Power Circuits: Strategies by Building Type

Circuiting decides which outlets, lights and equipment share a breaker, how many circuits each panel needs, and how panels are fed. It works in two directions:

• branch circuits group endpoints back to a nearby panel
• feeders carry power from that panel back to the main panel

Note: This overview is based on US practice — code references follow the model 2023 NEC. The 2026 NEC moved most load-calculation rules from Article 220 to new Article 120 with similar content; confirm the edition your jurisdiction has adopted. Wire sizes assume copper conductors with 75°C terminations.

1. The circuit hierarchy

Feeders run from panel to panel; branch circuits run from the last panel to the endpoints. Each feeder or branch circuit starts at a breaker sized to protect its own wire.

2. Core circuiting rules

Every circuit is sized so its wire, breaker and devices match, its continuous load stays under 80% of the breaker, and its voltage drop stays small.

Breaker, wire and device sizes (copper, 75°C terminations)

Sizing rules

• Continuous loads (3 hours or more): size the breaker and wire for 125% of the continuous load plus 100% of the non-continuous load. In practice a 20 A circuit carries no more than 16 A (1,920 VA at 120 V) of continuous load such as commercial lighting.
• Equipment share of a circuit: on a 15 or 20 A circuit, one cord-and-plug item may draw no more than 80% of the circuit rating, and fastened-in-place equipment no more than 50% where the circuit also serves lights or other receptacles (NEC 210.23).
• Receptacle loads (non-dwelling): count each single or duplex receptacle yoke at 180 VA. A 20 A, 120 V circuit (2,400 VA) therefore holds up to 13 receptacles by calculation; designers typically use 6–10.
• Voltage drop: the NEC recommends, but does not require, no more than 3% on a branch circuit or feeder and 5% total. Long runs often need wire one or two sizes larger than ampacity alone requires.
• Multiwire branch circuits: two or three circuits may share a neutral if their breakers are handle-tied or common-trip, so all are switched off together.
• Neutral at each circuit: the neutral and equipment grounding conductor run with their own circuit's conductors.

Load calculation basis (2023 NEC Article 220)

Non-dwelling receptacle loads take a demand factor: the first 10 kVA at 100% and the rest at 50%. Lighting values for offices, schools, retail and warehouses are from the 2026 NEC table, which has already factored in 125% for continuous load (Open Exam Prep); energy-code lighting power limits are usually lower than these.

3. Residential unit circuiting

The NEC requires dedicated 20 A circuits for four areas of a home: kitchen countertops, bathrooms, laundry and garages. Everything else is designed by load, with practical limits on how many outlets share a circuit.

3.1 Kitchens

Kitchens need the most circuits in the house: at least two for countertop receptacles plus one for each major appliance.

Countertop receptacles must be fed from at least two circuits so two high-wattage appliances (a kettle and a toaster, say) can run at once. Alternating circuits between adjacent countertop receptacles spreads the load.

3.2 Bathrooms

All bathroom receptacles need GFCI protection. Bathroom circuits are not required to have AFCI protection.

3.3 Bedrooms and living areas

Bedrooms have no dedicated-circuit requirement; they are circuited by load, with typical practice of 8–10 outlets per 15 A circuit or 10–12 per 20 A circuit (outlets include lights).

• AFCI: every 15 and 20 A, 120 V circuit serving bedrooms, living rooms, family and dining rooms, closets, hallways and similar rooms needs AFCI protection.
• Separate lighting and receptacles: many designers put each bedroom's receptacles on one circuit and group lighting from several rooms on another, so a tripped receptacle circuit does not leave the room dark.
• Dedicated loads: window or wall air conditioners, space heaters, home offices with heavy equipment, and media walls warrant their own circuits.
• Smoke alarms: hardwired, interconnected alarms with battery backup, usually on a regularly used lighting circuit so a tripped breaker is noticed quickly.

