Electrical · Glossary

Electrical glossary

The terms that matter most on a residential quote are: service (the capacity coming into the house), branch circuit (a single protected run from the panel), load calculation (the math that justifies the service size), AFCI and GFCI (arc-fault and ground-fault protection), AHJ (the local Authority Having Jurisdiction that inspects and approves the work), and listed (tested and certified by a recognized laboratory).

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Understanding six or seven terms is enough to read a quote critically. Service is the utility supply and its capacity in amps. Service entrance is the equipment between the utility and your main breaker — the piece that quietly makes panel quotes expensive. Branch circuit is one protected run from a breaker out to devices.

Load calculation is the document that determines whether your service is adequate. If a contractor argues for a capacity upgrade, this is the artifact you should ask for. AFCI detects arcing, the ignition mechanism behind a large share of electrical fires; GFCI detects current leaking to ground, the mechanism behind most electrocutions.

AHJ — Authority Having Jurisdiction — is the local official whose interpretation of the adopted code governs your project. Not the contractor, not NFPA. Listed means a product has been tested and certified by a recognized testing laboratory for its intended use. Grandfathered is informal shorthand for existing work permitted to remain because it complied when installed; it is not a synonym for safe.

How to choose

Learn 'load calculation' first: It is the term that converts a sales claim into a checkable document.
Know 'AHJ': When someone says 'the code requires,' the real question is what your AHJ enforces.
'Grandfathered' means legal, not safe: Ask whether a finding is a hazard or just a difference from current code.
'Listed' is not optional: Every breaker, device and cable should be listed by a recognized laboratory for the use it is being put to.
Ask for definitions on the spot: A good electrician explains terms without irritation. A refusal to explain is information about the contractor.

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All electrical glossary

Ampacity

Service Drop

Load Calculation

Main Breaker

Branch Circuit

Neutral Wire

Ground Fault

Arc Fault

Voltage Drop

Dedicated Circuit

Service Disconnect

Double Tap

Federal Pacific Panel

Zinsco Panel

Headroom Capacity

Electrification Ready

FAQ

Electrical glossary — common questions

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What does AHJ mean?

Authority Having Jurisdiction — the local government body or official responsible for enforcing the adopted code, typically your city or county electrical inspector. Their interpretation governs your project, which is why the correct question about any code claim is 'what does my AHJ enforce?'

What is a load calculation?

A standardized calculation of the total electrical demand of a dwelling, used to determine the required service size. It accounts for square footage, fixed appliances, heating and cooling, and any planned additions such as an EV charger. It is the evidence behind a service-upgrade recommendation.

What does 'listed' mean on an electrical product?

That a recognized testing laboratory has evaluated the product against a safety standard and certified it for a specific use. Installing unlisted equipment, or listed equipment outside its intended use, is a compliance and safety problem — and counterfeit breakers are a documented issue in online resale channels.

What's the difference between a breaker and a fuse?

Both are overcurrent protection. A fuse contains an element that melts and must be replaced; a breaker is a resettable mechanical switch. Fuse panels are still found in older homes; the presence of one is not automatically unsafe, but it should be evaluated by a licensed electrician, especially if fuses have been replaced with higher ratings than the wire allows.