Code Questions for Outdoor AC Combiner Box

I am designing a system that would have two Fronius 7.5 string inverters that would output 32.5 amps each. I plan on using an outdoor main breaker load center to double as both the combiner box and the AC disconnect switch. Just want to bounce a couple of questions off the code specialists.

1. Each inverter will be connected to a 40 amp breaker in the panel, therefore to comply with 20% back feed rule will I need to install at least a 200 amp rated panel but derate the main breaker to 100 amps? Thus the maximum load on any point of the panel bar would be 180 amps on a bar rated at 200 amps.

2. Are there other derating combinations that would work or less expensive ways to go? I know some utilities and inspectors require open bladed switches for AC disconnects but that is not required in my case. I have thought about a four bladed switch, but I do not want to shut down both inverters when servicing one or the other.


Thanks in advance.

Dan
23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    I am designing a system that would have two Fronius 7.5 string inverters that would output 32.5 amps each. I plan on using an outdoor main breaker load center to double as both the combiner box and the AC disconnect switch. Just want to bounce a couple of questions off the code specialists.

    1. Each inverter will be connected to a 40 amp breaker in the panel, therefore to comply with 20% back feed rule will I need to install at least a 200 amp rated panel but derate the main breaker to 100 amps? Thus the maximum load on any point of the panel bar would be 180 amps on a bar rated at 200 amps.

    2. Are there other derating combinations that would work or less expensive ways to go? I know some utilities and inspectors require open bladed switches for AC disconnects but that is not required in my case. I have thought about a four bladed switch, but I do not want to shut down both inverters when servicing one or the other.


    Thanks in advance.

    Dan

    Some Load Centers use busbars rated at 225 or 250 amps with a 200 amp main breaker. If you can find one of those, the 20% rule would give you a total of 270 or 300 amps to work with. 270 would allow you to only reduce the main breaker to 150, while 300 would let you continue to use the standard 200 amp main.
    Similarly, to reduce parts stocking and design costs, some 100 or 150 amp Load Centers use the same 200 amp busbars as the larger models.
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    inetdog wrote: »
    Some Load Centers use busbars rated at 225 or 250 amps with a 200 amp main breaker. If you can find one of those, the 20% rule would give you a total of 270 or 300 amps to work with. 270 would allow you to only reduce the main breaker to 150, while 300 would let you continue to use the standard 200 amp main.
    Similarly, to reduce parts stocking and design costs, some 100 or 150 amp Load Centers use the same 200 amp busbars as the larger models.

    that isn't quite how they meant the 20% rule. the bus is increased by 20% over the panel rating as a safety measure right from the start. that would allow for an extra amount of current over the panel rating, but not on top of the bus rating. a 100a panel can go 120a which means an extra 20a from pv could mingle with the 100a of a utility. if one were to take 20% more than the bus itself then you are exceeding the bus capability and are causing a safety concern for 120% of 120a is 144a and that is in no way acceptable.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    niel wrote: »
    that isn't quite how they meant the 20% rule. the bus is increased by 20% over the panel rating as a safety measure right from the start.

    I think that if you pose the question (or look at existing threads) on a forum such as MikeHolt, you will see that regardless of what may or may not have been meant, the 120% is applied to the busbar rating and not the nameplate rating of the panel or the rating of the originally supplied main breaker.

    If the busbar is rated for x amps, then the sum of all breakers which can supply current to the bus must not excede 1.2x.
    Combined with the requirement that the PV breaker(s) be or other backfeeding breakers be at the opposite end of the bus from the main breaker, that insures that no combination of loads attached to breakers anywhere in between the two can excede the busbar rating at any point along the way.
    Note that this is intended to cover the case of normal loads up to the load breaker ratings and not fault currents, which will be limited the load breakers themselves.

    The extra 20% allowance gives you credit for the fact that any load between the two ends can be getting current from both directions along the bus and therefore not exceeding the bus current rating at any point along the way.

    The backfeed situation differs from a transfer switch situation because it it likely that both sources will be delivering power to the bus at the same time.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    inetdog wrote: »
    Some Load Centers use busbars rated at 225 or 250 amps with a 200 amp main breaker. If you can find one of those, the 20% rule would give you a total of 270 or 300 amps to work with. 270 would allow you to only reduce the main breaker to 150, while 300 would let you continue to use the standard 200 amp main.
    Similarly, to reduce parts stocking and design costs, some 100 or 150 amp Load Centers use the same 200 amp busbars as the larger models.

