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Thread: Battery System Grid-Tie Back-feed Connection Rating

  1. #1

    Default Battery System Grid-Tie Back-feed Connection Rating

    I just got a dual XW6048 grid tie system inspected by the local AHJ. A friend who is also a solar contractor started the install but had vacation plans and needed a stand in for inspection. I was sweating it with fingers crossed but it got signed off.

    The issue in question was the battery system's connection in the utility fed AC panel. The 200 amp service entrance is down the driveway on a pole. The 200 amp utility-fed sub panel is in the garage. The inverter system's 100 amp grid-connection breaker is connected at the opposite end of the bussbars from the 200 amp main in the garage utility sub panel. The backed-up load panel is fed from the inverter load output with a 100 amp circuit breaker.

    This means that the 200 amp utility-fed sub panel (200 amp bussbar rating) is fed with a 200 amp breaker from the utility and a 100 amp breaker from the XW system. The inspector asked me about this with a raised eyebrow. I told him that the 100 amp back-feed breaker connection is larger than the inverter rating but this is needed to provide pass-through current when the utility is available. If you go by the inverter rating instead of the breaker rating the system still has a rating of 50 amps at 240. However, this system has an array capacity of about 5kw as so is only sized to back feed about 21 amps +/-. He accepted this explanation and said that contractors usually try to convince him that it is okay if the source breakers are at opposite sides of the bussbars.

    We talked about how if you go by back-feed breaker size, a back-up system would require a panel rated for at least 300 amps.

    A Siemens note on back feed current rating: (Thanks for posting this in another thread solar_dave)
    http://www.sea.siemens.com/us/intern...LCWPV-1209.pdf

    From the note:
    The 2008 NEC, Section 690.64(B)(2)states, “The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or connector.”
    *Interpretation: Main Breaker + PV Main must be less than or equal to 120% of the busbar rating.



    Anyone have any other ideas or experiences with getting a sign-off with this issue?

    Alex Aragon


    PS. Another pertinent note from a previous thread:
    Here is a link to a note from John Wiles about the "controversy" some inspectors may have regarding NEC code section 690.64(B)(2):
    http://www.nmsu.edu/~tdi/Photovoltai...onnections.pdf
    Last edited by SolaRevolution; April 14th, 2012 at 0:35 PDT.

  2. #2
    Join Date
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    Default Re: Battery System Grid-Tie Back-feed Connection Rating

    How much pass thru current is needed by your setup? I am surprised he let it slide but the back feed really is only 21 amps you are OK I guess. Is the down stream wire rated for 100amps? If not a bit scary if you get a short.

    I know on my grid tie back feed the most I could ever back feed is 46 amps, but with the 60 amp mains breaker it required a derate on the panel breaker to 175 amps. I am about to add another 20 amp circuit to my AC distribution box for some additional back-feed of 13.5 amps. So my total would be 59.5, but to keep the breaker rating at less than Max it will be upgraded to I think an 80 amp and the feed in wiring will be increased to the proper gauge. This will most like create the need to derate even further to 150 amps on the main. I am OK with that as the loads in that panel are 125 sub panel for my office/workshop building and a 60 amp sub panel for the Volt chargers, add up those loads and I am well within the loads limit.

    PS no problem on the PDF.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

  3. #3

    Default Re: Battery System Grid-Tie Back-feed Connection Rating

    The hous AC system really needs the full 200 amp utility feed rating to satisfy the Load Demand Calculations. I once had an inspector require a load demand profile based on NEC specs when I wanted to decrease the main utility breaker to have more backfeed capacity for a large GT system into a 200 amp main. The load demand actually required 225 amps. Oops. The inspector in that case accepted a tap to the sub feed between the 200 amp meter main breaker and the sub panel's 200 amp main breaker bassed on the 10 foot tap rule. NEC 240.21 (B)(1)-(2)

    The back-up panel has the entire kitchen, the 2hp well system, and...? Like I said, I did not perform most of the install, but I am pretty sure the reason for the 2nd inverter was to accomodate the large pass thru current. (Both XWs have their AC1 connections reduced to 50 amps in the programming)

    Alex
    Last edited by SolaRevolution; April 17th, 2012 at 8:38 PDT.

  4. #4

    Default Re: Battery System Grid-Tie Back-feed Connection Rating

    Quote Originally Posted by SolaRevolution View Post
    I just got a dual XW6048 grid tie system inspected by the local AHJ. ...

    I told him that the 100 amp back-feed breaker connection is larger than the inverter rating but this is needed to provide pass-through current when the utility is available. If you go by the inverter rating instead of the breaker rating the system still has a rating of 50 amps at 240. However, this system has an array capacity of about 5kw as so is only sized to back feed about 21 amps +/-. He accepted this explanation and said that contractors usually try to convince him that it is okay if the source breakers are at opposite sides of the bussbars.


    I am no expert, but I'm very interested in what is going on with this system (I have a single XW6048 and 4.7kw PV) and why the choice was made to have two XW converters.

    First, even though the XW6048 has been derated to 50 amps at 240v, isn't that just each one, so the total would be 100 amps at 240v for the system?

    When SolaRevolution (Alex) said "the entire kitchen," does that mean an electric stove too? If so, I can understand the possible need for the huge capacity, but then I wonder if it wouldn't have been more economical to get a gas stove.

    Also, by accepting the 100amp breaker, isn't that asking for a problem in the future when somebody else comes along and says they can just install more PV, since everything is already set up? Or put a generator into the system, which I don't think you even need a permit to do at that point, and have greater capacity that way?

  5. #5

    Default Re: Battery System Grid-Tie Back-feed Connection Rating

    The system designer has two inverters so that the back-up loads panel would be fed by a pass thru current greater than 60 amps. Is this necessary? I don't know. I did not design this system and did not do a load calculation. The back-up load panel is a GE with 20/40 breaker spaces and it is full. Although a single XW could probably handle the load demand at any given time, the NEC load calculations probably exceede 60 amps.

    While waiting for the inspector, the system owner had me install a switch with an LED and a warning buzzer connected to the AUX as a fault indicator so he would know when the utility goes out. This way he can manually turn off any unnecessary loads while running from the (20kWh) battery bank. He can turn off the buzzer but the switch remains lit until the fault clears. This will warn him of any faults in the XW system. The XW shows a fault when the utility drops out and does not clear until after the grid has come back on and remained stable for 5 minutes.

    I have put in multiple XWs in a few systems with with big battery banks just to have additional charging current for battery health and to minimize generator run time. (Not in this case)


    Alex

  6. #6
    Join Date
    Mar 2012
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    Western Nevada
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    Default Re: Battery System Grid-Tie Back-feed Connection Rating

    It is interesting that I had the same conversation with my inspector before I have even submitted my permit request. The SW inverters require a 60 amp breaker in the main control panel but my GE panel is a 200 amp service with a 200 amp main bkr. That would only allow me a 40 amp bkr to the inverter per NEC 705.12(D)(2). But inside the panel door was a spec sheet that clearly indicated that the buss was rated at 250 absolute amps max. The inspector gave me an extra 10 amps to 50. The only stuff I have on my critical panel fed by the inverter is a couple of fridgerators and a few light circuits. I don't know how he let you by with the 100 amp breaker giving you a total of 300 amps of possible source current even though your inverter cannot supply the 100 amps.

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