Plain-language glossary

The words nobody stops to define for you in your first week, explained the way I'd explain them on the job. These are the ideas — where a term is really governed by the code (colours, ampacity figures, fill), the entry says so and points you at the book rather than pretending the number lives here. Where a term is drawn somewhere on the site, the entry says "See it" and links the diagram.

Jump to a term

Some of these have a page of their own — the ones people actually search for, written out at length. Follow the "read more" link on the entry. And where a term is really about a physical thing, the fittings index and symbols index show you what it looks like.

First week

Twenty-one words you will hear before lunch on your first day. No numbers in any of them — just what the thing is and what job it does.

Hot (ungrounded conductor)

The conductor that carries the supply's voltage out to the load — the one at a different potential from the ground you stand on. It is the conductor a switch breaks, and the one a meter reads line voltage on. Which colour identifies it is a code rule. See it: Hot, neutral, ground · Single-pole switch

Neutral (grounded conductor)

The return half of the loop. It is held near ground potential at the supply, which is why it reads tame on a meter — but in a working circuit it carries the same current as the hot. "Grounded" at the supply is not the same as being "the ground". See it: Hot, neutral, ground

Ground (bonding conductor)

The conductor that ties every piece of metal a person might touch back to the source through a low-resistance path. It carries nothing in a healthy circuit; when a hot lands on the metal it carries the fault current home so the breaker opens. See bonding vs grounding for the two jobs it does. See it: Hot, neutral, ground — press the fault

Charge

Anything that uses the electricity: a lamp, a motor, a heater, a coil. On a schematic it is the component the hot side and the return side meet across, and it is the only thing in the loop that is meant to drop the full voltage. See it: How to read a schematic

Line vs load (terminals)

Line is the side fed from the supply; load is the side that feeds onward to whatever comes next. On a GFCI the distinction is the whole device: the supply goes to line, downstream devices hang off load, and getting the two backwards leaves the device's own face unprotected. Read more → See it: GFCI — and the swapped fault

Conducteur commuté

The hot after it has been through a switch — live only when the switch says so. In a lighting circuit it is the conductor that runs from the switch to the fixture. Same job as the hot, just under control. Read more → See it: Single-pole switch · Switch loop

Branch circuit

The conductors from the last overcurrent device — the breaker — out to the loads: the lights, the receptacles, the motor. It is the circuit you draw on this site. What a branch circuit may feed, and how it is protected, is code.

Feeder

The conductors between the service (or a main panel) and a sub-panel or a distribution point. A feeder carries several branch circuits' worth of current on its way to the panel that splits them up. Feeder, then panel, then branch circuits: that is the order from the street inward.

Service

The conductors and equipment that bring the utility's supply into the building and let it be disconnected and metered — the point where the utility's side ends and the building's side begins. Everything on this site happens downstream of it.

Breaker vs fuse

Both are overcurrent devices: they open the circuit when the current is too high for too long. A fuse does it once, by melting, and is replaced. A breaker does it by tripping a mechanism and is reset. Both sit in the hot; neither is a switch you should rely on to make a circuit safe to touch without a meter. See it: Symbols: fuse, breaker

Tension

The pressure that pushes current round a loop — the difference in potential between two points, measured in volts. A meter measures it across two things, never through one. On a schematic it is what the supply provides and what a load drops. See it: Ohm's law practice

Courant

The flow itself, measured in amps. It is the same all the way round a series loop, and it splits between parallel branches. Current is what heats a conductor, what trips a breaker, and what a GFCI counts out on the hot and back on the neutral. See it: Series / parallel practice

Résistance

How much a thing resists current for a given voltage, measured in ohms. A lamp is resistance that gives off light; a length of conductor is resistance you would rather not have. Ohm's law ties the three together: V = I × R. See it: Ohm's law practice

AC vs DC

Direct current flows one way, steadily — a battery. Alternating current reverses direction many times a second — the utility supply. The circuits drawn on this site are AC, but the continuity — what is connected to what — reads the same either way, which is why the diagrams never need to say.

