but there's also practicality and size and cost to consider. HOWEVER there's absolutely no problem as far as the wires ampacity as 10 gauge wire can handle 20 amps and NOT overheat BUT Voltage drop is another issue. That's why I noted of the three drops in your system, the 5 feet run of 10 gauge is the largest, although it will still suffice.

 

NOW ITS YOUR CALL not mine, like Fox News, we report, you decide lol

 

John T

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Thanks for the reponse. I now have panels on roof starting to wire it. I am going to run #10 from each set of series panels into camper then parallel them to larger wire. Plan to install a manual reset breaker inside camper in with the parralel #10 out with #4or 2. I am thinking 30a here and 70a on battery side of MPPT. Thanks again

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John, I really appreciate you crunching the numbers here, but I guess I want to reiterate that the "backbone" wiring really should be gauged at the system capacity (at the 125% rule) rather than what he has up top at the moment. It's certainly not a "golden rule", but it makes a lot of head sense. From the system he outlined, he's looking at a good.. what?.. $3500? So holding back $30-$50 on the wiring doesn't really equate in the long term scheme of things. Although.. $40 is $40.

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A couple of minor points.

 

  • You need to use the proper numbers when calculating voltage runs. Use the Isc rating of the panel, and use it appropriately for series/parallel setups. It makes a difference.
  • Make sure you account for the 125% for NEC
  • Use Vmp for calculations as appropriate. Not nominal 12 volts.
  • Make sure you use the boosted current from the solar controller....I always calculate with the max output of the controller. Even if initially you do not produce this. It is better to wire for that now,than have to rewire later.
  • Use 13.4 as your lowest voltage run. 14.8 as your highest. With a 12 volt nominal system. Unless you are using Lithium batteries.
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Yarome, fun chat, Yep I figured if he has 480 watts which corresponds to 20 amps at 24 volts series/parallel and if you use the 125% rule (how I always designed per the NEC) for 30 amp rated wire that's still 24 amps, so 20 amps max is well within specs of 10 AWG wire ampacity. BUT if you wanted to allow for expansion and since he has a 60 amp charge controller and if he added yet even more panels, then I would have to re calculate wire sizes and voltage drops, but I only answered his question based on what he asked and the equipment he currently has. Heck if its his money we can advise him to use much much bigger wire lol

 

If I planned a future upgrade I would go ahead and use bigger wire now then having to change it all over grrrrrrrrrrrrrrr but I just answered his question based on the info and sizes he quoted. Also I purposely used conservative figures even though due to inefficiency and the suns strength and its angle I doubt he would ever achieve 20 amps plus the charging voltage would be greater then 24 (perhaps 26 to 29+ volts which also reduces current, making the 20 amps I used as a baseline even more conservative. His actual max current might not be much over 16 to 18 amps in which case the voltage drop is reduced and that 30 amp rated wire is even more sufficient?

 

BUT I WAS ALWAYS A CONSERVATIVE DESIGNER and bigger is better (less voltage drop) and theres no harm in allowing for future expansion, so what he proposed will work (if my 0.41 volts max drop calculations are correct) but if he wants to add capacity in the future he might want to use bigger wire now and be done with it.

 

 

 

Jack, yep the figures I used as a baseline are purposely CONSERVATIVE and I doubt he will realize 20 amps, and at higher then 24 charging volts (more like 26 to 29+) current is reduced even more, SO my voltage drop calculations are MORE then what will actually be present. I just don't like to design right on the borderline so I used 20 amps and 12 or 24 volts ON PURPOSE.

 

Thanks yall, very fun chattin.

 

John T

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You really have to admit though.. in all other respects.. the OP has done his homework. I would not fault him a cent. We can nit pick but as long as you are following the %125 rule... you're golden. Your core systems are right on track and pretty much the BMW as far as charging systems go.

 

Good on ya ~Cheers

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The "design methodology" we used at our shop was strictly NEC where we 1) Computed the max continuous load,,,,,,,,,,,,,2) Sized the conductors for a minimum ampacity of 1.25 times that,,,,,,,,,,,,3) Calculated the Voltage Drop and increased the conductor size IF NECESSARY (as sometimes necessary on motor branch circuits) 4) Selected the overcurrent protection device to protect the conductors,,,,,,,,,,,, IN THAT ORDER. Ya know I kind of miss those days, now I can talk the talk but I no longer walk the walk lol

 

John T

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