AC Disconnect Switch question

cygone
cygone Registered Users Posts: 6 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
I am new to this board and am very grateful for discovering it. My building inspector has just approved my project and issued a building permit. I am installing 15, 240 watt Sharp panels with Enphase micro inverters just north of Santa Barbara, California. I live within three miles of the ocean and we sometimes are subjected to strong winds and rain. The building department is requiring I put an AC disconnect switch on the roof as well as another AC disconnect on the ground next to the circuit breaker panel. Essentially I will have two AC disconnect switches in a row.
I would like to find an AC disconnect box that is watertight for the roof. Any suggestions? Most of the boxes are fused, do I need to use a fused box in either location. Thanks for the help.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,658 admin
    #2
    Re: AC Disconnect Switch question

    No, as far as I know, the panel breaker is your branch circuit current limit.

    Wow--Up north, our utility (PG&E) has long dropped the external/lockable disconnect for safety reasons.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: AC Disconnect Switch question

    It doesn't make sense to have an AC disconnect on the roof; not accessible and therefor doesn't meet NEC regs as far as I know. Besides which, if you shut it off down below the inverters will output zero; same as if a wire should break or short or cause any other kind of interruption/variation to the inverter output connection to the main panel.

    Sorry to say it sounds like another case of an inspector who doesn't know what he's doing and is requiring something that is unnecessary. Unfortunately that doesn't mean you won't have to comply.

    Not very helpful, I know. Working one of these http://www.homedepot.ca/product/60-amp-outdoor-enclosed-non-fused-pullout-safety-switch-1-phase-2-wire-240v/941186 into the design probably wouldn't be easy either.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,658 admin
    #4
    Re: AC Disconnect Switch question

    Sounds like they are trying to help the firemen...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #5
    Re: AC Disconnect Switch question

    I agree. But why two disconnects. If the fire departments are well versed on pv systems, and the codes required disconnects at the same location, one disconnect should be more than enough. If I was on a roof and saw pv panels, I would ask the officer in charge to disconnect the power at the ground disconnect. Even if they are disconnected on the roof, the wires are still charged between the disconnect and the panels. Sounds like someone asking for more because they are unsure of the system. Just my 2 cents.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,658 admin
    #6
    Re: AC Disconnect Switch question

    I am not a fan of multiple switches/disconnects. More confusion (I thought I turned that off) and more things to fail/go wrong.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • drees
    drees Solar Expert Posts: 482 ✭✭✭
    #7
    Re: AC Disconnect Switch question

    Disco's on the roof are completely overkill. There will be no power on the roof when the main AC disco is off and locked!

    That said, I have them on my system because early Enphase sample single line diagrams included them! They no longer recommend roof mounted AC disconnects since they recommend disconnecting the first inverter in the string when working on the array as a safety measure. I will very likely remove my switches at some point in the future.

    I just used a regular outdoor rated junction box with 2 2-pole 20A switches. Not ideal because the switches are 65*C rated, but with temperature correction factors it's OK - I only have 6A going through them at most. Not sure what the temperature rating of your typical big AC disconnect is, but I'm sure it's not 90*C, either...
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  • cygone
    cygone Registered Users Posts: 6 ✭
    #8
    Re: AC Disconnect Switch question

    The requirement for the roof AC disconnect comes from the local fire department and not the building department. Fire department also required I leave a three foot path around the entire array including three feet at the ridgeline. Thanks so much for the quick response, finding this site has helped me quite a bit.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,658 admin
    #9
    Re: AC Disconnect Switch question

    Wow--That would have really put a crimp on my 6 year old GT system... It covers most of the "sunny" back side section of my roof (smaller home, 2 story makes for a relatively smaller sq footage of roof).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: AC Disconnect Switch question
    cygone wrote: »
    The requirement for the roof AC disconnect comes from the local fire department and not the building department. Fire department also required I leave a three foot path around the entire array including three feet at the ridgeline. Thanks so much for the quick response, finding this site has helped me quite a bit.

