# 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 I

^{2}P = V

^{2}/ RI = P / V

I = √P / R

I = V / R

V = P / I

I

_{T}- Series = I_{1}+ I_{2}+ ... I_{n}I

_{T}- Parallel = I_{1}+ I_{2}+ ... I_{n}1 /R

_{T}- Parallel = 1/R_{1}+ 1/R_{2}+ ... 1/R_{n}V

_{T}- Parallel = V_{1}+ V_{2}+ ... V_{n}R

_{T}- Series = R_{1}+ R_{2}+ ... R_{n}V

_{T}- Series = V_{1}+ V_{2}+ ... V_{n}W = I

^{2}x RPf = kW / kVA

I = W / V x Pf

E = W / I X Pf

W = V x I x Pf

Pf = W / E x I

P = I

^{2}RQ = I

^{2}XQ = V

^{2}/ XS = I

^{2}ZS = IV

Efficiency = Power Output / Power Input

L

_{T}- Series = L_{1}+ L_{2}+ ... L_{n}1 / I

_{T}- Parallel = I_{1}+ I_{2}+ ... I_{n}Z = V / I

Z = √R

^{2}+ X^{2}Z = √R

^{2}+ (X_{L}- X_{c})^{ 2}Capacitance (C) = Colombus (Q) / Volts (V)

1 /C

_{T}- Series = 1/C_{1}+ 1/C_{2}+ ... 1/C_{n}Inductive Reactance(X

X

_{L}) = 2 x Frequency (F) x Inductance (L)X

_{L}= 6.28 x F x LCapacitive Reactance (X

X

_{c}) = 1 / 2 x Frequency (F) x Capacitive (C)X

_{c}= 1/6.28 x F x CReactance (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

*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

*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 = 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

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