Common Formulas used on the Electrical Contractor Exam

I = V / R

R = V / I

V = R x I

P = V x I

P = R x I²

P = V² / R

I = P / V

I = √P / R

I = V / R

V = P / I

I_T - Series = I₁ + I₂ + ... I_n

I_T - Parallel = I₁ + I₂ + ... I_n

1 /R_T - Parallel = 1/R₁ + 1/R₂ + ... 1/R_n

V_T - Parallel = V₁ + V₂ + ... V_n

R_T - Series = R₁ + R₂ + ... R_n

V_T - Series = V₁ + V₂ + ... V_n

W = I² x R

Pf = kW / kVA

I = W / V x Pf

E = W / I X Pf

W = V x I x Pf

Pf = W / E x I

P = I² R

Q = I² X

Q = V² / X

S = I² Z

S = IV

Efficiency = Power Output / Power Input

L_T - Series = L₁ + L₂ + ... L_n

1 / I_T - Parallel = I₁ + I₂ + ... I_n

Z = V / I

Z = √R² + X²

Z = √R² + (X_L - X_c) ²

Capacitance (C) = Colombus (Q) / Volts (V)

1 /C_T - Series = 1/C₁ + 1/C₂ + ... 1/C_n

Inductive Reactance(X_L) = 2 x Frequency (F) x Inductance (L)
X_L = 6.28 x F x L

Capacitive Reactance (X_c) = 1 / 2 x Frequency (F) x Capacitive (C)
X_c = 1/6.28 x F x C

Reactance (X) = Capacitive Reactance (X_c) - Inductive Reactance (X_L)

Max Length = Voltage drop x Kemil / (1.732 or 2*) x Current x Resistance factor.
*If single-phase, use 2. If three-phase, use 1.73

Max Current = Voltage Drop x Kemil / (1.732 or 2*) x Length x Resistance factor
*If single-phase, use 2. If three-phase, use 1.73

Voltage Drop = 2 x (Length/Kemil) x Current x Resistance factor
Voltage Drop = 2 x Length x Current x Resistance factor / Kemil

Voltage Drop = 1.732 (Length/Kemil) x Current x Resistance factor
Voltage Drop = 1.732 x Length x Current x Resistance factor / Kemil