Watts to Amps Calculator

Watts to Amps Calculator — Free Commercial Electrical Tool

Your Results

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Amps (Current Draw)
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Min. Breaker Size
NEC 80% continuous load rule
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Min. Wire Gauge
NEC 310 copper at 75°C
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What This Watts to Amps Calculator Does

This free tool converts electrical power in watts (or kilowatts) to current in amps for DC, single-phase AC, and three-phase AC circuits. It's built specifically for commercial electrical work, whether you're sizing a breaker for new equipment, calculating circuit loads during a lighting retrofit, checking panel capacity before adding EV charging stations, or verifying wire gauge for a tenant buildout.

Beyond the basic conversion, the calculator applies the NEC 80% continuous load rule to recommend a minimum breaker size, and references NEC 310 copper wire ampacity tables to suggest a minimum wire gauge, two pieces of information every commercial electrician needs alongside the raw amp figure.

The Formulas: How Watts Convert to Amps

The conversion always requires knowing the voltage. You cannot convert watts to amps without it. The formula changes slightly depending on the circuit type.

DC Circuits

I = P ÷ V

I = amps, P = watts, V = volts. Used for battery systems, DC equipment feeds.

AC Single-Phase

I = P ÷ (V × PF)

PF = power factor. Applies to 120V and 240V circuits. Most commercial branch circuits.

AC Three-Phase

I = P ÷ (√3 × V × PF)

Line-to-line voltage. Used for 208V, 480V panels. Motors, large HVAC, EV chargers.

How to Use the Calculator

  1. Select Your Current Type

    Choose DC, AC Single-Phase, or AC Three-Phase. Most commercial building circuits are single-phase AC. Large equipment, motors, and EV charging infrastructure typically run on three-phase AC. DC applies to battery systems and some specialty equipment.

  2. Enter the Power Load

    Enter the wattage from the equipment nameplate, spec sheet, or fixture schedule. If you're working in kilowatts (common for HVAC and large motors), switch the unit dropdown to kW. For multiple loads on a circuit, add the wattages together before entering.

  3. Select Your Voltage

    Use the voltage preset dropdown to select the common commercial voltage for your circuit: 120V, 208V, 240V, 277V, or 480V. If your circuit is at a different voltage, select "Custom voltage" and enter it manually. The voltage should match the circuit, not the equipment nameplate voltage if they differ.

  4. Set the Power Factor (AC Circuits Only)

    For AC circuits, the power factor matters. Resistive loads like heaters use 1.0. Most commercial loads including LED drivers, motors, and mixed equipment fall in the 0.85–0.95 range. The default of 0.90 is a reasonable estimate for general commercial loads, use the actual value from your equipment spec sheet when available.

  5. Review Your Results

    The calculator returns the current draw in amps, a minimum breaker size based on the NEC 80% continuous load rule, and a minimum wire gauge per NEC 310 copper ampacity at 75°C. Always verify final breaker and wire sizing with a licensed electrician for permitted work.

Common Commercial Voltages Explained

Commercial electrical systems use different voltages depending on the application. Knowing which voltage applies to your circuit is essential for an accurate conversion.

VoltageSystem TypeCommon Applications
120VSingle-phase ACStandard receptacles, small appliances, lighting branch circuits
208VSingle-phase from 3-phase panelSmall HVAC, commercial kitchen equipment, some lighting panels
240VSingle-phase ACLarge HVAC units, electric water heaters, commercial dryers
277VSingle-phase from 480V 3-phase systemCommercial and industrial fluorescent/LED lighting, very common in warehouses, offices, retail
480VThree-phase ACLarge motors, industrial HVAC, EV charging infrastructure, manufacturing equipment

Common Commercial Equipment Load Reference

Use this table as a starting point for estimating circuit loads. Always verify against the actual equipment nameplate or spec sheet — loads vary significantly by model, efficiency rating, and operating conditions.

