Home electrical terms, in plain English.

0–9

120/240-volt split-phase

The standard single-family service: two 120-volt legs and a neutral from the utility transformer. Leg to neutral gives 120 volts for lights and outlets; leg to leg gives 240 volts for ranges, dryers, EV chargers and air conditioning.See also:Panel load calculator

120% rule

The NEC 705.12 limit on solar or battery power fed through a panel’s busbar: 125% of the inverter’s output current plus the main breaker rating can’t exceed 120% of the busbar rating. A 200-amp busbar with a 200-amp main leaves room for 32 amps of inverter current.See also:Solar panel capacitySolar-ready guide

208-volt three-phase

A 208Y/120-volt system found in condos, apartments and commercial buildings. Two legs give 208 volts, not 240, so a 240-volt appliance on it delivers about 13% less power at the same amps, and an EV charger charges that much slower.See also:EV charger calculatorWire size calculator

A

AFCI (arc-fault circuit interrupter)

A breaker or receptacle that trips when it detects arcing from a loose connection, a damaged cord or a nail through a cable. It protects against fire; ordinary breakers don’t respond to early arcing. The code requires AFCI on 120-volt, 15- and 20-amp circuits in most rooms of a house (NEC 210.12).See also:GFCI and AFCI guideBreaker trip troubleshooter

Airtight recessed light

A recessed light labeled as certified airtight, leaking less than 2.0 cfm of air at 75 pascals when tested to ASTM E283. California’s Energy Code requires it for recessed downlights, along with no screw-base sockets.See also:Recessed lighting layoutRecessed lighting

Aluminum branch wiring

Small aluminum wire used for outlet and light circuits in some homes wired between 1965 and the mid-1970s. CPSC found pre-1972 aluminum-wired homes 55 times more likely than copper-wired homes to have a connection reach fire-hazard conditions. The cable jacket is marked AL or ALUMINUM.See also:Home wiring by decadeRewires and remodels

Ampacity

The most current a wire can carry continuously without getting hotter than its insulation’s rating. NEC Table 310.16 lists it by size, metal and temperature column; 12 AWG copper is 20 amps in the 60 °C column.See also:Wire size calculator

Amps, volts and watts

Volts are the electrical pressure, amps are the flow, and watts are the work done. For a heater or other resistive load, watts = volts × amps, so a 1,500-watt heater on 120 volts draws 12.5 amps.See also:Breaker trip troubleshooter

Arc fault

An unintended spark across a gap, caused by damaged, overheated or stressed wiring: a loose terminal, a frayed cord, a screw through a cable. A small one may not draw enough current to trip a regular breaker, which is what AFCIs are for.See also:Breaker trip troubleshooterGFCI and AFCI guide

Automatic transfer switch (ATS)

A transfer switch that senses a utility outage, moves the house or selected circuits to a standby generator or battery on its own, and moves them back when utility power returns.See also:Backup power sizingSolar and battery ready

AWG (wire gauge)

American Wire Gauge, the size system for building wire. Smaller numbers are bigger wires. In copper, NEC 240.4(D) limits 14 AWG to a 15-amp breaker, 12 AWG to 20 amps and 10 AWG to 30 amps.See also:Wire size calculator

B

Backfeed

Power flowing into a panel from something other than the utility, such as a solar inverter or battery through its own breaker, or a generator through an interlock or transfer switch. A generator backfed through an ordinary outlet can energize the utility’s lines; an interlock or transfer switch is what prevents that.See also:Backup power sizingSolar panel capacity

Bonding

Connecting metal parts, like panel enclosures, water piping and gas piping, to the grounding system so they can’t sit at different voltages. It gives fault current a path back to the panel so the breaker trips.

