The Service and the Panel

The service: from the utility to the panel

The “service” is the set of wires and equipment that brings electricity from the utility to a building’s wiring. With overhead service, the service drop runs from the utility’s last pole to the house. With underground service, the wires come up from a buried line instead. Either way, they reach the meter socket, which holds the utility’s meter, and continue to the service equipment: usually a main breaker, or switches and fuses in older homes.

The meter counts kilowatt-hours. Many newer meters send your usage to the utility wirelessly. The utility is generally responsible for the equipment from the service connection at your house back to its distribution lines.

Inside the main panel

A panel (the code calls it a panelboard) is a metal cabinet with a cover. Behind the cover are live parts, energized metal you must never touch:

• The main breaker can switch off and protect everything at once. It’s sized for the home’s largest expected load.
• Two hot bus bars are heavy metal strips, one for each 120-volt leg of the service. Breakers clip onto them.
• Branch-circuit breakers each feed and protect one circuit. A single breaker connects one leg for 120 volts. A double-width breaker connects both legs for 240 volts, which is why a 240-volt circuit takes two breaker spaces.
• The neutral bar is where the white neutral wires of the circuits end.
• The ground bar collects the bare or green equipment ground wires. At the service, the neutral and the equipment grounds are tied to a grounding electrode, such as a rod in the earth, through the grounding electrode conductor (Lesson 5).

The card inside the panel door should say which breaker feeds which rooms. If yours is blank, Being an Adult 101 shows how to label it.

How a breaker knows when to trip

A circuit breaker is a switch that opens itself when too much current flows. Most home breakers have two separate trip mechanisms, because there are two kinds of trouble.

Overloads trip the thermal element. Current through a bimetal strip, two metals bonded together that expand by different amounts, heats it, and the strip bends. Bend it far enough and it releases a latch, and a spring snaps the contacts apart. Heating depends on current squared multiplied by time, so a small overload takes a long time to trip and a big one is quick. The safety standard for breakers (UL 489, as described by CPSC staff) requires 15- and 20-amp breakers to trip within 2 minutes at 200 percent of their rating and within 1 hour at 135 percent. That delay is on purpose. Wires heat up gradually too, so a short, small overload does them no harm, and the breaker doesn’t trip at the first brief surge.

Short circuits trip the magnetic element. A short is a low-resistance path, such as a hot wire touching a neutral or ground, and the current can jump to hundreds of amps in an instant. The current flows through a small electromagnet in the breaker. At about 8 to 12 times the rating, the magnet yanks the latch open right away, with no waiting for anything to heat up.

Either way, the handle ends up at OFF or stuck partway between ON and OFF. To reset it, you push it fully off, then on (Being an Adult 101 shows how). A breaker that trips again right away is telling you there’s a fault.

Fuses

Older homes may have fuses instead of breakers. A fuse contains a metal strip with a low melting point. Too much current heats it until it melts and opens the circuit for good, so it must be replaced, not reset.

The rule that matters most: replace a fuse only with one of the same rating. People sometimes swap a fuse that keeps blowing for a bigger one. That lets too much current flow and can overheat the wiring and start a fire. Never put anything other than a fuse in a fuse holder. CPSC suggests that, unless you are sure a circuit is a 20-amp circuit, a burned-out fuse be replaced with a 15-amp one, and that special non-interchangeable (Type S) fuse sockets be fitted so the wrong size won’t fit.

Service size

Panels and services are rated in amps. Berkeley Lab says the common sizes installed today are 100 and 200 amps, and older “legacy” sizes include 60 and 80 amps. In a 2026 Berkeley Lab study, 100-amp and 200-amp panels made up 55 and 29 percent of the panels in U.S. single-family homes. The University of Missouri Extension says homes heated with fuel need at least 100-amp service, and homes with electric heat need 150 to 200 amps.

Adding big loads, such as an electric-car charger or a heat pump, can push a home toward its limit. A full service upgrade is one answer. Others include load-management devices that switch off certain loads if a set limit is reached, which the 2023 edition of the National Electrical Code allows, sharing a circuit between two appliances, or choosing efficient appliances. An electrician does a load calculation to decide.