Single-phase vs three-phase

Single-phase is one alternating supply: one hot and a neutral, or two hots that swing opposite each other. Three-phase is three supplies staggered in time, which is how larger motors and buildings are fed. The relationships between the three are what wye and delta describe. See it: Wye and delta · 240 V receptacle — two hots, no neutral

Ballast / driver

The box inside a light fixture that turns the supply into what the lamp itself needs. A ballast does it for fluorescent and discharge lamps; a driver does it for LEDs. To the circuit, either one is simply the load — the switch and the return do not care what is inside.

Junction box

An enclosure where conductors are joined, pulled or split, with a cover you can remove. On a schematic the joins are dots; in the wall the dots live in a box. Every splice has to be in one — that is a code rule, and the reason is that a joint is the part of a circuit most likely to fail. See it: Symbols: junction box

Pigtail

A short length of conductor spliced to a group of others so that one wire, not three, lands on a device terminal. It keeps the through-current of a circuit in the splice rather than passing through a device's screw — pull the device and the rest of the circuit stays connected.

Whip

A short flexible run — conduit or cable — from a box to a piece of equipment that vibrates, moves or gets serviced: a motor, a unit heater, a fixture above a ceiling grid. Flexible so the equipment can be shifted without disturbing the fixed wiring.

EMT, rigid, PVC (raceway names)

The names of the common pipes conductors are pulled through. EMT is the thin-wall steel tube that bends on a hand bender; rigid is the thick-wall threaded steel; PVC is the plastic conduit glued together. Which one may be used where, and how full it may be, is code — this entry is names only. See it: Fittings index

Device vs fixture

A device carries or controls current but does not use it: a switch, a receptacle, a breaker. A fixture (or luminaire) is a load that uses it: a light. Both live in boxes; only fixtures show up on a schematic as something that drops voltage.

On the tools

The words that arrive with the bender, the panel and the motor room.

Common (switch terminal)

The odd terminal on a 3-way switch — the one that connects to either traveller depending on which way the blade is thrown. On the first switch it takes the incoming hot; on the last switch it becomes the switched hot to the load. See it: 3-way switch — the commons are labelled

Traveller

One of the two conductors running between the switches in a 3-way or 4-way circuit. The switches choose which traveller is live; the light is on when the whole run agrees on one. Also spelled "traveler". Read more → See it: 3-way switch · 4-way switch

SPDT (single-pole double-throw)

A switch with one input that connects to one of two outputs — never both, never neither. A 3-way switch is an SPDT: one common, two travellers. See it: Symbols: three-way switch

Seal-in (holding) contact

A contact on a relay or starter, wired in parallel with the Start button, that closes when the coil pulls in. Once closed it keeps the coil fed after the button is released — the circuit "seals in". The Stop button breaks the whole rung, the coil drops, and the contact opens again. Read more → See it: Motor start/stop station

Shrink

When you put a bend in a pipe, the pipe no longer reaches as far as it would straight — the bend "eats" length. Shrink is how much shorter the run gets, and you add it back when you measure so the fitting lands where you wanted it.

Gain

The opposite bookkeeping to shrink, for a 90° bend: the curve of the bend is shorter than going out to the corner and back, so the bend gains you a little length. You subtract the gain so the pipe is not long.

Take-up (deduct)

On a 90° stub, the distance from the back of the bend to the end of the pipe. It is a property of the bender and the pipe size and is stamped on the tool — you read it off the bender, you do not look it up in a code book.

Developed length

The true length of pipe consumed by a bend measured along the centre of the pipe as it curves, rather than the straight-line distance between where it starts and stops bending. It is what you actually cut, and it is always more than the straight measurement.

Springback

Steel is elastic before it is plastic, so a pipe you bend to an angle relaxes back a few degrees when you let go. You over-bend slightly to land on the angle you want. How much depends on the material and the size.

Kick

A single bend put in a pipe to nudge it over to one side — the offset done with one bend instead of two, usually to line a stub up with a box. A deliberate, controlled version of what a dog leg is by accident.

