Glossary

A: Acronyms A-Z. Click a letter to jump.

A

AC

Alternating Current. Power that reverses direction at the line frequency (60 Hz in US). §01

AC (cable)

Armored Cable (BX). Older commercial wiring with conductors in flexible armor (no separate ground). NEC 320. §07

AFB

Arc Flash Boundary. Distance at which incident energy drops to 1.2 cal/cm² (2nd-degree burn threshold). §18

AFCI

Arc-Fault Circuit Interrupter. Detects arcing faults on branch circuits. NEC 210.12. Required in dwelling bedrooms (and more). §04

AFE

Active Front End. VFD with active rectifier — produces < 5% THDi vs 30%+ for standard 6-pulse. §14, §15

AGM

Absorbent Glass Mat. A type of VRLA battery construction. §20

AHJ

Authority Having Jurisdiction. The person/office responsible for enforcing electrical code (typically city/county building dept). §30

AIC

Amperes Interrupting Capacity. The maximum fault current a device (breaker/fuse) can safely interrupt. §12

ANSI

American National Standards Institute. Source of voltage standards (C84.1) and protective device numbering (C37.2). §01, §17, §32

Apparent power (S)

Total power flowing in a circuit, including reactive component. Measured in VA / kVA. Conductors carry S, not just P. §01

ASHRAE

American Society of Heating, Refrigerating + Air-Conditioning Engineers. ASHRAE 90.1 = commercial energy code. ASHRAE 90.4 = data center energy. §31

ATS

Automatic Transfer Switch. Device that transfers load between utility and standby power source. §19

AWG

American Wire Gauge. Conductor size. Smaller number = larger conductor. §04, §07

B

BIL

Basic Impulse Level. Equipment's withstand voltage rating for lightning impulses. §09, §23

BMS

Building Management System (HVAC + lighting), or Battery Management System (Li-ion ESS). Context-dependent. §20, §25

Bonding jumper

Conductor that ensures electrical continuity between metallic parts. NEC 250. §13

Branch circuit

Conductors between final OCPD and outlet/equipment. NEC 210, 430. §04

BTU/hr

British Thermal Unit per hour. Heat unit. 1 kW = 3,412 BTU/hr. §01

Bus

Conductor that distributes power to multiple connections. Switchgear bus, transformer bus, etc. §05

Busway

Factory-built bus bars in metal enclosure. NEC 368. §08

C

Cable tray

Open support system for cables. NEC 392. §08

cal/cm²

Calories per square centimeter. Unit of incident energy in arc flash. §18

CB

Circuit Breaker. Reusable overcurrent device. §11

Class I/II/III

Hazardous location classification: gas (I), dust (II), fibers (III). NEC 500. §21

CMR

Communications Riser cable. NEC 800. §32

Code letter

Motor nameplate letter (A-V) indicating locked-rotor kVA per HP. §14

COPS

Critical Operations Power Systems. NEC 708. Highest-tier emergency. §19

CRAH

Computer Room Air Handler. Cools data center IT halls. §14

CSI

Construction Specifications Institute. MasterFormat division system. Division 26 = Electrical. §28

CT

Current Transformer. Steps down current for metering or protection. §17, §22

CWP

Condenser Water Pump. Pumps water to/from cooling tower. §14

D

DC

Direct Current. §20

DCFC

DC Fast Charging. Level 3 EV charging (50-350+ kW). §26

Demand factor

Ratio of max demand to total connected load. Always ≤ 1.0. NEC 220. §03

DCFC

DC Fast Charging — see also Level 3. §26

Diversity factor

Ratio of sum of individual peaks to system peak. Always ≥ 1.0. §03

Δ (delta)

Three-phase winding configuration where windings form a triangle. No neutral. §09

DOL

Direct-On-Line. Motor starting method — connects motor directly to line. 6-8× FLA inrush. §14

DR

Demand Response. Utility program paying for load reduction during peaks. §27

DRC

Daylight Responsive Controls. Required by ASHRAE 90.1 within 15 ft of windows. §31

