Sizing strings

[Jnewell]

Need help designing strings with micro inverters, 20a max for the breaker due to 120%. Is it just watts of the panels divided by 240? Not going over 16a for a 20 a circuit? Without using over the rated output of the micro as a guide for panel size?

 

[ggunn]

Need help designing strings with micro inverters, 20a max for the breaker due to 120%. Is it just watts of the panels divided by 240? Not going over 16a for a 20 a circuit? Without using over the rated output of the micro as a guide for panel size?

Wattage rating of the solar modules is irrelevant. Most microinverters use AWG #12 wiring, which limits you to a 20A breaker per "string" (they are in parallel so they are really branches instead of strings) which in turn limits you to 16A total rated current per branch. You can have any number of micros per branch you want as long as the total current is 16A or less; there doesn't have to be the same number per branch, the modules do not have to all be in the same orientation, and shading a module does not affect the rest.

 

[Carultch]

Voltage per string is relevant for sizing strings of "wild PV", that is modules wired in series without any power electronics that re-format the power.

Power per string is relevant for power optimizer strings in series, with DC-to-DC converters deployed at the module-level, and wired in series. The inverter and optimizer will specify the max power per string, and you add up the nominal power of each module to not exceed this value.

Current per "string", or rather branch, is what is relevant for combining microinverters, where they are combined in parallel and all have the same voltage as that particular branch circuit of the AC grid (usually 208V or 240V). The microinverter is current-limited, such that even if you connect it to a panel that can produce more power, the microinverter will not allow it to exceed its rated power. It is designed to "leave sun on the roof" in the form of heat, by shifting the panel voltage away from the Vmp/Imp "sweet-spot", when there is more power than it can use.

 

[ggunn]

One more thing that you may or may not be aware of is that even if the attached module cannot produce as much power as the microinverter is capable of delivering, the NEC directs us to design the wiring for the maximum microinverter rated current.

 

[Jnewell]

Very helpful, what about string inverters? Is the concept the same as in sizing for ac output of the one inverter?

 

[Phil Timmons]

Very helpful, what about string inverters? Is the concept the same as in sizing for ac output of the one inverter?

Yeah, this olde skool. At least in PV land. A simple "string" is just a DC series circuit. So the CURRENT (Amps) is the same through every wire and every PV Module (aka Panel) in the string (or DC series circuit).

Each voltage rise across each PV Module adds up -- maybe 30 to 40 Volts each. THAT becomes the limiting factor. Your string inverter will have some rated MAX DC series voltage per string. DO NOT Exceed it. Many sense an "over voltage" and void the warranty if exceeded. 600 Volts is a typical max string voltage -- but we just put in some new commercial SMA with 1000 VDC per string, so you will need to check. I think the 600 Volts limit was a nod to old Code Limit.

So the "design" comes down to figuring out the MAX DC Voltage your string or strings will make -- and this varies by your location, latitude, and usually the COLDEST Day of the year. Bright, clear, sunny, winter day with snow reflecting can product a surprising amount of PV. So you add the Voltages across the string(s) and stay below the MAX DC Voltage for your inverter (and wire insulation rating).

Most manufactures of PV Modules and Inverters have on-line interactive calculators to assist you with portion of the design. Do you have a breed-brand-model number(s) for either the PV or Inverters to check?

 

[ggunn]

Very helpful, what about string inverters? Is the concept the same as in sizing for ac output of the one inverter?

Not at all. String inverters are totally different and in addition to what Phil Timmons said there are rapid shutdown requirements that most likely need to be taken into account; microinverters are inherently rapid shutdown compliant.

If you are considering getting a PV system for your home I highly (!) recommend hiring a reputable experienced PV system integrator to design and build it rather than trying it DIY.

 

[BillK-AZ]

Very helpful, what about string inverters? Is the concept the same as in sizing for ac output of the one inverter?

String inverters are not all the same, inverters that use optimizers on each PV module (such as SolarEdge) are basically series strings of optimizers. String voltage and current is controlled by the inverter via communications to each optimizer (SolarEdge). Optimizers are basically variable DC/DC converters that isolate PV module voltage from string voltage. String current for calculating wire size is based on optimizer output current, not PV module current. There are minimum and maximum number of series optimizers and power limits per string. Always follow the manufacturer's requirements.