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Thread: Sizing a Utility Transformer for PV and Building Load

  1. #1
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    Sizing a Utility Transformer for PV and Building Load

    Hello,

    Say I have a 2 building loads of 208Y/120A at 400amps. The transformer cans are 3 50KVA at 208Y/120V. I want to interconnect at the splice blocks the secondary side of the transformer so it will offset both mains and their respective meters. I'll be using inverters that have a 40A max. Can I size up to 320A or 400A for the solar? Is there anything else that I should be careful of? I will be seeking out to a professional just trying to get a heads up before I approach them.

    Thanks

  2. #2
    Join Date
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    Quote Originally Posted by baserunner4 View Post
    Hello,
    ... I want to interconnect at the splice blocks the secondary side of the transformer so it will offset both mains and their respective meters. ...

    What does that mean? Where are the meters? You cannot backfeed two meters with one solar system. What is the metering regime for this solar system?

    Other than that, if you have a supply side connection and a 400A service then you could backfeed 320A nameplate inverter current, yes.

  3. #3
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    Quote Originally Posted by jaggedben View Post

    What does that mean? Where are the meters? You cannot backfeed two meters with one solar system. What is the metering regime for this solar system?

    Other than that, if you have a supply side connection and a 400A service then you could backfeed 320A nameplate inverter current, yes.

    I didn’t think of that. I think it would be a new service. The current meters are after the main circuit breakers.

  4. #4
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    Quote Originally Posted by baserunner4 View Post
    Hello,

    Say I have a 2 building loads of 208Y/120A at 400amps. The transformer cans are 3 50KVA at 208Y/120V. I want to interconnect at the splice blocks the secondary side of the transformer so it will offset both mains and their respective meters. I'll be using inverters that have a 40A max. Can I size up to 320A or 400A for the solar? Is there anything else that I should be careful of? I will be seeking out to a professional just trying to get a heads up before I approach them.

    Thanks
    400A OCPD and 320A inverter output. NEC 705.12(A), "The sum of the ratings of all overcurrent devices connected to power production sources shall not exceed the rating of the service." The utility will have to allow virtual net metering for this to offset the meters.

  5. #5
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    Nov 2015
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    I should say one more thing on this subject. Utilities rate things differently then we do based on the NEC. So while the distribution transformer might have a nameplate rating of 150kVA and the utility owned conductors might be rated for 400A by the NEC the utility might not rate this as a 400A service. Get the service rating from the utility and use that to base the back feed on.

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