Dog leg

A pipe run that wanders off the straight line because the bends do not share a plane — the offset twisted, or a second bend was rolled without meaning to. It is a mistake, not a technique, and it is why you level your bends.

Déclassement

Reducing the current you allow a conductor to carry because conditions make it run hotter than the ideal — high ambient temperature, or several current-carrying conductors bundled together sharing their heat. It is a multiplication you apply to a starting value; the starting values and the factors themselves come from your code book, not from here. Read more →

Courant admissible

The current a conductor can carry continuously without its insulation getting hotter than it is rated for. It is a temperature limit expressed as a current. The actual figures for a given conductor and insulation are code-book and manufacturer data — look them up there. Read more →

Continuité des masses ou mise à la terre

Grounding connects a system to the earth, mainly to hold it at a stable reference and handle things like lightning and line surges. Bonding ties all the metal that could become energized together so it is at the same potential and a fault has a low-resistance path back to the source that trips the breaker. People use the words loosely; they are not the same job. Read more → See it: Hot, neutral, ground

Neutral vs identified conductor

The neutral is the grounded circuit conductor that carries the unbalanced return current. "Identified" means it is marked so anyone can tell it apart from the ungrounded conductors — the specific marking and colour are set by code. Not every white-looking wire is a neutral, and not every neutral is only a neutral, which is exactly why the identification rules exist. See it: Switch loop — no neutral in the box

Étoile et triangle

Two ways to connect a three-phase set of windings. Wye ties one end of each of the three together at a common point (which can give you a neutral); delta connects them end to end in a loop (no neutral from the windings themselves). The choice changes the voltage relationships you see between lines and to neutral. Read more →

Facteur de puissance

The fraction of the current that is actually doing work. Motors and other inductive loads pull current that swings out of step with the voltage; that out-of-step part moves energy back and forth without doing useful work but still has to be carried. Power factor is the ratio of real power to the total, between 0 and 1. Read more →

Inrush

The brief, large current a load draws in the first instant it is switched on — a motor starting, a transformer energizing, a bank of lamps lighting cold. It can be many times the running current and dies away in a fraction of a second, which is why protection has to tolerate a short spike without tripping. Read more → See it: Contactor

AFCI ou DDFT (GFCI)

A GFCI watches for current leaking off the circuit — going somewhere other than back on the neutral, such as through a person — and opens fast. An AFCI watches for the electrical signature of arcing, the sputtering fault that starts fires in damaged wiring. One is about shock, the other about fire; some devices do both. Read more → See it: GFCI

Home run

The cable or conductors that run from the panel to the first box of a circuit, before the circuit branches out to the rest of its devices. The "run home" to the panel.

Dead leg

A length of conductor or a tap that is not carrying current in the present configuration — either spare, isolated, or fed from a source that is off. Worth naming because "dead" is a claim you verify with a meter, never an assumption.

Mule tape

A flat woven pulling tape, marked with its rated strength and often with footage, used to pull conductors into conduit. Flat so it does not dig in, strong so it does not snap mid-pull, and low-stretch so you know how much you are actually pulling.

LB and C bodies (condulets)

Cast fittings with a removable cover that give you access to pull or turn conductors in a rigid run. The letter describes the shape: an LB turns the run back at a right angle (the opening faces the "back"), a C body is a straight-through pull point. There are also LL, LR and T shapes for other directions. See it: Fittings index

Panel room and job site

The words that arrive with the meter, the lockout kit, the panel door and the first motor you are handed. Each one has a page under On the job that goes further.