Busbar

The metal bars inside a panel that the breakers clip onto. The busbar rating on the panel label caps what the panel can carry, and it sets how much solar or battery the 120% rule allows.See also:Solar panel capacityPanel load calculator

C

C-10 license

California’s Electrical Contractor classification, issued by the CSLB. A C-10 contractor “places, installs, erects or connects any electrical wires, fixtures, appliances, apparatus, raceways, conduits, solar photovoltaic cells” and related equipment.See also:Hiring an electrician

CEC (California Electrical Code)

Title 24, Part 3 of California’s building standards. The 2025 edition is based on the 2023 NEC and took effect January 1, 2026. Not to be confused with the California Energy Commission, which also goes by CEC.See also:Permit checkerGFCI and AFCI guide

Conduit fill

How much of a conduit’s cross-section the wires may take up, set by NEC Chapter 9. Crowding more current-carrying wires into one conduit also means derating their ampacity.See also:Wire size calculator

Continuous load

A load where the maximum current is expected to continue for 3 hours or more, like EV charging. The wire and breaker are sized at 125% of it, so a 48-amp charger needs a 60-amp circuit.See also:EV charger calculatorWire size calculator

CSLB (Contractors State License Board)

The California agency that licenses contractors, including C-10 electrical contractors. You can look up any license number on the CSLB website before you hire.See also:Hiring an electrician

D

Dead-front

The inner cover of a panel, with the breaker handles poking through. It keeps hands off live busbars and wires. Read labels and flip breakers through it; don’t take it off.See also:Breaker trip troubleshooter

Demand factor

A percentage the NEC load calculation applies to groups of loads because they don’t all run at full power at once. It’s why a house can have more connected load than its service rating and still pass.See also:Panel load calculator

Dimmer compatibility

Whether a given LED lamp or fixture and a given dimmer work together without flicker, buzz or dropout. California’s 2025 Energy Code requires wall-mounted dimmers for lighting in habitable spaces, so it comes up on every remodel.See also:Outlets, switches and dimmersRecessed lighting layout

Double tap

Two wires under a breaker terminal that’s listed for only one. Home inspectors flag it; the usual fix is a pigtail or a separate breaker.See also:Troubleshooting

Dual-function breaker

A breaker that combines AFCI and GFCI protection. Square D sells them as Dual Function (DF) QO and Homeline breakers. Kitchens and laundry areas are on both code lists, so they often get one.See also:GFCI and AFCI guideBreaker trip troubleshooter

E

EMT (electrical metallic tubing)

Thin-wall steel conduit, the kind you see exposed in garages and commercial spaces. Individual wires such as THHN are pulled through it.See also:Garage electricalWire size calculator

Equipment grounding conductor

The bare or green wire that runs with a circuit and ties metal boxes, appliance frames and the ground pin back to the panel. It carries current only during a fault, giving it a low-resistance path so the breaker trips fast.

EVEMS (EV energy management)

A listed system that watches the house’s load and throttles EV charging so the total stays within what the service can carry. The 2023 NEC lets EV circuits be sized by the managed load (625.42), which can avoid a panel upgrade.See also:EV charger calculatorPanel load calculatorEV charger installation

EVSE

Electric vehicle supply equipment, the wall unit most people call the charger. The charger itself is inside the car; the EVSE supplies power safely and tells the car how much current it may draw.See also:EV charger calculatorHome EV charging guideEV charger installation

F

Federal Pacific (FPE) panel

A panel made by Federal Pacific Electric, recognizable by its Stab-Lok breakers. It’s one of the older brands insurers and home inspectors flag.See also:Problem panel checkerPanel upgrades

Final inspection

The inspector’s last visit on a permit, after devices and fixtures are installed and powered. Passing it closes the permit.See also:Permit checker

G

Generator inlet

A weatherproof inlet on the outside wall that a portable generator’s cord plugs into. It feeds the panel through an interlock kit or transfer switch, so the generator can’t backfeed the utility.See also:Backup power sizingSolar and battery ready

GFCI (ground-fault circuit interrupter)

An outlet or breaker that trips when current leaks out of the circuit, through water or a person. Class A devices trip at 6 milliamps and hold below 4. The code requires them in bathrooms, kitchens, garages, outdoors, laundry areas and near sinks (NEC 210.8).See also:GFCI and AFCI guideBreaker trip troubleshooterOutlets, switches and dimmers

Ground fault

Current taking a path to ground it shouldn’t, through water, a damaged appliance or a person. GFCIs catch the small ones a regular breaker never notices.See also:Breaker trip troubleshooter

Grounding electrode

The connection between the electrical system and the earth: ground rods, the rebar in the foundation, or a metal water pipe. It handles lightning and utility surges. Tripping a breaker is the equipment grounding conductor’s job, not the electrode’s.

