Useful Electrical Formulas


Ohm’s Law

I =E/R

or E = IR
where I = current (amperes)
E = electromotive force (volts)
R = resistance (ohms)

Conductor Resistivity
R = ρ*L/a

where ρ = specific resistance (or resistivity) (ohm metres, Ω·m)
L = length (metres)
a = area of cross-section (square metres)
Temperature correction
Rt = Ro (1 + αt)
where Ro = resistance at 0ºC (Ω)
Rt = resistance at tºC (Ω)
α = temperature coefficient which has an average value for copper of 0.004 28
R2 = R1 * (1 +αt1 ) /(1+ αt2 )

where R1 = resistance at t1 (Ω)
R2 = resistance at t2 (Ω)

α Values Ω/ΩºC
copper 0.00428
platinum 0.00385
nickel 0.00672
tungsten 0.0045
aluminum 0.0040

Alternating Current

R.M.S. value of sine curve = 0.707 maximum value
Mean value of sine curve = 0.637 maximum value
Form factor of sinusoidal = RMS Value /Mean Value = 1.11

Frequency of alternator =pN/60 cycles per second
Where p = number of pairs of poles
N = rotational speed in r/min
Instantaneous value = Maximum value × Sin (2πft)
Note: calculator must be in radian mode

Slip of Induction Motor

((Slip speed of field - speed of rotor)/Speed of field) * 100

Current in AC Circuit

Current = impressed volts/impedance

Power Factor

p.f. = true watts/(volts x amperes)

also p.f. = cos Φ, where Φ is the angle of lag or lead