wire gauge for panel to controller

atownmike55

Hi. Wonderful forum. 
Installing 2 - 210 W MONO panels tomorrow on my 2019 Keystone 5th.  I have an existing Renogy 40AMP Rover charge controller and two existing 100 AH Lithium batteries.  Have successfully used a Renogy 100 W suitcase for a year.    Inverter / charger Multiplus will be stage two in a month or two.  Three questions:

1)  Renogy tech guy says to run panels in parallel.  I had planned on series.  

2) Current solar plug for the suitcase is 8 gauge wire running 10 feet to battery bank.  Going to connect with a bar, 10 guage from the new panels that will have a run of about 10 feet from panels to where it will junction to the solar plug and then onto the controller.  Total run to charge controller from the panels is about 18 feet then.  Question:  Will 10m guage be safe or do I need to go 8 guage?

3) Should the batteries be in series or parallel?  Currently they are in series.

Thanks for weighing in.
Mike

atownmike55

Sorry  Current solar plug for the suitcase is 10 gauge wire running 8 feet to battery bank

BB.

Welcome to the forum Mike,

In solar/battery power, details matter.
https://www.renogy.com/rover-elite-40a-mppt-solar-charge-controller/

• MPPT Controller: 15-100 VDC input
• 520 Watt solar max input for 12 volt bank (vendor spec)
• You have battery temperature compensation is turned off (no temp compensation for Lithium Ion batteries)

And need to know exactly what solar panel(s) you will be installing...

Do you have existing solar panels that you wish to continue to use? If yes, need Vmp and Imp rating for those panels

What are the Vmp/Imp ratings for your 210 Watt panels?

For MPPT solar charge controllers, the recommended Vmp-array (from very cold to very hot climates) runs around:

• 15 volt max battery charging * 1.3 minimum MPPT controller voltage requirement = Vmp-array-std~19.5 volts minimum suggested for MPPT
• 100 VDC max MPPT controller input * 0.67 arctic freezing weather = Vmp-array-std ~ 67 volts max (i.e., -40F/C condtions)

Such is why site temperatures are important... If you are on a nice Caribbean island where it is 75F all year long--Then you may have other options for series/parallel array options.

Where is the system located (nearest major city)? And/or what are the min/max temperatures for your location (Voc/Vmp change with temperature; aka panel Voc voltage rise as temperature falls, and Vmp falls as temperature rises).

Unless there is something very different about your panels (high Vmp/Voc voltages)--Yes, you can probably put them in series without any problems, and you can now run smaller AWG wiring and/or longer wiring as needed (such as park RV/Camp in shade, put panels in sun).

200 Watt panels are fairly large format... And can be difficult to move with one person. Is this a fixed installation (rack mount, RV mount) or is this a portable installation (stake to ground--prevent wind from blowing them over and making expensive modern art)?

For current vs AWG, this simplified NEC table is pretty conservative:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

And you can use a voltage drop calculator for AWG vs Wire Length (one way run for this website) vs Current:

https://www.calculator.net/voltage-drop-calculator.html

Typically for solar panel and general wiring, you design for around 1% to 3% max voltage drop. For battery to charge controller wiring, design for ~0.05 to 0.10 volt drop max (short/heavy cables from controller to battery bank). And for 12 VDC wiring suggest a max of 0.5 VDC drop from battery to loads.

-Bill

mike95490

atownmike55 said:

3) Should the batteries be in series or parallel?  Currently they are in series.

What voltage is your system  ?  12V   24v ?   That needs to be addressed first ?

atownmike55

Hi guys.
Thanks for the quick reply.  

Yes, the two 210 V Newpowa Mono s are now fixed on the roof.  I am south of you 3 hours in San Luis Obispo area.  Will be traveling mainly in the west and southern states.  I have a 2000w generator for backup but would like to use it seldom.  I have the option of connecting them in series or parallel and that is now my biggest question.  I just went with 8 guage wire.

So this is a 12 volt system. The panels generate:

Voltage at Pmax (Vmp)	16.77V
Current at Pmax (Imp)	12.48A

Thanks for the help on choosing series or parallel for the panels.
Mike

BB.

Connecting the two panels in series is the correct choice for you.

10 AWG would have been fine... 8 AWG is nice too.

The size of the array (and the battery bank) really depends on your electrical loads... If it is for lightning and other smaller loads, a smaller array can work fine. If you use an Induction cook top, 12 VDC refrigerator, Instapot, etc.. (i.e,. electrical vs propane), then the more panels you stick on the roof, the happier you will be.

