以下のガイドはまだ日本語に翻訳されていないため、英語で表示しています。計算機自体はすべて日本語に対応しています。
この計算機の使い方
- Choose what to solve for: amps, watts or volts.
- Choose the circuit type. Household outlets are single-phase AC.
- Enter the two known values and, for AC, the power factor (1.0 for resistive loads like heaters; 0.8–0.95 for motors).
計算式
Real power depends on voltage, current and, in AC circuits, the power factor between them. Three-phase power uses line-to-line voltage and a factor of √3.
DC: P = V × I
Single-phase AC: P = V × I × PF
Three-phase AC: P = √3 × V × I × PF
Example
- A 1,800 W heater on 120 V (PF 1): I = 1,800 / 120 = 15 A.
- That's the full rating of a 15 A circuit; continuous loads should stay at or below 80% (12 A).
よくある質問
What is the 80% rule?
In the US NEC, continuous loads (3+ hours) shouldn't exceed 80% of a breaker's rating. A 20 A circuit should carry no more than 16 A continuously.
What is power factor?
The ratio of real power (W) to apparent power (VA). Inductive loads such as motors draw more current than their wattage alone suggests.
What voltage should I use?
Common supplies are 120/240 V in North America, 230 V in Europe and much of the world, and 100 V in Japan. Three-phase systems are typically 208, 400 or 480 V line-to-line.