Cheat sheet
Electrical formulas
Ohm’s law, power, series and parallel resistance, and US conductor colors.
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Ohm’s law
| Find | Formula |
|---|---|
| Voltage (V) | V = I × R |
| Current (A) | I = V ÷ R |
| Resistance (Ω) | R = V ÷ I |
Power
| Find | Formula |
|---|---|
| Power (W), DC or resistive AC | P = V × I = I² × R = V² ÷ R |
| Single-phase AC real power (W) | P = V × I × PF |
| Three-phase AC real power (W) | P = √3 × V(line-to-line) × I × PF |
| Apparent power (VA) | S = V × I (single-phase), √3 × V × I (three-phase) |
| Horsepower | 1 hp = 746 W |
PF is the power factor, between 0 and 1. For purely resistive loads such as heaters, PF = 1.
Series and parallel resistance
| Arrangement | Total |
|---|---|
| Series | R = R1 + R2 + R3 + … |
| Parallel | 1/R = 1/R1 + 1/R2 + 1/R3 + … |
| Two in parallel | R = (R1 × R2) ÷ (R1 + R2) |
US conductor colors
| Conductor | Color |
|---|---|
| Equipment grounding | Green, green with yellow stripes, or bare |
| Grounded (neutral) | White or gray |
| Ungrounded (hot), 120/240 V | Commonly black and red, plus blue on three-phase |
| Ungrounded (hot), 277/480 V | Commonly brown, orange and yellow |
The NEC sets the grounding and neutral colors. Hot conductor colors are a convention and may be set by local code or the job specification.
Sources
- NFPA 70, National Electrical Code (NEC), Article 200 (grounded conductors) and 250.119 (equipment grounding conductors).
- Standard circuit theory. Check your local code and the authority having jurisdiction for anything installed.