Electrical Engineering Glossary

The adiabatic equation, k-squared S-squared equals I-squared t, determines whether a conductor can withstand the thermal energy produced by a short-circuit current before the protective device clears the fault. IEC 60364-5-54 Clause 543.1 applies this equation to protective conductors, where k is a material constant, S is cross-sectional area, I is fault current, and t is disconnection time.

IEC 60364-5-54 Clause 543.1 | BS 7671:2018 Regulation 543.1.3

Ambient temperature derating adjusts a cable's current-carrying capacity when the surrounding air or soil temperature differs from the standard reference value. IEC 60364-5-52 Table B.52.14 provides correction factors for air temperatures from 10 to 80 degrees Celsius. Higher ambient temperatures reduce the cable's ability to dissipate heat, requiring a lower operating current to prevent insulation degradation.

IEC 60364-5-52 Table B.52.14 | BS 7671:2018 Table 4B1

An arc flash is a dangerous release of energy caused by an electric arc between conductors or from a conductor to ground. IEEE 1584-2018 Clause 4 defines the incident energy calculation model used to determine arc flash hazard levels, working distances, and required personal protective equipment categories for personnel safety in electrical installations.

IEEE 1584-2018 Clause 4 | NFPA 70E Article 130.5

The arc flash boundary is the distance from an arc source at which the incident energy falls to 1.2 calories per square centimetre — the threshold for the onset of a second-degree burn on unprotected skin. IEEE 1584-2018 Clause 4.7 provides the calculation methodology. Workers closer than this boundary must wear arc-rated personal protective equipment appropriate to the calculated incident energy level.

IEEE 1584-2018 Clause 4.7 | NFPA 70E Article 130.5

Breaking capacity is the maximum fault current that a protective device can safely interrupt without sustaining damage or posing a hazard. IEC 60947-2 Clause 2 defines ultimate breaking capacity (Icu) and service breaking capacity (Ics) for circuit breakers. The selected device must have a breaking capacity equal to or greater than the prospective fault current at its installation point.

IEC 60947-2 Clause 2 | BS 7671:2018 Regulation 434.5.1

Buried cable derating accounts for reduced heat dissipation when cables are installed directly in the ground or in underground ducts. IEC 60364-5-52 Table B.52.15 provides correction factors based on soil thermal resistivity and ground temperature. Dry or rocky soil with high thermal resistivity requires greater derating than moist soil, as heat cannot transfer efficiently from the cable surface.

IEC 60364-5-52 Table B.52.15 | BS 7671:2018 Table 4B3

Busbar current rating is the maximum continuous current a busbar system can carry without exceeding specified temperature rise limits above ambient. IEC 61439-1 Clause 10.10 defines temperature rise verification methods for switchgear assemblies. Busbar rating depends on material, cross-section, surface finish, enclosure ventilation, and proximity to other current-carrying conductors.

IEC 61439-1 Clause 10.10 | BS EN 61439-2 Clause 10.10

Cable ampacity is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating under specified installation conditions. NEC Article 310.16 tabulates ampacity values for common conductor sizes, insulation types, and temperature ratings. Ampacity must be derated for ambient temperature, conductor grouping, and conduit fill per NEC Section 310.15.

NEC/NFPA 70:2023 Article 310.16 | IEC 60364-5-52 Tables B.52.2–B.52.13

The cable grouping factor is a derating multiplier applied when multiple loaded cables are installed in close proximity, reducing each cable's current-carrying capacity due to mutual heating. BS 7671 Table C.3 provides grouping factors for various arrangements including bunched cables, cables on trays, and cables in enclosed conduits. More cables in a group require greater derating.

BS 7671:2018 Table C.3 | IEC 60364-5-52 Table B.52.17

A cable installation method classifies how and where cables are physically installed, directly determining their current-carrying capacity. IEC 60364-5-52 Table B.52.1 defines reference methods including clipped direct to a surface, enclosed in conduit, on cable tray, and direct burial. Each method has corresponding ampacity tables reflecting different heat dissipation characteristics.

