Electrical Engineering Glossary: 36 Terms with Worked Examples

The terms that appear across the calculators and guides, each defined in plain language, followed by an example with real numbers and a link to the calculator where the idea is put to work. Where a definition depends on a standard, the standard is named.

Last reviewed 10 November 2026

A

Ambient temperature and grouping correction factors

Ca, Cg

Ca corrects the tabulated capacity when the ambient differs from the reference (30 °C in air, 20 °C in ground in IEC 60364-5-52); Cg reduces it when several loaded circuits share a route and heat each other. Both are taken from the tables of the applicable standard.

Example: For PVC cable at 40 °C, IEC 60364-5-52 gives Ca = 0.87; two bunched circuits give Cg = 0.80, so a 27 A cable is good for 27 × 0.87 × 0.80 = 18.8 A.

Apparent power

S (kVA)

The product of RMS voltage and current (times √3 in a three-phase circuit), whatever that current is doing. It sets the current that cables, transformers, alternators and UPS units must carry, which is why they are rated in kVA.

Example: A three-phase 400 V circuit carrying 100 A: S = √3 × 400 × 100 = 69.3 kVA.

Related calculator: kVA to kW Converter (and kVAr)

B

Breaking capacity

Icn, Icu, Ics

The highest fault current a device can interrupt safely: Icn for MCBs to IEC 60898-1, and the ultimate (Icu) and service (Ics) capacities for circuit breakers to IEC 60947-2. It must be at least the prospective short-circuit current where the device is installed, unless the manufacturer documents back-up protection by an upstream device.

Example: Where the prospective fault current is 12 kA, a 6 kA or 10 kA MCB is not adequate; a 15 kA device or higher is needed.

Related calculator: Transformer Short-Circuit Current Calculator

C

C-rate and Peukert exponent

C, k

C-rate is the discharge current relative to capacity: 1C empties the rated Ah in about an hour. Peukert's law, t = H × (C / (I × H))^k, describes how the delivered capacity falls at higher rates; k = 1 is an ideal battery, real batteries (especially lead-acid) have k above 1, taken from the manufacturer's discharge tables.

Example: A 100 Ah battery rated at the 20 h rate, with k = 1.2, discharged at 10 A (0.1C): t = 20 × (100 / 200)^1.2 = 8.7 h, not the 10 h a simple Ah ÷ A gives.

Related calculator: Battery Runtime Calculator with Peukert

Current-carrying capacity (ampacity)

The maximum current a conductor can carry continuously in its installation conditions without exceeding its insulation temperature limit (70 °C for PVC, 90 °C for XLPE). It is the tabulated value multiplied by the applicable correction factors.

Example: A tabulated 100 A with ambient factor 0.87 and grouping factor 0.80 gives Iz = 100 × 0.87 × 0.80 = 69.6 A.

D

Depth of discharge

DoD

The fraction of a battery's rated capacity that is used before recharging. Deeper cycling gives more usable energy per cycle but usually fewer cycles; the allowable DoD depends on chemistry and the manufacturer's cycle-life data.

Example: A 12 V, 200 Ah battery at 50 % DoD gives 12 × 200 × 0.5 = 1200 Wh of usable energy.

Related calculator: Battery Runtime Calculator with Peukert

Derating

A reduction of the output equipment can safely deliver because conditions are worse than the rating basis, such as altitude and intake temperature for engines or ambient temperature for alternators and cables. Derating figures come from the manufacturer or the relevant standard.

Example: A 45 kW requirement at a site with a 10 % derate needs a set rated 45 / (1 − 0.10) = 50 kW.

Related calculator: Generator Sizing Calculator

Design, rated and permissible current (Ib ≤ In ≤ Iz, I2 ≤ 1.45 Iz)

Ib, In, Iz, I2

IEC 60364-4-43 overload protection requires the circuit design current Ib ≤ device rating In ≤ cable capacity Iz, and the conventional tripping current I2 (the current that surely trips the device within the conventional time) ≤ 1.45 Iz. For MCBs to IEC 60898-1, I2 = 1.45 In, so the second condition follows from In ≤ Iz; other devices, such as gG fuses, have a higher I2 and need checking against their own standard.

Example: Ib = 28 A on a cable with Iz = 36 A: a 32 A MCB gives 28 ≤ 32 ≤ 36, and I2 = 1.45 × 32 = 46.4 A ≤ 1.45 × 36 = 52.2 A.

