RV Install, wiring advice/help

BB. wrote: »

Lots of questions--I will take a stab them.

• 135 watt panel * 0.77 system derating * 1/14.5 volts charging = 7.2 amps charging
• 7.2 amps / (4*75AH batteries) = 0.024 or 2.4 % rate of charge

Normally, we would recommend that a battery bank be charged with 5% (to 13%) maximum rate of charge (for flooded cell batteries).

Ideally, you should have at least one more panel if this is intended for long term camping/RV'ing use. If just weekends and you will use Utility power and/or backup genset for charging--you can use the solar panels to just extend your power some.



From our host's store, they say:



So there are two questions...

1. 10 AWG wire is good for 30 amps or more (per NEC). So Isc of 8.37 amps is well under 10 AWG wire--so that is fine.
2. Is there too much resistance for this wire run

For #2 we use a voltage drop calculator, the one-way wire length, 7.63 amps and find the voltage drop for, say 30 ft wire 10 AWG run is:

• 0.6 volts for 10 AWG, 30 ft @ 7.73 amps

That is actually a bit high of voltage drop (3.5% or so)... Ideally, most people would aim for less than 2% voltage drop. However, if my 30' run was too long of guess for the wire run--then you are perfectly OK with a 15' run.

Second, you have choosen a BlueSky MPPT controller... Have you already purchased it yet? If not, you may want to think about another brand/model.

The BlueSky has a maximum PV input voltage of 35 volts--For your setup, ideally with two Kyocera 135 watt solar panels, you should be connecting them in series for a Vmp of ~35 volts (Voc will be higher).

This gives you several advantages... Lower current flow through the copper wiring for the PV array (lower voltage drop) and if you camp in very hot areas--your battery charging voltage from the array will be better (Vmp drops with high temperatures, and can limit the maximum charging voltage / charging current to the battery bank).

You may want to look at the Morning Star 15 amp MPPT or (even 45 amp MPPT if you plan on adding more panels in the future) charge controllers.

Or look at the Rogue 30 amp MPPT charge controller.

Note: All three of these charge controller have differing Varray-Voc maximum input voltage--Depending on how many panels you plan on installing and which controller you pick--will affect your wiring choices.



See above--depends on length of wire run and if you will be running more than one panel in parallel. Also, if you can run a different MPPT charge controller, you can (and should) run your two solar panels in series.



The maximum "optimum" wattage I would recommend for a 25 amp 12 volt charge controller would, roughly, be:

• 25 amps * 14.5 volts charging batt * 1/0.77 system derating = 471 watts
• 3x 135 watts = 405 watts

So, yes, no problem with 3x 135 watt Kyocera panels in parallel on this controller.



Depends on the maximum amount of solar panels you will be running... But for 25 amps, using NEC 1.25 safety factor:

• 25 amps * 1.25 = 31.25 amp rate fuse/wiring minimum

So, you are looking at the next standard fuse/breaker size of 30-40 amps and 10-8 awg wire.

You also want a low voltage drop between the controller and the battery bank--but your wire run is so short that is not an issue here.



Really depends on the charging current/maximum load... With four batteries in parallel, I would assume that the interconnect cable must be rated to carry at least 1/2 of the expected maximum current (x 1.25 safety factor). The batteries should share 1/4 of the current, but frequently that does not happen (why I picked 1/2 rather than 1/4 for cable sizing).



Generally, the inverter manuals should discuss their wiring requirements... But working from back of the envelope calculations... 1,500 watt inverter running 12 volt battery bank, assume 80% efficient, and 10.5 cutout voltage for inverter (dead battery voltage) with NEC 1.25 derating for fuses / wiring:

• 1,500 watts * 1/10.5 volts * 1/0.80 eff * 1.25 = 223 amps rated circuit

There is a fuse rated at 250 Amps or 250 Amp Circuit Breaker so that would be the wire rating too.

If your maximum draw will be less than 1,500 watts, then you could size the wiring/fusing for less.



If you eventually get a another charge controller--look for those with remote battery temperature sensors (the small MorningStar 15 amp MPPT controller really needs one).

In general, RV and small cabin use should plan on using as little DC/AC power as possible. Refrigerators should be propane powered. Use florescent or LED lighting, efficient appliances (like laptop computers and such). People do use microwave ovens--but as you can see, that is a tremendous load on a 12 volt battery bank...

For a 4x75AH battery bank, the maximum continuous power you should draw from them is around C/2.5 or:

• 4x75 amps / 2.5 = 120 amps

For an inverter load that would be:

• 120 amps * 10.5 volts (low battery) * 0.80 inverter eff = 1,008 watts max 120 VAC load for this bank

If you can get your loads down to 300 watts or so maximum--The MorningStar 300 watt True Sine Wave 12 volt inverter (600 watt surge) would be ideal for your setup.

Anyway, my 2 cents worth--and the above are just suggestions and rules of thumb I like to look at when designing a system. There are good reasons to not follow rules of thumb--but we need to discuss those in a bit more detail.

I hope I was helpful.

-Bill