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7/13/2015 PVCombinerBoxBuyer'sGuide|HomePowerMagazine
http://www.homepower.com/articles/solarelectricity/equipmentproducts/pvcombinerboxbuyersguide?v=print&print=true 1/3
PV Combiner Box Buyer's Guide
LastUpdated:Jan17,2012PublishedIn: August/September2009
CombinerboxesareanintegralpartofmanyPVinstallations,servingasthemeetingplacewherethewiringfromarrayseriesstringscometogetherinparallelconnections.
InallbutthesmallestPVsystems,modulesarewiredtogetherinseriesstrings,wherethepositiveleadsofonemoduleareconnectedtothenegativeleadsofthenextmodule.Thisresultsincumulativevoltageoutput,withcurrent(amps)stayingthesame.Theboxwheretheoutputwiresfrommultipleseriesstringsarejoinedisthecombinerbox.Ratedforoutdooruse,itcontainsovercurrentprotectiondevices(OCPDs)andthenecessarybusbarsandterminalsforcombiningtheinputs.
Manyinstallersusedtobuildtheirowncombinerboxesforlackofavailability,butnowthereareplentyofcommercialproductstochoosefrom.Somemanufacturerswillcustombuildcombinerboxestomeetspecificsystemrequirements.Article690.4(D)ofthe2008NationalElectricalCodeincludesPVcombinerboxesasequipmentthatmustbeidentifiedforappropriateuseandlistedbyanapprovedtestinglaboratory,suchasUnderwritersLaboratories(UL)orETLlabs.Thedaysofhomemadecombinerboxesarequicklydisappearing.
Ifyouliveinalocalewherethe2008NECisineffect,yourelectricalinspectorwilllikelyrequireonethatislistedbyanapprovedtestinglaboratory,asmandatedbyNECArticle690.4(D).
GridDirectSystems
Everyinverterandchargecontroller,whetherinautilityinteractiveorstandalonesystem,hasaDCinputvoltagewindowthatmustbeadheredto.Inbatterylessgridtiedsystems,invertersrequirerelativelyhighDCinputvoltages,from150to600V(typically7to13moduleswiredinseries).
SmallgridtiedPVsystems(lessthan5kW)oftenhaveonlyoneortwoseriesstringsofmodules.Withfewwiresandnoneedforseriesfusing,aPVcombinerboxisntnecessary:Stringscanbeterminateddirectlyintheinverter(seetheWhySeriesFusing?sidebar).However,largerresidentialsystems(between5and10kW)oftenhavethreeormoreseriesstrings,andwilllikelyneedfusing.Systemsabove5kWalsotypicallyhavemorewirestodealwith,whichcanmeanmoreinstallationtime,largerconduit,andincreasedwirecostsifallthewiresneedtoberuntotheinverter.Acombinerboxoffersaplacetohouseseriesfusesandparalleltheseriesstringinputs,reducingthenumberofarrayoutputwiresneededtoruntotheinverterlocation.
Somegriddirectinvertermanufacturersincludeseriesfusingwithintheinverteroritsattacheddisconnectbox,whichmayeliminatetheneedforaseparatecombinerbox.However,usingaPVcombinerboxlocatedclosetothePVarraygivesyoueasyaccesstothewiresforeachseriesstring.ThiscanbehandyfortroubleshootingamalfunctioningPVarraywithouthavingtorunbackandforthtotheinverterarea.
GridDirectSystems
LetslookatagridtiedPVsystemwiththreeseriesstringsof10moduleseach.Threeseriesstringsmeanthreesetsofwiresthreepositivesandthreenegatives,withonegroundwirecomingfromourPVarray.EventhoughsomeinvertershavemultipleDCinputs,theymaynotincludeseriesfusingcapability.Thenitslikelythatseriesfusingwillbeneeded.Evenifallthenecessaryinputfusingisprovidedwithintheinverter,acombinercanbeusedtoeliminaterunningsevenwiresallthewayfromthePVarraytotheinverterpossiblyreducingwiringcostsandinstallationtime.
BatteryBasedSystems
Batterybasedsystems(offgridandgridtiedwithbatterybackup)havelowerDCsystemvoltagesthangriddirectsystems(althoughhighervoltagechargecontrollersareexpectedtohitthemarketsoon).Historically,thisvoltagerangedfrom12to48Vnominaltomatchbatteryvoltages,butstepup/downMPPTchargecontrollersnowallowarrayswithhighervoltagestochargelowervoltagebatterybanks,makinghigherarrayvoltagesmorecommon(withopencircuitvoltagesusuallylimitedto150V).Evenwiththeuseofstepup/downchargecontrollers,typicallyonlyonetosixmodulesareplacedinaseriesstring,whichmeanscombiningmorestringsinparalleltogetthedesiredpoweroutput,whichnecessitatesacombinerbox.
Inaddition,chargecontrollerstypicallyhaveonlyasinglesetofinputwireterminals.Foroffgridsystems,itsoftenwise
By:LenaWilensky
PublishedIn: Issue#132,August/September2009
7/13/2015 PVCombinerBoxBuyer'sGuide|HomePowerMagazine
http://www.homepower.com/articles/solarelectricity/equipmentproducts/pvcombinerboxbuyersguide?v=print&print=true 2/3
tohaveenoughroominthecombinerforaddingmorestringsofmodulesinthefuture.Ifthecombineranditsoutputwiresaresizedaccordingly,andthechargecontrollerhasbeensizedtohandletheadditionalamperage,wiringmorePVmodulesintothesystemcanbeaseasyasterminatingthenewwiresinthePVcombinerbox.
