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NEC 210.20, 240.4 & 240.6 — load amps, multipliers, standard breaker, 75°C copper wire
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Breaker panels & verification tools
Square D branch devices, a main panel option, and meters for dead checks before you swap breakers.
Square D QO120 20A Single-Pole Breaker
- Visi-Trip indicator when tripped
- QO series — common residential load centers
- UL listed replacement breaker
Square D QO140L200PG 200A Load Center
- 200A main breaker included
- 40 spaces — whole-home upgrades
- Plan feeder sizes with your calc results
Fluke 117 True RMS Multimeter
- Non-contact voltage before touching bus
- CAT III 600V for panel-side checks
- Confirm dead circuits when changing breakers
Klein Tools NCVT-3P Voltage Tester
- 12–1000V AC non-contact detection
- Built-in flashlight for dark panels
- Quick tripped vs energized checks
Klein Tools MM400 Multimeter
- Voltage and continuity on branch circuits
- Catches loose neutrals after breaker work
- Compact for crowded enclosures
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How to size a circuit breaker
Circuit breaker sizing is not only matching breaker to wire gauge. You need the load current, whether it is continuous, and the correct NEC multipliers before you use a standard size chart.
The governing sections are NEC 210.20 for branch overcurrent, NEC 240.4 for conductor protection, and NEC 240.6 for standard ratings. Motor branch circuits add NEC Article 430 — especially 430.52 for short-circuit / ground-fault protection.
Continuous load rule — NEC 210.20(A)
Per NEC 210.20(A), branch overcurrent devices must be at least 125% of continuous load plus 100% of non-continuous load on the same circuit. Continuous means expected to run three hours or more.
80% rule
The 125% breaker requirement is the same idea as the “80%” framing: 16A continuous needs at least a 20A breaker; a 20A breaker is often limited to 16A continuous load.
Step-by-step
Step 1 — Load current. Single-phase: I = W / V. Three-phase: I = W / (V × 1.732). Or use nameplate amps.
Step 2 — Multiplier. Continuous ×1.25; non-continuous ×1.0; mixed: (continuous ×1.25) + (non-continuous ×1.0).
Step 3 — Standard size. Round up to the next NEC 240.6(A) rating.
Step 4 — Wire ampacity. Per NEC 240.4, the conductor must be protected per its ampacity table (this calculator uses the embedded 75°C copper ladder from the live tool).
Common mistake
A 20A breaker on 12 AWG is common, but a continuous 18A load needs at least 18 × 1.25 = 22.5A → 25A breaker minimum. The 20A device would be undersized for that continuous load even if the wire is large enough.
Motor circuits — NEC 430.52
Motors draw high inrush. NEC 430.52 allows inverse-time breakers up to 250% of motor FLA for branch short-circuit / ground-fault protection. Running overload is separate (430.32). This calculator’s “motor” option applies that live 250% factor to your entered current — verify FLA on the nameplate.
Common sizing questions
1,500W space heater at 120V?
12.5A; continuous → 15.6A minimum → 20A next standard. Wire: 12 AWG copper.
240V dryer ~5,400W?
22.5A; non-continuous → 30A breaker typical. Wire: 10 AWG per common installs (neutral for controls per assembly).
240V A/C?
Use nameplate MCA and MOP when provided (NEC 440). For motor FLA work, see the motor FLA calculator.
Breaker larger than wire?
Generally no for branch circuits (NEC 240.4). Motors and taps have specific exceptions.
Breaker Sizing Companions
Amp rating from load: Circuit Breaker Sizing Calculator and the Circuit Breaker Sizing Chart (125% continuous lookup). Protection type (standard / GFCI / AFCI / dual-function): Circuit Breaker Types Explained. Trips after the size looks correct: Why Does My Breaker Keep Tripping?.
Related calculators
Breaker Sizing Companions
Amp rating from load: Circuit Breaker Sizing Calculator and the Circuit Breaker Sizing Chart (125% continuous lookup). Protection type (standard / GFCI / AFCI / dual-function): Circuit Breaker Types Explained. Trips after the size looks correct: Why Does My Breaker Keep Tripping?.
Breaker Sizing vs Wire Sizing — Two Different Calculations
The breaker protects the conductor — not the load directly. You size the overcurrent device to the load current (with continuous and motor multipliers), then verify the conductor ampacity is equal to or greater than the breaker rating per NEC 240.4. Wire sizing for voltage drop is a separate performance check on long runs. All three — load, breaker, and wire — must agree before you pull cable.
A common and dangerous mistake is upsizing the breaker to stop nuisance trips without upsizing the wire. That defeats overcurrent protection and is a fire hazard. If the breaker keeps tripping, find the cause: overloaded circuit, weak connection, motor issue, or undersized conductor — do not simply swap in a larger breaker. Pair this calculator with our voltage drop calculator and breaker sizing guide for a complete branch circuit plan.