PV Circuit Segments

[Danjo]

I’m still trying to find the best placement for my batteries, charge controller, and fuse box. The best place I can find for the charge controller and fuse box is right behind the driver seat.

The batteries, however, seem to want to be on the passenger side about 9 feet back, so maybe a 35 foot round trip distance from the charge controller.

The RT for the fuse box is also 35 feet.

The drop from panel to charge controller is short, maybe 5 feet.

So what should be used for the conductor length calculation? Is the drop from the panels to the charge controller a separate calculation from the controller to battery bank? Is the battery to fuse box it’s own?

Thanks for your help

[dzl_]

First let me make sure I understand your situation.


Leg 1: PV panel to SCC = 10ft (5x2)
Leg 2: SCC to Batttery = 35ft
Leg 3: Battery to Fuse Block = 35ft
Leg 4: Last leg (fuse block to consumers)


Definitely not ideal from a voltage drop / efficiency standpoint. I've got some followup questions.


What is the voltage of your batteries, and what is the voltage coming off your PV array?
How many amps will each leg carry?


In answer to your question, yes, each leg can/should be calculated separately.

[Danjo]

Originally Posted by dzl_

First let me make sure I understand your situation.


Leg 1: PV panel to SCC = 10ft (5x2)
Leg 2: SCC to Batttery = 35ft
Leg 3: Battery to Fuse Block = 35ft
Leg 4: Last leg (fuse block to consumers)


Definitely not ideal from a voltage drop / efficiency standpoint. I've got some followup questions.


What is the voltage of your batteries, and what is the voltage coming off your PV array?
How many amps will each leg carry?


In answer to your question, yes, each leg can/should be calculated separately.

Thanks. The cable lengths are as in your reply.

12 volt AGMs, two 200 Ah in parallel.

I havent connected the panels together because I’m waiting to route to the controller location, but I have six 12 volt, 100 watt panels 2 in series, 3 in parallel, so in a perfect world, 300 watts at 24 volts.

So what is that, 12 Amps Out the controller?

I plugged parameters into the Blue Sea Systems Circuit Wizard and it says 4 AWG is sufficient for the 35 foot RT.

[dzl_]

Voltage Drop
Voltage Drop = Drop in voltage due to resistance of the wire

V = I x R


To calculate power loss, you need to know the current (amps) and the resistance of the circuit (resistance of wire x length of circuit).

Power Loss
Power Loss = power (watts) lost as heat due to resistance of the wire

P = I² x R


You need the same information to determine power loss as you use for voltage drop, the only difference is that current is squared.


Here is a good calculator for calculating voltage drop that'll save you from doing the math.


I do not have a recommendation for a Power Loss calculator.

[dzl_]

Originally Posted by Danjo

Thanks. The cable lengths are as in your reply.

I haven't connected the panels together because I’m waiting to route to the controller location, but I have six 12 volt, 100 watt panels 2 in series, 3 in parallel, so in a perfect world, 300 watts at 24 volts.

So what is that, 12 Amps Out the controller?

So just to clarify, 6 x 100 Watt panels will always be 600 Watts no matter how they are configured. The amperage and voltage will differ based on arrangement but wattage is always the same.

And if your batteries are 12v, the output of your controller will be 12v, 600W / 12v = 50 Amps

So Assuming 4AWG:
Voltage drop = 50A x (35ft x 0.00024) = 0.42V (3.5% of 12.8V)
Power Loss = (50A x 50A) x (35 x 0.00024) = 21W (3.5% of 600W)

That isn't horrible on its own (not ideal either), but you have to consider how it all adds up. You are losing 21W to heat between the controller and the battery, and then possibly another 21W (maybe more maybe less depending on amperage and wire size) between the battery and the fuseblock, and then another unknown amount between the fuseblock and the appliance.

Also regarding the solar panels, '12v' solar panels are not actually 12v, its a little misleading, your 12v panels are probably between 16-18V, so two in series will be roughly 32-36V. You want to look at the datasheet or the sticker on the back, that'll have accurate numbers for voltage and amperage.

I plugged parameters into the Blue Sea Systems Circuit Wizard and it says 4 AWG is sufficient for the 35 foot RT.

I love the blue sea circuit wizard, its great for standard voltages (12, 24). For your solar panels you'll need to figure out the actual voltage and use a calculator like the one I linked above.

One final thing (that I alluded to above), the calculators can only make recommendations based on the data you give them, 4 AWG may be fine for the 35ft circuit but you need to make sure you are accounting for the total round trip distance that current will travel (all the relevant legs).


I'm struggling to keep this semi-simple but I think I'm failing at that.

West Marine and Blue sea have some good short intro articles on the topic:
Marine DC Wiring Basics
Wire Size and Ampacity
Choosing the correct Wire Size For DC Circuits

Blue Sea also has this handy reference chart which applies to 12V DC