Subpanels

A subpanel is a smaller panel fed from the main panel by a heavy circuit called a feeder. It brings breakers closer to where the circuits are needed, such as a garage, a workshop or an addition. Adding one is a common alternative when the main panel is out of breaker spaces. Berkeley Lab found that at least 20 percent of panels of all sizes have no breaker space left.

Check yourself

Tap an answer to check it, or open “Show the answer.”

1. What does the thermal (bimetal) part of a breaker respond to?

   • Short circuits only
   • Overloads, with a delay that depends on how big the overload is
   • Ground faults of 5 milliamps
   • Lightning
   Show the answer

   Overloads, with a delay that depends on how big the overload is. The bimetal strip heats with current squared multiplied by time, so a small overload trips slowly and a big one quickly.

2. Why does a dead short trip a breaker almost instantly?

   • The bimetal melts
   • The huge current pulls the magnetic trip open
   • The neutral bar disconnects
   • The meter shuts off
   Show the answer

   The huge current pulls the magnetic trip open. A short circuit can draw 8 to 12 times the rating or more, which trips the electromagnet at once.

3. Under UL 489, as CPSC describes it, a 20-amp breaker carrying 40 amps must trip within…

   • 1 second
   • 2 minutes
   • 1 hour
   • It never has to trip
   Show the answer

   2 minutes. 40 amps is 200 percent of 20 amps, and the standard requires a trip within 2 minutes at 200 percent.

4. A fuse keeps blowing. What’s the right response?

   • Put in a bigger fuse
   • Put a coin behind it
   • Find out why: the circuit is overloaded or has a fault
   • Tape it
   Show the answer

   Find out why: the circuit is overloaded or has a fault. A bigger fuse lets the wires overheat. Unplug some loads, and if it still blows, call an electrician.

5. How many breaker spaces does a 240-volt circuit usually take?

   • One
   • Two, one on each hot leg
   • Three
   • None; it bypasses the panel
   Show the answer

   Two, one on each hot leg. A 240-volt circuit connects across both 120-volt legs, so it uses a double-width breaker.

Go deeper

• U.S. Consumer Product Safety Commission staff (Lee, Trotta and King): New Technology for Preventing Residential Electrical Fires, Arc-Fault Circuit Interrupters CPSC staff paper: how thermal-magnetic breakers and AFCIs work, with the UL trip limits.
• OpenStax College Physics 2e, 20.6 Electric Hazards and the Human Body Thermal hazards, fuses and circuit breakers.
• Lawrence Berkeley National Laboratory: Sizing Electric Service Panels and Utility Infrastructure for Residential Electrification (technical brief, July 2024) How electricians and utilities size services and panels.
• U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518) CPSC’s guide to warning signs in home wiring.
• Being an Adult 101: Reset a Tripped Circuit Breaker Reset a breaker and label your panel.

Sources

• OSHA 29 CFR 1910.399: Definitions (Electrical)
• OSHA 29 CFR 1910.304: Wiring Design and Protection
• Lawrence Berkeley National Laboratory: Sizing Electric Service Panels and Utility Infrastructure for Residential Electrification (technical brief, July 2024)
• Lawrence Berkeley National Laboratory: Characterizing Electrical Panel Capacity, Breaker Space, and Loads in U.S. Single-Family Homes (2026)
• U.S. Energy Information Administration: Electricity Explained, Measuring Electricity
• University of Hawaiʻi Maui College, Building Maintenance & Construction (open textbook), 5.3 Electrical Terminology
• University of Missouri Extension G1409: Is Your Wiring System Safe and Energy Efficient?
• OpenStax College Physics 2e, 20.6 Electric Hazards and the Human Body
• U.S. Consumer Product Safety Commission staff (Lee, Trotta and King): New Technology for Preventing Residential Electrical Fires, Arc-Fault Circuit Interrupters
• U.S. Consumer Product Safety Commission: CPSC Guide to Home Wiring Hazards (Publication 518)
• OSHA 3075: Controlling Electrical Hazards (2002, revised)
• U.S. Consumer Product Safety Commission: Home Electrical Safety Checklist (Publication 513)
• U.S. Fire Administration: Fire Extinguishers