Breaker Size Calculator
40.0A load × 125% continuous rule = 50.0A required, pair with 8 AWG copper
Per the NEC 125% continuous-load rule as of 2026. The breaker protects the wire, never upsize one without the other. Final sizing is confirmed by a licensed electrician.
The 125% rule, explained
Breakers protect wire, and the code's core rule for anything that runs three hours or more, EV chargers, water heaters, space heating, is sizing at 125% of the load's current. A 40-amp continuous draw therefore needs a 50-amp breaker on 50-amp-rated wire. That's why a '48-amp' EV charger lives on a 60-amp circuit.
Non-continuous loads (most appliances) size at 100%. The calculator applies the right rule and rounds up to the next standard breaker size.
The mistake this tool prevents
The classic DIY error is 'fixing' a tripping breaker by installing a bigger one on the same wire. The breaker stops tripping, and the wire, now unprotected past its rating, becomes the fuse. Breaker and wire are a matched pair; the output above shows both for exactly that reason. If a breaker trips repeatedly, see why does my breaker keep tripping.
The 125% rule in the wild
Continuous loads, anything sustaining three-plus hours, heat conductors toward equilibrium, so code demands the famous margin. The everyday cases: a 40A-charging EV needs 50A protection (40 × 1.25), a 5,500W water heater at 240V draws 23A and wants 30A, baseboard heat sizes the same way. Miss the rule and the breaker nuisance-trips at best, runs hot at worst.
Non-continuous loads size at 100%, a range's occasional peak doesn't heat-soak the circuit. The calculator applies the right rule from your checkbox; inspectors apply it from the code book; both reach the same number.
When the answer isn't a bigger breaker
A breaker that trips under load is telling you about the circuit, and the fix menu starts smaller than replacement: redistribute loads across circuits, add a dedicated circuit for the offender, or repair the heating connection that's tripping thermally below rating. The bigger-breaker 'fix' on unchanged wire is the one move that converts an annoyance into a hazard.
Where a larger breaker is legitimate, the wire already supports it, or the wire upgrades too, this calculator plus the wire-size tool produce the matched pair an inspector will pass.
Breaker types: the sizing question after the sizing question
Amperage settles half the specification; type completes it. Standard thermal-magnetic breakers serve general circuits. AFCI protection is code-required in most living areas, GFCI where water lives, dual-function where both apply. HACR-rated breakers serve HVAC equipment; high-magnetic variants tame motor inrush that nuisance-trips standard units.
Panel brand locks the ecosystem, breakers must match the panel's listing, and the approved-parts detail is where bargain-bin breakers fail inspections. The calculator gives the number; the install pairs it with the right device from the right list.
Reading your panel's spare capacity honestly
Before adding any breaker, two inventories matter: physical space (open slots, or tandem-breaker room where the panel's listing allows) and electrical budget (the load calculation against service size). A panel can have five open slots and zero spare amps, or a full face and comfortable headroom, the two axes are independent.
The five-minute check: photograph your panel with the door open and count. If space is short but capacity is fine, a sub-panel solves it cheaply; if capacity is the wall, the service conversation begins. Either answer beats discovering it mid-project. Panels also telegraph their history here: a face full of tandem breakers usually means the space question was answered cheaply once already, and the next addition deserves the sub-panel or upgrade conversation rather than a third workaround.
Frequently asked questions
9,600W at 240V is 40 amps; as a continuous load × 125% that requires 50 amps, a 50A breaker on 6 AWG copper. This is the standard 40A-charging setup.
Only if the wire is 30-amp rated (10 AWG copper). Upsizing a breaker on 12 AWG wire removes the wire's protection, it's the specific mistake that causes fires.
15, 20, 25, 30, 35, 40, 45, 50, 60, 70A and up. You round up from the required amperage to the next standard size, the calculator does it for you.
Standardization, 15 through 60 in fives, then coarser steps, keeps panels, wire tables, and inspections interoperable. You always round up from required amps to the next standard size; the gaps are intentional.
General-use circuits share by design; fixed appliances above ~50% of circuit capacity want dedication, and code names many (ranges, dryers, EV chargers) explicitly. When in doubt, the appliance's install manual states its circuit demand.
Most 240V spas specify a 50A GFCI breaker (some 40A or 60A, the spec plate rules). The GFCI requirement is absolute for water-adjacent loads, and spa panels combine the breaker with the code-required disconnect. Size from the manufacturer's plate, never from the tub's advertised horsepower, marketing horsepower and electrical demand are barely acquainted, and the plate is what the inspector reads. When in doubt, photograph the spec label and send it with your estimate request.