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Thread: Photovoltaic System Design

  1. #1
    Join Date
    Jul 2012
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    Oakland,California
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    Photovoltaic System Design

    I recently graduated from a Photovoltaic Solar Sytem Design class (crunch course) and now working to actially design a system. I need help in designing the system as follows:

    45 Suniva 260 watt module

    What the proper design?

    do I design it in 5 strings of 9 modules with an 3 phase 11.7 inverter with a combiner box in between module and inverter? What wire's should I use: From modeule to combiner box, combiner box to inverter and from inverter to utility subpanel. What about the size of the cisrcuit breaker?

    or desin it in 3 strings of 10 modules with a 6.0 KW inverter plus 2 strings of 8 modules with a 5KW inverter? How will i show it as a line diagram with all wires indicated?

  2. #2
    Join Date
    Jul 2009
    Location
    Mesa Arizona
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    Apparently your crash course did not include defining all the necessary variables in such a design. Are you sure this is not homework?

    Some of the missing information:

    The lowest temperature at the installation site figures into the calculation of the number of series PV modules that are allowable. High temperature figures into wire sizing.

    Also needed is the voltage limits of the inverter if you have picked one. If the inverter is not specified, then many need to be considered. The AC breaker depends on the specifications of the inverter for the service voltage. Wire size depends on the circuit breaker and temperature.

    Distances: wire length figures into some of the calculations, especially the AC cable.

    You do not specify the voltage of the 3-phase service.

    Location: electrical codes vary.

    In general, if there are four or more parallel PV strings to an inverter, consider the use of a DC combiner. Conduit fill derating is a factor when there are more wires in a conduit.

    Smaller inverters are usually single phase. If you need to use multiple smaller inverters, use three the same size.

    Most inverter manufacturers have websites with utilities for some of these calculations.

    There is likely no single optimum design, but there are many proper designs. A full design must also cover many utility issues, grounding, labeling, consideration of the capability of the point of connection and its ratings, how to mount the PV modules, etc.

  3. #3
    Join Date
    Jul 2012
    Location
    Oakland,California
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    26
    Quote Originally Posted by BillK-AZ View Post
    Apparently your crash course did not include defining all the necessary variables in such a design. Are you sure this is not homework?

    Some of the missing information:

    The lowest temperature at the installation site figures into the calculation of the number of series PV modules that are allowable. High temperature figures into wire sizing.

    Also needed is the voltage limits of the inverter if you have picked one. If the inverter is not specified, then many need to be considered. The AC breaker depends on the specifications of the inverter for the service voltage. Wire size depends on the circuit breaker and temperature.

    Distances: wire length figures into some of the calculations, especially the AC cable.

    You do not specify the voltage of the 3-phase service.

    Location: electrical codes vary.

    In general, if there are four or more parallel PV strings to an inverter, consider the use of a DC combiner. Conduit fill derating is a factor when there are more wires in a conduit.

    Smaller inverters are usually single phase. If you need to use multiple smaller inverters, use three the same size.

    Most inverter manufacturers have websites with utilities for some of these calculations.

    There is likely no single optimum design, but there are many proper designs. A full design must also cover many utility issues, grounding, labeling, consideration of the capability of the point of connection and its ratings, how to mount the PV modules, etc.
    Sorry for the missing info:
    The place is in Oakland, California. The lowest record low is 24 f and the average high is 75 f.
    My initial thought was use a 11/7 kw inverter 3 phase. I was thinking of using #8 wire neutral and #10 wire for grounding and i did a few calculation and came up with a 40a for the circuit breaker (based on the inverter output).

  4. #4
    Join Date
    Jul 2012
    Location
    Oakland,California
    Posts
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    Design result

    Most of my design just was all residential and no more than 18 panels. This is my first commercial type design involving 45 panels. Any critique is greatly appreciated.

    Record low=24f Average High=75f. Module facing SW. using (45) Suniva 260W modules where 30 modules are mounted a flat roof with a Rayport-B ballast system at 10 degrees tilt and the other 16 modules mounted on a proposed 30'x12' steel canopy with unirac mounting system.

    My proposed system design is 5 strings of 9 modules, a combiner box tied to a 11.4KW Fronius IG Plus, 3-phase (208V/240V/277V) grid tied inverter.

    what and how many wires should I use from module to combiner box, wires going from combiner box to inverter and wires from inverter to new utility sub-pabel. My thoughts are to use #8 AWG - THHN/THWN-2 for conductors, #8 AWG - THHN/THWN-2 for Neutral and #10 AWG - THHN/THWN-2 for ground. Should wires be in a 3/4" or 1" EMT conduit? Should I derate the wires?
    Last edited by mnlara; 07-30-12 at 03:54 AM.

  5. #5
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
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    Quote Originally Posted by mnlara View Post
    Most of my design just was all residential and no more than 18 panels. This is my first commercial type design involving 45 panels. Any critique is greatly appreciated.

    Record low=24f Average High=75f. Module facing SW. using (45) Suniva 260W modules where 30 modules are mounted a flat roof with a Rayport-B ballast system at 10 degrees tilt and the other 16 modules mounted on a proposed 30'x12' steel canopy with unirac mounting system.

    My proposed system design is 5 strings of 9 modules, a combiner box tied to a 11.4KW Fronius IG Plus, 3-phase (208V/240V/277V) grid tied inverter.

    what and how many wires should I use from module to combiner box, wires going from combiner box to inverter and wires from inverter to new utility sub-pabel. My thoughts are to use #8 AWG - THHN/THWN-2 for conductors, #8 AWG - THHN/THWN-2 for Neutral and #10 AWG - THHN/THWN-2 for ground. Should wires be in a 3/4" or 1" EMT conduit? Should I derate the wires?
    That's the problem, IMO, with "crash courses". They take your money and show you how to do a few specific things, but when it comes time to generalize, you can't do it. Wire sizing needs to be calculated from derated ampacities, conduit size determined from tables in the NEC... but if you can't figure out how many conductors to run, then you may have wasted your money.

  6. #6
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
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    3,247
    If this is residential, then consider using the caluclators for a particlar inverter manufacturer.
    http://www.power-one.com/renewable-energy/pv-inverters http://stringtool.power-one.com/

    Can the service handle 10kW worth of PV? Will it be the load or supply side of the service disconnect?
    Ron

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