Electrical Formula Cheat Sheet Quick Reference

Ohm's Law

• V = I × R
• I = V / R
• R = V / I
• P = V × I
• P = I² × R
• P = V² / R

Single-Phase Power

• kVA = V × I / 1000
• kW = kVA × PF
• I = kVA × 1000 / V
• I = kW × 1000 / (V × PF)
• kW = V × I × PF / 1000

Three-Phase Power

• kVA = V × I × 1.732 / 1000
• kW = kVA × PF
• I = kVA × 1000 / (V × 1.732)
• I = kW × 1000 / (V × 1.732 × PF)
• kW = V × I × 1.732 × PF / 1000

Motor Formulas

• HP = kW / 0.746
• kW = HP × 0.746
• Efficiency = Output / Input × 100
• Torque (lb-ft) = HP × 5252 / RPM
• Sync RPM = 120 × Freq / Poles
• Slip = (Sync RPM - Actual) / Sync RPM

FLA: Use NEC Tables 430.247–430.250 for motor full-load current, not nameplate.

Transformer Formulas

• kVA = V × I / 1000 (1φ)
• kVA = V × I × 1.732 / 1000 (3φ)
• I = kVA × 1000 / V (1φ)
• I = kVA × 1000 / (V × 1.732) (3φ)
• Turns Ratio = V_pri / V_sec
• V_pri × I_pri = V_sec × I_sec

NEC Quick Rules

• Continuous load: 125% (210.20, 215.3)
• Motor conductor: 125% FLC (430.22)
• Motor OCPD: varies by type (430.52)
• Voltage drop: 3% branch, 5% total (210.19 Info Note)

Standard breaker sizes (240.6):
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

Conduit & Box Fill

• 1 wire: 53% fill (Ch.9 Table 1)
• 2 wires: 31% fill (Ch.9 Table 1)
• 3+ wires: 40% fill (Ch.9 Table 1)
• Nipple (≤24"): 60% fill (Ch.9 Note 4)

Box fill (314.16): Each conductor = 1× largest conductor volume. Clamps = 1×. Devices = 2×. EGC = 1× largest.