IEC 60364-5-52 Table B.52.1 | BS 7671:2018 Table 4A2

Cable insulation class defines the maximum continuous operating temperature and voltage rating of a cable's insulating material. IEC 60502-1 Table 1 classifies insulation types including PVC at 70 degrees Celsius and XLPE at 90 degrees Celsius. The insulation class directly determines current-carrying capacity, short-circuit withstand, and suitability for specific installation environments.

IEC 60502-1 Table 1 | BS 7671:2018 Table 52.1

The mV/A/m rating expresses a cable's voltage drop characteristic as millivolts per ampere per metre of cable route length. BS 7671 Table C.7 tabulates these values for copper and aluminium conductors with PVC and XLPE insulation. This rating simplifies voltage drop calculation to a single multiplication: Vd equals mV/A/m multiplied by design current multiplied by cable length divided by 1000.

BS 7671:2018 Table C.7 | IEC 60364-5-52 Clause 525.1

Cable sizing is the engineering process of selecting the minimum conductor cross-sectional area that safely carries the design current while satisfying voltage drop limits, short-circuit withstand requirements, and installation derating factors. The procedure is defined in IEC 60364-5-52 Clause 523 and produces a conductor size in square millimetres.

IEC 60364-5-52 Clause 523 | BS 7671:2018 Appendix 4

Cable tray fill is the proportion of usable cross-sectional area inside a cable tray occupied by installed cables. NEC Article 392 limits fill ratios based on cable type and arrangement — single-layer or stacked — to ensure adequate ventilation, maintain current-carrying capacity, and provide space for future cable additions without exceeding thermal limits of existing conductors.

NEC/NFPA 70:2023 Article 392 | IEC 61537 Clause 8

Conduit fill is the percentage of a conduit's internal cross-sectional area occupied by the cables routed through it. NEC Article 344.22 and Chapter 9 Table 1 limit fill to 40 percent for three or more conductors to allow adequate heat dissipation, prevent mechanical damage during cable pulling, and ensure future maintenance access.

NEC/NFPA 70:2023 Chapter 9, Table 1 | AS/NZS 3000:2018 Clause 3.9.4

A correction factor is a multiplier applied to a cable's base current-carrying capacity to adjust for actual installation conditions that differ from standard reference values. IEC 60364-5-52 Clause 523 defines the application of correction factors for ambient temperature, cable grouping, soil thermal resistivity, and thermal insulation that collectively determine the effective cable ampacity.

IEC 60364-5-52 Clause 523 | NEC/NFPA 70:2023 Section 310.15

Cos phi is the cosine of the phase angle between fundamental voltage and current waveforms, representing the displacement power factor of an AC circuit. IEC 60831-1 references cos phi in capacitor bank ratings for power factor correction. Improving cos phi from 0.7 to 0.95 reduces current draw by approximately 26 percent and lowers network losses.

IEC 60831-1 Clause 4 | IEEE 1459-2010 Clause 3.2

A derating factor is a multiplier less than one applied to a cable's tabulated current-carrying capacity to account for adverse installation conditions. BS 7671 Appendix 4 provides correction factors for ambient temperature, grouping with other cables, thermal insulation contact, and soil thermal resistivity that reduce the permissible current to prevent overheating.

BS 7671:2018 Appendix 4 | IEC 60364-5-52 Clause 523.1

Discrimination, also called selectivity, is the coordination between series-connected protective devices so that only the device nearest the fault operates, leaving upstream circuits unaffected. IEC 60947-2 Annex A defines methods for verifying full and partial discrimination using time-current characteristic curves and manufacturers' selectivity tables for circuit breaker combinations.

IEC 60947-2 Annex A | BS 7671:2018 Regulation 536.4

A diversity factor is the ratio of the sum of individual maximum demands to the actual maximum demand of the combined load group. BS 7671 Table 1A provides recommended diversity allowances for domestic installations by circuit type. Applying diversity prevents oversizing supply cables and switchgear by recognising that not all loads operate simultaneously.

