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I’m a little wary of plug-in solar in the US. Some of the bills propose allowing 1200 watt panels which can overload wiring depending on what else is on the circuit and how in the wall wiring is run. Limiting plug-in panel wattage to, say, 400 watts might be necessary

  • officermike@lemmy.world
    5·
    16 hours ago

    I am not an electrical engineer, but based on OP’s description, it sounds like a solar panel that connects to an outlet in an existing circuit. Say you have a solar panel plugged into the first outlet on a 15-Amp circuit, with solar producing 1200 Watts of available power. Then you have a 10-Amp load plugged into the next outlet in the circuit, and another 10-Amp load plugged in further down the circuit. That 15-Amp circuit has wiring rated for 15 Amps. You have 20 amps of load, but the solar panel is providing half of that downstream from the circuit breaker. The breaker sees only 10 amps of load and doesn’t trip, though you have wiring downstream from the solar panel that’s carrying 20 Amps. This will start a fire.

    • pdqcp@lemmy.dbzer0.com
      1·
      3 hours ago

      I’m not sure why all the downvotes, but overloading is true, see here how to avoid it:
      https://balkon.solar/news/2025/03/17/how-does-plug-in-pv-in-germany-work/

      Preventing Circuit Overload (Source + Load on One Circuit)

      The key safety concern is that a plug-in solar inverter feeds into a final sub-circuit “downstream” of the main breaker. Normally, a circuit breaker senses all current on its circuit and trips if the load is too high. But with a PV source injecting current at a wall socket, part of the load is supplied locally. The breaker only “sees” the net current from the grid – not the portion supplied by the solar module . This means the wiring could carry more amps than the breaker’s rating without tripping. For example, on a 16 A circuit a 600 W micro-inverter (≈2.6 A) plus a 16 A appliance could theoretically pull ~18.6 A through the wires while the breaker sees only ~16 A . Over time, such an unseen overload can overheat cables, especially in old or unfavorable installations.

      The 800 W limit (specified as 800 VA) was chosen as a conservative safe value so that typical 1.5 mm² house wiring can handle the extra current margin. Even under full sun, a 800 W unit rarely sustains peak output (often max ~500–550 W ≈ 2.4 A), and most circuits can tolerate that small extra current. The limit is rising to 800 W (≈3.5 A) as EU regulations consider <800 W “not significant” generation. At 800 W, German guidance still deems the slight increase manageable, but extra safety margins are advised. For example, in older homes with uncertain wiring, it’s recommended to replace the 16 A breaker with a 13 A one. A 13 A MCB will trip sooner, ensuring that the sum of grid + solar current can’t overheat the cables (13 A from grid + ~3 A PV ≈ 16 A total). Another strategy is having an electrician connect the balcony PV on a dedicated circuit with its own breaker, so it doesn’t combine with heavy appliance loads on the same line. Then even larger system would be possible.

      In practice, users are advised to avoid overloading scenarios proactively: connect the mini-PV to a circuit that isn’t already near capacity, and never daisy-chain multiple PV units or plug them into extension strips . Only one unit per outlet/circuit is allowed, which prevents additive overcurrent from two inverters. By following these limits and using the existing circuit protection, Germany keeps wiring stress “within tolerance” . Notably, tests have shown that issues (excessive heating) would only start to appear above roughly 700 W sustained feed-in combined with a fully loaded 16 A circuit in worst conditions.

      • officermike@lemmy.world
        2·
        1 hour ago

        I suspect it’s easier for them to say “you’re wrong” without backing up that assertion than to accept that this isn’t a completely safe way to implement solar without involving an electrician.

      • officermike@lemmy.world
        3·
        14 hours ago

        I’ve had more than a few classes on circuits throughout my schooling, from high school physics to my mechanical engineering college coursework. Please enlighten me as to where my logic is flawed.

        Two sources wired in parallel can supply more current than either individual source can supply on its own. The wiring on each parallel branch will have identical voltage, and the converged branch will carry the sum of the currents. Total load of 20 amps exceeds the capacity of the wiring, breaker doesn’t see the full load and doesn’t trip.

    • usernamesAreTricky@lemmy.ml
      6·
      15 hours ago

      In Germany, it’s limited to more like 800 watts (and I think some other safety regulations). As I understand it, it’s generally worked without this being much of an issue despite millions of plug in solar installs (primarily for balcony solar)

      • officermike@lemmy.world
        1·
        14 hours ago

        I am accounting for that, that’s the whole point I’m making. Breaker is supplying 10 of its available 15 Amps, solar provides another 10 Amps, load downstream is drawing 20. Wiring between solar and load is carrying 20 Amps, but potentially rated for only 15.