    How do you tell if a say 150 amp rated panel really has a 200 amp bus bar inside? I'm sure the manufacturers don't disclose this.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box

    i might be more interested in where it gives this ok in the nec rules to add another 20% on the bus rating.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    How do you tell if a say 150 amp rated panel really has a 200 amp bus bar inside? I'm sure the manufacturers don't disclose this.

    Actually, they do if you can find the right place to ask. One way to do it is to find the part number on the busbar assembly and look it up as a replacement part.
    In terms of finding out what manufacturer and what panel to look at, from a cold start, that is not as easy. But I can try to find some references for you.
    SMA SB 3000, old BP panels.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    niel wrote: »
    i might be more interested in where it gives this ok in the nec rules to add another 20% on the bus rating.

    It is not adding 20% to the bus rating, it is allowing the sum of the supply breakers to go up to 120% of the bus rating, which is different. There is an NEC section dealing with backfeeding which states the 120% rule, and I will try to find the section reference for you.
    SMA SB 3000, old BP panels.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    inetdog wrote: »
    It is not adding 20% to the bus rating, it is allowing the sum of the supply breakers to go up to 120% of the bus rating, which is different. There is an NEC section dealing with backfeeding which states the 120% rule, and I will try to find the section reference for you.

    From 2011 NEC, but similar language in different sections in earlier codes:
    705.12 Point of Connection.
    (D) Utility-Interactive Inverters.

    (2) Bus or Conductor Rating. 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 conductor.

    Exception: Where the photovoltaic system has an energy
    storage device to allow stand-alone operation of loads, the
    value used in the calculation of bus or conductor loading
    shall be 125 percent of the rated utility-interactive current
    from the inverter instead of the rating of the overcurrent
    device between the inverter and the bus or conductor.

    This can be found, along with a helpful discussion, on the MikeHolt form in this thread.

    Where it is difficult to meet the 120% rule, the common alternative is to connect the backfeed subpanel on the POCO side of the main panel/ service entrance (referred to as a "line side tap") There are a variety of rules governing such a connection, including what size and kind of wire and what kind of panel (Service Rated) has to be used for the backfeed, and a licensed electrician should be able to find all of the information. However, although the NEC specifically allows for a line side tap, some POCO's may take issue with it, since it is so intimately involved with their service connection point.
    SMA SB 3000, old BP panels.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    How do you tell if a say 150 amp rated panel really has a 200 amp bus bar inside? I'm sure the manufacturers don't disclose this.


    Take a look at the MikeHolt forum link that I posted in my reply to Neil. One suggestion there is
    CUTLER HAMMER MAKES A 225 RATED BUSS WHICH WOULD ALSO WORK
    Unfortunately that is still a few amps short of what you need.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    inetdog wrote: »
    From 2011 NEC, but similar language in different sections in earlier codes:



    This can be found, along with a helpful discussion, on the MikeHolt form in this thread.

    Where it is difficult to meet the 120% rule, the common alternative is to connect the backfeed subpanel on the POCO side of the main panel/ service entrance (referred to as a "line side tap") There are a variety of rules governing such a connection, including what size and kind of wire and what kind of panel (Service Rated) has to be used for the backfeed, and a licensed electrician should be able to find all of the information. However, although the NEC specifically allows for a line side tap, some POCO's may take issue with it, since it is so intimately involved with their service connection point.


    Thanks for the info. It is helpful.

    On a line side tap configuration, where is the neutral/ground bond? Does it stay in the main distribution panel or does it need to move out to the line side tap? Is the line side tap considered a disconnect or more important a "first disconnect"? If it is a first disconnect then, in my case, I would have to replace my main panel since it does not have separate neutral and ground bars.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    Thanks for the info. It is helpful.

    On a line side tap configuration, where is the neutral/ground bond? Does it stay in the main distribution panel or does it need to move out to the line side tap? Is the line side tap considered a disconnect or more important a "first disconnect"? If it is a first disconnect then, in my case, I would have to replace my main panel since it does not have separate neutral and ground bars.

    In my limited understanding, the panel which is connected to the line side tap has to be rated for use as a service disconnect. That may not be the case with the panel you have.
    Since the exact same neutral wire is bonded to ground in your main service panel, it must not be again bonded to the ground in your backfeed panel. If you have to add an additional insulated terminal bar to that panel to accomplish this, then that will be necessary. The panel manufacturer should offer as a field mounted accessory a separate ground bar.
    Whether you can use a ground bar other than one from the panel manufacturer is a question for an electrician or inspector.
    I do not see any reason to add a separate ground bar to your main panel, since that it a permitted location for the bond.