BS 7671:2018 Table 1A | AS/NZS 3000:2018 Appendix C

Earth fault loop impedance is the total impedance of the fault current path from the source through the phase conductor, fault, and protective conductor back to the source. BS 7671 Regulation 411.4 requires that this impedance be low enough to ensure the protective device disconnects the supply within the specified time to prevent electric shock.

BS 7671:2018 Regulation 411.4 | IEC 60364-4-41 Clause 411.4

Earthing resistance is the opposition to current flow between an earth electrode and the general mass of earth, measured in ohms. BS 7430:2011 Clause 7 specifies design methods, measurement techniques, and acceptable resistance values for different installation types. Low earthing resistance ensures protective devices operate within required disconnection times during earth faults.

BS 7430:2011 Clause 7 | IEC 60364-5-54 Clause 542

Electrode configuration describes the physical arrangement of conductors where an arc flash may occur, significantly affecting the arc's behaviour, direction, and energy distribution. IEEE 1584-2018 Clause 4.3 defines five configurations — VCB, VCBB, HCB, VOA, and HOA — each with different correction factors that modify the calculated incident energy and arc flash boundary distance.

IEEE 1584-2018 Clause 4.3 | IEEE 1584-2018 Table 1

EV charging load is the electrical demand imposed by an electric vehicle supply equipment unit on the installation. IEC 61851-1 defines charging modes from Mode 1 through Mode 4, with power levels ranging from 3.7 kilowatts single-phase to over 350 kilowatts DC fast charging. Circuit design must account for continuous duty and diversity among multiple chargers.

IEC 61851-1 Clause 6 | BS 7671:2018 Section 722

Fault loop impedance Zs is the measured or calculated total impedance of the earth fault current path from the supply transformer through the phase conductor, fault, and protective conductor back to the source. BS 7671 Regulation 411.4.5 specifies maximum Zs values for each protective device to ensure fault disconnection within 0.4 or 5 seconds depending on circuit type.

BS 7671:2018 Regulation 411.4.5 | IEC 60364-4-41 Clause 411.4

Incident energy is the thermal energy per unit area arriving at a specific working distance from an electric arc, measured in calories per square centimetre. IEEE 1584-2018 Clause 4.4 provides the empirical calculation model. Incident energy determines the arc rating of required personal protective equipment — higher energy requires higher-rated PPE to prevent burn injuries.

IEEE 1584-2018 Clause 4.4 | NFPA 70E Table 130.7(C)(15)(a)

An IP rating classifies the degree of protection provided by an electrical enclosure against intrusion of solid objects and moisture. IEC 60529:2013 defines the two-digit code where the first digit indicates protection against solids from zero to six, and the second digit indicates water protection from zero to nine. IP ratings influence cable entry design and equipment selection.

IEC 60529:2013 Clause 4 | BS EN 60529:1992+A2 Table 2

A K-factor transformer is designed to handle the additional heating caused by harmonic currents from non-linear loads without derating. IEEE C57.110-2018 provides methods for determining transformer capability under nonsinusoidal load currents. The K-factor quantifies the harmonic heating effect — higher K-factor ratings indicate greater tolerance to harmonic-rich load profiles common in modern commercial buildings.

IEEE C57.110-2018 Clause 4 | UL 1561 Section 25

Let-through energy, expressed as I-squared-t in ampere-squared-seconds, is the thermal energy a protective device allows to pass through to the downstream circuit during fault clearance. IEC 60947-2 specifies I-squared-t values for circuit breakers at various fault levels. This energy must not exceed the withstand capability of cables and equipment, verified using the adiabatic equation.

IEC 60947-2 Clause 4.3 | BS 88-2.2 Clause 8

Maximum demand is the greatest electrical load expected to occur simultaneously on a supply system, measured in amperes or kilovolt-amperes. BS 7671 Appendix 1 provides diversity allowances and assessment methods for domestic and commercial installations. Accurate maximum demand estimation prevents oversizing of supply cables, switchgear, and upstream transformer capacity.