I

IC-rated

A recessed light rated for direct contact with ceiling insulation (IC stands for insulation contact). Lights without the rating need clearance from insulation.See also:Recessed lighting layoutRecessed lighting

In-use cover

A deep “bubble” cover for an outdoor receptacle that stays closed and weatherproof with a cord plugged in. Receptacles in wet locations need this style of cover.See also:Outlets, switches and dimmers

Interlock kit

A sliding plate or bracket, listed for a specific panel, that makes it impossible to have the main breaker and the generator breaker on at the same time.See also:Backup power sizing

J

J1772 (SAE J1772)

The standard plug for Level 1 and Level 2 charging in the U.S. The Department of Energy says all commercially available EVs here can use it, Teslas through the adapter they come with.See also:EV charger calculatorHome EV charging guide

JA8

Joint Appendix 8 of California’s Energy Code, the high-efficacy standard light sources must be certified and marked to. In new homes, additions and alterations, installed luminaires and lamps must comply.See also:Recessed lighting layoutKitchen remodel guide

K

kcmil

Thousand circular mils, the size unit for wire bigger than 4/0 AWG. 250 kcmil is 250,000 circular mils; 4/0 is 211,600.See also:Wire size calculator

Knob-and-tube

The earliest house wiring: single conductors run on porcelain knobs and through porcelain tubes, with no ground wire.See also:Home wiring by decadeRewires and remodels

L

LED driver

The electronics that convert line voltage to the low-voltage DC an LED needs. Its design decides how a fixture dims and whether it flickers.See also:Recessed lighting

Level 1 and Level 2 charging

Level 1 is a 120-volt outlet and adds about 5 miles of range per hour. Level 2 runs on 208 or 240 volts and adds about 25 miles per hour; a typical home unit at 30 amps delivers 7.2 kW, per the Department of Energy.See also:EV charger calculatorHome EV charging guideEV charger installation

Load calculation

The NEC Article 220 arithmetic that estimates a home’s demand, to size a new service or to check whether an existing panel can take one more big load like an EV charger or heat pump.See also:Panel load calculatorPanel upgrade guide

Low voltage

Power-limited wiring for doorbells, thermostats, landscape lights, network cable and cameras, supplied through a transformer or power supply. It follows different rules and uses different cable than the 120-volt wiring in the walls.See also:Cameras and doorbells

M

Main breaker

The breaker that shuts off everything in the house. Its rating, such as 100, 125 or 200 amps, is usually the size of the service.See also:Panel load calculatorPanel upgrades

Main-breaker derate

Swapping the main breaker for a smaller one, say 200 amps down to 175, to make room for solar under the 120% rule. It only works if the home’s load calculation still fits under the smaller main.See also:Solar panel capacitySolar-ready guide

MC cable (metal-clad)

Insulated wires inside interlocking metal armor, with a separate green grounding wire. Common in commercial work and in exposed runs.See also:Commercial electrical

Meter-main combo

One enclosure that holds the utility meter socket and the main breaker, often with branch breakers too. Replacing one involves the utility, because the meter socket is part of it.See also:Panel upgrade guideUtility finder

N

NACS (SAE J3400)

The charging connector Tesla developed, now standardized by SAE as J3400. The Department of Energy notes several automakers announced they would adopt it starting as early as 2025.See also:EV charger calculatorHome EV charging guide

NEC (National Electrical Code)

NFPA 70, the model electrical code the National Fire Protection Association publishes every three years (2020, 2023, 2026). California adopts it as the California Electrical Code; the current 2025 CEC is based on the 2023 NEC.See also:GFCI and AFCI guide

NEMA 14-50

A 50-amp, 125/250-volt, 4-wire receptacle (two hots, neutral and ground), the usual outlet for plug-in EV chargers. In a garage or outdoors it needs GFCI protection, because 210.8(A) covers 125- to 250-volt receptacles there.See also:EV charger calculatorEV charger installation

Net Billing Tariff

The CPUC’s rooftop-solar rules for PG&E, SCE and SDG&E customers who applied to interconnect on or after April 15, 2023. Exported power is credited at values from the CPUC’s Avoided Cost Calculator, usually below the retail rate, and customers take an electrification time-of-use rate.See also:Solar panel capacitySolar-ready guideUtility finder

Neutral

The grounded conductor, usually white or gray, that carries current back to the source on a 120-volt circuit. It’s connected to ground only at the main service; in a sub-panel, neutral and ground stay separate.

