Load current is only the start of breaker selection
This calculator divides entered power by voltage to estimate current for a simple single-phase load, then applies the sizing percentage you enter. The default 125% factor is illustrative and is not a universal breaker-selection rule.
Actual overcurrent protection must coordinate the load, conductors, equipment, duty classification, voltage and phase, fault ratings, environment, applicable electrical code, manufacturer instructions, and local authority requirements.
How to use the circuit breaker calculator safely
- Enter power and voltage: Use verified equipment data and the correct circuit configuration.
- Set the factor: Enter only a factor supported by the applicable design rule; do not assume 125% always applies.
- Calculate the screening current: Review load current and adjusted design current, not a proposed breaker size.
- Complete a code-compliant design: Have a qualified professional evaluate conductors, protection, duty, equipment, and local requirements.
Formula and variables
This simplified relationship assumes the entered watts represent the applicable real or apparent load for a single-phase screening calculation.
Load current = power ÷ voltage; adjusted current = load current × sizing factor ÷ 100- P — Power
- Entered load power (W)
- V — Voltage
- Entered circuit voltage (V)
- I — Load current
- Simplified calculated current (A)
- F — Sizing factor
- User-selected adjustment (%)
Illustrative resistive-load screening
A nominal 2,400 W load is entered at 240 V with an illustrative 125% factor.
- Power
- 2,400 W
- Voltage
- 240 V
- Factor
- 125%
- Load current = 2,400 ÷ 240 = 10 A
- Adjusted current = 10 × 1.25 = 12.5 A
Result: Screening values: 10 A load current and 12.5 A adjusted current.
These numbers do not select a standard breaker rating or establish conductor ampacity.
Understanding your results
Do not treat the result as a breaker recommendation
A safe installation requires a coordinated code-based design.
- Motor and nonlinear loads can require different calculations.
- Conductor ampacity and termination ratings can constrain protection.
- Standard breaker ratings, interrupting rating, poles, and voltage classification still require evaluation.
Assumptions
- The calculation is a simplified single-phase P/V screening model.
- Power and voltage inputs describe the same operating condition.
- The sizing percentage is deliberately selected by the user.
Limitations
- Does not calculate three-phase current, power factor, efficiency, reactive power, inrush, harmonics, demand, or load diversity.
- Does not determine conductor size, derating, temperature correction, voltage drop, grounding, fault current, interrupting rating, or protective-device trip curve.
- Does not determine whether a load is continuous or apply jurisdiction-specific electrical code.
- Not electrical installation advice and not a substitute for a licensed or otherwise qualified professional.
Common mistakes
- Assuming the default 125% factor applies to every load.
- Selecting the next standard breaker without checking conductor and equipment rules.
- Using watts divided by volts for a three-phase, motor, or power-factor-sensitive load.
- Ignoring nameplate, manufacturer, and local authority requirements.
Practical use cases
Electrical education
Demonstrate the basic relationship between watts, volts, and amperes.
Preliminary screening
Estimate a starting current value before a complete professional design.
Frequently asked questions
What breaker size should I buy?
This calculator does not answer that question. Breaker selection depends on applicable code, conductor ampacity, equipment, load type and duty, available fault current, and other installation details.
Why is the default factor 125%?
It is a common illustrative adjustment in some contexts, but it is not universally applicable. Use only the factor required for your specific load and governing rules.
Does this work for three-phase loads?
No. Three-phase calculations require a different relationship and may also require power factor and efficiency.
Sources and review
- 1910.304 — Wiring design and protection — Occupational Safety and Health Administration. Accessed 2026-07-13.
- NFPA 70 — National Electrical Code — National Fire Protection Association. Accessed 2026-07-13.
Reviewed 2026-07-13.