Equipment TypeTypical LoadTypical VoltageEst. Amps
LED Recessed Downlight (6-in)10–15W120V0.08–0.13A
LED Troffer (2x4, 40W)40W120–277V0.14–0.33A
LED High Bay (150W)150W277V~0.54A
LED High Bay (300W)300W277V~1.08A
Rooftop HVAC Unit (5-ton)~6,000W208/240V25–29A
Rooftop HVAC Unit (10-ton)~11,000W480V 3-phase~14.8A
Level 2 EV Charger (7.2kW)7,200W208/240V30–35A
DC Fast Charger (50kW)50,000W480V 3-phase~67A
Commercial Refrigerator200–500W120V1.7–4.2A
Commercial Dishwasher2,000–5,000W240V8.3–20.8A
3-Phase Motor (5 HP)~3,730W480V 3-phase~5.0A
3-Phase Motor (25 HP)~18,650W480V 3-phase~24.9A

The NEC 80% Rule for Commercial Circuits

The National Electrical Code (NEC 210.20 and 215.3) requires that a circuit breaker or fuse used for continuous loads — loads expected to operate for 3 hours or more — must not be loaded above 80% of its rated ampacity. This means a 20-amp breaker can only safely carry 16 amps of continuous load.

For commercial facilities, most lighting circuits, HVAC equipment, and production loads qualify as continuous. The calculator automatically applies the 80% rule to its breaker recommendation: it takes the calculated amp draw, divides by 0.80, and rounds up to the next standard breaker size. This is the minimum breaker you should install for that load — always consult a licensed electrician before finalizing any breaker or panel work.

Frequently Asked Questions

No — it's not possible to convert watts to amps without voltage. Watts measure power (the rate of energy use), while amps measure current (the flow of electrons). The relationship between them always requires voltage as the third variable. The formula is I = P ÷ V for DC, with power factor added for AC circuits. If you don't know the circuit voltage, check the panel label, the equipment nameplate, or contact your facility's electrical contractor.
Power factor (PF) is the ratio of real power (watts, the work actually done) to apparent power (volt-amps, what the circuit delivers). A power factor of 1.0 means 100% of the current drawn is doing useful work. Inductive loads like motors, transformers, and older ballasts have lower power factors — they draw more current than their wattage alone would suggest, because some current is used to maintain magnetic fields rather than do work. For commercial facilities, this matters because a lower power factor means higher current draw on the circuit, requiring larger wire and breakers even for the same wattage.
Single-phase power uses two conductors (hot and neutral) and is common for lighting, receptacles, and smaller equipment up to about 10kW. Three-phase power uses three hot conductors and delivers power more efficiently for larger loads — it's standard for commercial HVAC, motors above 5 HP, EV charging infrastructure, and manufacturing equipment. Three-phase systems require less wire for the same power delivery and provide smoother operation for motors. Most commercial buildings have both: a 480V three-phase service for large equipment and 120/208V single-phase branch circuits for lighting and receptacles.
NEC 210.20 requires that overcurrent protection devices serving continuous loads be rated at no less than 125% of the continuous load current — which is equivalent to limiting the load to 80% of the breaker's rating. The rule exists because breakers generate heat under load, and sustained operation at or near rated capacity can cause thermal degradation over time, leading to nuisance trips or, in extreme cases, failure. For commercial facilities where lighting and HVAC run continuously during operating hours, this rule almost always applies.
For a lighting retrofit, add up the wattage of all new fixtures on a given circuit, then use this calculator to find the total amp draw at your circuit voltage (typically 120V or 277V for commercial lighting). Compare that to your existing circuit breaker size — if the new total load exceeds 80% of the breaker rating, you either need to redistribute fixtures across circuits or upgrade the breaker and wire. FSG's lighting retrofit team handles load calculations and panel coordination as part of every commercial retrofit project.
Wire gauge must be sized to handle the circuit's maximum current without overheating. The calculator references NEC 310 copper wire ampacity values at 75°C, which is the standard rating used for most commercial wiring in conduit. As a general guide: 14 AWG handles up to 15A, 12 AWG up to 20A, 10 AWG up to 30A, 8 AWG up to 50A, and 6 AWG up to 65A. For long runs, voltage drop may require upsizing the wire beyond what ampacity alone dictates. Always have final wire sizing reviewed by a licensed electrician.

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Disclaimer: Results are estimates for informational and planning purposes only. Breaker sizing, wire gauge selection, and all electrical work must be performed or reviewed by a licensed electrical contractor in accordance with the National Electrical Code and applicable local codes. FSG does not guarantee the accuracy of these estimates and is not liable for decisions made based on calculator output.