Current Rating Calculator
Solve for current, power or voltage for DC, single-phase and three-phase loads. In Advanced mode, pick the next standard fuse or breaker rating, check the cable ampacity, see trip ranges and a current ladder, and use real examples and reference tables.
Ib ≤ In ≤ Iz
The design current Ib must not exceed the device rating In, and In must not exceed the cable ampacity Iz. This protects the cable from overload.
Why 125 % for continuous loads
A load running three hours or more is treated as continuous. The NEC sizes the device at 125 % of it, so it runs at no more than 80 % of its rating.
Fuse vs breaker
Fuses and breakers both interrupt overcurrent. Breakers reset and have trip curves (B, C, D). Fuses must be replaced after they blow.
A guide, not a design
Real sizing also depends on ambient temperature, grouping, installation method, motor starting and short-circuit rating.
How to Use This Calculator
The current of a load follows I = P ÷ V, with a factor for phases, power factor and efficiency. Choose what to solve, enter the rest, then size the protective device.
1. Choose the supply system
Single-phase AC, balanced three-phase AC or DC. The correct formula is used automatically.
2. Choose what to solve for
I (the load current), P (the power a given current delivers) or V (the voltage that gives a given power and current).
3. Enter your values
Type a number, tap a preset (1.5 to 100 kW, 12 to 400 V), or drag the slider under each field. Sliders are logarithmic for wide ranges.
4. Pick the units
Power in W, kW, MW or hp, voltage in V or kV, current in mA, A or kA. Values convert when you change a unit.
5. Calculate and read the result
Press Calculate & show result. It scrolls to the result card with the answer, the steps, apparent power and the chart of current against power.
6. Size the fuse or breaker
Advanced is on by default: set power factor, efficiency, sizing rule, standard and cable ampacity. You get the next standard rating, design checks, a current ladder and trip range. Use Basic for a simpler view.
What This Tool Offers
Built on I = P ÷ V and extended to real AC loads and protective-device selection.
Solve in any direction
Find current, power or voltage for DC, single-phase and three-phase loads, with each step shown.
Power factor and efficiency
Handle motors and inductive loads with power factor and efficiency, and see apparent and reactive power.
Fuse and breaker sizing
The next standard IEC MCB, IEC fuse or NEC breaker rating, with an optional 125 % continuous-load rule.
Cable and design checks
Checks Ib ≤ In ≤ Iz, utilisation of the device and the maximum continuous load it can carry.
Charts and diagrams
Current versus power for several voltages, a current ladder, breaker trip ranges and a labelled circuit.
Real examples and references
Kettles, heaters, EV chargers, motors and ranges, plus standard rating tables and trip-curve data.
Real-world loads
Typical appliances with current and the next standard protective device computed on this page (cable ampacities are illustrative). Click Load to try one.
| Load | Supply | Current I | Design Ib | Device |
|---|
Standard fuse and breaker ratings
| Standard | Common ratings (A) |
|---|
Breaker trip curves (IEC 60898-1)
| Type | Instantaneous trip | Typical use |
|---|---|---|
| B | 3 – 5 × In | Lighting, resistive loads, long cable runs |
| C | 5 – 10 × In | General purpose, small motors, mixed loads |
| D | 10 – 20 × In | High inrush: transformers, large motors |
Maximum continuous load by rating
With the 80 % rule for a continuous load. Real power at unity power factor, using the supply system and voltage currently set.
| Rating | Max continuous current (80 %) | Max power at your voltage |
|---|
Constants and unit reference
| Quantity | Value |
|---|---|
| 1 hp (mechanical) | 745.7 W |
| Three-phase factor | √3 ≈ 1.732 |
| 1 kW | 1,000 W |
| 1 kV | 1,000 V |
| Apparent power | S = V · I (× √3 for three-phase), in VA |
Validation and sources
Continuous-load 125 % rule: NEC 210.20(A) and 215.3
Standard ampere ratings: NEC 240.6(A); IEC 60898-1 (MCBs); IEC 60269 (fuses)
Cable coordination Ib ≤ In ≤ Iz: IEC 60364-4-43
Each result is cross-checked by recomputing P = k · V · I · PF · η
Standard ratings, trip ranges and code references are listed from general knowledge and may differ by edition and country. Always follow the electrical code that applies to your installation and confirm with a qualified electrician or engineer.
Formulas
P = k · V · I · PF · η · V = P / (k · I · PF · η) (k = 1 or √3)
Design current Ib = I × 1.00 or 1.25 · Device rating In = next standard value ≥ Ib
Max continuous current = 0.8 · In (if the 125 % rule is used)
Magnetic trip = m × In with m = 3–5 (B), 5–10 (C), 10–20 (D)
Results are first-order estimates for planning, not a replacement for an electrical design.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| I_rating | Current rating | A |
| P | Power | W |
| V | Voltage | V |
What it means
The current rating of a fuse or circuit breaker is based on the expected load current. For a resistive load, I = P/V. The protective device is chosen to be slightly larger than the full‑load current to avoid nuisance tripping, but small enough to protect the circuit. For motors, the rating is typically 125‑150% of the full‑load current to allow for starting surges. Example: A 1000W heater at 230V draws I = 1000/230 ≈ 4.35A. A 5A fuse would be selected (the next standard size above 4.35A). This provides adequate protection and prevents nuisance blowing. Proper sizing is essential for safety and reliability.
Worked example
Fuse/Breaker Rating – Practical Example
Real‑World| Parameter | Value |
|---|---|
| P | 1000 W |
| V | 230 V |
| Formula | I = P / V |
Common mistakes
Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.Applications
Fuse/breaker current rating I_rating = P/V gives the nominal current for a resistive load. Engineers select a rating slightly above this to avoid nuisance tripping. This ensures protection of conductors and equipment. This formula is used in electrical installation design.
- Overcurrent protection device selection
- Circuit design and safety compliance
- Load current estimation for rating
- Protection coordination and selectivity
- Educational understanding of protection sizing
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