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  1. #1
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    480V vs 208V inverters

    Some info needed on commercial systems. I do mainly resi.

    IN GENERAL
    For commercial, what is the most common AC Voltage in the USA?
    How common is 208 Vac Wye?
    If, as I read, 208 is the most common commercial voltage why are "all" solar inverters 480V/277V?
    Is it standard practice in comm. solar to install a transformer to get to your particular AC V?

    SPECIFICALLY
    Right now, I am tasked with re-powering an (E) 28 kW-DC, 15 yr old system with an (E) 208 V Wye Xantrex inverter (30 kVA) and separate ground mounted isolation transformer.
    I am having a helluva time finding a good inverter match.

    To make it harder, The DC circuit is combined on roof into a single #2/0 conductor circuit.
    So, I need a single inverter (not two).
    I also need a single DC input on that inverter that can handle that amperage. Many 30 kW inv.s seem to have multiple inputs.

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    Have you evaluated what it would cost to replace the single feed circuit with a multiple wire circuit instead and divide the array output at the combiner location?
    It may be better in the long run to incur this incremental expense to allow more choice in the configuration of the rest of the system. Otherwise your limitation to either a single MPPT input or several MPPT inputs slaved to single master for Vmp setting may make it impractical to use available modern inverters.
    Note that some inverters with two or more sets of input terminals allow you to configure them either as two separate MPPT inputs or parallel them into a single MPPT input.

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    I would say 208V services are much more common than 480 in terms of number of buildings with each. I have always been a bit baffled by the lack of offerings in 208 inverters - both string and central. IIRC, the best string you can do is a fronius 15KW symo. That will do 1kv strings, and is probably the only 208 string inverter out there that does 1kv strings.

    If you didnt see it, in this thread https://forums.mikeholt.com/showthread.php?t=197410 from a little while back I ranted against using 480 inverters with transformers to connect to 208 services. Obviously though, depending on the size and wire run, just the wire savings could make it worth having a tranny.

    I concur with Goldie about seeing what it would take to split that combined DC up. You would have more inverter options, and likely multiple MPPT's
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by Zee View Post
    Some info needed on commercial systems. I do mainly resi.

    IN GENERAL
    For commercial, what is the most common AC Voltage in the USA?
    How common is 208 Vac Wye?
    If, as I read, 208 is the most common commercial voltage why are "all" solar inverters 480V/277V?
    Is it standard practice in comm. solar to install a transformer to get to your particular AC V?

    SPECIFICALLY
    Right now, I am tasked with re-powering an (E) 28 kW-DC, 15 yr old system with an (E) 208 V Wye Xantrex inverter (30 kVA) and separate ground mounted isolation transformer.
    I am having a helluva time finding a good inverter match.

    To make it harder, The DC circuit is combined on roof into a single #2/0 conductor circuit.
    So, I need a single inverter (not two).
    I also need a single DC input on that inverter that can handle that amperage. Many 30 kW inv.s seem to have multiple inputs.
    A light commercial application will likely be built with a 120/208V service, and this is the default service voltage unless there's an advantage to having 277/480V. Even larger commercial/industrial applications, will most likely have a 120/208V grid somewhere on the premises, due to 120V needed for receptacle loads. 277/480V would be the main service voltage if most loads need 277V or 480V, while a step-down transformer might handle ~10% of the building loads that need 120V.

    In spite of this fact that 120/208V services are more common, inverters for these grids are less common because of economic reasons. Generally, voltage is cheap while current is expensive. Voltage is "paid for" with insulators like plastic and ceramics, while current is "paid for" with metal conductor/semiconductors and heat sinks. A 208V 3-phase inverter will be about 40% the power of an otherwise equivalent 480V 3-phase inverter. Case and point, compare SolarEdge's 14.4kW inverter with their 33.3kW unit. Same physical size, same output amps, same input amps & string qty, same housing, same heat sink. The 33.3 kW unit has more economy of scale on its cost, and the 33.3 kW has the capability of higher DC operating voltage, which gives you an advantage on the DC side of system design. For this reason, 480V grids are the standard solution for an array not electrically connected to a building, which puts economic pressure to manufacturer 480V inverters in greater numbers.

    It used to be that a transformer was part of the inverter. The winding ratio would allow you to build roughly the same inverter, and adapt it to ether grid voltage per how its internal transformer is built. Or by changing the taps on an adjustable transformer, as was the practice with string inverters. With transformerless inverters, that is no longer the case. You would need an external transformer to step from a 480V inverter to a 208V building grid.
    Last edited by Carultch; 05-09-19 at 11:02 PM.

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    The "standard" voltage rating of the services you run into will pretty much depend on the size of the facilities you work with. Typically once the current at a service hits a certain point, and this varies by utility, the utility will force a change in the service. It usual progresses something like 240/120V split phase to 240/120V 3ph or 208/120V 3ph then to 480/277V 3ph. After that you get into primary voltage service for really high ampacities.

    If you deal mostly with smaller commercial buildings and smaller PV systems then for you most services will be 240/120V split phase with some 240/120V or 208/120V 3ph once in a while. If you deal mostly with larger buildings and campuses then you will mostly see 480/277V 3ph with the occasional 240/120V or 208/120V 3ph and then you use a transformer.

    Worst thing in the world is trying to connect a large PV system to a 208V service. Crazy number of parallel conductors, maybe a cable duct between the transformer and the service SWBD. If it's a supply side interconnection, and it almost always is, then you have to land all that in way too small an area.
    Last edited by pv_n00b; 05-10-19 at 06:30 PM.

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    Thank you gentlemen for the excellent and diverse info!

    Looks like I have to "uncombine" the DC input AND add a 480 to 208 VAC transformer.

    Although,
    I can skip transformer by getting 2 @ 15 kW (Solectria or Fronius Symo) inverters that are available in 208 VAC output.

    Still comparing prices, features, LABOR etc

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    So after weeks of delay and digging, it looks like i cannot easily un-combine the 11 DC strings.
    So i need an inverter with a single DC input.

    The Fronius SYMO fits the bill, due to what it mentions as "direct pass through avaialable" for pre combined strings)
    But, largest SYMO is 24 kW.
    Array size is 31.79 kW (10 or so years old, Mitsubishi 170W mod.s)

    SE facing at approx 30 degree pitch.

    1. What will I lose annually in AC output? and... by "I", I mean customer

    2. Yaskawa (AKA Solectria) has a 28TL inverter which I prefer as 28 kW inverter would take 32 kW array - no prob.
    They mention "fuse bypass available" but I am not sure what that means..... and how many amps allowed on DC input?

    FWIW - The array rated Imp is 76A on 1/0 conductors.

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    Never mind above questions. Fronius SYMO inverter it is.

    Is there a commonly used 24 kW, 480 to 208 VAC transformer in the solar biz?
    Any particular features or any xformer with above rating will do?

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