small system design

The short circuit current for a solar panel is the maximum current (under full sun) the panel can output. This is for deciding the wiring and breaker ratings for the solar array...

More or less, I ignore the Isc rating, and go to the data sheet and look for the series fuse rating... The old rule of thumb of when you have 3 or more panels in series, you fuse/breaker each series string so that if one panel gets shorted (or the wiring gets shorted), the other panels do not feed "too much" current into the short and start a fire.

Remembering that Power = Voltage * Current, in the case of a short circuit, the panel voltage is zero volts... And zero volts * any current = zero Watts--So Isc does not figure into the power numbers during operation.

An MPPT charge controller internally is a "buck mode" down converting switching power supply. It is sort of like5 the DC equivalent of an AC transformer. It takes high voltage/low current of the array, and efficiently down converts to low voltage/high current to charge the battery bank (and run your DC loads)... Math wise (ignoring losses)... Say you have a 330 Watt panel with Vmp~30 volts:

• 330 Watts Pmp / 30 volts Vmp = 11 Amps Imp
• MPPT input is 330 Watts or 30 volts at 11 Amps
• MPPT output down converts to
• 330 Watts / 14.5 volts battery charging = 22.8 Amps battery charging at xx.x volts

The computer inside the MPPT charge controller figures out the "optimum" solar panel operating points (Vmp and Imp for the specific temperature, sun, and battery conditions). So you don't have to tell the controller Vmp for your specific solar array--It figures it out and runs that way.

Once the battery charging current begins to drop (and/or your DC loads drop), the controller transitions from "bulk charging" (trying to get as many Watts as it can from the solar array) to Absorb and eventually Float charging. The controller simply takes less current from the solar array to supply less current to the battery bus.

Think of the MPPT (maximum power point tracking) controller the equivalent of an automatic transmission for a car, vs driving the car around with with no transmission and just a clutch (PWM controller -- Pulse Width Modulation). With a PWM controller, it is just an "on/off" switch. Works very efficiently with the "right Vmp solar panel", but is not efficient with higher voltage Vmp panels/arrays.

-Bill

Yes, assume all is working correctly, the controller will "clip" the output to 40 Amps.

And the "realistic" peak current (occurs rarely, and not for very long typically). For "cost effective" derating, using 77% derate:

• 2 x 345 Watt panels * 0.77 panel+controller derating * 1/14.5 volts nominal charging = 36.6 Amps... fixed "usual best case" peak current during charging

I/we always try to be on the "conservative side" a bit... In this example, the "lost harvest" due to clipping with this array is not very much (a few hours a year? Only when the battery is less than ~80% discharged, and light DC loads, cool/clear sky, etc.).

Just keep the controller in a well ventilated (cool if possible) area--No hot closet, poor circulation, with southern sun in through a window.

Temperature (every 10C/18F increase in room temperature is a ~1/2 reduction in life for electronics). Also, even thermal cycling from cold to hot every day, is not great either. See "HALT Testing":

https://en.wikipedia.org/wiki/Highly_accelerated_life_test
-Bill

For a DIY system, poster 2manytoyz has a lot of pictures (scroll 1/2 way down page):

http://2manytoyz.com/

-Bill

If you have 1 series string of panels (and don't plan on adding more parallel strings later), no over current protective device is required from the array to the charge controller.

HOWEVER, many folks like using a properly rated switch (or even properly rated breaker) near the charge controller so they can cycle the solar array power off and do work without worrying about a "hot" PV circuit as they work on the wiring in the area.

And higher voltage DC wiring with decent current is nothing to sneeze at... 100 VDC @ 15 amps can be "more exciting" than 100 VAC @ 15 amps (DC current tends to sustain arcs much better than AC current).

A nice Youtube example:
-Bill

Cable from Panel Frames to ground rod--Really for lightning control. My own array (installed 15 years ago, GT system), the installer just ran the array ground to the AC grounding screw in the inverter--No extra ground rod). We have had lightning over the years, but rare, and not real close (hills, tall trees, power poles, etc.). Your choice on that (and whatever code says).

You do want to run a 6 AWG ground wire from the "array grounding" to the house grounding so that if there is a short circuit somewhere--It will pop a breaker (6 AWG provides current path back to AC main panel and Neutral/Ground bonding point).

-Bill

If your AC branch circuit power is "local" to the solar power system is fed through a GFI... You are fine. The GFI will trip if there is any H/N to ground/locally earth grounded frame. You do not need a ground wire (of some sort) from solar frame back to the home ground.

If, for example, you had some yard lights mounted on the solar array that are powered by straight 120/240 VAC power. You would want EITHER a ground wire from the main panel (same AWG or heavier vs the L/N wiring), And/Or GFI protection (either GFI breaker in Main panel, GFI outlet to yard, or GFI locally to local ground.

The idea that "somehow" the circuit will be turned off is there is a short (solid green wire ground prevents the local "metal" from becoming "hot" wrt ground) and for any outside (near pool, near water, near sink inside) that will trip if there is a Hot connection through a "person" to earth ground (reduce the chances of electrocution).

-Bill

I try for "functional explanations around grounding".

Running an outlet "near" the array--Then the only source that can "electrify the array" with power from the AC mains is that branch circuit. So running a ground wire from the nearby outlet to the array would certainly keep the "solar mounting frame" "safe" if you (for example) were using an electric drill or hanging Christmas tree lights on the frame. If there was no way to get AC house power to the frame under any normal cases (100 feet away, no plans to run any power to the mounting frame, then grounding to the house is not really needed. A ground rod at the base of the framework, is more than good enough (for static discharge and lightning).

For the Battery Bank... Again two part test... One is that any metal in the area (electrical panels AC or DC, plumbing like sinks, AC inverter chassis, etc.) be tied to the same "single ground point" as the Battery (negative typically) point (as well as the AC grounding from the house). So as that any reasonably conceivable electrical/insulation failure would avoid "electrifying" some random large piece of metal (and would trip a fuse/breaker instead).

The other part is ground rod/cold water pipe. But if the battery is already grounding to the "house ground", the house usually has cold water pipe (and newer code) ground rod. So a separate ground rod is not needed...

HOWEVER, lightning is a Radio frequency phonomyn. Lightning does not like right angle turns, does not like to "follow simple (6 AWG) cable for much more than 10-20 feet if there is something else (like earth) nearby--It will jump away. Also, if you have lightning ground coming down from an array in the "middle of the roof, and down through the center of the house, the lightning will naturally try to go to the "walls" of the house (all the current flowing in one direction down a wire, creates a magnetic field, that "repels" the electrical current from flowing down the middle of the conductor towards the edges (i.e., copper pipe is used for large AC electrical conductors), and if you have multiple grounds from roof to ground rods--The ground conductors are to be distributied equally around the outside of the house (like at the four corners).

If your AC mains/DC power system is at the wall of a house, a ground rod right next to the exterior wall works great. If your power system is in the "middle" of a building, that is not great--the lightning will try to reach away from the center to other wiring/plumbing/etc. at the "edges of the building".

I think a ground rod to battery negative bus, short, at least 6 AWG, close to outside wall, with ground rod driven next to outside watt/foundation is not a bad idea in lightning prone country (and tie DC ground bus to rest of house AC grounding too for short circuit tripping of breakers/fuses).

-Bill