To show you a bit more about the math... Say you want two days of energy usage (no sun/cloudy weather) and run the Lithium bank from 90% to 20% state of charge (70% of capacity) for longer battery life:

• 200 AH * 12 volt battery bank * 0.70 of capacity * 1/2 days usage * 0.85 AC inverter eff = 714 WH of 120 VAC power per day

Just to pick a different location, say your array is mounted flat to roof and you are in Fresno Ca:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Fresno
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.42	3.48	5.03	6.51	7.64	8.30
Jul	Aug	Sep	Oct	Nov	Dec
8.00	7.27	5.89	4.45	2.96	2.28

And you camp from February through October--3.48 hours of sun per day (Feb):

• 714 WH per day * 1/0.61 end to end AC off grid system eff * 1/3.48 hours of sun per day = 336 Watt array "break even" for February

714 WH per day should be enough to run LED lights, cell phone charger, a small laptop computer... But if you are running a DC refrigerator, induction cook top, etc... Then you probably want more power than that.

Not saying the above is "right or wrong" for you and your needs... Many RV folks are OK with just one day of battery storage (not much room in RV, driving a lot using engine charging, etc.). And that is fine too. But you really need to look at everything--For example, getting 12 volts from the tow vehicle to the RV trailer to charge the battery bank needs some heavy wiring or using an Inverter in the truck to make 120 VAC--Ship to the Trailer--And an 120 VAC to 12 VDC battery charger (sending high current 12 VDC any distance needs very heavy wiring).

For example, say you want to charge your bank in 2 hours of driving:

• 200 AH * 14.8 volts charging * 1/2 hours of charging * 0.70 bank capacity = 1,036 Watt load (on 12 volt RV bus)
• 1,03614.8 Watts of charging load / 14.8 volts charging = 70 Amps @ "12 VDC"
• 1,036 Watts * 1/120 VAC * 1/0.85 AC inverter/charger eff = 10.2 Amps @ "120 VAC"
  

Much easier to send 1,036 Watts @ 120 VAC vs @ 12 VDC... Of course, now you need an AC inverter on the truck. And a 70 Amp AC battery charger, etc... (use the AC charger with your AC genset for backup)...

Just an example of how the math works... You may run your genset for 4 hours to recharge--And then use a smaller AC charger, etc...

Choices...

-Bill

atownmike55

Wow !  Thank you for such a thorough explanation.  I will hook up today.
Mike

atownmike55

HI Bill.  Am going with a Multi Plus 2 install now.  Increasing PV from 210 W x 2 to 219 w X 4 on roof of 5th wheel.  Increasing 100Ah lithium batts from 2 to 4.  I would like to put my batteries under my bed as it is over the front bay where the Multi Plus 2 - Dc/Dc charger will be.  I am doing this to keep the batteries warmer than the front bay which can freeze.  My question is, do you think a  6 foot run of both positive and neg batt cables to the inverter is too far with power loss.   what size battery cables would you use in this application?
thanks so much in advance for you wisdom.
mike

BB.

Which version of AC inverter--The 2,000 or 3,000 Watt?

https://www.solar-electric.com/victron-multiplus-ii-inverter-charger-12-3000-120-50-2-120v.html

Guessing at 3,000 Watts, always start with the manual:

https://www.solar-electric.com/lib/wind-sun/Manual-MultiPlus-II_2x_120V.pdf (page 10-11 of manual for DC electrical)

4.2. Connection of battery cables In order to utilize the full capacity of the product, batteries with sufficient capacity and battery cables with sufficient cross-section should be used. The DC cables must be copper and rated 90ºC (194ºF). See table.

12vdc/3000watt/120 adc charging
Recommended battery capacity (Ah) 400–1200
Recommended DC fuse 400 A
Recommended cross section (mm2 ) per + and - connection terminal *, **
0 – 5 m *** 2x AWG 1/0 (5 meters => ~16.5 feet of cable length)
5 – 10 m*** 2x AWG 2/0

Recommended cable lugs Size
AWG 2/1 Molex part no. 19221-0243
AWG 1/0 Molex part no. 19221-0240

* Follow local installation rules.
** Do not locate battery cables in a closed conduit
*** “2x” means two positive and two negative cables.
Remark: Internal resistance is the important factor when working with low capacity batteries. Please consult your supplier or the relevant sections of our book ‘Energy Unlimited’, downloadable from our website.

I always like to suggest not to "oversize" the AC inverter... It can waste a bit more power (larger AC inverters usually take more "tare" or "on with no loads" current that smaller inverters. Battery bank size, wire AWG, weight, costs etc. are all higher with the larger AC inverter.

