Watts to Amps Calculator
Watts ↔ Amps
Convert between watts and amps for any lighting load.
LED drivers typically run 0.9 to 0.98. Use 1.0 for resistive loads.
Watts to Amps Calculator: The Complete Guide
Whether you're sizing a circuit for a new LED retrofit, specing parking lot fixtures, or planning a warehouse high bay installation, knowing how to convert between watts and amps is essential. This free calculator handles the math for any commercial lighting load. Understanding the formulas behind it will make you a smarter buyer, a safer installer, and a more confident decision-maker.
What Are Watts, Amps, and Volts?
Three units describe electrical power, and you need all three to do useful conversions between them.
Watts (W) measure power. Power is how much electrical energy something uses or produces per second. A 100 watt LED high bay consumes 100 joules of energy every second of operation.
Amps (A), short for amperes, measure current. Current is the rate of electrical charge flowing through a circuit. Higher amperage means more electrons moving through the wire each second.
Volts (V) measure voltage, or the electrical pressure that pushes current through a circuit. A useful mental model: voltage is like water pressure and amperage is like the volume of water flowing through the pipe.
These three units are tied together by the foundational electrical formula:
Rearrange that equation and you can solve for whichever value you don't have. The calculator above handles all the variations for you.
How to Convert Watts to Amps
The exact formula depends on your circuit type. Commercial LED lighting projects almost always involve AC power, but DC matters for solar applications, low voltage track lighting, and emergency lighting battery banks.
DC Circuits
For direct current applications like batteries, solar arrays, and low voltage lighting:
AC Single-Phase Circuits
For standard residential and light commercial installations:
AC Three-Phase Circuits
For industrial and large commercial loads (480V three-phase is the workhorse):
How to Convert Amps to Watts
Flipping the formulas to go from current back to power:
- DC: Watts = Amps × Volts
- AC Single-Phase: Watts = Amps × Volts × Power Factor
- AC Three-Phase: Watts = √3 × Amps × Volts × Power Factor
The calculator at the top of this page handles both directions. Just tap the toggle.
What Is Power Factor and Why Does It Matter?
Power factor (PF) is a number between 0 and 1 that describes how efficiently electrical current is being converted into useful work. In AC circuits, current and voltage don't always rise and fall in perfect sync. The mismatch between the two creates reactive power that doesn't do useful work but still flows through the system.
A power factor of 1.0 means perfect alignment. Every amp delivered is doing useful work. Pure resistive loads like incandescent bulbs and heating elements have a PF of 1.0.
LED drivers, motors, and most modern electronics introduce some mismatch. Common power factor ranges by load type:
- Premium LED drivers: 0.95 to 0.99
- Standard LED drivers: 0.90 to 0.95
- Older fluorescent ballasts: 0.50 to 0.80
- Resistive loads (heaters, incandescent): 1.0
- Induction motors (unloaded): 0.30 to 0.60
For commercial LED lighting estimates, 0.9 is a safe default. Most quality LED drivers manufactured today exceed 0.9 PF, and many drivers used in PrimeLights fixtures exceed 0.95.
Common Commercial Voltages and Where They're Used
| Voltage | Phase | Common Application |
|---|---|---|
| 120V | Single | Residential, small office, retail plug loads, general purpose receptacles |
| 208V | Single or Three | Light commercial buildings, restaurants, mixed-use spaces |
| 240V | Single | Residential HVAC, EV chargers, electric water heaters, large appliances |
| 277V | Single | Commercial lighting on 480Y/277V systems (most common for offices, warehouses, retail) |
| 480V | Three | Industrial equipment, manufacturing, large commercial HVAC, motor loads |
Why 277V Dominates Commercial Lighting
Most commercial buildings have a 480Y/277V service. The 480V three-phase powers HVAC and heavy equipment, while each phase to neutral provides 277V which is what runs the lighting. Using 277V over 120V cuts current draw by more than half for the same wattage. That translates to smaller wire sizes, less voltage drop on long runs, fewer circuit homeruns back to the panel, and lower installation cost. For new construction and major retrofits, 277V is almost always the right call.
LED Lighting Amperage Reference Table
Quick lookup for common LED wattages at common voltages. All values are single-phase AC assuming a 0.9 power factor. Round up when sizing circuits.
| Fixture Wattage | 120V | 208V | 240V | 277V | 480V |
|---|---|---|---|---|---|
| 50W | 0.46 A | 0.27 A | 0.23 A | 0.20 A | 0.12 A |
| 100W | 0.93 A | 0.53 A | 0.46 A | 0.40 A | 0.23 A |
| 150W | 1.39 A | 0.80 A | 0.69 A | 0.60 A | 0.35 A |
| 200W | 1.85 A | 1.07 A | 0.93 A | 0.80 A | 0.46 A |
| 240W | 2.22 A | 1.28 A | 1.11 A | 0.96 A | 0.56 A |
| 300W | 2.78 A | 1.60 A | 1.39 A | 1.20 A | 0.69 A |
| 400W | 3.70 A | 2.14 A | 1.85 A | 1.60 A | 0.93 A |
| 500W | 4.63 A | 2.67 A | 2.31 A | 2.01 A | 1.16 A |
Why Amperage Matters for Commercial Lighting Projects
Getting the amperage right is more than a math exercise. It directly impacts:
Circuit and breaker sizing. The National Electrical Code (NEC) generally limits continuous lighting loads to 80% of a breaker's rating. A 20 amp breaker can safely carry 16 amps of continuous lighting load. If you're loading more than that, you need to split the run or upsize the circuit.