Drawout breaker

CB removable from cubicle for service. Common in switchgear. §05, §22

E

EGC

Equipment Grounding Conductor. Bonds equipment metal to ground. NEC 250.122. §13

ELC

Electrical Loss Component (ASHRAE 90.4 metric). §31

EMT

Electrical Metallic Tubing. Thin-wall conduit. NEC 358. §04, §07

EPR

Ethylene Propylene Rubber. Premium MV cable insulation. §07

ESS

Energy Storage System. Battery (or other) backup. NEC 706, NFPA 855. §25

EV

Electric Vehicle. §26

EVEMS

EV Energy Management System. Allows demand reduction via load sharing. NEC 625.42. §26

EVSE

EV Supply Equipment. The "charger" (actual battery charger is in the car). NEC 625. §26

F

FLA

Full Load Amperes. Motor current at rated mechanical output. §01, §04

FLC

Full Load Current. NEC table value (NEC 430.247-250). Use FLC, not FLA, for NEC sizing. §04, §14

FOIT/FOUT

Flash Of Invisible/Unstyled Text. Web typography artifact (off-topic but appeared in identity). N/A

Fuse

Sacrificial overcurrent device. Cleared by element melting. §11

G

GEC

Grounding Electrode Conductor. From service neutral to ground electrode. NEC 250.66. §13

GFCI

Ground-Fault Circuit Interrupter. Personnel protection (5 mA). NEC 210.8. §13

GFP / GFPE

Ground Fault Protection of Equipment. NEC 230.95 — required at 480V services ≥ 1000A. 1,200A pickup max. §13

GIS

Gas-Insulated Switchgear. SF6 gas-insulated, compact. §22

GFP

(See GFP / GFPE.)

Group LOTO

Multiple workers each apply own lock to a multi-lock hasp. §29

H

Harmonic

Frequency component above the fundamental (60 Hz). 5th harmonic = 300 Hz. Generated by nonlinear loads. §15

HP

Horsepower. Mechanical output rating. 1 HP = 0.746 kW. §01, §14

HRG

High-Resistance Grounded. Industrial grounding scheme limiting ground fault to ~ 5 A. §13

I

I (current)

Electrical current, measured in Amperes (A). Color-coded red in Power Atlas formulas. §01

ICCB

Insulated-Case Circuit Breaker. Higher-tier than MCCB. §12

IECC

International Energy Conservation Code. Building energy code. §31

IEEE

Institute of Electrical and Electronics Engineers. Issues color books, IEEE 1584, IEEE 519, etc. §32

IFC

Issued for Construction. Drawing version released for construction. §28

Incident energy

Arc flash energy at working distance, measured in cal/cm². 1.2 cal/cm² = 2nd-degree burn threshold. §18

IS

Intrinsically Safe. Hazardous location protection method — energy too low to ignite. §21

IT

Information Technology. Server load in data centers. §02

K

K-factor transformer

Transformer rated for harmonic loads. K-13 = data center standard. K-4 = light harmonics. §09, §15

kVA

Kilovolt-Ampere. Apparent power unit. 1000 VA. §01

kVAR

Kilovolt-Ampere Reactive. Reactive power unit. §01, §15

kW

Kilowatt. Real power unit. 1000 W. §01

L

L1, L2, L3

Line conductors of a 3-phase system (also Phase A, B, C). §01

L1, L2 charging

EV charging levels. L1 = 120V. L2 = 208/240V. §26

LFP

Lithium Iron Phosphate. Safer Li-ion chemistry. §20

LOTO

Lockout/Tagout. Energy isolation procedure. NFPA 70E + OSHA 1910.147. §29

LPL

Lightning Protection Level. IEC 62305 risk classification. §23

LPD

Lighting Power Density. W/sq ft limit per ASHRAE 90.1. §31

LRA

Locked-Rotor Amperes. Motor starting current (6-8× FLA). §14

LRkVA/HP

Locked-Rotor kVA per HP. Determined by motor Code Letter. §14

LSI / LSIG

Long-time / Short-time / Instantaneous (+ Ground). Adjustable trip settings on electronic CBs. §11

LV

Low Voltage (≤ 600V per NEC 110.2). §02

M

MBJ

Main Bonding Jumper. Joins service neutral to ground at service equipment. ONE per service. NEC 250.28. §13

MC

Metal-Clad cable. Conductors in metal armor. NEC 330. §07

MCA

Minimum Circuit Ampacity. Sets the wire size. For motors: 1.25 × FLC (NEC 430.22). §04

