| Scenario | Voltage | Phase | Power | PF | Eff. | Continuous | Safety | Derate | Base A | Adj. A | Breaker |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Irrigation pump | 230 V | Single | 1.50 kW | 0.85 | 0.90 | Yes | 10% | 5% | 8.53 | 12.31 | 15 A |
| Garden lighting | 120 V | Single | 600 W | 0.98 | 1.00 | No | 10% | 0% | 5.10 | 5.61 | 10 A |
| Greenhouse heater | 230 V | Single | 3.00 kW | 1.00 | 1.00 | Yes | 10% | 0% | 13.04 | 17.93 | 20 A |
Example values are illustrative. Confirm breaker types and wiring for outdoor environments.
- Power conversion: Watts = kW × 1000, and Watts ≈ HP × 746.
- Single-phase current: I = P ÷ (V × PF × Eff)
- Three-phase current: I = P ÷ (√3 × V × PF × Eff)
- Adjusted current: Iadj = I × ContinuousFactor × (1 + Safety%) × (1 + Derate%)
- Breaker selection: Choose the next standard breaker size ≥ Iadj.
- Enter your supply voltage and choose single or three-phase.
- Enter the equipment power in W, kW, or HP.
- Set power factor and efficiency from the nameplate.
- Keep continuous load checked for long running devices.
- Add a safety margin and any derating add-on if needed.
- Press calculate to see breaker size and detailed values.
- Use CSV or PDF buttons to save the result.
For motors, consider starting current and local electrical rules.
Outdoor loads and breaker intent
Garden circuits often combine irrigation pumps, low-voltage drivers, heaters, and timer panels. A properly sized breaker protects conductors from overheating while tolerating normal operating current. Outdoor installations also face moisture, longer cable runs, and intermittent equipment starts, which can raise effective current and nuisance trips.
From nameplate power to operating current
This calculator converts input power to watts and estimates current using supply voltage, power factor, and efficiency. Motor-driven garden equipment rarely behaves like a pure resistive load. A lower power factor and less-than-perfect efficiency increase amperage, so using realistic PF and efficiency values helps you avoid undersized protection.
Continuous duty and design margins
Loads that run for long periods should be treated as continuous, increasing the design current by 25%. The safety margin adds extra headroom for measurement uncertainty, aging equipment, and seasonal load growth. If the installation is hot, crowded, or uses bundled conductors, an additional derating add-on helps reflect reduced heat dissipation.
Standard breaker selection and usable capacity
After applying duty, safety, and derating factors, the tool selects the next standard breaker size at or above the adjusted current. It also estimates usable watts for that breaker using the same assumptions, helping you check whether a planned device fits the circuit without constant tripping.
Example dataset for typical garden equipment
Example: 230 V, single-phase, 1.50 kW pump, PF 0.85, efficiency 0.90, continuous, safety 10%, derate 5%. Base current ≈ 8.53 A, adjusted ≈ 12.31 A, recommended breaker 15 A. Always verify cable size, protective devices, and local rules.
1) What power factor should I use for a pump?
Use the nameplate value when available. If unknown, many small induction motors fall between 0.75 and 0.90 at rated load. Using a conservative PF reduces the risk of undersizing the breaker.
2) Why does efficiency affect breaker sizing?
Efficiency accounts for losses inside the device. For the same delivered output, a less efficient motor draws more electrical input power, which increases current. That higher current must be protected by an appropriately sized breaker.
3) When should I check the continuous load option?
Enable it when the equipment can run for extended periods, such as irrigation pumps, greenhouse fans, or heaters operating for hours. The continuous factor adds capacity headroom to reduce overheating and nuisance trips.
4) What does the derating add-on represent?
Derating reflects harsher conditions like high ambient temperature, crowded conduits, or limited ventilation. These conditions reduce conductor cooling, so an added percentage increases the design current and can push the selection to the next breaker size.
5) Is the recommended breaker always correct for motors?
It is a solid planning estimate, but motors can draw high starting current. Depending on motor type and local practice, you may need a different curve breaker or starter protection. Confirm with equipment documentation and applicable standards.
6) Why does the tool suggest a standard breaker size?
Real installations use standardized breaker ratings. Selecting the next standard size above the adjusted current helps ensure availability and compliance. It also accounts for minor variations in load and supply voltage.
7) Can I use this for lighting circuits in the garden?
Yes. Enter the total lighting wattage, voltage, and set PF and efficiency near 1.00 for mostly resistive drivers. Include safety margin for future fixtures. Ensure outdoor-rated protection and proper grounding for wet locations.