Quick Reference
Essential formulas, units, and reference tables for electrical fundamentals.
Ohm's Law
V = I × R
Variables
- V = Voltage (Volts (V))
- I = Current (Amperes (A))
- R = Resistance (Ohms (Ω))
Example
If I = 2A and R = 5Ω, then V = 2 × 5 = 10V
I = V ÷ R
Variables
- I = Current (Amperes (A))
- V = Voltage (Volts (V))
- R = Resistance (Ohms (Ω))
Example
If V = 12V and R = 4Ω, then I = 12 ÷ 4 = 3A
R = V ÷ I
Variables
- R = Resistance (Ohms (Ω))
- V = Voltage (Volts (V))
- I = Current (Amperes (A))
Example
If V = 24V and I = 6A, then R = 24 ÷ 6 = 4Ω
Power Formulas
P = V × I
Variables
- P = Power (Watts (W))
- V = Voltage (Volts (V))
- I = Current (Amperes (A))
Example
If V = 120V and I = 2A, then P = 120 × 2 = 240W
P = I² × R
Variables
- P = Power (Watts (W))
- I = Current (Amperes (A))
- R = Resistance (Ohms (Ω))
Example
If I = 3A and R = 10Ω, then P = 9 × 10 = 90W
P = V² ÷ R
Variables
- P = Power (Watts (W))
- V = Voltage (Volts (V))
- R = Resistance (Ohms (Ω))
Example
If V = 120V and R = 60Ω, then P = 14400 ÷ 60 = 240W
Common Electrical Units
| Symbol | Unit | Measures |
|---|---|---|
| V | Volt | Voltage (electrical pressure) |
| A | Ampere | Current (flow of charge) |
| Ω | Ohm | Resistance (opposition to flow) |
| W | Watt | Power (rate of energy transfer) |
| Hz | Hertz | Frequency (cycles per second) |
| F | Farad | Capacitance (charge storage) |
| H | Henry | Inductance (magnetic storage) |
Wire Color Codes (US NEC)
| Color | Function | Notes |
|---|---|---|
| Black | Hot (Line) | Primary hot conductor |
| White | Neutral | Return path for current |
| Green | Ground | Equipment grounding conductor |
| Red | Hot (second phase) | Second hot in 240V circuits |
| Blue | Hot (3-phase) | Used in 3-phase systems |
Common Residential Voltages
🇺🇸 United States
- 120V — Standard outlets (lighting, small appliances)
- 240V — Large appliances (dryer, oven, A/C)
- 60 Hz — AC frequency
🇬🇧 United Kingdom / 🇦🇺 Australia
- 230–240V — Standard outlets
- 50 Hz — AC frequency
⚡ Safety Quick Rules
- Always de-energize before working. Use lockout/tagout (LOTO) procedures and verify zero energy with a tested meter.
- Treat every conductor as live until you personally verify it is de-energized with a reliable multimeter.
- Wear appropriate PPE. Insulated gloves, safety glasses, and arc-rated clothing matched to the hazard level.
- Never work alone on energized equipment. Always have a safety partner and know the location of the nearest emergency shutoff.
- Respect ampacity ratings. Never use undersized conductors or bypass fuses and breakers.
Common Formulas at a Glance
| Formula | Solves For | Notes |
|---|---|---|
| V = I × R | Voltage | Ohm's Law |
| P = V × I | Power | Watt's Law |
| R_total = R₁ + R₂ + … | Series Resistance | Series circuits |
| 1/R_total = 1/R₁ + 1/R₂ + … | Parallel Resistance | Parallel circuits |
| E = P × t | Energy (kWh) | P in kW, t in hours |
| f = 1 / T | Frequency | T = period in seconds |