Electrical Load Calculator
Estimate current draw (amps) from watts, voltage, and power factor for 1-phase or 3-phase systems.
Your Load Estimate
- Enter watts — Use the total real power (W) for the load or equipment.
- Set voltage — Use the nominal system voltage.
- Pick phase — Three-phase uses √3 relationships.
- Enter power factor — If unknown, 1.00 is a conservative simplification for planning.
- Breaker planning — Continuous loads often use a 125% planning factor before selecting a standard breaker size.
How Do I Calculate Electrical Load?
An electrical load estimate converts power (watts) into current (amps) based on system voltage, phase, and (optionally) power factor. It's useful for planning circuits, comparing equipment, and getting a rough sense of service or feeder demand.
For many resistive loads (heaters, incandescent lighting), watts are close to real power. For motors and some electronics, power factor can matter, which is why this calculator shows both kW and kVA when applicable.
Example: a 3,000W load at 240V single-phase draws about 12.5A (before any start-up or duty-cycle considerations). For breaker planning, use this result as a baseline and then verify with equipment specs and local requirements.
Electrical Load vs Watts/Amps/Volts: Which Tool Should You Use?
These tools both output amps, but they're meant for different scales of work.
Use Electrical Load Calculator to total multiple devices/circuits (a mini “load schedule”), especially when you need a combined amp draw.
Use Watts/Amps/Volts for quick conversion of a single device or a single circuit.
After you estimate total amps, you can move on to Breaker Size, Wire Size, and Voltage Drop for planning.
Example Load List (Sum of Multiple Items)
| Item | Watts | Amps @ 120V (approx.) |
|---|---|---|
| LED lighting (whole area) | 300 W | 2.5 A |
| Refrigerator | 600 W | 5.0 A |
| Microwave | 1200 W | 10.0 A |
| Small appliance load | 900 W | 7.5 A |
| TOTAL | 3000 W | 25.0 A |
Example only. Real loads vary and many appliances have different starting/continuous behavior.
Load Calculation Formulas
This calculator totals multiple items into a single estimated current. It supports both simple single‑phase estimates and three‑phase approximations.
Three‑phase (PF≈1) = Amps ≈ Watts ÷ (√3 × Volts)
Total load = Total watts = sum of all item watts before converting to amps
If you're only converting one device, the Watts/Amps/Volts calculator is simpler. If you're adding many loads, a clear item list is the key.
Related Calculators
Calculate total load before sizing your breaker and wire. Browse the electrical calculator collection.
FAQ
What if I don't know power factor?
If you do not know PF, using 1.0 provides a simple planning estimate. Many motor loads have PF below 1, which increases current for the same watts.
Why does three-phase reduce amps for the same watts?
Three-phase power distributes load across three conductors simultaneously. The √3 (1.732) multiplier reflects how three-phase voltage combines — the result is that three-phase delivers the same power at roughly 58% of the current of single-phase. This is why commercial buildings use three-phase — smaller wire gauges and lower heat generation for the same power capacity.
How do I calculate total electrical load for a house?
The NEC standard method sums all lighting (3 VA/ft²), small appliance circuits (1,500 VA each), laundry circuits, and all fixed appliances at nameplate VA. The first 10,000 VA is taken at 100%, the remainder at 40%. Divide by service voltage to get amps. A 2,000 ft² home with typical appliances typically comes out to 150–200A service requirement.
What is the difference between kW and kVA?
kW (kilowatts) measures real power — the actual work being done. kVA (kilovolt-amperes) measures apparent power — the total power drawn from the supply including reactive power from motors and transformers. The ratio of kW to kVA is the power factor (PF). Pure resistive loads like heaters and incandescent lights have PF = 1.0, so kW = kVA. Motors typically run at 0.8–0.9 PF.
Should I add a safety margin to my calculated amps?
Often, yes. Real systems have start-up surges, duty cycles, and future loads. Use the calculated amps as a baseline, then consider leaving headroom rather than sizing everything to the exact minimum.