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Old 26-03-2013, 15:58   #1
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solar wire sizing

Is there any problem with sizing solar wire from the panel to the controller using the panels MPPT voltage if using MPPT controller? Big difference in sizing for voltage drops.
It is OK if others want to do it different on THEIR boat
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Old 26-03-2013, 18:20   #2
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If sizing for safety: The total maximum current of all panels connected to the network without benefit of fusing ... Sizing for safety usually results in wire on the small side of reasonable. It is a jacket temperature issue.

If sizing for efficiency: one really needs a "policy" regarding the acceptable loss. Yachties tend to be foolish here and greatly oversize wires, wasting money and displacement while achieving a very small benefit.

If sizing for voltage drop seen by the consuming device (eg: SSB, refrigeration): Use the difference between the expected supply voltage and the voltage specification of the device.

Complicated enough? That's how us retired engineers earned the big bucks and retired early ...
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Old 26-03-2013, 18:28   #3
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More to your point perhaps is that wires are sized for current not voltage, in general. I would size them using just the nominal 12V if a voltage is needed in the calculation. Losing 1V in the wiring would annoy me. Half a volt (5% of the energy) not so much.
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Old 26-03-2013, 18:45   #4
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Re: solar wire sizing

You can make the installation easier, while still retaining efficiency, by using different sized wires. I ran 12 gauge from the panels through the deck, then I switched to something larger (4 or 6 gauge, I can't remember) from there to the MMPT regulator. This gave me less than a 3% voltage drop, despite running through the engine compartment. Maybe overkill, but the heavier wire cost less than $100. I only have so much room for solar panels, and I wanted to make the most of them.
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Old 26-03-2013, 19:03   #5
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Re: solar wire sizing


between the panel and controller size for the following:

Insulation should be rated for Voc (this should not be a problem at all unless you series a bunch of panels).

Circuit protection/wire size should be rated for Isc (short circuit current).

Then if you are looking at voltage drop, use the Vmpp as you suggest. This will be the actual operating voltage. If you are using 60+ cell modules then you can accept greater voltage loss, so you have to do the cost/benefit to decide if you will accept 1, 2, 3, or 5% loss of power (voltage).
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Old 27-03-2013, 00:47   #6
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Re: solar wire sizing

Use the current that will actually be in the wires. This is one advantage of series connection, or high voltage panels, the wires can be smaller.
Voltage drop is nearly always the factor that determines the wire size.

It also means the wire from the controller to the batteries may need to be thicker even if the run is shorter. Once again just use the actual current (now at 12v) to calculate the wire size.

Make sure you allow for the voltage drop on both the positive and negative sides. On many calculators you have to double the distance to arrive at the correct voltage drop. They often do not make this clear.
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