Ohm's Law explained
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit.
V = I × R (Voltage = Current × Resistance)
P = V × I (Power = Voltage × Current)
Plumbing analogy that helps:
- Voltage (V) = water pressure, the force pushing electrons through the circuit.
- Current (I, amps) = flow rate, how many electrons pass per second.
- Resistance (R, ohms) = pipe width, how much the wire/load restricts flow.
- Power (P, watts) = work done, voltage × current.
Watts, amps, volts, what's the difference?
Voltage (V)
Electric pressure. Use the voltage on your device's label or your supply; the calculator has 120V, 220V and 240V presets.
Current (A)
Electron flow rate. A breaker's amp rating is the current its circuit is built to carry.
Power (W)
Work done by electricity each second. Volts × amps: a 30A load at 240V uses 7,200W.
Common watts-to-amps conversions
| Watts | Amps @ 120V | Amps @ 240V |
|---|---|---|
| 100W | 0.83 A | 0.42 A |
| 500W | 4.17 A | 2.08 A |
| 1,000W | 8.33 A | 4.17 A |
| 1,500W | 12.50 A | 6.25 A |
| 2,000W | 16.67 A | 8.33 A |
| 3,000W | 25.00 A | 12.50 A |
| 5,000W | 41.67 A | 20.83 A |
| 7,200W | 60.00 A | 30.00 A |
| 10,000W | 83.33 A | 41.67 A |
Wire gauge (AWG) capacity guide
Wire must be sized for the current it carries. Too small = overheating and fire hazard. The table is the National Electrical Code's limit for small copper conductors (NFPA 70, section 240.4(D)). The NEC is a model code that states and cities adopt with changes, so your local code, and an electrician, govern. Larger circuits depend on the cable type and its insulation rating, so they are not listed here.
| Gauge | Max breaker (copper) | Circuit |
|---|---|---|
| 14 AWG | 15 A | 15 A branch circuits |
| 12 AWG | 20 A | 20 A branch circuits |
| 10 AWG | 30 A | 30 A branch circuits |
Circuit breaker sizing
Circuit breakers protect wiring from overcurrent. The breaker amp rating must match the wire gauge. Watts are volts × amps; the copper sizes are the NEC's small-conductor limits:
- 15A breaker → 14 AWG copper → 1,800W max at 120V (1,440W continuous)
- 20A breaker → 12 AWG copper → 2,400W max at 120V (1,920W continuous)
- 30A breaker → 10 AWG copper → 3,600W (120V) / 7,200W (240V)
- 50A breaker → 12,000W at 240V; the conductor size depends on the cable, so have it sized
The 80% continuous load rule: the National Electrical Code sizes a circuit for a load that runs 3 hours or more so that the load uses no more than 80% of the breaker rating. So a 20A circuit at 120V should carry at most 1,920W continuously (20 × 120 × 0.80). This is a model code; local code governs.
Electrical safety basics
- Don't exceed circuit capacity: add up the watts of everything on one circuit and keep long-running loads within the continuous figure above.
- Use GFCI outlets where water is nearby, such as kitchens, bathrooms, garages, and outdoor spaces. A GFCI cuts the power quickly when current leaks to ground.
- Don't daisy-chain extension cords or power strips. Each adds resistance and heat.
- Extension cord rating matters: the cord's amp or watt rating is printed on it; use one rated at or above the load.
- Call an electrician for: adding new circuits, panel upgrades, anything inside the breaker box, or if you feel any tingling when touching metal (ground fault).
Frequently asked questions
How do I convert watts to amps?
Divide watts by volts: Amps = Watts ÷ Volts. A 1,200W microwave at 120V = 1,200 ÷ 120 = 10 amps. At 240V, the same wattage = 5 amps (half the current because voltage doubles). This is the power relation P = V × I, rearranged to I = P ÷ V. Use our calculator above for any values.
How many amps is 1500 watts at 120V?
1,500 watts ÷ 120 volts = 12.5 amps. A 1,500W space heater on a standard 120V outlet draws that much, within a 15-amp circuit's rating but with little room for anything else. At 240V, 1,500W = 6.25 amps. The National Electrical Code sizes circuits for loads running 3 hours or more at 80% of the breaker, 12 A on a 15 A breaker, one more reason to keep other loads off the heater's circuit; your local code governs.
What is Ohm's Law?
Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R): V = I × R. Voltage equals current times resistance. Rearranged: I = V ÷ R (current = voltage ÷ resistance), R = V ÷ I (resistance = voltage ÷ current). Power extension: P = V × I = I² × R = V² ÷ R. It is named after the German physicist Georg Ohm and underlies nearly every circuit calculation.
What wire size do I need for 20 amps?
Under the National Electrical Code's small-conductor rule, copper on a 20 A breaker must be at least 12 AWG; 14 AWG copper is limited to 15 A. Long runs may need a larger size to keep voltage drop down, which our voltage drop calculator works out. Aluminum needs a larger size than copper for the same current. The NEC is a model code, so your local code governs.
Why does the US use 120V and Europe uses 220V?
Largely history: American systems settled on a lower voltage early, and much of Europe standardized later on a higher one. For the same power, a higher voltage needs less current (I = P ÷ V), so thinner wires will do, but it is more dangerous to touch. Many chargers accept a range of input voltages; the range is printed on the label.
How many watts can a 15 amp breaker handle?
15A × 120V = 1,800 watts maximum. For a continuous load, one running 3 hours or more, the National Electrical Code limits you to 80% = 1,440 watts. Short bursts (microwave, vacuum) can use the full rating; long ones (space heater) should stay under the lower figure. If your breaker trips repeatedly, you're exceeding the rating: reduce the load or have an electrician add a circuit.
How many amps does a refrigerator use?
It varies with size, age and whether the compressor is running. The nameplate inside the fridge lists its rated current; divide its rated watts by 120 V if only watts are given. A running compressor draws far less than the brief surge when it starts, and over a day it cycles on and off, so the average is lower than either.
What is the difference between watts and amps?
Amps (amperes) measure current, the flow rate of electrons. Watts measure power, how much work the electricity does. Volts measure pressure, the force pushing electrons. Analogy: amps are like gallons per minute of water, volts like water pressure, watts like how much work the water can do. The relationship: Watts = Volts × Amps. You can't directly compare watts and amps without knowing voltage.
How do I calculate resistance?
Use Ohm's Law: Resistance = Voltage ÷ Current (R = V ÷ I). Example: a circuit with 120V and 5A current has 120 ÷ 5 = 24 ohms of resistance. Alternative formula using power: R = V² ÷ P (resistance = voltage squared ÷ watts) or R = P ÷ I² (resistance = watts ÷ current squared). Units: ohms (Ω). Tools like a multimeter measure resistance directly, use the Ω setting on a disconnected device.
Can I plug a 1500 watt heater into a regular outlet?
A standard US outlet on a 15A circuit at 120V supplies up to 1,800W. A 1,500W heater draws 12.5A, within that. BUT: don't run the heater plus other loads on the same circuit (TV, microwave, etc.) because combined draw can exceed 15A and trip the breaker. Also avoid extension cords, they add resistance and can overheat at high current. Plug directly into a wall outlet, and keep other appliances off the same circuit.