NM-B cable (Romex)

Nonmetallic-sheathed cable, the plastic-jacketed cable inside most house walls. Romex is a brand name, not the cable type. Its conductors are rated 90 °C, but NEC 334.80 limits its ampacity to the 60 °C column.See also:Wire size calculatorRewires and remodels

Nuisance trip

A trip with no hazard behind it, or none anyone can find. Some are real faults that come and go; some are equipment that doesn’t get along with a GFCI or AFCI, like the heat pumps behind the HVAC exception to 210.8(F).See also:Breaker trip troubleshooterGFCI and AFCI guide

O

Overload

More current than a circuit is rated for, for long enough to heat the breaker’s thermal trip. Heaters, hair dryers and kitchen appliances sharing a circuit are the usual cause.See also:Breaker trip troubleshooter

P

Panelboard

The code’s name for a breaker panel: the enclosure with the busbars and breakers that divide incoming power into branch circuits.See also:Panel upgradesPanel upgrade guide

Permit

Written permission from the city’s building department to do electrical work, followed by inspections. New circuits, panel changes and EV chargers typically need one.See also:Permit checkerHiring an electrician

PoE (Power over Ethernet)

One network cable carrying both data and low-voltage power to a camera, access point or doorbell, so the device doesn’t need an outlet nearby.See also:Cameras and doorbells

Power sharing

Two or more EV chargers linked to split one circuit’s capacity, so a second car doesn’t need a second full-size circuit. The 2023 NEC allows multiple EVSE to be sized by the managed load under an energy management system (625.42(A)).See also:EV charger calculatorEV charger installation

PSPS (Public Safety Power Shutoff)

A planned outage a utility such as SCE or SDG&E can call when it believes strong winds create an imminent and significant risk of power lines starting a wildfire. The CPUC sets the notification rules.See also:Backup power sizingUtility finder

R

Rough-in

The stage when boxes, cable and conduit are in but the walls are still open. The rough inspection happens here, before insulation and drywall.See also:Permit checkerRewires and remodels

S

SER cable

Service-entrance cable, style R: insulated hots and neutral plus a ground in one jacket, often aluminum. Used for sub-panel and range feeders; run through thermal insulation, it’s held to the 60 °C ampacity column.See also:Wire size calculator

Service drop

The overhead wires from the utility pole to the house.See also:Utility finder

Service entrance

Where utility power enters the house: the service-entrance conductors, the meter socket and the service disconnect, which is usually the main breaker.See also:Panel upgrade guide

Service lateral

Underground utility conductors to the house, the buried version of a service drop.See also:Utility finder

Short circuit

A hot conductor touching neutral or ground with almost nothing in between. Current spikes and the breaker trips instantly.See also:Breaker trip troubleshooter

Split-bus panel

An older panel with no single main breaker. Several breakers at the top act as the service disconnect, and one of them feeds a lower section of branch breakers.See also:Problem panel checkerPanel upgrades

Sub-panel

A second panel fed from a breaker in the main panel, typical for garages, ADUs and additions. Its neutral and ground bars stay separate.See also:Wire size calculatorPanel upgradesGarage electrical

Super off-peak

The cheapest window on some time-of-use plans. On SDG&E plans that have it, including EV-TOU-5, it runs midnight to 6 a.m. and 10 a.m. to 2 p.m. on weekdays, and midnight to 2 p.m. on weekends.See also:Utility finderEV charger calculator

Supply-side connection

Connecting solar or storage between the meter and the main breaker (NEC 705.11) instead of through a breaker on the busbar, which sidesteps the 120% rule’s busbar limit. Also called a supply-side tap.See also:Solar panel capacitySolar-ready guide

Surge protective device (SPD)

