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

IT - Series = I1 + I2 + ... In

IT - Parallel = I1 + I2 + ... In

1 /RT - Parallel = 1/R1 + 1/R2 + ... 1/Rn

VT - Parallel = V1 + V2 + ... Vn

RT - Series = R1 + R2 + ... Rn

VT - Series = V1 + V2 + ... Vn

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

LT - Series = L1 + L2 + ... Ln

1 / IT - Parallel = I1 + I2 + ... In

Z = V / I

Z = √R2 + X2

Z = √R2 + (XL - Xc) 2

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

1 /CT - Series = 1/C1 + 1/C2 + ... 1/Cn

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

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

Reactance (X) = Capacitive Reactance (Xc) - Inductive Reactance (XL)

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

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