Sizing of cable from panels to battery bank

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Sizing of cable from panels to battery bank

Postby Hastings » Fri Oct 26, 2018 2:54 pm

Hi. Wondering who can help me.
Working out how big cable from panels to battery bank over 12 mtrs with a 60 amp controller next to panels.
5/250 watt panels divided by 18 v = 55 amps, yes?
So using an online wire calculator, I need 0 gauge cable. Yes?
If so, how do you connect 0 gauge into the very small controller?

Thank you
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Re: Sizing of cable from panels to battery bank

Postby jed » Fri Oct 26, 2018 3:15 pm

Hi Hastings,
You put the solar controller close to the battery so the 50 amp distance is very short.
If the controller is rated for 50amp then it will fit a suitable cable.

The sizing of the cable/s from the panels to the controller is a different question and depends on whether they are in series or parallel, the voltage/current rating of the panels, and the rating of the controller.
You also likely need to add fuses.

Suggest some reading at RPC website.
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Re: Sizing of cable from panels to battery bank

Postby davidg » Fri Oct 26, 2018 6:17 pm

AUS we use mm2 cables not AWG that's US.
Some car suppliers use AWG (US produced) but its wrong to use for electrical installations in AUS, even in cars now we are mostly in mm2 nowadays.

Best option for PV Solar wiring is run from the panel to an MPPT charge controller, which negates the need for extra low voltage panels (18V) which is really really old school and more expensive type of panels, the MPPT charge controller should be as close as practical to the battery/battery bank, under 2 metres ideally. Keeping in mind that once you exceed in most cases 3 parallel strings of cable to the MPPT charge controller each string needs to be fused/CB's for each cable run (AS4509 STD).

Based on this I'd use 16mm2 cable (biggest cable that fits properly anyway)from the MPPT charge controller through a suitable CB to the battery system, which is exactly what I did for a very remote installation recently, the V drop that system has is 0.1V at a good charge current being pushed into them (48V nominal pack).

Hastings wrote:Working out how big cable from panels to battery bank over 12 mtrs with a 60 amp controller next to panels. 5/250 watt panels divided by 18 v = 55 amps, yes?

Go for the biggest cables terminals will fit properly, is generally the best option.

Example from recent DC coupled SPS system

IMG_20181013_132153a.jpg
DC Switchboard
IMG_20181013_132153a.jpg (28.85 KiB) Viewed 509 times


IMG_20181013_134036a.jpg
General system overview, almost complete
IMG_20181013_134036a.jpg (22.85 KiB) Viewed 509 times


CB's for each string at the top of the DC SB and CB for the feed (Solar in) to the MPPT charge controller and then CB between MPPT charger controller (Solar out) and the battery pack.
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Re: Sizing of cable from panels to battery bank

Postby Hastings » Sat Oct 27, 2018 8:58 am

Thank you to Jed and Davidg for their information.
Davidg, What did you mean about 18v panels being really old. What v are they now.
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Re: Sizing of cable from panels to battery bank

Postby davidg » Sat Oct 27, 2018 10:36 am

Hastings wrote:Davidg, What did you mean about 18v panels being really old. What v are they now.

There not old, in terms of manufacture. There old in terms of design and more expensive per watt

You can use regular type panels like those used on house and commercial buildings in their place which run at higher voltages, less expensive per watt in comparison and are available in higher watt ratings.

To use them though you need to use a real MPPT charger controller in a DC based or coupled system as the voltage of lets say a single 275W panel will be approx 38-39VOC.

The advantage for the panel/string is they will start supplying charge earlier in the morning typically and will stop later around the end of the day and at more consistent higher currents as well, the way MPPT charge controllers work is vastly more efficient and more productive compared to traditional charge controllers, but they do require a voltage that is somewhat higher than the highest voltage the battery/pack will ever require in any situation to charge, it can be a lot higher in fact, for instance the system I posted about, is 3 panels per string giving an approx 117VDC VOC, the battery pack is a nominal 48VDC the Midnite Solar MPPT charge controller takes the voltage the current as it is and converts it the required voltage along with improved charge current to charge the battery at the voltage required and maximize the current as well. Typically being about 20% and in some cases might be 40% more efficient, the general suggestion though is 20 to 30% improvement. That's a big jump in performance for a charging system.

I use a smaller portable MPPT charge controller (100V max input) 40A max output with only 1 or 2 panels (38.9 VOC 250-270W) on a 12V 160AH battery the current one panel in good sunlight the charge currents it pumps out is impressive with programmed control. If I'm using 2 panels I have used them in series making the output over 70VOC the charge voltage and current is precise and not only charges the battery but also allows for loads connected it to be supplied effectively as though they are powered directly by the sun. Loads small inverter for computer/s and power tool chargers and also a couple of 12V power tool chargers, all sorts of other small loads as needed, beats the hell out of a generator in far away places.
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An OTT Genset for a house.
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Re: Sizing of cable from panels to battery bank

Postby Hastings » Sat Oct 27, 2018 2:34 pm

Thank you Davidg
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Re: Sizing of cable from panels to battery bank

Postby lantern » Mon Oct 29, 2018 8:50 am

2 x L x I x R ÷ A

L = Length of cable.
I = Current in Amps.
R = 0.017 resistance of Copper
A = Cross Sectional area of cable in (mm2)

Is what I have used in the past.
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Re: Sizing of cable from panels to battery bank

Postby Hastings » Wed Oct 31, 2018 6:54 am

Thanks Lantern
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