Plant electrical calculators
Single- and three-phase power, motors, power factor, voltage drop, transformers and batteries: 14 everyday plant calculations.
Basics and energy
Ohm's law and DC power
Also valid in AC for a purely resistive load. A resistor that dissipates more than its rating overheats and burns. Electrical work: qualified person and lockout only.
Resistors in series and parallel
In parallel, the equivalent is always smaller than the smallest resistor. Measure resistance de-energised, with the circuit isolated. Electrical work: qualified person and lockout only.
Heating element: resistance and current
Per element, resistance assumed constant (it changes a little with temperature). A 240 V element on 208 V loses about 25 % of its power. Electrical work: qualified person and lockout only.
Energy used (kWh)
Load factor = average power actually drawn ÷ connected power. For a motor, measure the input power rather than trusting the nameplate.
Battery / UPS runtime
Estimate only: real capacity drops at high discharge current (Peukert effect), in the cold and with age. Allow a 20 to 25 % margin and run a discharge test. Electrical work: qualified person and lockout only.
AC power and distribution
Single-phase power
Measure the power factor with a power analyser; otherwise use the nameplate value (valid at full load only). Electrical work: qualified person and lockout only.
Three-phase power
U = line-to-line voltage, I = line current (average of the three phases). Balanced load assumed; if the currents differ by more than 10 %, find out why. Electrical work: qualified person and lockout only.
Power factor correction (capacitors)
Do not aim above 0.95 to 0.98: overcompensation at light load and a resonance risk with drive harmonics. Capacitors stay charged after switch-off: wait for discharge and verify absence of voltage. Electrical work: qualified person and lockout only.
Transformer: full-load and short-circuit current
Short circuit at the secondary terminals with an infinite upstream source: a conservative value to check breaking capacity. Arc-flash and coordination studies need a full calculation. Electrical work: qualified person and lockout only.
Cable voltage drop
IEC 60364-5-52 Annex G method: ρ in service 0.0225 (copper) or 0.036 (aluminium) Ω·mm²/m, λ = 0.08 mΩ/m. Usual guide: 3 to 5 % in total; the local electrical code governs. Electrical work: qualified person and lockout only.
Motors and drives
Motor full-load current
P = mechanical shaft power (nameplate). Estimate only: set overload protection from the nameplate current and size conductors from the electrical code tables. Electrical work: qualified person and lockout only.
Synchronous speed and slip
p = number of poles (2 × pole pairs). Typical full-load slip of an induction motor: 1 to 5 %; it rises with load, so speed tells you about load.
Motor starting current
k comes from the nameplate or motor data sheet (often 5 to 8). With star-delta, starting torque also drops to one third. Starting current lasts a few seconds. Electrical work: qualified person and lockout only.
Variable speed drive: reduced power (affinity laws)
Ideal cube law: centrifugal pump or fan with no static head. With static head and drive (2 to 4 %) and motor losses, real savings are lower: measure them.
Learning and checking tools. The ranges shown are general guidance: always follow your plant's procedures, permits and set points. Nothing is sent anywhere: the calculations run in your browser.