breakers and a final setup check

Listing information in support of your question is always OK (brand, model, link, etc.).

idiggplants wrote: »

ok, so i wanted to run our final setup by everyone here just to make sure everything would work ok. this setup is for a remote cabin that we visit monthly. we have monitored our usage and use a maximum of 50ah in a 3 day visit.

So--How big of battery bank do you want? Do you want to support 50 AH @ 12 volt for three days of no sun and 50% maximum discharge? That would be a 100 AH @ 12 volt battery--Very doable and not too big (theft, when battery eventually dies, cheap to replace, don't need to oversize solar array, etc.).

• we plan starting with the following:
• 200w of panels
• 30a charge controller
• 1 rv deep cycle battery
• not sure if i can mention the brand here, if not, sorry... but it is renogy. 2x 100w mono panels and the charge controller is the "30 amp pwm charge controller with lcd display"

From a rule of thumb point of view, recommend a 5% to 13% rate of charge:

• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 94 Watt array minimum (weekend cabin, seasonal usage)
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 188 Watt array nominal (full time usage 9+ months a year)
• 100 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 245 Watt array "typical cost effective" maximum

So a 200 Watt solar array would be very nice for a ~100 AH @ 12 volt battery (deep cycle).

Next, you did not say where the cabin would be located, and if this is summer/winter or when usage. Assuming 50 AH and 200 Watt of panels, roughly the hours of sun needed to "break even" on usage would be:

• 50 AH (per 3 days) * 1/3 days of power * 14.5 volts charging * 1/0.61 DC system eff * 1/200 Watts of panels = 1.98 hours of sun per day "break even"

So, ~2 hours of sun would take you pretty deep into winter for many locations...

this is a project my father is funding. if it were me, we would go with 300 or 400w right off the bat. but he will require seeing that 200w is not enough to make that call.

I would be hard pressed to see the need for going with a larger array/battery bank unless your loads are more than lighting/cell phone/radio/battery charging/small water pump.

I assume security/theft is always in the back of your mind with a weekend cabin. Keep the valuables to a minimum. Will you have/need a backup power sources (small Honda eu1000i 900 Watt genset) if backup power is needed (use vehicle battery/alternator or hike in, etc.)?

regardless, im interested in wiring and breakers.

first off, fuses are cheap. is it necessary to use breakers?

Fuses are cheap, but you need to have spares (big fuses + holders, such as the 200 amp type can be more expensive than breakers, especially if you have to replace one fuse over the life of the system).

Many times, it is nice to use a breaker as your main battery disconnect for the loads... You simply turn of the loads and lock up (typically leave the charge controller connected to the battery bank through its own fuse/breaker). Having a fuse + DC Switch does not offer much over a single switchable breaker. Cost both out and see what works best for you.

we will have all 2-4 panels mounted on a pole. with about 40' of distance(closer to 30-35, but for plannings sake, lets call it 40) between the panels and controller. panels will be ran parallel. is it ok to get a 4 way splitter at the panels, and solder that into 6gauge wire and run that back to the controller.. and put a breaker/fuse there? or do all the panels need to be fused separately?

In theory, you only need fuse/breaker for solar panel string (parallel connections) if you have 3 or more panels. Many people still like a breaker (or breaker per panel) for working on the system (turn off power to controller for servicing, debugging the solar array--Turn off one breaker at a time to find "bad" solar panel/wiring).

and then, what size fuses/breakers do we need?

Popping breakers/fuses is always a pain, and you can design the system so that it never happens in normal operation. Basically heavy enough wire to carry the current expected.

• 100 Watt panel / ~17.5 volt Vmp = 5.714 amps Imp (guessing)

Maximum current (worst case):

• 5.714 Amps * 1.25 Solar Derating * 1.25 NEC derating = 8.9 amps max current

So, you should fuse/breaker/wire run for ~9 amps per parallel solar panel. And 4 parallel panels is "too big" for a 30 amp PWM charge controller.

Wiring wise, you don't want too much voltage drop from the solar array to the charge controller. 40' one way wire run (different voltage drop calculators use one way or round trip wire run), Say 11 amps for two panels in parallel, and no more than 0.5 volt drop:

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

8 AWG:
Voltage drop: 0.55
Voltage drop percentage: 3.14%
Voltage at the end: 16.95

So, 8 AWG is close (probably close enough) for two panels--Double to 4 panels (22 amps):

4 AWG
Voltage drop: 0.44
Voltage drop percentage: 2.51%
Voltage at the end: 17.06

You are talking about some pretty heavy copper cables here (theft/cost). At some point you may want to look at a MPPT charge controller. Get a single 250 Watt panel with Vmp~30 volts and Imp~8.33 amps, and you are looking at:

12 AWG
Voltage drop: 1.06
Voltage drop percentage: 3.53%
Voltage at the end: 28.94

And using a single MorningStar 15 amp MPPT type charge controller (less for panels+wiring, more for MPPT controller+accessories--Always get the remote battery temperature sensor for the MS 15 amp MPPT controller).