A device at the panel that clamps voltage spikes. Since the 2020 NEC, 230.67 requires a Type 1 or Type 2 SPD on every service supplying a dwelling, including when the service equipment is replaced; the 2023 edition sets a 10 kA minimum rating.See also:Panel upgrade guidePanel upgrades

Switchboard

A large freestanding distribution assembly for commercial buildings and big multifamily services, not something a typical house has.See also:Commercial electrical

T

Tamper-resistant receptacle

A receptacle with internal shutters that open only when both prongs of a plug push in at once. The NEC requires them in new homes; Leviton cites about 2,400 children a year severely shocked by objects pushed into outlets.See also:Outlets, switches and dimmers

Tandem breaker

A half-width breaker that fits two circuits in one panel space. It’s allowed only in panels whose label says so, and only in the positions the label allows.See also:Panel load calculatorPanel upgrades

THHN/THWN-2

The common single-conductor building wire pulled through conduit. The letters describe the insulation: thermoplastic, heat-resistant, nylon-jacketed, and W for wet-rated.See also:Wire size calculator

Title 24

California’s Building Standards Code. Part 3 is the California Electrical Code; Part 6 is the Energy Code, which sets lighting rules like JA8 and required dimmers.See also:Permit checkerKitchen remodel guide

TOU (time-of-use) rate

A utility rate that costs more at peak hours and less off-peak. Solar customers on the Net Billing Tariff take electrification TOU rates: E-ELEC (PG&E), TOU-D-PRIME (SCE) and EV-TOU-5 (SDG&E).See also:Utility finderEV charger calculator

Transfer switch

A switch that moves selected circuits, or the whole house, between utility power and a generator or battery, never both at once.See also:Backup power sizingSolar and battery ready

Transformer (doorbell)

A small transformer that steps 120 volts down to low voltage for a doorbell chime. A video doorbell may need a bigger one than the original, so check its instructions before installing.See also:Cameras and doorbells

Trim-out

Installing devices, fixtures and cover plates after drywall and paint, followed by the final inspection.See also:Permit checker

U

UF-B cable

Underground feeder cable with a solid, sunlight- and moisture-resistant jacket, rated for direct burial. Its ampacity is limited to the 60 °C column (NEC 340.80).See also:Wire size calculatorGarage electrical

V

VA and kVA

Volt-amperes, volts times amps, the unit NEC load calculations use. 1 kVA is 1,000 VA, about 4.2 amps at 240 volts.See also:Panel load calculator

Voltage drop

The voltage lost to a wire’s resistance over distance. NEC informational notes suggest no more than 3% on a branch circuit or feeder and 5% combined; they’re recommendations, not requirements.See also:Wire size calculator

W

Weather-resistant receptacle

A receptacle marked WR, built to hold up to sun and corrosion. The NEC requires them in damp and wet locations, which takes in outdoor outlets.See also:Outlets, switches and dimmers

Z

Zinsco panel

A panel sold under the Zinsco name. Like Federal Pacific, it’s one of the older brands insurers and home inspectors flag.See also:Problem panel checkerPanel upgrades

