Ohm's Law explained
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It's the single most important equation in electrical engineering.
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. US outlets: 120V. EV chargers/dryers: 240V. Europe: 230V.
Current (A)
Electron flow rate. Household circuits: 15 or 20 amps. Big appliances: 30–50 amps.
Power (W)
Work done by electricity. LED bulb: 10W. Microwave: 1,000W. EV charger: 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.
| Gauge | Max amps | Typical use |
|---|---|---|
| 18 AWG | 7 A | Lamp cords, low-voltage |
| 16 AWG | 13 A | Light-duty extension cords |
| 14 AWG | 15 A | Standard 15A household circuits, lighting |
| 12 AWG | 20 A | 20A circuits, kitchen outlets, garage |
| 10 AWG | 30 A | Electric dryer, water heater, RV hookup |
| 8 AWG | 40 A | Large appliances, sub-panels |
| 6 AWG | 50 A | Electric range, EV charger (L2) |
| 4 AWG | 70 A | Service entrance (small homes) |
| 2 AWG | 95 A | Service entrance (typical home) |
| 1/0 AWG | 125 A | Service entrance (large home, 200A) |
US vs international voltage
| Region | Voltage | Frequency |
|---|---|---|
| USA / Canada | 120V (240V for large appliances) | 60 Hz |
| Japan | 100V | 50 or 60 Hz (by region) |
| United Kingdom | 230V | 50 Hz |
| Europe (most) | 230V | 50 Hz |
| Australia | 230V | 50 Hz |
| China | 220V | 50 Hz |
| India | 230V | 50 Hz |
| Mexico | 127V | 60 Hz |
| Brazil | 127V or 220V (by region) | 60 Hz |
Circuit breaker sizing
Circuit breakers protect wiring from overcurrent. The breaker amp rating must match the wire gauge:
- 15A breaker → 14 AWG wire → 1,800W max (1,440W continuous)
- 20A breaker → 12 AWG wire → 2,400W max (1,920W continuous)
- 30A breaker → 10 AWG wire → 3,600W (120V) / 7,200W (240V)
- 50A breaker → 6 AWG wire → 12,000W at 240V — common for EV chargers and electric ranges
The 80% continuous load rule: any device running more than 3 hours should use only 80% of breaker capacity to prevent nuisance tripping. So a 20A circuit should handle max 1,920W continuous (20 × 120 × 0.80).
Electrical safety basics
- Don't exceed circuit capacity — combined device wattage must stay under 80% of the breaker's total (1,440W for 15A, 1,920W for 20A).
- Use GFCI outlets in kitchens, bathrooms, garages, and outdoor spaces. Trips in milliseconds to prevent shock.
- Don't daisy-chain extension cords or power strips. Each adds resistance and heat.
- Extension cord gauge matters — 16 AWG cord rated for 13A can't safely carry 15A. Use 14 AWG for 15A loads.
- 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 Ohm's Law for power: 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 typical 1,500W space heater on a standard 120V outlet draws 12.5 amps — safe on a 15-amp circuit but will trip a 15A breaker if you add much more. At 240V (dryer outlet), 1,500W = 6.25 amps. Never run a 1,500W appliance plus other loads on the same 15A circuit — the combined draw can exceed the 80% continuous load rating (12A safe limit).
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. Named after Georg Ohm (1827). Fundamental to all electrical calculations.
What wire size do I need for 20 amps?
12 AWG copper wire is standard for 20A circuits. 14 AWG is only rated for 15A (undersized and dangerous for 20A). For longer runs (>50 ft), upsize to 10 AWG to minimize voltage drop. Always check local code (NEC). Aluminum wire requires larger gauge for same ampacity (10 AWG aluminum = 12 AWG copper). 20A circuits typically feed kitchen outlets, garage, and bathroom receptacles.
Why does the US use 120V and Europe uses 220V?
Historical accident. Edison's DC system in the 1880s used ~110V because carbon filament bulbs worked well at that voltage. The US standardized as AC was adopted. Europe standardized later on 220V for efficiency — higher voltage means lower current for the same power, which means thinner wires. Higher voltage is more efficient for transmission but more dangerous to handle. Modern appliances often support both (100–240V switching power supplies).
How many watts can a 15 amp breaker handle?
15A × 120V = 1,800 watts maximum. However, the continuous load rule limits you to 80% = 1,440 watts for loads running more than 3 hours. For occasional use (microwave, vacuum), full 1,800W is fine. For continuous use (space heater, EV charger), stay under 1,440W. If your breaker trips repeatedly, you're exceeding the rating — reduce the load or upgrade the circuit to 20A (requires 12 AWG wire).
How many amps does a refrigerator use?
Modern refrigerators draw 3–6 amps (400–700W) while running on 120V, but they cycle on/off. Startup surge can be 10–15 amps briefly. A typical 20 cu ft fridge averages 1–2 amps over 24 hours. Large side-by-side or commercial models can draw 6–10 amps. Manufacturers list the FLA (Full Load Amps) on the nameplate inside the fridge. Dedicated 15A or 20A circuit is recommended to prevent tripping from startup surges.
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?
Yes, standard US outlets are 15A at 120V = 1,800W max. A 1,500W heater draws 12.5A — within limits. 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.