    The backfeed panel and the main panel are connected in parallel, usually by running the wires for the backfeed panel via an approved connection method to the wires which also end at the main breaker of the main panel (including the incoming neutral and the local grounding electrode conductor.) You will run four wires between the panels: L1, L2, N and Ground. (Or the Ground may be the conduit connecting the two panels.)
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box

    Well, I need a little clarification. You said a line side tap has to use a panel that is RATED as a service disconnect. OK, but it is not actually THE service disconnect. So then, like you say, the EGC then still bonded with the neutral (ie grounded conductor) in the original main disconnect and only there. OK!

    Following your logic, then the only way to disconnect a line side tap is to pull the meter. Seems a little dangerous for the average homeowner!! But if its allowed............
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    Well, I need a little clarification. You said a line side tap has to use a panel that is RATED as a service disconnect. OK, but it is not actually THE service disconnect. So then, like you say, the EGC then still bonded with the neutral (ie grounded conductor) in the original main disconnect and only there. OK! Seems a little dangerous for the average homeowner!! But if its allowed............

    That's so close to correct.


    Since there has to be either a main breaker or fewer than 5 load breakers inside the feedback panel to allow it and the main panel to jointly serve as "the" service disconnect, you do not have any NEC problems with disconnecting both the various loads and the backfeed circuits. The line side tap wires themselves are (logically if not physically) no more exposed than the wires going from the meter to the main panel's main breaker. But, yes, to actually connect the line side tap in the first place and to do any work on the line side of the backfeed panel you (the licensed electrician) would have to pull the meter. (Actually, have the POCO out to do that, since the seal will have to be broken and then replaced when the work is done.)

    PS: to simplify the labeling requirements and keep from confusing the AHJ, it is probably best not to put any actual load breakers inside the backfeed panel, even if you could use a few extra circuits. :-)

    Another place where the line side tap is legal under NEC, if POCO approves it, is inside the meter socket housing. Same problem there, only more so.

    Not a lot of electricians who have not yet done GTI work are initially aware of the ins and outs of the task.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box

    Thanks, inetdog, for all the help. I've got it now.

    Dan
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box

    Just as an FYI my AHJ refused a line side tap in a residential install, they said it was acceptable in a commercial install. YMMV.

    In my case this created the need for a second 200 amp panel with derated main breaker to 175 amps, a service entrance upgrade to 400 amps with the associated line pull to the transformer.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box

    This is just a follow-up to let everyone who responded what I finally decided to do to connect the second inverter to my system.

    First I nixed the idea of a line side tap. Too expensive and I probable would run into problems with local utility.

    I have decided to install a 150amp rated outside sub panel that will be supplied by an 80amp breaker in the main panel. The main panel is 200amps but I will derate it with a 150amp main breaker so that I can feed the 80amps from the solar system. Thus total possible load from input devices in the main panel would be 230amps (ie 150 plus 80). The 200amp bus bar should be able to carry 240amps per the 120% rule. (ie 200 x 1.2 = 240). Main panel now should be good to go.

    Now since the sub panel is being supplied by 80amps from the main panel and also by two 40amp breakers from the inverters, the total possible load on the buss bar in the sub panel would be 160amps max. (ie 80 + 40 + 40). Using the 120% rule the 150amp bus bar could carry a 180amp load without failing (ie 150 x 1.2 = 180). My possible load from all input devices in the sub panel is 160amps, thus I should be good to go.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    DanS26 wrote: »
    The main panel is 200amps but I will derate it with a 150amp main breaker so that I can feed the 80amps from the solar system. Thus total possible load from input devices in the main panel would be 230amps (ie 150 plus 80). The 200amp bus bar should be able to carry 240amps per the 120% rule. (ie 200 x 1.2 = 240). Main panel now should be good to go.

    Sounds very good, as long as you have verified that the 150 amp main breaker is really adequate to supply your expected loads when the PV is not contributing. I am not sure whether, in this situation, the NEC requires that the main breaker carry a particular calculated expected load, or just that the service be adequate for the calculated load. The calculated load can be either higher or lower than your maximum real life load, depending on your usage patterns.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Code Questions for Outdoor AC Combiner Box
    inetdog wrote: »
    Sounds very good, as long as you have verified that the 150 amp main breaker is really adequate to supply your expected loads when the PV is not contributing. I am not sure whether, in this situation, the NEC requires that the main breaker carry a particular calculated expected load, or just that the service be adequate for the calculated load. The calculated load can be either higher or lower than your maximum real life load, depending on your usage patterns.

    The highest observed load in my house was a little less than 20,000 watts or 83.3 amps assuming my loads are evenly balanced, so derating the main breaker should be no problem even with high motor/compressor starting loads. I use a monitoring system so I do know the actual loads.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
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