Commonly Used Electrical Calculation Formulas
author： Guangyi Substation
source： Guangyi Substation
elease time：2025-04-26
【Summary Description 】 Commonly Used Electrical Calculation Formulas
Commonly Used Formulas
P = U × I R = U ÷ I P – Power (W)
P = I 2 ×R R = P ÷ I 2 R – Resistance (Ω)
P = U 2 ÷R I = U ÷ R I – Current (A)
U = I × R U – Voltage (V)
X C = 1/(2πƒC) X L = 2πƒL X C --- Capacitive Reactance (Ω) X L ---- Inductive Reactance (Ω)
Q L = U 2 / X L Q L =I 2. X L Q L --- Reactive Power (Vr) ƒ----- Frequency (Hz)
Q C = U 2 / X C Q C =I 2. X C Q C --- Reactive Power (Vr) L-----Inductance
Total impedance (Ω) of a series circuit consisting of resistance, inductance, and capacitance
Z = R 2 + (X L –X C ) 2 Z = U ÷ I I = U ÷ Z
Motor rated current I N
Three-phase I N = P N 3×U N ×COSΦ×η Single-phase I N = P N ÷U N ×COSΦ×η
P N --------- Motor Rated Power (W) U N ------ Motor Rated Voltage (V)
COSΦ – Power Factor, η – Motor Efficiency
Full-voltage motor start
I ST /I N ≤3/4 + P S /4 P S
I ST — Motor starting current (A) I N — Motor rated current (A)
P S — Power transformer capacity (kVA) P N — Motor rated power (kW)
COSΦ = 1 + (Q / P) 2 COSΦ—Power Factor, P—Active Power
Q----Reactive Power
Three-phase symmetrical load
△Connection P = 3 × U_phase × I_phase × cosΦ = × U_line × I_line × cosΦ
Y-connection P = 3 × U_phase × I_phase × cosΦ = × U_line × I_line × cosΦ
Determining the Rated Current of Contactor Main Contacts
I = Pn × 10 3 /K Un Pn Motor Power Un Rated Voltage K ≈ 1–1.4
Ohm's Law:
Series connection: U = U1 + U2 + U3 + U4, R = R1 + R2 + R3 + R4
I = I1 = I2 = I3 = I4
In parallel: U = U₁ = U₂ = U₃ = U₄, I = I₁ + I₂ + I₃ + I₄
1/R = 1/R₁ + 1/R₂ + 1/R₃ + 1/R₄
Two identical resistors connected in parallel: R = R1 × R2 / (R1 + R2)
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