Detuned reactor

p %

A reactor in series with each capacitor step that tunes the combination below the lowest significant harmonic, so the bank looks inductive at harmonic frequencies and cannot resonate with the supply. It is specified by p = X_L / X_C; 7 % and 14 % are common choices.

Example: On 50 Hz, p = 7 % tunes to 50 / √0.07 ≈ 189 Hz, below the 5th harmonic (250 Hz). The capacitor voltage rises to 400 / (1 − 0.07) ≈ 430 V, so capacitors with a higher voltage rating are used.

Related calculator: Power Factor Correction Capacitor Calculator

Displacement vs true power factor

cos φ, PF

Displacement power factor is cos φ, the phase shift between the fundamental voltage and current. True power factor is P / S including harmonics, so it is lower than cos φ when electronic loads such as drives and rectifiers distort the current.

Example: With a sinusoidal voltage, true PF = cos φ / √(1 + THDi²). A drive with cos φ = 0.95 and 40 % current THD has true PF ≈ 0.95 / √1.16 = 0.88.

Related calculator: Power Factor Correction Capacitor Calculator

F

Full-load current

FLC, I_N

The current a motor or other equipment draws at its rated output and rated voltage. For a motor it is found from shaft output, efficiency and power factor, or read from the nameplate.

Example: A 15 kW, 400 V three-phase motor at 92 % efficiency and PF 0.85: I = 15 000 / (√3 × 400 × 0.85 × 0.92) = 27.7 A.

Related calculator: Motor kW / HP / Amps Converter

I

Illuminance (lux) and luminous flux (lumen)

E (lx), Φ (lm)

Luminous flux, in lumens, is the total visible light emitted by a source. Illuminance, in lux, is the flux falling on a surface per square metre (1 lx = 1 lm/m²). Recommended task illuminances are given in standards such as EN 12464-1.

Example: 5000 lm spread evenly over 10 m² gives 5000 / 10 = 500 lx, the value EN 12464-1 lists for writing and reading in offices.

Installation method

How a cable is installed, which determines how easily it loses heat. IEC 60364-5-52 defines reference methods (A1, A2, B1, B2, C, D1, D2, E, F, G), from conduit in a thermally insulated wall to cable on open trays.

Example: In the IEC 60364-5-52 tables, a 2.5 mm² copper PVC cable with two loaded conductors is rated 19.5 A by method A1 but 27 A clipped direct (method C).

Inverter efficiency

η_inv

The ratio of AC output power to DC input power. The battery must supply the load divided by this efficiency, and efficiency usually falls at very light load, so the manufacturer's curve matters for small loads on large inverters.

Example: A 1000 W AC load at 92 % efficiency takes 1000 / 0.92 = 1087 W from the battery, i.e. 1087 / 24 = 45.3 A from a 24 V bank.

Related calculator: UPS & Inverter Sizing and Runtime Calculator

K

Kilowatt-hour and tariff

kWh

A kilowatt-hour is the energy of 1 kW used for one hour, the unit on electricity bills. The tariff is the price per kWh, which may be flat, tiered by monthly consumption or time-of-use, often with fixed or demand charges on top.

Example: A 1.5 kW air conditioner running 8 h a day for 30 days uses 1.5 × 8 × 30 = 360 kWh; at 0.12 per kWh that costs 43.20 in the local currency.

Related calculator: Electricity Cost & Appliance Energy Calculator

L

Line and phase voltage

V_LL = √3 × V_LN

In a three-phase star (wye) system, line-to-line voltage is measured between two phases and line-to-neutral (phase) voltage between a phase and neutral; the first is √3 times the second.

Example: A 400 V line-to-line supply gives 400 / √3 = 231 V line-to-neutral, the 230/400 V system of IEC 60038.

Related calculator: Three-Phase & Single-Phase Current Calculator

M

Maintenance factor

The ratio of illuminance at the end of the maintenance period to the initial illuminance, allowing for lumen depreciation, lamp failures and dirt on luminaires and room surfaces. CIE 97 builds it as the product LLMF × LSF × LMF × RSMF.

Example: With MF = 0.8, keeping a maintained 500 lx means designing for 500 / 0.8 = 625 lx when new.

Maximum power point tracking

MPPT

A charge controller or inverter function that keeps the PV array at the voltage where it delivers maximum power and converts that power to the battery or grid voltage. Output current is therefore set by array power divided by battery voltage, not by the array current.

