Ohm’s law:
V = I × R
I = V ÷ R
R = V ÷ I
 

Single-phase AC power:
P (watts) = V (volts) × I (amps) × Power factor
 

Three-phase AC power:
P 3 ∅ (watts) = V (volts) × I (amps) × Power factor × 1.732
 

Power factor:
PF = cosine (phase angle) × 100
PF = Power (watts) ÷ [ V (volts) × I (amps)]
PF = Real Power (watts) ÷ Apparent Power (VA)
 

Apparent power:
Apparent Power (VA) = V (volts) × I (amps)
 

Voltage Drop:

Vd = I x R

where I = current in amps, R = resistance of the cabling

Note: R = (2 x K x Q x L) / CM, where K is the resistivity of copper (temperature dependent but usually around 10.5 to 12.9), Q  is an AC adjustment factor for 2/0 or larger cable (this can usually be ignored because it's very close to 1), L is the length of the cable run, and CM is the circular mil cross-sectional area, which can be found in properties of materials tables. 

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Peak voltage:
V peak = V rms × 1.414
 

RMS voltage:
V rms = V peak × 0.7071
 

Inductive reactance:
X L (ohms) = 2 π ƒ L
where X L is the inductive reactance, π is pi (3.14), ƒ is the frequency in hertz, and L is the inductance in henrys