I am not a big fan of installing large battery banks under sleeping platforms... But lets face it, RVs have limited amount of space and it is critical that Li Ion batteries be kept >~40F for use. Smoke (and propane) alarms to alert you to any issues early.

Please use fuses/circuit breakers/wiring of good quality where needed (pretty much every wire/branch circuit that leaves the battery bus needs a fuse/breaker to limit short circuit current (over current protection is to prevent wiring fires--Size protection to wire capacity).

Protect wiring from sharp edges and chaffing/vibration. Protect against loose metal objects falling on bus bars...

-Bill

atownmike55

Great counsel. thank you.  So if I read this correctly, if I have a battery bank and need to run 7 feet of battery cable to the Multiplus 2 (yes a 3000W), I should use 1/0 wire.  Not sure about the "2X".  Do I need to have 2 positive cables and 2 negative from the battery bank going to the inverter ?
thanks again.  Am a neophyte 

BB.

Yep--two + cables in parallel and two - cables in parallel...

Just as an example... The maximum current pull from the inverter would be (generic math):

• 3,000 Watts * 1/0.85 AC inverter efficiency * 1/10.5 volts battery cutoff = 336 Amps

If there is a chance you will be using near 3 kWatts for longer periods of time (>a few seconds of starting surge current--Which can be 2x the 3 kWatt rating of the inverter):

• 336 Amps * 1.25 NEC "continuous current derating" = 420 Amp rated branch wiring and overcurrent protection...

The manual talks about 400 Amp fuse and wiring--Good enough...

The basic NEC wire ampacity ratings:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Two parallel high temp insulation rated wiring: 1/0 = 170 amps. 2x170a= 340 Amps

So all of that works out OK. You could use 1x heavy cable (every 3 AWG change in number, is 2x more (or less) circular array).

You probably will much prefer 2x parallel + cables vs 1x "extra thick" copper cable (350 cirkmill) or 0.373 vs 0.681 inches of stranded copper wire (plus insulation thickness).


-Bill

atownmike55

Bill, 
How would I join the 2 positive cables together?  At the battery disconnect switch then onto the MultiPlus2?   And the negative pair ?

Also, I will be running a 6/4 cable to the inverter from the panel area.  I will disconnect the incoming shore power there and join it with box ?  What do you suggest to join the four strands?

The majority of my 14 foot run will be underneath the 5th wheel tucked up into the frame.  Do you suggest I use an armored 6/4 wire (aluminum or steel) or some sort of conduit ?

Many thank yous

Mike

BB.

You would (normally) terminate each cable end with the crimp connectors (as recommended by the mfg.) and then you would take the 2x + connector ends and "stack them" on the positive breaker/fuse connection and on the + inverter DC input connection. Ideally, both cables should be the same length (if one cable in a pair is longer, the extra resistance will cause more current to flow on the shorter/lower resistance cable and could cause it to overheat under heavy loads). Same with the negative pair of cables.

One of the recommended Molex crimp lugs with bolt hole:

https://www.molex.com/molex/products/part-detail/ring_and_spade_ter/0192210243

https://www.molex.com/molex/products/family/battery_cable_lugs (random example)

"Exposed" or enclosed wiring... This is where it gets complicated. The above Victron recommendation is for cables in "free air". If you put them in conduit, that limits air circulation and can cause them to "run hot" and potentially melt the insulation. One answer is to go with heaver AWG cable (more expense, larger conduit).

The NEC (US National Electric Code) has all sorts of deratings for temperature, conduit fill, conduit type, moisture exposure, etc... Needless to say, the answer is "complicated".

And you don't want high current (400 amp fuse/breaker) cables to be exposed to where they can can be damaged (road debris) or a modern issue living in wilderness--Rodents/rats/etc. eating the insulation.

It has been too many decades for me to lookup/interpret how to use the NEC to make these calculations (I am not an electrician).

As for the AC shore power connection... I am not sure I would suggest armored cable in a location that gets wet/mud/road salt/vibration and such.

EMT (the light weight metal conduit)--Not going to last long with road salt (in my humble opinion).

Non-Metal PVC Conduit is good for wet/outside/buried... Might be a good choice. You have to pull the wiring.

Or direct burial 6/3+Ground "Romex" type cable? Good for wet/sunlight.

https://www.homedepot.com/p/Southwire-125-ft-6-3-Black-Stranded-Romex-SIMpull-CU-NM-B-W-G-Wire-63950002/202316279

Again--Not my area of expertise. Just my personal suggestions. Others here have a lot more experience and may have better solutions.

-Bill