Example: A 1200 W array charging a 24 V battery through an MPPT controller gives about 1200 / 24 = 50 A before losses, even if the array current at 100 V is only about 12 A.

Related calculator: Off-Grid Solar PV System Sizing Calculator

MCB trip curves B, C and D

IEC 60898-1 sets the instantaneous (magnetic) trip band of an MCB: type B between 3 and 5 × In, type C between 5 and 10 × In, type D between 10 and 20 × In. B suits resistive loads and long cables, C general and motor circuits, D high-inrush loads such as transformers.

Example: A C32 breaker trips instantaneously somewhere between 5 × 32 = 160 A and 10 × 32 = 320 A.

P

Peak sun hours

PSH

Daily solar irradiation on the panel plane expressed as the equivalent number of hours at 1000 W/m². It is numerically equal to the irradiation in kWh/m² per day, taken from a source such as PVGIS or NASA POWER for the site and the design month.

Example: 5.5 kWh/m²/day is 5.5 PSH, so a 2 kWp array yields about 2 × 5.5 = 11 kWh DC per day before losses.

Related calculator: Off-Grid Solar PV System Sizing Calculator

Power factor

PF = P / S

The ratio of real power to apparent power, between 0 and 1. A low power factor means more current for the same useful power, so larger cables, higher losses and often tariff penalties.

Example: A 100 kW load at PF 0.8 draws 100 / 0.8 = 125 kVA; at 400 V three-phase that is 180.4 A instead of 144.3 A at PF 1.

Related calculator: Three-Phase & Single-Phase Current Calculator

Power factor correction capacitor bank

Qc (kVAr)

Capacitors that supply reactive power locally so it no longer flows through the network, raising the power factor seen upstream. The required size is Qc = P × (tan φ1 − tan φ2); banks are fixed or switched automatically in steps.

Example: Raising 100 kW from PF 0.70 to 0.95: Qc = 100 × (1.020 − 0.329) ≈ 69 kVAr.

Related calculator: Power Factor Correction Capacitor Calculator

Prime and standby ratings (ISO 8528-1)

PRP, ESP, COP

ISO 8528-1 defines continuous (COP), prime (PRP), limited-time running and emergency standby (ESP) ratings. Prime power allows unlimited hours at variable load; emergency standby is for supply during a mains outage for a limited number of hours per year (up to 200 h in the standard), with average-load limits set by the standard and the manufacturer.

Example: If a datasheet lists 500 kVA standby and 450 kVA prime, a remote site using the set as its main source must be sized against 450 kVA.

Related calculator: Generator Sizing Calculator

Prospective short-circuit current

Ik, PSCC

The current that would flow at a point if the conductors were short-circuited there with negligible fault impedance. It is limited by the impedance of the source, transformer and cables upstream, and it sets the breaking capacity required of protective devices.

Example: A 1000 kVA, 400 V transformer with uk = 6 % has I_FL = 1 000 000 / (√3 × 400) = 1443 A, so Ik ≈ 1443 / 0.06 = 24.1 kA at its terminals, taking the upstream network as infinite.

Related calculator: Transformer Short-Circuit Current Calculator

R

Reactive power

Q (kVAr)

Power that oscillates between the source and the magnetic or electric fields of the load each cycle. It does no net work but still needs current. Motors and transformers draw inductive reactive power; capacitors supply it.

Example: A load of 100 kVA and 80 kW has Q = √(100² − 80²) = 60 kVAr.

Related calculator: kVA to kW Converter (and kVAr)

Real (active) power

P (kW)

The part of the power that is actually converted into heat, light or mechanical work. Energy meters integrate it over time as kWh, and P = S × cos φ.

Example: 100 kVA at PF 0.8 delivers P = 100 × 0.8 = 80 kW; running for 5 h that is 400 kWh.

Related calculator: kVA to kW Converter (and kVAr)

S

Soft starter

A thyristor controller that ramps the motor voltage up during starting. Starting current falls in proportion to voltage and starting torque with its square, so a current limit trades inrush against available torque.

Example: A motor whose DOL current is 6 × FLC, limited to 3 × FLC, starts at about 50 % voltage and therefore about 0.5² = 25 % of its DOL starting torque.

Related calculator: Generator Sizing Calculator

Specific fuel consumption

SFC (L/kWh, g/kWh)

Fuel used per unit of electrical energy produced. It is lowest near the set's rated load and rises per kWh at light load, so the manufacturer's consumption figures at 25, 50, 75 and 100 % load are the reliable source.

