Calculations for inverter wiring, breakers & fuses

Everything I have read re calculations for sizing of cables, fuse & breaker from batteries to the inverter use Continuous Watts instead Surge Watts.  It seems like this is a mistake.  Your thoughts?


  • BB.BB. Super Moderators, Administrators Posts: 28,315 admin
    Generally, for short term surge current, most breakers and fuses (and wiring) has a thermal mass that gives you a few minutes of "surge current" support before they trip (or overheat the wiring).

    There are "fast blow" devices that will probably trip if you hit them with high surge current. It can be difficult to find, but there are usually "trip curves" available for major devices that can better answer your question (and you may find that device XX will trip somewhere between 2 seconds and 2 hours--They are typically very broad bands).

    One thing to watch for is voltage drop... For example, we typically suggest a flooded cell lead acid battery needs recharging around 11.5 volts under moderate load. And a typical AC inverter will "cut out"/shutdown at 10.5 volts.

    I suggest designing your DC electrial system (wiring, fuses/breakers, etc.) with a 0.5 volt of maximum drop at rated power--This allows for a ~2x surge current (typical inverter surge rating of a few seconds to a few minutes) and 1.0 function drop before the inverter will shutdown with a less than full charge battery bank.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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