Where the code-based definitions come from

1. ICC Store: 2025 California Electrical Code, Title 24, Part 3Based on the 2023 NEC; effective January 1, 2026 (CEC, NEC). Read September 22, 2026.
2. WireSizes.com: NEC Table 310.16 (2023)Ampacities and the 240.4(D) small-conductor limits (ampacity, AWG). Read September 22, 2026.
3. NEC Chapter 9, Table 8 (reprint)Circular-mil areas, 4/0 = 211,600 and 250 kcmil = 250,000 (kcmil). Read September 22, 2026.
4. Charles R. Miller, EC Magazine: Sizing Conductors, Part VINM cable held to the 60 °C rating; 90 °C for derating only (NM-B). Read September 22, 2026.
5. Southwire: UF-B Copper Cable spec60 °C ampacity per 340.80; direct burial; sunlight and moisture resistant (UF-B). Read September 22, 2026.
6. Mike Holt, EC&M: Commercial Loads — Part 1Continuous load is 3 hours or more; 125% sizing (continuous load). Read September 22, 2026.
7. Mike Holt, EC&M: Don't Let Voltage Drop Get Your System Down3% and 5% are recommendations, not requirements (voltage drop). Read September 22, 2026.
8. Mark C. Ode, EC Magazine: Special-Purpose GFCIsClass A GFCIs trip at 6 mA and not below 4 mA (GFCI). Read September 22, 2026.
9. NEC 210.8(A) text (2020 edition)125- through 250-volt receptacles in the listed dwelling locations (GFCI, NEMA 14-50). Read September 22, 2026.
10. ExpertCE: NEC 210.12 ExplainedAFCI rooms and circuits (AFCI). Read September 22, 2026.
11. CPSC Publication 5133: Arc Fault Circuit InterruptersOrdinary breakers don’t respond to early arcing (AFCI, arc fault). Read September 22, 2026.
12. ESFi: Understanding Your Home Electrical SystemArc faults come from damaged, overheated or stressed wiring (arc fault). Read September 22, 2026.
13. Mike Holt: 2023 Code change analysis, 210.8Kitchen, sink-area and appliance GFCI changes (GFCI, dual-function breaker). Read September 22, 2026.
14. NFPA TIA 70-23-3210.8(F) HVAC exception, expiring September 1, 2026 (nuisance trip). Read September 22, 2026.
15. Schneider Electric FAQ FA125415Square D QO, QOB and Homeline Dual Function (DF) breakers (dual-function breaker). Read September 22, 2026.
16. CPSC Publication 516: Repairing Aluminum Wiring1965 to mid-1970s; 55 times more likely; AL / ALUMINUM jacket marking (aluminum branch wiring). Read September 22, 2026.
17. Mayfield Renewables: Code Corner, NEC 705.12125% of inverter current plus the main ≤ 120% of the busbar; 200 A / 32 A example (120% rule, busbar). Read September 22, 2026.
18. Leviton: NEC Code Requirements for Surge230.67: Type 1 or Type 2 SPD for dwelling services since 2020; 10 kA minimum in 2023 (SPD). Read September 22, 2026.
19. Leviton: Tech Talk — GFCI and Tamper-Resistant ReceptaclesHow TR shutters work; about 2,400 children a year severely shocked (tamper-resistant receptacle). Read September 22, 2026.
20. Leviton 1450R product page50 A, 125/250 V, NEMA 14-50R, 3-pole 4-wire, for EV charging (NEMA 14-50). Read September 22, 2026.
21. Eaton: NEC 2023 home electrification (Tom Domitrovich, Feb 26, 2024)625.42(A) energy management and 625.42(B) adjustable settings for EVSE (EVEMS, power sharing). Read September 22, 2026.
22. U.S. DOE Alternative Fuels Data Center: Charging Electric Vehicles at Home and StationsLevel 1 about 5 miles per hour, Level 2 about 25; 30 A = 7.2 kW; J1772 and J3400 (Level 1/2, J1772, NACS). Read September 22, 2026.
23. California Energy Commission: 2025 Residential Lighting FAQsJA8 compliance; airtight downlights under 2.0 cfm at 75 Pa (ASTM E283), no screw bases; dimmers in habitable spaces (JA8, airtight, dimmers). Read September 22, 2026.
24. CPUC: Net Energy Metering and Net BillingAvoided Cost Calculator export values, usually below retail; E-ELEC, TOU-D-PRIME, EV-TOU-5 (Net Billing Tariff, TOU). Read September 22, 2026.
25. CPUC: Net Billing TariffApplies to interconnection applications on or after April 15, 2023 (Net Billing Tariff). Read September 22, 2026.
26. SDG&E: Extended Super Off-Peak HoursWeekdays 12–6 a.m. and 10 a.m.–2 p.m.; weekends 12 a.m.–2 p.m.; plans including EV-TOU-5 (super off-peak). Read September 22, 2026.
27. CPUC: Public Safety Power ShutoffsUtilities may de-energize lines for imminent, significant wildfire risk from strong winds (PSPS). Read September 22, 2026.
28. CSLB: C-10 Electrical Contractor classificationThe C-10 definition quoted here (C-10, CSLB). Read September 22, 2026.

General information, not a design or legal advice. Spot a definition that’s wrong or missing? Text a photo or a note and it gets fixed.