Example: At an illustrative 0.25 L/kWh, a set delivering 80 kW burns 80 × 0.25 = 20 L/h, or 480 L in 24 h.

Related calculator: Diesel Generator Fuel Consumption Calculator

Star-delta starting

Y-Δ

The motor starts with its windings connected in star, then switches to delta near full speed. In star, line current and torque are both about one third of their DOL values. The motor must have six terminals and be rated in delta for the supply voltage.

Example: A motor with a DOL starting current of 6 × 27.7 = 166 A draws about 166 / 3 ≈ 55 A during the star stage, plus a short transient at the changeover.

Related calculator: Generator Sizing Calculator

Starting kVA and locked-rotor current

I_LR, I_A/I_N

Locked-rotor current is what an induction motor draws at standstill when switched direct on line (DOL), typically five to eight times full-load current at a low power factor. Starting kVA is the corresponding apparent power, and it is usually what sizes a generator feeding motors.

Example: A 27.7 A motor with I_A/I_N = 6 draws about 166 A at start, i.e. √3 × 400 × 166 ≈ 115 kVA.

Related calculator: Generator Sizing Calculator

T

Transformer impedance voltage (per-unit impedance)

uk %, Z %

The percentage of rated voltage that must be applied to one winding to circulate rated current with the other winding short-circuited. It equals the per-unit impedance, so the maximum fault current is about rated current divided by uk; typical and minimum values depend on size and on the standard (IEC 60076-5).

Example: uk = 4 % limits the fault current to 1 / 0.04 = 25 × rated current; for a 630 kVA, 400 V unit that is 25 × 909 A ≈ 22.7 kA.

Related calculator: Transformer Short-Circuit Current Calculator

Transformer no-load loss, load loss and efficiency

P0, Pk, η

No-load (core) loss P0 is drawn whenever the transformer is energised, whatever the load. Load (copper) loss Pk is quoted at rated current and scales with the square of the load fraction. Efficiency is output ÷ (output + P0 + x² × Pk), using the manufacturer's loss figures.

Example: A 1000 kVA unit with P0 = 1.1 kW and Pk = 10 kW at 50 % load and PF 0.9: losses = 1.1 + 0.5² × 10 = 3.6 kW, output 450 kW, η = 450 / 453.6 = 99.2 %. The no-load loss alone is 1.1 × 8760 = 9636 kWh a year.

Related calculator: Transformer Loading & Efficiency Calculator

U

Utilisation factor and room index

UF, K

The utilisation factor is the fraction of lamp lumens that reaches the working plane. It is read from the luminaire manufacturer's tables against the room index K = L × W / (Hm × (L + W)), where Hm is the mounting height above the working plane, and the surface reflectances.

Example: A 10 m × 8 m room with Hm = 2.2 m has K = 80 / (2.2 × 18) = 2.02. With UF 0.6, MF 0.8 and 4000 lm luminaires, 500 lx needs N = 500 × 80 / (4000 × 0.6 × 0.8) = 20.8, so 21 luminaires.

V

VA and W rating of a UPS

VA, W

A UPS or inverter has two output limits: apparent power in VA and real power in W, related by its rated output power factor. The load must stay within both.

Example: A 1500 VA / 1350 W UPS (output PF 0.9) feeding 1000 W at PF 0.7: 1000 / 0.7 = 1429 VA, within both limits but close to the VA limit.

Related calculator: UPS & Inverter Sizing and Runtime Calculator

Variable-frequency drive

VFD

A converter that rectifies the supply and synthesises a variable-frequency, variable-voltage output, so the motor can start with roughly rated current and full torque. It draws harmonic current that the supply, especially a generator, must tolerate.

Example: Using the planning value of 1–1.5 × running kVA, a motor running at 19.2 kVA adds a starting step of about 19–29 kVA, against roughly 115 kVA on DOL.

Related calculator: Generator Sizing Calculator

Voltage drop

ΔV

The voltage lost along a cable because of its resistance and reactance, usually expressed as a percentage of the nominal voltage. Recommended or required limits vary by code, commonly 3 % or 5 %.

Example: 32 A three-phase at PF 0.85 over 50 m of 16 mm² copper at 70 °C: ΔV = √3 × 32 × 50 × (0.001289 × 0.85 + 0.00008 × 0.527) = 3.15 V, or 0.79 % of 400 V.

Related calculator: Cable Voltage Drop Calculator