Calculate the correct circuit breaker size, wire gauge, voltage drop, and power loss for any electrical load. NEC-compliant with AWG wire reference table.
The **Breaker Size Calculator** determines the correct circuit breaker rating, minimum wire gauge, voltage drop, and conductor power loss for any electrical load. Enter the total wattage, voltage, power factor, and wire run length, and the calculator recommends a standard breaker size following the NEC 80% continuous-load rule.
Proper breaker and wire sizing is critical for electrical safety. An undersized breaker trips nuisance, while an oversized breaker fails to protect the wiring, creating a fire hazard. Similarly, undersized wires cause excessive voltage drop and heat, while oversized wires waste material and cost.
This calculator covers 120 V to 480 V systems, supports power factor correction for motor loads, and includes a complete AWG wire reference table with ampacity, resistance, and cross-sectional area. The voltage-drop calculation follows common NEC guidance so you can compare breaker choice and conductor size in the same pass. That keeps the current, conductor, and voltage-drop checks together so you can screen a circuit before you look up the final installation details. It is helpful when you want to compare a few breaker and wire options before checking the exact code table that applies to the installation.
Breaker sizing, conductor sizing, and voltage-drop checks are tightly connected, and treating them as separate calculations is where many mistakes start. This calculator gives you a single estimate for load current, recommended breaker rating, wire gauge, and voltage drop so you can screen a circuit design before checking the final installation details against the applicable code and local requirements.
Current: I = W / (V × PF) Breaker Size: I / margin, rounded up to standard size Voltage Drop: Vd = I × 2 × (Ω/1000ft) × length/1000 Voltage Drop %: Vd / V × 100 Wire Loss: P = I² × R_wire
Result: Recommended 25 A breaker, 1.98 V drop (1.6%), 10 AWG wire
A 2 400 W resistive load at 120 V draws 20 A. At 80% NEC margin, a 25 A breaker is required. Voltage drop over 50 ft of 12 AWG is about 2.0 V (1.6%), within the 3% limit.
The breaker protects the conductor, while the conductor also has to satisfy voltage-drop and installation constraints. A circuit can have an acceptable breaker size and still need a larger wire because of run length or operating conditions. That is why it is useful to evaluate load current and wire behavior together instead of stopping at the first breaker number that seems to fit.
Many undersized circuits come from using the nameplate current without applying the continuous-load margin. Heating loads, EV charging, and other long-duration circuits often need more headroom than intermittent loads. The calculator helps expose that difference early, but the final installation should still be checked against the exact equipment instructions and governing code tables.
Electrical work still depends on conductor material, insulation temperature rating, bundling, ambient temperature, termination limits, and local code adoption. Use the result to narrow the likely breaker and wire range, then verify the final selection with the applicable electrical standard or a qualified electrician before installation.
For continuous loads that run for three hours or more, the overcurrent device is commonly sized at 125% of the load current. That is the same as saying the continuous load should not exceed 80% of the breaker rating.
Motors have a lagging power factor (typically 0.8–0.9) and high starting (inrush) current. Use the motor nameplate amps and applicable NEC tables.
Equipment receives insufficient voltage, causing poor performance, overheating, or failure. NEC recommends ≤3% for branch circuits.
Yes, but aluminium has higher resistance and lower ampacity than copper. Use appropriate NEC tables and rated connectors.
A breaker that switches both hot legs of a 240 V circuit simultaneously. Required for 240 V appliances in North America.
Not directly — breaker size is based on load current. However, long runs may require thicker wire to keep voltage drop within limits.