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Sub-Panel Sizing for 6kw System

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  • Sub-Panel Sizing for 6kw System

    Going to install a solar system at a future date. Need to upgrade the existing 45 year old 200 amp Zinsco MSP first and decide on the new MSP breaker sizes.

    The planned 6kw solar system would have two 8 panel arrays and one string inverter, SMA Sunny Boy 6000TL-US with a 25 amp max a/c output, connected to a subpanel in the garage. Their is already a 125 amp subpanel that has 90 amps of total breakers installed (3 20's, 2 15's,) and is serviced by the MSP with a 50 amp breaker. The wiring to the subpanel appears to be 8 awg with ~30 foot run.

    Given the below calcs, the 125 SP would need to be upgraded. If the SP is upgraded to 200 amps, can a 50 amp MSP breaker service it with the additional inverter? The existing 50 amp breaker has never tripped in 15 years. Then again, it is an older Zinsco breaker that was installed about 25 years ago when the garage was built. The garage 125 amp SP services 40 amps to a water well pump, garage lights, door opener and some outlets. No welding or heavy duty shop equipment in garage.

    120 rule for a 125 amp subpanel: 125 x 1.2 = 150 - 50 (50 amp MSP breaker) = 100 breaker amps available.
    120 rule for a 200 amp subpanel: 200 x 1.2 = 240 - 50 (50 amp MSP breaker) = 190 breaker amps available


    Any ideas or suggestions appreciated.

  • #2
    I run a very similar setup.

    I have my PV install on a "subpanel" with a 125 amp rated bus bar. I feed it with a 30 amp breaker from the main panel (200 amp rated and main breaker). I also have a 30 amp "main" breaker installed in the subpanel that is connected to the feeder (so the feeder is protected in both directions as this panel is served by #10 wire). My PV system is rated at 30 amps and I have additional loads in the subpanel just like you.

    I don't think you will have a problem with the subpanel other than you probably need to convert it from a main lug to main breaker panel if you add PV to it - so check if this is possible with your subpanel - I had to add a "hold down kit" to mine for the 30 amp "main" breaker. I also had to relocate the 30 amp breaker in the main panel to the end of the bus.

    The only concern would be how your 50 amp breaker in the main panel is treated being back-fed, since this is over the 40 amps allowed by your 200 amp main breaker. Since this can never back-feed more than the 25 amps of your inverter, maybe you are okay - not sure what the NEC says on that - you may want to put a call in to your building department and ask. You could also source a 200 amp panel with a higher rated bus bar when you replace your main panel.

    Disclaimer: Not an electrician - just a random guy on the internet.
    5.7 kW DC enphase M250 + SW285 solar patio cover

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    • #3
      You don't include the breakers for the loads in the calculation. You need a 40A breaker for the inverter, you have a 50A breaker for service = 90A on a 125A breaker box. You can even upgrade the main breaker to 100A and still be fine.
      Solar Queen
      altE Store

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