SMA DC Disconnect

lukeylukey Registered Users Posts: 21
I am having conflicting statements in regards to the SMA DC disconnect. I am planning an array with 2 strings of 14 BP SX 3195 and the Sunny Boy 5000. I talked to an installer and he said that the amperage of the strings are too high for the DC disconnect. Now if I have my two strings wired in series and going directly into the DC disconnect I will have 8.6 ISC per string, coreect. In the SMA manual it says that there are four fused inputs for 4 strings DC with each input being rated at 15 amps. Now calculating the saftey factor of 1.56 x 8.6 ISC per string I get 13.416amps and the 15 amp fuse should be good enough, correct???

Would appreciate any input.

Luke

Comments

  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: SMA DC Disconnect
    I am planning an array with 2 strings of 14 BP SX 3195 and the Sunny Boy 5000.

    Could be a miscommunication... "...2 strings of 14 BP SX 3195..." = 5,460 W STC, which is probably too much power for the Sunny Boy 5000.

    HTH,
    Jim / crewzer
  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: SMA DC Disconnect

    Hi, do you have a link to a spec sheet?  Is is this one, by any chance?

    http://www.sunwize.com/info_center/pdfs/sma_dc-discnctswitch.pdf

    I notice in the above document that is states, "The DC disconnect is designed for use with the SB3300U or the 3800U inverters.  Use of the DC Disconnect for other applications is not recommended nor supported by SMA America."

    It could be that the output connections are not rated for enough current.  Just because there are 4 x 15A fuses on the source side does not mean that the load side can handle 60A, in my opinion.  I think of a general circuit breaker panel on a house, where it could be a 100A panel, but if you total up all the breakers on the loads, that sum could be much more than 100A.

    If I looked at the wrong document above, you can disregard me.

    Another option would be to look on eBay, or buy new, a Square-D HU361, or H361, if you want fuses, too.  That's rated for 30A, nicely accomodating your 26.

    p.s. STC rating 10% above the inverter spec wouldn't be so bad, unless you happen to be in remarkably good conditions, like a cold mountaintop above the cloud line. It would help to know where your location is.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: SMA DC Disconnect
    p.s. STC rating 10% above the inverter spec wouldn't be so bad

    Roderick seems to be correct about this issue. A quick visit to SMA's sizing tool indicates that they're OK with up to 5,500 W into the Sunnyboy 5000. The tool also appears to allow for up to 16 BP SX 3195's per string.

    Another two-circuit DC disconnect option might be a pair of OutBack PSSB's. The 150 VDC breakers can be wired in series to provide high-voltage protection.

    See: http://www.outbackpower.com/pdfs_wiring_diagrams/PSSBWiringDiagram.pdf
    and: http://www.outbackpower.com/pdfs_manuals/PSSB%20manual.pdf

    HTH,
    Jim / crewzer



  • lukeylukey Registered Users Posts: 21
    Re: SMA DC Disconnect

    Hey guys,

    Thanks for your opinions. The DC disconnects come as standard accessory to the SMA 5000/6000/7000 and are compatible with the inverter for free. Check out SMA homepage for the info.

    Luke
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: SMA DC Disconnect

    I think you're going to have to do a bit more research into this issue... the data sheet for the SMA Sunnyboy 5000 says the max DC current input spec is 21 A.  21 A / 1.56 = 13.46 A Isc.

    See: http://download.sma-america.com/smaprosa/dateien/4752/SB5000US-7000US-042007.pdf

    :?
    Jim / crewzer

    But... the user manual says the "Maximum input short circuit current" spec (page 10-5) is 36 A. 36 A / 1.56 = 23 A Isc, so that should be plenty for your two strings of modules (2 X 8.6 A Isc = 17.2 A Isc; or 17.2 A Isc x 1.56 = 26.832 A design current).  I would think that the supplied 15 A fuses would be fine for your application (8.6 A Isc x 1.56 = 13.416 A design current; one fuse for each of the two strings).

    See: http://download.sma-america.com/smaprosa/dateien/4752/SB5000US-7000US-11-SE1607.pdf

    You may need to show this page & spec to your installer. I suspect the "21 A" spec above is supposed to map to an array's STC Imp spec.

    HTH,  8-)
    Jim / crewzer

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