Electrical Formulas

Quick-reference formula cards for electricians — 20 essential formulas with worked examples and NEC references.

Electrical Fundamentals

Core electrical laws and relationships

Ohm's Law & Power Wheel

V = I × R
beginner2 examples

Voltage Drop Formulas (1Φ & 3Φ)

VD = 2 × L × I × R / 1000
beginner2 examples

Parallel Resistance Formula

1/Rt = 1/R₁ + 1/R₂ + 1/R₃ + ...
beginner1 example

Power Systems

AC power, three-phase, and power quality formulas

AC Power Formulas

P = V × I × cos(φ)
intermediate1 example

Three-Phase Power Formulas

P = √3 × V_LL × I_L × PF
intermediate2 examples

Transformer Formulas

I = kVA × 1000 / (V × √3)
intermediate1 example

Short Circuit Current Formula

Isc = kVA × 1000 / (V × √3 × %Z/100)
advanced1 example

Capacitor kVAR Sizing Formula

kVAR = kW × (tan φ₁ − tan φ₂)
intermediate1 example

Arc Flash Incident Energy (IEEE 1584)

E = 4.184 × Cf × En × (t/0.2) × (610^x / D^x)
advanced1 example

Circuit Sizing

NEC conductor, conduit, and overcurrent sizing formulas

Conduit Fill Formula

Fill% = (Σ A_conductor / A_conduit) × 100
beginner1 example

NEC Demand Factor Formulas

Lighting: first 3,000 VA @ 100% + remainder @ 35%
intermediate1 example

Lumen Method Formula

N = (E × A) / (Φ × CU × MF)
intermediate1 example

Cable Pulling Tension Formula

T = W × L × f
advanced1 example

NEC 125% Continuous Load Rule

OCPD ≥ 1.25 × I_continuous + 1.0 × I_non-continuous
beginner1 example

Motor Circuits

Motor circuit design and efficiency formulas

Motor Circuit Formulas

Conductor ≥ 1.25 × FLC
intermediate1 example

Motor/Equipment Efficiency

η = (P_out / P_in) × 100%
beginner1 example

Renewable Energy

Solar, battery, and energy storage formulas

Battery Sizing Formula

Ah = (P × t) / (V × DoD × η)
beginner1 example

Solar String Sizing Formula

N_max = V_inv_max / Voc_cold
intermediate1 example

Conversion & Reference

Unit conversions and quick reference formulas

AWG to mm² Conversion Formula

A(mm²) = 0.012668 × 92^((36−AWG)/39)
beginner1 example

Resistance Temperature Correction

R₂ = R₁ × [1 + α(T₂ − T₁)]
intermediate1 example

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