Wire gauge selection. Higher amperage requires thicker wire. Undersizing wire creates voltage drop and overheating risks. As a rough guide: 14 AWG handles up to 15 amps, 12 AWG up to 20 amps, 10 AWG up to 30 amps, 8 AWG up to 40 amps.
Voltage drop calculations. Long runs at higher amperage cause significant voltage drop. NEC recommends keeping voltage drop under 3% on branch circuits and 5% total from service to outlet.
Driver compatibility. When daisy-chaining LED fixtures, the total connected amperage must stay within both the driver and circuit limits.
Energy cost projections. Utilities bill in kilowatt-hours, not amps, but amp-level analysis helps you size circuits accurately, avoid overspending on infrastructure, and benchmark loads against utility rebate program thresholds.
How Many Fixtures Can I Run on One Circuit?
A common practical question. Using the NEC 80% continuous load rule:
20 amp circuit at 277V single phase can handle:
- 16 amps × 277V × 0.9 PF = 3,989 watts of continuous lighting load
- About 26 fixtures at 150W each, or 13 fixtures at 300W each
20 amp circuit at 120V single phase can handle:
- 16 amps × 120V × 0.9 PF = 1,728 watts of continuous lighting load
- About 11 fixtures at 150W each, or 5 fixtures at 300W each
This is why commercial buildings standardize on 277V for lighting wherever possible. The same circuit infrastructure delivers more than twice the lighting capacity at no additional cost.
Need help spec'ing your LED project?
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Shop PrimeLightsFrequently Asked Questions
How many amps does a 100 watt LED draw?
On a standard 120V single-phase circuit with 0.9 power factor, a 100 watt LED draws approximately 0.93 amps. On a 277V commercial circuit, the same fixture draws only 0.40 amps. At 480V three-phase, just 0.13 amps per phase.
What's the difference between single-phase and three-phase power?
Single-phase delivers power through two wires (hot and neutral) and is standard for residential and light commercial loads. Three-phase delivers power through three hot wires offset by 120 degrees, providing more efficient power transfer for larger commercial and industrial loads. Most commercial lighting installations connect single-phase from a three-phase service to balance loads across the panel.
Do I really need to know the power factor?
For rough estimates, no. Using a PF of 1.0 yields a worst-case amperage that's higher than actual draw, so wire and breaker sizing based on it stays conservative. For accurate load calculations on large projects or when applying for utility rebates, yes. Most quality LED drivers operate between 0.9 and 0.98 power factor.
What's the maximum lighting wattage on a 20 amp circuit?
Following NEC 80% continuous load rules: about 1,920 watts at 120V (PF 1.0) or 1,728 watts at 0.9 PF. At 277V, up to 4,432 watts at PF 1.0 or 3,989 watts at 0.9 PF. Always design with the worst-case PF in mind, then verify against your actual fixture specs.
Can I use this calculator for non-lighting loads?
Yes. The formulas work for any electrical load. The calculator doesn't know whether you're sizing for LED fixtures, motors, heaters, or computers. Just make sure you use the right power factor for the load type. Motors and HVAC equipment have very different power factors than LED lighting.
Why do high-voltage circuits use less amperage?
Power equals voltage times current. To deliver the same wattage, doubling the voltage halves the current. This is why utility transmission lines run at hundreds of thousands of volts. Moving power efficiently over long distances requires minimizing current and the I²R losses (heat) in the wire.
Are these calculations NEC compliant?
The math is correct, but real-world NEC compliance involves additional factors: continuous load adjustments, derating for ambient temperature and conductor bundling, voltage drop limits, breaker coordination, and grounding requirements. Use this calculator for planning and quick estimates. For installation, work with a licensed electrician.
What is VA and how does it differ from watts?
VA (volt-amperes) measures apparent power, while watts measure real power. The relationship is: Watts = VA × Power Factor. For a load with PF 1.0, VA and watts are identical. For lower power factors, VA exceeds watts. Generators, transformers, and UPS systems are typically rated in VA because they must handle apparent power, not just real power.
How do I calculate amps for a parking lot LED retrofit?
Total the fixture wattages, then divide by your circuit voltage and power factor. For example, 30 pole-mounted LED fixtures at 240W each totals 7,200 watts. On a 277V single-phase circuit at 0.9 PF, that's 7,200 ÷ (277 × 0.9) = 28.9 amps. You'd need to split that across at least two 20 amp circuits (each loaded under 16 amps continuous), or one 30 or 40 amp circuit depending on local code interpretation and contractor preference.
Does my LED driver's listed wattage include power factor losses?
Reputable manufacturers list both input watts (what the driver pulls from the wall) and output watts (what reaches the LEDs). For amperage calculations, always use input watts, since that's what the circuit must supply. The difference between input and output watts represents driver efficiency, typically 88% to 94% for quality LED drivers.
