Formula used
- Connected Watts = sum of (watts each × quantity).
- Diversified Watts = Connected Watts × (Demand % ÷ 100).
- Adjusted Watts = Non‑continuous + (Continuous × 1.25), then × (1 + Safety %).
- Apparent Power (VA) = Adjusted Watts ÷ Power Factor.
- Running Current (A) = VA ÷ Voltage.
- Voltage Drop (V) ≈ I × R × (2 × length). R uses a rough copper table.
Breaker and conductor suggestions use a conservative quick guide. Always verify with local regulations, insulation ratings, temperature, and installation method.
How to use this calculator
- Enter your supply voltage and a realistic power factor.
- Add each patio device with watts and quantity.
- Mark motors or pumps and set a start factor.
- Set demand factor if loads won’t run together.
- Set continuous share for long-running equipment.
- Add a safety margin for uncertainty and growth.
- Optionally enter run length to estimate voltage drop.
- Click calculate to view results above the form.
- Download CSV or PDF for sharing and recordkeeping.
Example data table
| Device | Watts each | Qty | Subtotal watts | Motor? | Start factor |
|---|---|---|---|---|---|
| String lights | 10 | 30 | 300 | No | — |
| Fountain pump | 120 | 1 | 120 | Yes | 3× |
| Outdoor heater | 1500 | 1 | 1500 | No | — |
Replace example values with your actual nameplate wattage and quantities.
Start with connected wattage and nameplate labels
List every outdoor device that may run from the patio circuit, including lights, pumps, heaters, speakers, and seasonal tools. Use the nameplate wattage or the product manual, not marketing claims. Multiply watts by quantity to build connected load, then keep a record for future additions. Document settings, assumptions, and revisions so crews can reproduce the calculation later.
Apply demand and diversity to match real operation
Many patios do not run every device at the same time. A demand factor reduces connected watts to a diversified value that better reflects typical use. Choose a higher percentage when equipment overlaps, and a lower percentage when loads are clearly staggered, such as irrigation timers versus evening lighting. Review seasonal usage patterns to avoid oversizing during occasional holiday lighting events.
Treat continuous loads with extra capacity
Equipment that runs for long periods, such as landscape lighting or a circulation pump, should be treated more conservatively. The calculator applies a common continuous adjustment by increasing the continuous portion before converting to current. This helps avoid nuisance trips and excessive heat in conductors and terminations.
Estimate current, breaker sizing, and conductor guidance
Adjusted watts are converted to apparent power using the power factor, then divided by voltage to estimate running amperage. The suggested breaker is the next common size above the calculated current. The conductor suggestion is a quick, conservative reference and must be verified for insulation rating, ambient temperature, and installation method.
Check voltage drop on longer outdoor runs
Long cable runs can reduce delivered voltage, lowering heater output and stressing motors. The calculator estimates drop using a copper resistance table and the round-trip length. If the percentage is above your limit, shorten the run, increase conductor size, or consider a closer receptacle location for heavy equipment.
FAQs
What does power factor change in the result?
Power factor converts real watts into apparent power. Lower values increase calculated VA and amperage, which can push you to a larger breaker or conductor. Use 1.00 for resistive heaters and a lower value for motor-driven equipment.
How should I pick a demand factor?
Use 100% if most devices run together. Use 60–80% when usage overlaps partially, like lights plus a small pump. Use lower values only when operation is clearly staggered and you can enforce that schedule.
Why does the calculator boost continuous loads?
Loads that run for extended periods can heat wiring and terminations. Many design practices treat continuous portions more conservatively, often using a 125% adjustment. This reduces nuisance trips and supports safer long-duration operation.
How accurate is the startup surge estimate?
It is a rough indicator based on your start factor. Actual inrush depends on motor type, controller, and voltage at the device. Use it to flag risk, then confirm with manufacturer data or a clamp meter test.
What voltage drop limit should I use?
A common planning target is 3% for branch circuits. Sensitive electronics or motor loads may benefit from lower drop. If you exceed your target, increase conductor size, reduce length, or relocate the receptacle closer.
Can I use this for multi-circuit patios?
Yes. Run the calculator separately for each circuit, or group loads by their receptacle and breaker. Keep heavy heaters and pumps on dedicated circuits when possible to reduce overlap and simplify troubleshooting.