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller

is the 30 amp charge controller enough for 4 panels, or only 3? at max output i know it could only handle 3, but since we arent located on the equator, im thinking we will never see max output, so i was thinking a 4th panel would be fine, and worse case, might blow a fuse occasionally, not the controller. im prepared to admit that im completely wrong here, obviously.

If you are in the mountains, cool/dry air at altitude--Your system will do very well when compared to a system at the equator in peak output power (obviously, hours of winter sun, not so much).

Comments/questions?

-Bill

idiggplants wrote: »

i also hadnt thought about diagnosing problem panels. maybe it would be better to run 3 10g wire pairs so we can see what each one is doing without climbing the pole. thats some considerable cost increase though. 3 10g wires vs 1 6g.

but breaker sizing, it sounds like we want a 10a breaker per 100w panel, or a single 30a for 3 panels?

It is one breaker (fuse) per + lead for each parallel connected solar panel. You can have the three breakers out at the pole and run a single 6 AWG wire back, or put the breakers back at the battery shed and run 3 pairs of 10 AWG cables.

just to be clear, these breakers are protecting the charge controller, right? or the panels? im used to breakers protecting the wire against too much power. im not quite sure i understand the idea that you dont need a breaker for 3 or less panels but for 4 or more you do..

The breakers at the panels are there to protect one (for example) shorted panel from being feed excess current from the other two (or more) parallel panel strings.

If you look at the data sheet, you should find a series fuse rating (something like 10 amps for these panels--I am guessing). When one panel is shorted, then two panels in parallel can exceed the fuse rating of the third sorted panel. In general, 2x panels just exceeds the fuse rating of the panels (it is close--4 or more panels in parallel do exceed the series fuse rating--i.e., 3x panels feeding a shorted 4th panel).

Not all solar panel specs will list the series fuse rating.

-Bill

OK and Isc is 5.75 amps.

If you have three panels and one shorts, you have 2*Isc feeding the shorted panel/panel wiring or ~11.5 amps into a panel rated for 15 amps Isc--So for these panels, you do not need a series fuse until you have 4 or more parallel panels (Isc of 15 amps is a pretty high rating for a Isc~5.75 rated panel--A good thing).

Look at for 11 parallel solar panels--Now you have 10*Isc of source current that can feed ~57.5 amps into a single shorted panel--No fuse/breaker per panel (or string of series panels), it would be easy for the solar array to overheat the wiring and start a fire.

Certainly, these are not common faults/failures (shorted panel wiring)--But if you get an animal chewing the wiring or rock/hail through a panel, it can happen.

The other major concern is "arc fault" failures (intermittent connector/wiring connections that start a small DC arc in the connection)--These are very hard to protect against and are also a (rare) but not unknown type of failure.

Here is a thread on a solar panel fire (non-UL rated panels, poorly designed and manufactured)--And example of what can happen in extreme situations:

Panel Fire Question

-Bill

Yep... The circuit breaker should be a minimum of:

5.75 amps * 2 * 1.25 NEC derating = 14.4 ~ 15 amps minimum...

Of course 6 AWG cable and your charge controller can manage a higher current rating.

You have the choice of 1x 15 amp breaker per panel, OR a >=15 amp breaker between the array and charge controller Vsolar input connection (single beaker is really just a handy on/off switch--One breaker per parallel panel is better for diagnosing problems or adding more panels later).

Note, for your setup, when you get to 4 or more parallel panels, you should have 1x 15 amp breaker per parallel solar panel connection.

-Bill

Put the breakers in a metal box at the base of the pole (or at ladder height)... Not perfect--Especially if you are worried about thieves.

And this is one of the reasons that MPPT charge controllers can be nice... You can put one or two large 250+ watt panels up (no fuses). Or even run some panels in series (depending on panel Vmp rating and controller Vpanel input ratings).

In theory, you can run one pair of ~11 AWG cables for each panel back to the shed or one pair of 8 AWG back to the charge controller--The cost of copper is the same (a little bit more for plastic insulation, etc.). Of course, you cannot get 11 AWG wire normally. Pretty closely, every 3 AWG decrease in wire gauge is 2x more copper cross section (and 2x the price per foot).

Using 5.7 amps @ 17.5 volts on 12 AWG wiring for 40 feet:

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

Voltage drop: 0.72
Voltage drop percentage: 4.11% (a bit too much drop)
Voltage at the end: 16.78

Two runs of 10 AWG cable vs one run of 8 AWG is not that much more expensive--But you need to account for the ability to pull more cable later (if you want to add couple more parallel panels, for example). Large buried conduit, leave pull rope in tube for upgrades, etc.

-Bill