Power & Electrical calculator
Event Power Calculator: Watts to Amps
Amps equal watts divided by volts: a single-phase 1,800 W load on a 120 V circuit draws 15 A. Enter the load, voltage, and phase below to get the current draw and the smallest standard breaker that carries it under the 80% continuous load rule.
US: 120 V branch circuits, 208/240 V for larger loads. EU: 230 V.
1.0 for resistive loads; 0.8–0.95 for amplifiers, motors, and switch-mode supplies.
Formulas
Single-phase current
I = P / (V × PF)- I:
- current in amps
- P:
- power in watts
- V:
- voltage in volts
- PF:
- power factor (1.0 for resistive loads)
Three-phase current
I = P / (√3 × V(L-L) × PF)- V(L-L):
- line-to-line voltage (e.g. 208 V in a 120/208 wye service)
How it works
Every load on a show (amplifiers, moving lights, LED walls, chain motors, catering urns) draws current in proportion to its power and in inverse proportion to the supply voltage. The same 3,600 W load pulls 30 A at 120 V but only 15 A at 240 V, which is why heavy loads run on higher voltages.
Three-phase services split the load across three legs. The √3 factor (about 1.732) accounts for the 120° phase offset between legs when you calculate from line-to-line voltage. Balanced loading across all three legs is what keeps feeder and generator capacity usable.
Breakers are designed to carry 80% of their rating continuously (3 hours or more, which describes most show loads). A 20 A circuit should carry no more than 16 A of continuous load, which is why two 1,800 W loads do not fit on one 20 A circuit even though the raw arithmetic looks tempting.
Worked example: An amplifier rack rated 4,200 W on a 208 V single-phase drop, PF 0.9
- 1.Current: I = 4200 / (208 × 0.9) = 22.4 A.
- 2.Check the 80% rule: 22.4 / 0.8 = 28.1 A minimum breaker rating.
- 3.The smallest standard breaker above 28.1 A is 30 A.
The rack draws about 22.4 A and needs a 30 A circuit.
Common event loads: watts to amps
| Load | 120 V single-phase | 208 V single-phase | 208 V three-phase |
|---|---|---|---|
| 1,000 W | 8.3 A | 4.8 A | 2.8 A |
| 1,800 W | 15.0 A | 8.7 A | 5.0 A |
| 3,600 W | 30.0 A | 17.3 A | 10.0 A |
| 5,000 W | 41.7 A | 24.0 A | 13.9 A |
| 10,000 W | 83.3 A | 48.1 A | 27.8 A |
Field notes
- Amplifier nameplate ratings are worst-case. For music program, most manufacturers publish one-eighth or one-third power current draw; that is the realistic planning number.
- Motor loads (chain hoists) have inrush current several times their running draw; do not size their circuits from running watts alone.
- When loading a three-phase service, balance the legs. A 400 A three-phase service with everything piled on one leg is a 400 A problem on that leg.
Frequently asked questions
How many amps does a 1,000 watt light draw?
On a 120 V circuit, 1,000 W ÷ 120 V = 8.3 A. On 230 V (EU), the same fixture draws about 4.3 A. Tungsten fixtures are resistive, so power factor is effectively 1.0.
How many watts can I put on a 20 amp circuit?
At 120 V, a 20 A breaker supports 2,400 W peak, but only 1,920 W of continuous load under the 80% rule. Treat show loads as continuous.
What power factor should I assume for audio amplifiers?
Modern switch-mode amplifiers with power factor correction run near 0.9–1.0. Older non-PFC designs can dip to 0.7–0.8. When in doubt, use the manufacturer’s published current draw at one-eighth power for program material.
Why does three-phase math use 1.732?
1.732 is √3. In a balanced three-phase system the three legs are 120° apart, and √3 is the geometric factor relating line-to-line voltage to the power delivered across all three legs.