MCB

Main Circuit Breaker. Panel's main disconnect. §05

MCCB

Molded-Case Circuit Breaker. Standard panelboard breaker. §12

MCC

Motor Control Center. Modular cabinet of motor starters. §14

MCOV

Maximum Continuous Operating Voltage (SPDs). Sustained voltage SPD can withstand without operation. §24

MEL

Master Equipment List. Document of all loads requiring power. From other disciplines to electrical. §02

MLO

Main Lug Only. Panel without integral main breaker — fed protected from upstream. §05

MLC

Mechanical Load Component (ASHRAE 90.4 metric). §31

MOV

Metal Oxide Varistor. SPD technology. Wears out with each surge. §24

MOCP

Maximum Overcurrent Protection. Caps the breaker size. NEC 430.52 (motors). §04

MV

Medium Voltage (1 - 35 kV). NEC 110.2. §02

MVA

Megavolt-Ampere. 1,000 kVA. §01, §12

MVA method

Fault current calculation method using MVA additions. §12

N

NEC

National Electrical Code (NFPA 70). The electrical installation standard. Updated every 3 years. §32

NEMA

National Electrical Manufacturers Association. Sets equipment standards (enclosures, motor frames, starter sizes). §14

NFPA

National Fire Protection Association. Issues NEC, NFPA 70E, NFPA 110, NFPA 780, NFPA 855. §32

NM-B

Non-Metallic sheathed cable type B (Romex). Residential interior. NEC 334. §07

NMC

Nickel Manganese Cobalt. Higher-energy Li-ion chemistry. Higher thermal runaway risk than LFP. §20

O

OCPD

OverCurrent Protective Device (breaker or fuse). NEC 240. §11

ODP

Open Drip-Proof motor enclosure. §14

ONAN

Oil-immersed Natural Air Natural cooling (transformer). §09

OFCI

Owner-Furnished, Contractor-Installed. Common in DC industry. §28

OSHA

Occupational Safety + Health Administration. Federal workplace safety. 29 CFR 1910 + 1926. §32

P

P (real power)

Real (active) power, measured in Watts (W). Color-coded green in Power Atlas. §01

PCC

Point of Common Coupling. Boundary between user and utility. IEEE 519 limits apply here. §15

PDU

Power Distribution Unit. Data center: 480 → 415Y/240V step-down + integral panel. §05

PF

Power Factor. cos θ = P/S. 1.0 = pure resistive; 0.8 = lagging inductive. §01, §15

PFC

Power Factor Correction. Capacitor banks (or inverter-based) to bring PF closer to 1.0. §15

PMS

Power Management System. Centralized controller for complex emergency systems. §27

PMSM

Permanent Magnet Synchronous Motor. Modern HVAC + EV motor. §14

PPE

Personal Protective Equipment. NFPA 70E categorizes by arc flash incident energy. §18

PT / VT

Potential / Voltage Transformer. Steps down voltage for metering or protection. §17, §22

Per-unit

Quantities normalized to a base value. Used in fault and load flow analysis. §01, §12

PUE

Power Usage Effectiveness. Total facility power / IT power. 1.0 = perfect, 2.0 = inefficient. §31

PV

Photovoltaic. Solar electricity. NEC 690. §25

PWM

Pulse Width Modulation. VFD output technique. §14

R

R (resistance)

Electrical resistance, measured in Ohms (Ω). §01

Rapid shutdown

NEC 690.12 — PV array DC voltage must drop to safe level within 30 sec of activation. §25

RCM

Reverse Current Monitor. Some utility intertie protection. §25

RFI

Request for Information. Contractor question requiring formal engineer response. §28

RMC

Rigid Metal Conduit. Heaviest gauge raceway. NEC 344. §07

RMS

Root Mean Square. Equivalent DC heating value of an AC waveform. Standard AC measurement. §12

RPP

Remote Power Panel. Data center row-level branch panel, fed from PDU. §05

S

S (apparent power)

Apparent (total) power, measured in VA / kVA. Color-coded purple in Power Atlas. S² = P² + Q². §01

SAD

Silicon Avalanche Diode. SPD technology — faster + tighter than MOV. §24

SCC

Short-Circuit Current. Same as available fault current. §12

SCCR

Short-Circuit Current Rating. UL 508A rating for industrial control panels. §05

SCR

Silicon Controlled Rectifier (semiconductor) OR Short Circuit Ratio (IEEE 519). Context-dependent. §14, §15

SE / SER

Service Entrance cable. Residential. NEC 338. §07

SF (motor)

Service Factor. Multiplier on motor's continuous overload rating. 1.0 or 1.15 typical. §14

SFP

Sound-Field Particles (acronym sometimes used in arc flash for plasma). N/A in Power Atlas.