BS 7671:2018 Appendix 1 | AS/NZS 3000:2018 Appendix C

Mineral insulated cable uses compressed magnesium oxide powder as insulation between copper conductors and a seamless copper sheath, providing inherent fire resistance and exceptional temperature tolerance. BS EN 60702-1 specifies performance requirements. MICC maintains circuit integrity during fires, making it essential for emergency lighting, fire alarm, and smoke extraction circuits in life-safety applications.

BS EN 60702-1 Clause 5 | BS 7671:2018 Table 4J1A

Motor locked rotor current is the steady-state current drawn by an electric motor when the rotor is mechanically prevented from turning while rated voltage is applied to the stator. IEC 60034-12 Table 2 specifies locked-rotor current ratios by motor design classification. This value represents the maximum sustained current the motor can draw and determines protection device selection and cable withstand requirements.

IEC 60034-12 Table 2 | NEC/NFPA 70:2023 Table 430.7(B)

Motor starting current is the transient inrush current drawn by an electric motor during acceleration from standstill to rated speed. IEC 60034-12 classifies motor starting characteristics and defines locked-rotor current ratios. Starting current typically ranges from five to eight times full-load current and determines the sizing of upstream cables, protection devices, and supply transformers.

IEC 60034-12 Table 2 | NEC/NFPA 70:2023 Article 430

In a balanced three-phase system the neutral current is theoretically zero, but triplen harmonics from non-linear loads cause substantial neutral currents that may exceed phase currents. IEC 60364-5-52 Clause 524 addresses neutral conductor sizing for circuits supplying harmonic-generating equipment, requiring neutral conductors sized to carry the expected harmonic currents safely.

IEC 60364-5-52 Clause 524 | BS 7671:2018 Regulation 523.6

Power factor is the ratio of real power in watts to apparent power in volt-amperes, indicating how effectively electrical energy is converted into useful work. IEEE 1459-2010 defines measurement methods for sinusoidal and non-sinusoidal conditions. A low power factor increases current draw, causes higher losses, and may incur utility penalty charges on commercial supplies.

IEEE 1459-2010 Clause 3 | IEC 60831-1 Clause 4

Prospective fault current is the maximum current that would flow at a given point in an electrical installation if a short circuit or earth fault of negligible impedance occurred. BS 7671 Regulation 434.1 requires this value to be determined so that protective devices and cables are rated to safely interrupt and withstand the fault energy without damage.

BS 7671:2018 Regulation 434.1 | IEC 60364-4-43 Clause 434

Protective device coordination is the systematic study ensuring series-connected overcurrent devices operate in the correct sequence during faults. IEC 60947-2 Annex A provides verification methods using time-current curves and selectivity tables. Proper coordination ensures only the device nearest the fault trips, maintaining supply to unaffected circuits and minimising disruption.

IEC 60947-2 Annex A | BS 7671:2018 Regulation 536.4

RCD operating time is the interval between the occurrence of a residual current exceeding the rated threshold and the automatic disconnection of the supply by the residual current device. IEC 61008-1 Clause 8.6.2.2 specifies maximum operating times: 300 milliseconds at rated residual current and 40 milliseconds at five times rated residual current for general-purpose RCDs.

IEC 61008-1 Clause 8.6.2.2 | BS 7671:2018 Regulation 531.2

Short-circuit current is the abnormally high current that flows when a low-impedance fault path forms between live conductors or between a live conductor and earth. IEC 60909-0 Clause 1 defines standardised methods for calculating initial symmetrical short-circuit current, peak current, and breaking current used for equipment rating and protection coordination.

IEC 60909-0 Clause 1 | BS 7671:2018 Regulation 434.5

Solar PV cable sizing determines the minimum conductor area for photovoltaic string and array cables that carry DC current from panels to the inverter. IEC 62548 specifies requirements for voltage drop, current-carrying capacity under elevated roof temperatures, and short-circuit protection. PV cables must withstand continuous operation at maximum power point current multiplied by a 1.25 safety factor.