    Never heard of this before and can't see any reason for it either. I wonder what it is based on? firerescue712, have you got any ideas? :confused:

    Let's see; three feet above the array, three feet below the array, five feet for the array ... I just ran out of roof! :roll:
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  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    #11
    Re: AC Disconnect Switch question
    firerescue712 wrote: »
    ... If the fire departments are well versed on pv systems ...

    In my city, one of 60 in the U.S. with a fire department that's Class 1 ISO certified, the department's standard procedure is to cover any rooftop PV modules with tarps at any structure fire call.

    Best regards,

    Bill
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #12
    Re: AC Disconnect Switch question
    cygone wrote: »
    I am new to this board and am very grateful for discovering it. My building inspector has just approved my project and issued a building permit. I am installing 15, 240 watt Sharp panels with Enphase micro inverters just north of Santa Barbara, California. I live within three miles of the ocean and we sometimes are subjected to strong winds and rain. The building department is requiring I put an AC disconnect switch on the roof as well as another AC disconnect on the ground next to the circuit breaker panel. Essentially I will have two AC disconnect switches in a row.
    I would like to find an AC disconnect box that is watertight for the roof. Any suggestions? Most of the boxes are fused, do I need to use a fused box in either location. Thanks for the help.
    Unfortunately, building inspectors are sometimes a bit behind the curve on solar technology. This requirement comes from conventional systems where even when the system is shut down, the DC conductors on the roof and in the conduits are still live, and firefighters are at risk. In microinverter systems, of course, when the main breaker on the service is turned off, then the only live conductors are the short cables from each module to its attendant microinverter, and a roof mounted AC disco doesn't do anything at all about that. The inspector and whoever is telling him what to do apparently do not understand that UL1741 requirements have already killed those conductors once the firefighters shut down the power to a building in the event of a fire.

    Plead your case to the inspector, but be prepared to have to do it anyway.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: AC Disconnect Switch question

    It should also be pointed out that with micro-inverters there is no high Voltage DC on the roof.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #14
    Re: AC Disconnect Switch question
    cygone wrote: »
    The requirement for the roof AC disconnect comes from the local fire department and not the building department. Fire department also required I leave a three foot path around the entire array including three feet at the ridgeline. Thanks so much for the quick response, finding this site has helped me quite a bit.

    Yeah our code guys are requiring 3 ft at the ridge line now, glad mine was in before that one hit. I think it is the fire departments as well.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #15
    Re: AC Disconnect Switch question

    The reason for the three foot openings are to allow for cutting a hole in the roof to vent heat and gasses in the event of a fire. We are taught to cut a minimum or a 2' x 4' opening in the roof. It also allows access for the firefighters to walk (while carrying axes, ladders, saws, pikes poles) around the solar panels without having to crawl over them or wedge themselves between the rows of panels. I am still stumped by the requirement for the disconnect on the roof, though. I hope this explains the 3 foot requirements. It is nice to give back a little amount of the vast knowledge you have shared with me, Cariboucoot.
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  • newenergy
    newenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    #16
    Re: AC Disconnect Switch question

    Some cities always require roof mounted disconnects and some only when there are roof mounted inverters (usually microinverters). It is a silly requirement as roof mounted inverters will shut off when the power is off and with ground mounted inverters it only shuts the power off after the disconnect - the array is still hot. A roof mounted disconnect will probably just give the firefighters a false sense of security.

    I don't know about a watertight disconnect. Generally it's hard to mount a disconnect on the roof and we go with an outdoor pull out (A/C type) disconnect on microinverter systems

    Steel_NF_Large.jpg

    on a central inverter the roof disconnect has to be DC and it's usually more like:

    Basics_SolarElect_ArrayDisc.jpg.

    (pictures way not to the same scale - top disconnect is much smaller than the bottom one)

    Even on the microinverters, some cities will require a knife blade type disconnect.
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