SLD

Single-Line Diagram. Schematic showing 3-phase system as single lines. The system map. §02

Soft Starter

Reduced-voltage motor starter using SCRs. Ramps voltage on start. §14

SPD

Surge Protective Device. NEC 285. §24

SWGR

Switchgear. Free-standing switching equipment, drawout breakers. §05

T

T-code

Hazardous location temperature code. T1 (450°C max) - T6 (85°C max). Equipment T-code < gas auto-ignition. §21

Tap conductor

Smaller conductor branched from larger feeder. NEC 240.21 special rules. §06

TC / TC-ER

Tray Cable / Tray Cable Exposed-Run rated. NEC 336. §07, §08

TCC

Time-Current Curve. Plots OCPD trip time vs current on log-log axes. Used for coordination studies. §11, §17

TDD

Total Demand Distortion. % distortion vs maximum demand current. IEEE 519 uses TDD. §15

TEFC

Totally Enclosed Fan-Cooled. Most common motor enclosure. §14

THD

Total Harmonic Distortion. % distortion vs fundamental. §15

THDi

Total Harmonic Distortion of current. §15

THHN

Thermoplastic, Heat-resistant (90°C dry only), Nylon-jacketed. NEC 310. §07

THWN-2

Thermoplastic, Heat- + Wet-resistant (90°C wet/dry), Nylon. The default modern conductor. §07

Tier (data center)

Uptime Institute classification: Tier I (basic) — Tier IV (fault tolerant). Atlas DC1 = Tier III equivalent. §02

TPG

Temporary Protective Ground. Used during MV/HV LOTO. §29

Triplens

3rd, 9th, 15th harmonics — add in neutral instead of cancel. §15

TX

Transformer (abbreviation in SLDs). §02, §09

U

UF / UF-B

Underground Feeder cable. Direct-burial residential. NEC 340. §07

UL

Underwriters Laboratories. Independent product testing. UL 489 (CBs), UL 508A (industrial control panels), UL 1741 (inverters), etc. §32

UPS

Uninterruptible Power Supply. Battery-backed power, no transfer time (online double-conversion type). §19

USE-2

Underground Service Entrance, 90°C wet/dry. NEC 338. §07

V

V (voltage)

Voltage, measured in Volts. Color-coded blue in Power Atlas. §01

VAR

Volt-Ampere Reactive. Unit of reactive power. §01, §15

V_LL

Line-to-Line voltage. Between two phase conductors. §01

V_LN

Line-to-Neutral voltage. Between phase and neutral. V_LL = √3 × V_LN. §01

VFD

Variable Frequency Drive. AC motor speed controller. §14

VPR

Voltage Protection Rating (SPDs). What voltage gets through during a surge. Lower is better. §24

VRLA

Valve-Regulated Lead-Acid. Sealed batteries common in UPS. §20

W

W

Watt. Real power unit. §01

Wireway

Sheet metal trough with hinged cover for conductor support. NEC 376. §08

Working space

NEC 110.26 — clearance required around electrical equipment. Most-cited rule. §30

X

X (reactance)

Inductive (X_L) or capacitive (X_C) reactance. Imaginary component of impedance. §01

X/R ratio

System reactance/resistance. Higher = more asymmetric fault current. §12

XFMR / TX

Transformer abbreviations. §09

XHHW-2

XLPE insulation, Heat- + Wet-rated 90°C wet/dry. Premium conductor. NEC 310. §07

XLPE

Cross-Linked Polyethylene insulation. Tougher than PVC. §07

XP

Explosion-Proof (Class I Div 1 motor enclosure). §21

Z

Z (impedance)

Total opposition to AC current. Z = R + jX. Color-coded iron/dark in Power Atlas. §01

%Z

Per-unit transformer impedance. Low %Z = high downstream fault current. §09

Zone (haz loc)

IEC system for hazardous locations. Zone 0 / 1 / 2 (gas), Zone 20 / 21 / 22 (dust). §21

ZSI

Zone-Selective Interlocking. Modern CB technique for selective coordination at full speed. §11, §18

Glossary v1.0 · ~ 200 entries · cross-referenced to sections