TheSpecs
TheSpecificationstableonthefollowingpagessummarizesthemainfeaturescommontomostPVcombinerboxes.WeincludedproductsthatarecurrentlyavailabletoconsumersintheU.S.market.
MaximumDCVoltage(Max.VDC)
EachcombinerisratedtoaccommodateuptoaspecificDCvoltage.ResidentialgriddirectsystemsgenerallyaccommodateDCvoltagesupto600V,whilemostbatterybasedPVsystemDCvoltagesarelower,typicallymaxingoutat150V.
MaximumNumberofPVInputCircuits
CombinerboxescomewithacertainnumberofterminalswherePVsourcewirescanbeattachedandanequivalentnumberofOCPDspaces.ResidentialPVcombinersusuallyneednomorethan12inputs.Griddirectsystemsrequirefewerinputs,whilebatterybasedsystemscanrequiremore.Additionally,batterybasedPVsystemscommonlyhavemultiplechargecontrollers,andeachmayhaveitsowncombinerbox.
MaximumOCPDRating
Theovercurrentprotectiondevices(OCPDsfusesorcircuitbreakers)chosenfortheparticularcombinercannotexceedthisrating.PVmodulenameplateslisttherequiredseriesfuserating.
OCPDType
CombinerboxescomewithOCPDs,usuallyeitheraformoftouchsafefuseholdersorDCratedcircuitbreakers.Sincegridtiedsystemscangenerateupto600VDCwellabovethe150VDCratingofmostavailablecircuitbreakersfusesarealmostalwaysnecessary.OCPDsfoundincombinerboxesaregenerallylimitedtobatterybasedsystems.Mostfuseholdersarenotratedtobeopenedunderload,sotheycannotbeusedasaDCdisconnect.ThisisanespeciallyimportantconsiderationforhighvoltageDCsystems:Tryingtoopenafuseholderwithoutfirstopeningthecircuit(whichcanbesafelydoneviathemainDCdisconnect),willlikelydrawanelectricalarcthatcancauseafireandburnand/orshockyou.
WireRange:SizeofInput&OutputTerminals
Wireterminalsareratedtofitarangeofwiresizes,inbothbusbarsandtheOCPDs.Becausetheoutputcircuitiscarryingthecombinedampacityofalltheincomingseriesstringsandalsomayhavetobeoversizedduetovoltagedropforlongerwirerunsinlowervoltagesystems,itmaybenecessarytoconnectarelativelylargediameterwiretotheoutputterminals.
MaximumNumberofOutputCircuits.
Sometimesitisappropriatetousemultipleinvertersorchargecontrollerswithasinglearrayliketwooffourstringsintooneinverterandtheothertwointoasecondinverter.Forthis,youllneedeithermultiplecombiners,oronecombinerthatcanaccommodatetwoseparateoutputcircuits.Ofallthecombinerslisted,onlyahandfuloffertwooutputcircuits.
MaximumContinuousOutputCurrent
Eachcombinerhasaratingfortheamountofoutputcurrentthatitcansafelyhandle.DependingonthesizeandnumberofOCPDsinuse,itmaybepossibletooverloadthecombinersbusbars.Forexample,inacombinerwithsixteen20Acircuitbreakers,320Acouldflowintotheoutputcircuit,overloadingthebusbarifitisratedat100A.Toensurethisdoesnothappen,useacombinerwithahighenoughoutputrating.
DataMonitoringCapabilities
Whileindividualstringmonitoringisgenerallyusedincommercial,ratherthanresidentialscale,systems,some
7/13/2015 PVCombinerBoxBuyer'sGuide|HomePowerMagazine
http://www.homepower.com/articles/solarelectricity/equipmentproducts/pvcombinerboxbuyersguide?v=print&print=true 3/3
homeownersareinterestedintrackingtheperformanceofeachstringofPVmodulestoensurethesystemisworkingoptimally.Combinerboxeswithdatamonitoringcapabilityareoftenreferredtoassmartorintelligentcombiners,andalloweasy,quickinstallationofdatamonitoringsystems.
WeatherproofRating
MostcombinerboxesareinstalledneartheirPVarrays(thatis,intheweather),andrequireappropriateoutdoorratings.InstallationsareexpectedtolastatleastthelifetimewarrantyofPVmodules(about25years),soinstallingdurable,longlivedequipmentthatwillstanduptotheenvironmentinwhichitisplacediscritical.
EveryapprovedelectricalenclosurehasaNationalElectricalManufacturersAssociation(NEMA)ratingassociatedwithit.TheratingsonPVcombinersincludeNEMA3and3R(ratedforoutdooruseinlimitedorientations)andNEMA4and4X(includesadditionalprotectionsoboxescanbemountedinanyorientation,fromverticaltohorizontal).Wiringmustentertheboxthroughacordgripthatwillproperlysealthepenetrationandupholdtheweatherproofratingofthatcombinerbox.
Listing
TheNECrequiresthatallelectricalequipmenthaveavalidsafetytestlistingtoensuresafetyforthegeneralpublic.IntheUnitedStates,mostofthistestingisperformedbyUL,anindependentnonprofit,althoughthereareotherindependentlaboratoriesthatwilltesttoULstandards.OnesuchlabisIntertek,whichofferstheETLlistedmark.AllofthePVcombinerboxesinthetablehaveacurrentorpendinglisting.
Access
LenaWilenskyiscoownerofNunatakAlternativeEnergySolutionsinCrestedButte,Colorado.ShemanagestotaketimeoutfromplayinginthemountainstodesignandinstallPVandremotepowersystems,andinstructPVclassesforSolarEnergyInternational.
lrecinosGreatarticle,thanks!!
1year3monthsago