What good is XW grid-tie with the 120% backfeed rule??

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  • RCinFLARCinFLA Solar Expert Posts: 1,458 ✭✭✭✭
    Re: What good is XW grid-tie with the 120% backfeed rule??

    The old SW series had a breaker mounted in the inverter line to the ACin-ACout bus. Ignoring the present fact that SW series doesn't meet 1741 requirements, it would seem an inverter line breaker, rated at inverter max current, would help to ensure the backfeed limit is backed up with a physical breaker. This would seem to allow a 60 amp main panel breaker feed to Aux panel but still have a 35 amp breaker protected grid backfeed. Main panel bus would still have to be rated to 35 amps plus main panel breaker input amperage but that is better then having to reduce the 60 amp feedthrough to aux panel.

    Looking through the XW manual I did not see this unit based breaker on XW inverter line. The XW 240/120 vac complicates things a bit allowing a higher single sided current (120vac) then a balanced 240 vac load. I am not sure of the interpretation of NEC with regard to an electronic regulated limit on inverter grid backfeed output. I would tend to agree that this may not be the safest situation without a secondary failsafe mechanical breaker.
  • Ralph DayRalph Day Solar Expert Posts: 977 ✭✭✭✭
    Re: What good is XW grid-tie with the 120% backfeed rule??

    I have an SW4048 and with the added GTI (grid tie interface ) it is 1741 compliant. It's an add on box with lots of gadgets and stuff inside. Bought but unused as of yet...

    Ralph
  • RCinFLARCinFLA Solar Expert Posts: 1,458 ✭✭✭✭
    Re: What good is XW grid-tie with the 120% backfeed rule??

    The GTI adaptor was a not too successful 'bandaid'.

    Early SW models allowed selling without GTI adaptor. I think the software was changed in the 2004-2005 timeframe to prevent it without GTI to allow them to continue selling them.

    I have a SW4048 that allows selling stand alone, think it is rev 4.01 SW.

    Anyway the point was SW's have a 35 amp breaker in the inverter branch. With 60 amp branch breaker feed the 35 amp breaker will prevent any more into or out off inverter/charger.

    You can still suppliment ACin to ACout current which could create another delima in there could be 60 + 35 amps to ACout to pop breakers. This will not normally happen but it relies on current sense transformer measurement and electronic control so, in a way, you are back to same 'catch22' as XW.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: What good is XW grid-tie with the 120% backfeed rule??
    aj164 wrote: »
    With a battery-backed Xantrex XW system on the grid, the output of the system typically goes to a sub-panel that you want backed up in case the grid goes down.

    The breakers in the XW distribution box are 60A, but the maximum size breaker I can put on the grid 200A service panel is 40A (due to NEC 120% backfeed rule).

    Something doesn't seem right about this. At night, with the grid on and inverter off, it means I'm actually limited to 40A at the sub-panel. (I don't want the inverter supplementing any part of the load from batteries as long as the grid is available.)

    Is there another way to do this? Meter load-side tap?

    Thanks,

    AJ

    OK, this was the original question. I tried to add additional information, but I knew that Solar Guppy had already stated the answer previously. Since he is the expert, I'll quote (italics for his quote) from his 9/1/2009 post in the "Some help tweaking an XW6048" thread:

    The panel should have 60 amps, not 40 if you want the load side of the XW to have the 12kw rating.

    For the back feed requirement, that's meet by the software in the XW, it limits the back feed to 6kw maximum ( 30 amps ) and that is what is used for the back feed calculation for the main breaker panel, not the 60 amps.

    The confusion is straight GT inverters only sell energy, the XW can have power flow in both directions, for Sell its 30 amps, for load support, its 60 amps
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