Off-grid, RV & cabin
Enter your circuit's continuous current. Get the minimum fuse or breaker rating, rounded to a standard size.
How it works Your panels can only charge what your battery can hold. The backup runtime estimator in our Battery & Storage section can help you size the bank for your actual daily load.
Step 1
The actual sustained current the circuit is expected to carry, not a peak or momentary spike.
Step 2
This is the standard safety margin used for circuits running 3 hours or more.
Step 3
Always round up, never down, to keep the circuit properly protected.
For a circuit carrying a continuous 50 amps, applying the standard 125% safety margin gives a minimum rating of 62.5 amps, which rounds up to a standard 70-amp fuse or breaker. This margin and rounding approach applies to any continuous-duty off-grid circuit, from battery-to-inverter cabling to charge controller output. The OSHA's electrical safety standards lays out the same rules from the regulator's side.
The 125% rule accounts for the fact that continuous loads, those running 3 hours or more, generate sustained heat in wiring and connections over that extended period, so both wire and breaker sizing need headroom beyond the exact expected current to operate safely without nuisance tripping or overheating. A breaker sized exactly to the load current would run right at its thermal limit continuously, which isn't a safe long-term operating point. You can check the knock-on effects with the connector and wiring inspection cost estimator.
Always round up to the next standard fuse or breaker size, never down. Rounding down would mean the breaker trips under normal continuous load, causing frustrating nuisance trips during expected operation, while rounding up keeps the circuit protected with an appropriate margin while still tripping well before a genuine overload or fault condition develops.
The wire feeding the circuit must be rated to safely carry at least the breaker's rated current, not just the actual load current. A breaker sized correctly for the load but paired with undersized wire defeats the purpose of the protection, since the wire could overheat and pose a fire risk before the breaker trips to interrupt the circuit. The wire gauge sizing calculator in our inverters and equipment tools handles this exact pairing between breaker rating and wire size.
Check the battery voltage selection tool to confirm your wire gauge matches this breaker's rating, and see the 12V vs 24V vs 48V comparison if you're still deciding on system voltage, since that choice directly affects circuit current. NFPA 70, the National Electrical Code, is the ultimate source for the 125% rule and the standard breaker sizes used throughout this tool, and OSHA's electrical safety standards cover the workplace-safety side of the same wiring principles. Once a system is wired, the connector and wiring inspection cost estimator in our maintenance and lifespan tools budgets for the periodic checks that keep it safe over time.
Questions
For a circuit carrying a continuous 50 amps, applying the standard 125% safety margin gives a minimum rating of 62.5 amps, which rounds up to a standard 70-amp fuse or breaker.
The 125% rule accounts for the fact that continuous loads, those running 3 hours or more, generate sustained heat in wiring and connections, so both wire and breaker sizing need headroom beyond the exact expected current to operate safely without nuisance tripping or overheating.
Always round up to the next standard fuse or breaker size, never down. Rounding down would mean the breaker trips under normal continuous load, while rounding up keeps the circuit protected without nuisance tripping during expected operation.
Yes, the wire feeding the circuit must be rated to safely carry at least the breaker's rated current, not just the actual load current. A breaker sized correctly for the load but paired with undersized wire defeats the purpose of the protection, since the wire could overheat before the breaker trips.
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