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Old 08-08-2014, 14:05   #1
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Cable Size

I am installing three solar panels, two 135 watt at 17.9 volts and one 100 watt at 17.5 volts. All connected in parallel. I am assuming parallel is the way to go. Same manufacturer new panels, just need the 100 to fit into my bimini nicely.

Mid Nite Solar Kid MPPT controller states to size the wire at less than 2% voltage drop.

At a run of 23 ft my calculations come out to 4 awg cable. This seems heavy however the voltage loss on 6 8 or 10 reduces the voltage more than 1 volt at 20 amps.
Am I missing something or is 4 the way to go

Fletch
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Old 08-08-2014, 14:15   #2
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I could see increasing the wire gauge to accommodate the two panels, however 4 seems excessive. Also, the return wire gauge should be of equal size amps out wire... Just food for thought.

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Old 08-08-2014, 14:20   #3
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Re: Cable Size

How about a smaller(#10) till you get below deck and then #4 to the buss?
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Old 08-08-2014, 14:31   #4
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Re: Cable Size

Hi Camp,

If the run is 23 feet (therefore 46 feet of wire), then #4 is what you need to keep the loss under 2%. If 23 feet is the total wire (11 1/2 foot run), then #8 is what you need.
For the 23 foot run, if you can withstand 3% drop, then #6 will do it (actually, a 2.5% drop = .5 v drop). I would think this would work fine (and save you some bux!) [fwiw, .5 volts at 21 amps = 10.5 watts (out of 370)]

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Old 08-08-2014, 16:31   #5
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Re: Cable Size

The definition of a run is the distance out and back. Therefore I am not sure if your run is 23 feet or 46 feet as you described it.

For the first of the three circuits you described, let's assume your run is 48 feet. If your potential is 17.9 volts and your power is 135 watts, then your current would be 7.54 amperes.

Entering your acceptable loss of 2%, your potential, your current and your run (48 feet equals 14.6 meters) into the calculator gives you a cable size of 6 AWG in order to obtain no less than a 2% drop.

DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk

The total run (from source to the load and back to source) must be the same gauge wire. Otherwise if you have two different wire gauges in a circuit, you must calculate for the wire of the smaller gauge. It is irrelevant if the majority of the circuit is the larger gauge wire.
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Old 08-08-2014, 17:59   #6
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Re: Cable Size

It makes no sense to say that you need the same size conductor for the whole run. You would factor in the resistance for each portion of the run for each conductor size.
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Old 08-08-2014, 20:29   #7
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Re: Cable Size

Thanks for the advice.

The distance is between 20 and 23 feet from the solar panels to the battery bank, Round trip 20 to 43 feet,

There are three panels totalling 405 watts ( 3X135).

Fletch
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Old 08-08-2014, 22:40   #8
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Re: Cable Size

Since you've got an MPPT controller, why not wire the panels in series. You'll have 1/3 (approx) of the current, with correspondingly much lower wire gauge requirements. Just check that the resulting voltage is not above the limit for your controller.
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Old 09-08-2014, 04:35   #9
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Re: Cable Size

Greetings and welcome aboard the CF, CeeJay.
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Old 10-08-2014, 02:40   #10
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Re: Cable Size

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Originally Posted by David M View Post
The definition of a run is the distance out and back.....

DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.ukok
This calculator seems to use the length from source to load, not the round trip length there and back. So you would end up with a cable that is the wrong size.

All other cable size calculators do use the distance there and back because there is equal voltage drop on both circuits.
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Old 11-08-2014, 10:15   #11
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Re: Cable Size

Taking a quick look at the Midnite solar site Mid Nite Solar Kid MPPT I would say to go with series wired panels as mentioned by Cee jay this would increase the voltage lowering the amps and allowing smaller wire sizes The kid seems to allow up to 150 VDC input) . You may run into problems because of the different wattages however, someone with more experience tying different panels together may chime in. There is also several long running threads on parallel vs serial.
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Old 12-08-2014, 00:31   #12
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Re: Cable Size

Use series only if there is zero chance of a shadow on any of the panels. In other words with 3 panels on a boat use parallel as there is bound to be some shading, however small. Series doesn't work well on boats with a few exceptions, mostly large powerboats with unobstructed upper decks. Mainesail agrees with this and has posted as much.
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Old 12-08-2014, 13:46   #13
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Re: Cable Size

Forgive my lack of solar knowledge but, the power from a cell is what it is. That being said, I'm finding it hard to understand, how wiring the panels in series could reduce the power produced by a sunny panel if in series through a shaded panel. Yes, a total power reduction but, I don't see an additional reduction due to being run through a shaded panel.
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Old 12-08-2014, 14:20   #14
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Re: Cable Size

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Originally Posted by sailinglegend View Post
This calculator seems to use the length from source to load, not the round trip length there and back. So you would end up with a cable that is the wrong size.

All other cable size calculators do use the distance there and back because there is equal voltage drop on both circuits.
the sizing tool is correct as it adds a factor of 2. ( if you work it out from first principles)

dave
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Old 12-08-2014, 15:27   #15
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Re: Cable Size

Shading on the module cell, if the panels are wired in series, will interrupt the current path and thus reduce input from all the panels, depending upon where the shading is. There is some validity to the idea of wiring in parallel (to a common buss independent of the individual modules would be best ie: don't daisy chain the modules) if the array layout is subject to differential shading from the mast, sails, or other obstructions during the course of the day. This would allow the modules that were not being shaded to still provide input. If you did not have a shading consideration and are using a MPPT controller that allowed up to 150Vdc input, then it would allow a much smaller wire sizing because of the higher series voltage, then reducing the higher voltage to charging level voltage required by the nominal system battery bank voltage. Running a smaller gauge wire, such as #10 to the buss and then continuing on with the calculated remaining wire size for the distance will result in an acceptable less than 2% Vd.
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