Solar - questions on cabling and fuses

[ERON]

I am also in the process of installing a solar system on my boat. And I am also using 6 awg wire. I have a 30 + foot run from my MPPT. And 6 -8 awg is what was recommended by multiple charts I looked at. From all the reading I have done, you need a fuse as near as you can to the battery bank and it is highly recommended that you have a circuit breaker that is correctly rated for your panels between your panels and controller, for it is important to have the option to shut off your panels. The breaker pictured earlier in the thread is the kind I got. Regarding 6 awg fitting in your mppt , it depends on the mppt. I decided on the epever tracer bn model. I am looking forward to get my panels running so I dont need to start my diesel to charge my batteries when I am out and about. Feels like Freedom!

[Kelkara]

Originally Posted by Gary Mc

However most tables and on line calculators use the distance to the load so they are already accounting for the return leg in their calculations.
Hope this helps.

I'm going to disagree with "most tables" ... here are the first four I could think of that I might go to when checking out a wire on my boat ... Blue Sea, ABYC, Ancor, West Marine.

[aa1bflyboy]

Originally Posted by Gary Mc

Those numbers look a little high. For 15 amps at 7 meters using #10 I get a 2.5% drop and with #6 I get a 1% drop.
I would go with #8 my self.

#8 would have been my recommendation as well and what is my intent to use for my Aloha.

[ForeverDes]

Using this calculator, 6ga keeps the voltage drop under 1%, 8ga is just a little over 1%, and 10 gauge is 2.07% voltage drop for a 20' one way run at 27v and 14 amps.
I'd use 6ga because a. it's a one time cost, and b. I always try to oversize from specs.

But that's just personal opinion. Even 10 ga should be safe and have an acceptable efficiency.

[noelex 77]

Many seem to be recommending tables designed for 12v on the 27v solar panel wiring the OP is contemplating. This is not correct. Higher voltages make a substantial difference.

My brain works more intuitively on metric cable sizing, but if google conversions are correct 6awg cable has a resistance of around 0.0013 ohms per metre and the cabling distance is 6.1m one way 12.2m total. Therefore the total resistance is 0.0013x12.2= 0.0159. Therefore the voltage drop will be 14x 0.0158= 0.222v. This is a percentage voltage drop of 0.222/27 x100 = 0.82%. This is more than acceptable, especially considering that this percentage loss will only be present during peak output of the solar panels.

The final consideration is the maximum current carrying capacity of the wiring. 6awg cable is adequate for 80-135A depending on the wire specifications/location/bundling, so 14A is well within the current carrying limitations of the wire.

Conclusion: unless future expansion of solar production is anticipated 6awg is fine.

The cable from the conroller to the batteries is a different calculation. The controller will change the 27v to the battery voltage (and the current will increase), so the “12v” voltage drop tables can be used for this section of wiring using the higher current value.

[mitiempo]

Originally Posted by mvweebles

.......are the lugs on a modern MPPT large enough for 6awg cable? Of not, how are these connections made up? Obviously, stripping some strands would physically work, but wondering if there's a better practice.
Peter

Victron controllers do not have lugs. They have a slot that the connection is made in by tightening a screw. Most put bare wire in these type of connections but it is not the best way to do this. Ferrules are the best solution and are designed for this type of connection. A 6 awg ferrule will fit in a Victron 100/30 or 100/50 controller.

[noelex 77]

As there are only two solar panels per controller, fusing on the wiring between the solar panel and the controller is not needed in this case. However, a means of easily disconnecting the solar panels is nice to have. The solar panels should be disconnected from the controller (for most controllers) before the batteries otherwise damage can occur. Thus a circuit breaker between the solar panels and the controller is often fitted as it functions as a convenient switch. Pure switches rather than circuit breakers can be used (if fusing is not required) but for systems with high voltage panels switches with appropriate ratings are hard to find. Enclosures etc also need to be considered.

Many modern marine solar systems (although not the OP’s installation) have DC voltages in the potentially lethal range (above 50v). DIN rail circuit breakers are available to manage these high DC voltages and there are accessories such as enclosures, cable glands etc that are designed for high voltages. Most marine switches and fuses are limited to 32v DC which is fine for this particular installation, but not for many systems.

Circuit protection (fuse or circuit breaker) is always needed between the battery and the controller, but the voltages here are (usually) conventional 12 or 24v.

[GordMay]

The industry standard formula for calculating minimum Wire Size, for a given Voltage Drop is:
CM = (K x A x L) ÷ VD
Where: CM = The Circular Mil cross sectional area of the wire
(See Table 1, and ABYC Section E-8, Table III)
K = 10.75 Representing the Mil-Foot Resistance of Copper Wire @ 780 F.
L = The Total Length of the wire in Feet
(This is the ‘Round Trip’ length of both Positive + Negative wires)
VD = Permitted Drop in Volts
(Ie: 3% @ 12.5V = 0.375 ED or 3% @ 25V = 0.750 ED)

Wire Size Charts from “Ohm’s Law & Boats” ➥ https://www.cruisersforum.com/forums....html#post1256

1. ➥ https://www.cruisersforum.com/galler...r&imageuser=79
2. ➥ https://www.cruisersforum.com/galler...r&imageuser=79