IEC 62548 Clause 7 | BS 7671:2018 Section 712

Step voltage is the potential difference between two points on the ground surface separated by a distance of one metre in the direction of maximum voltage gradient during an earth fault. IEEE 80 Clause 8 provides calculation methods for step voltage in substation grounding design. Excessive step voltage can cause dangerous current flow through a person's legs and body.

IEEE 80 Clause 8 | IEC 60364-4-41 Clause 411.7

Symmetrical components is a mathematical technique that decomposes unbalanced three-phase voltages and currents into three balanced sets: positive sequence, negative sequence, and zero sequence. IEC 60909-0 Clause 7 applies this method for calculating unbalanced fault currents. The technique simplifies analysis of asymmetrical faults by allowing each sequence network to be solved independently.

IEC 60909-0 Clause 7 | IEEE C37.010 Clause 5

Thermal resistance in cable engineering is the measure of opposition to heat flow from a conductor through its insulation and surrounding medium to the ambient environment, expressed in kelvin-metres per watt. IEC 60287-1-1 provides calculation methods for steady-state current ratings based on thermal resistances of insulation layers, coverings, and the external installation medium.

IEC 60287-1-1 Clause 2 | IEC 60287-3-1 Clause 1

Total harmonic distortion measures the degree to which a voltage or current waveform deviates from a pure sinusoid, expressed as a percentage of the fundamental component. IEEE 519-2022 establishes harmonic limits at the point of common coupling to prevent power quality degradation. High THD causes overheating in transformers, nuisance tripping of protective devices, and interference with sensitive electronic equipment.

IEEE 519-2022 Table 2 | IEC 61000-3-2 Clause 6

Touch voltage is the potential difference between simultaneously accessible conductive parts that a person could contact during an earth fault. IEC 61140 establishes permissible touch voltage limits — typically 50 volts AC in normal conditions and 25 volts AC in wet locations. Protective earthing and bonding systems must ensure touch voltages remain below these thresholds throughout fault duration.

IEC 61140 Clause 7 | BS 7671:2018 Regulation 411.3.1.1

Transformer impedance, expressed as a percentage, represents the fraction of rated voltage required to circulate full-load current through the short-circuited secondary winding. IEC 60076-1 Clause 10 defines measurement procedures and tolerances. Impedance directly determines the maximum prospective short-circuit current at the transformer secondary terminals and affects voltage regulation under load.

IEC 60076-1 Clause 10 | BS 7671:2018 Regulation 434.5.1

A transformer tap changer adjusts the transformer turns ratio by connecting to different tapping points on the winding, allowing regulation of the output voltage. IEC 60076-1 Clause 5 specifies tapping range requirements and rated quantities. On-load tap changers can adjust voltage during operation while off-circuit tap changers require the transformer to be de-energised for adjustment.

IEC 60076-1 Clause 5 | IEC 60214-1 Clause 4

A Type B RCD detects and disconnects for all types of residual current including AC sinusoidal, pulsating DC, and smooth DC fault currents. IEC 62423 defines the requirements for Type B devices, which are essential for circuits supplying equipment that can generate DC fault currents, such as variable frequency drives, EV chargers with DC charging capability, and photovoltaic inverters.

IEC 62423 Clause 1 | BS 7671:2018 Section 722

Voltage drop is the reduction in electrical potential along a conductor caused by its impedance when current flows through it. IEC 60364-5-52 Clause 525 limits permissible voltage drop to ensure equipment operates within rated tolerances. It is calculated using the formula Vd equals mV per amp per metre multiplied by design current and cable length.

IEC 60364-5-52 Clause 525 | BS 7671:2018 Regulation 525.1

Voltage regulation is the percentage change in voltage from no-load to full-load conditions, indicating how well a supply system maintains steady voltage as load varies. IEC 60038:2009 defines standard nominal voltages and permissible tolerances, typically plus or minus 10 percent. Good voltage regulation ensures all connected equipment operates within its rated voltage range for reliable performance.

IEC 60038:2009 Clause 4 | BS 7671:2018 Regulation 525.1