Nameplate (motor)

The metal plate riveted to a motor that states what it is: horsepower, voltage, full-load amps, speed, frame, service factor, insulation class, duty. It is the manufacturer's statement about that motor, and it is what you wire and protect to. Read it before you touch the motor; photograph it before you leave. Read more → See it: Reading a motor nameplate

Facteur de service

A multiplier on the nameplate that says how far past its rated load a motor can be pushed without immediate damage — 1.0 means none, above 1.0 means some margin. It is a margin for short overloads and hot afternoons, not a rating to run at: insulation life drops fast when you live in the service factor. It is a property of that motor, not a permission from the code; whether you may use the margin, and how it affects protection, is the book's call. Read more → See it: Reading a motor nameplate

FLA (full-load amps)

The current the motor draws at its rated load, as printed on the nameplate. The code book has its own full-load-current values, used for sizing conductors and some protection, and the two are different figures for different jobs: nameplate for the overload setting, the book's for the wire. Do not swap them. Read more → See it: Reading a motor nameplate

CAT rating (measurement category)

The rating on a meter and its leads that says how close to the supply it is built to survive a transient — the nearer the service, the bigger the surge a fault can throw and the higher the category the meter needs. It is about what the meter withstands, not what it measures. Match the category to where you are standing, not to the voltage you expect. Read more → See it: Meter safety and CAT ratings

Live-dead-live

The three-step proof that a circuit is dead: test the meter on a known live source, test the circuit, test the meter on the live source again. The two live readings prove the meter and the leads were working the whole time; without them a dead reading proves nothing. See it: Meter safety and CAT ratings · Lockout / tagout

Lockout / tagout

Isolating a source of energy, putting your own lock on the isolating device so nobody can re-energise it, tagging it with who and why, and proving it dead before work starts. Your lock, your key, on until your hands are out. The procedure you follow is your employer's and your jurisdiction's; the idea is the same everywhere. Read more → See it: Lockout / tagout walk-through

Panel schedule

The card in the panel door that lists every breaker by number and says what it feeds. Odd numbers down the left, even down the right, top to bottom. It is the map you read before you open anything — and the thing you update when you add a circuit, or the next person is working blind. Read more → See it: Reading a panel schedule

Two-pole breaker

A breaker that takes two adjacent spaces on one side of the panel and lands on both legs of the supply, with its handles tied so both trip together. It is how a 240 V load gets fed. Because the legs alternate down the bus, two spaces in a row are always on different legs — which is the whole reason it works. See it: Reading a panel schedule · 240 V receptacle

Control transformer

The small transformer in a starter enclosure that steps the supply down to the control voltage the coil and the pushbuttons run at. Its primary terminals are marked H1, H2; its secondary X1, X2 — the rails of the ladder diagram. Its size and protection come from the manufacturer and the book. Read more → See it: Transformer basics · Reading a ladder diagram

Rapport de transformation

The number of turns on a transformer's primary winding divided by the number on its secondary. Voltage follows the ratio one way and current follows it the other: twice the turns on the secondary gives twice the volts at half the amps, because the power through the transformer is the same on both sides (less a little heat). Read more → See it: Transformer basics

NO / NC (normally open, normally closed)

The state of a contact with no force on it — nobody pressing the button, the coil de-energised, the float hanging. A normally open contact passes nothing until something acts on it; a normally closed one passes current until something acts on it. "Normal" means at rest, not the usual running state, which is the trap. Read more → See it: Reading a ladder diagram · Symbols: contacts

Pilot device

Anything that tells a starter what to do without carrying the motor current itself: pushbuttons, selector switches, limit switches, float and pressure switches, pilot lights. They live on the control side of the ladder, at control voltage, and they are what a pushbutton station is full of. See it: Two-wire vs three-wire control

Patch panel

The rack-mounted strip where every data cable from the building lands on its own numbered jack, so the connection to the switch is a short patch cord you can move rather than a cable you have to re-pull. The label on the panel is the label on the wall plate — or it should be. Read more → See it: Low-voltage and data basics

Plenum-rated

A cable jacket rated for use in a space that carries air for the building — the gap above a suspended ceiling that the HVAC breathes through. Plenum jacket gives off less smoke and burns slower. Whether a ceiling space is a plenum, and what may run through it, is the code's and the mechanical drawings' call. See it: Low-voltage and data basics

A few of these — shrink, take-up, developed length, kick — are bending terms. If you want the actual bending math worked out, that is what bentright.com is for.

Voir aussi

• Continuité des masses ou mise à la terre
• AFCI ou DDFT (GFCI)
• Courant admissible
• Facteur de puissance
• Étoile et triangle
• More in Reference