3.4 Laundry rooms

3.5 Other required and recommended circuits

3.6 Example panel schedule

A typical all-electric three-bedroom, two-bath home on a 200 A, 120/240 V service. Spaces are numbered in sequence for clarity (real panels number odd on the left and even on the right); a two-pole breaker takes two spaces. Ratings are illustrative and must come from the load calculation and nameplates.

GFCI on 240 V circuits reflects the 2023 NEC's coverage of 125–250 V receptacles in kitchens, laundry areas, garages and outdoors; hardwired equipment and local amendments vary, so confirm with the adopted code.

Circuit lists for kitchens, bathrooms, laundry and garages, and AFCI and GFCI locations: Open Exam Prep.

4. Circuiting endpoints to subpanels

Branch circuits connect endpoints (receptacles, lights, equipment) back to the nearest panel. Good circuiting groups outlets so that one tripped breaker affects as little as possible, loads stay under 80% of the breaker, and wiring runs stay short.

Grouping strategies

Typical outlets per circuit (practice, not code limits)

Homeruns and routing

• A homerun is the run from the last box of a circuit back to the panel. Plans show it as an arrow labeled with the panel and circuit number (for example, "LP-2A-14").
• Loop outlets in the order they sit along walls or rows, and bring the homerun from the box closest to the panel.
• Combine up to three circuits in one homerun conduit with a shared neutral (a multiwire branch circuit) only with a handle-tied breaker; otherwise give each circuit its own neutral.
• More than three current-carrying conductors in one raceway or cable bundle triggers ampacity derating, which often forces a larger wire size.
• Keep normal, emergency and low-voltage wiring in separate raceways; emergency (life-safety) circuits must be kept entirely separate from normal wiring.
• Choose panel locations so most branch circuits stay under about 75–100 ft; beyond that, upsize the wire for voltage drop or add a panel closer to the loads.

5. Feeders from remote subpanels to the main panel

A feeder carries power from the main panel (or switchboard) to a subpanel. It is sized from a load calculation of everything the subpanel serves, protected by a breaker at the main panel, and always carries its own equipment grounding conductor.

Sizing and protection

• Ampacity: at least 100% of the non-continuous load plus 125% of the continuous load (NEC 215.2), after demand factors from the load calculation.
• Overcurrent protection: a breaker in the upstream panel sized to protect the feeder conductors; the subpanel's bus rating must equal or exceed it.
• Voltage drop: aim for 2–3% on the feeder so feeder and branch together stay under 5% (recommended, not required).
• Spare capacity: size feeders and subpanels with 20–25% spare capacity and spare breaker spaces for future loads.
• Ground-fault protection: feeder disconnects of 1,000 A or more on 480Y/277 V systems need equipment ground-fault protection (NEC 215.10).

Typical feeder sizes (copper, 75°C, before voltage-drop upsizing)

Grounding and neutral separation

• The neutral is bonded to ground in one place only: the service (main) disconnect, or the source of a separately derived system such as a transformer.
• At every subpanel, the neutral bar is isolated from the enclosure and the equipment grounding conductors land on a separate bar bonded to the enclosure. A four-wire feeder (two hots, neutral and ground; or three hots, neutral and ground) is required.
• Each step-down transformer is a new system: bond neutral to ground at the transformer or its first disconnect, and connect it to a grounding electrode.

Remote and separate-building subpanels

• A separate building (detached garage, ADU, pool house, portable classroom) generally gets one feeder, a disconnect at or near where the feeder enters, its own grounding electrode, and a four-wire feeder with the neutral not bonded at the building (NEC 225, 250.32).
• Underground feeders need burial depths and protection per NEC 300.5.
• Long feeders are often upsized for voltage drop; a 100 A feeder over about 150 ft frequently moves up one or two wire sizes.

Feeder sizing, grounding and separate-building rules: Open Exam Prep.

6. Commercial and retail circuiting

Commercial buildings separate circuits by voltage and function: lighting and large mechanical loads at 480Y/277 V, receptacles and small equipment at 208Y/120 V through step-down transformers, and each tenant fed from its own panel and meter.

Panel strategy. A typical retail tenant has one 208Y/120 V panel for receptacles and equipment and, in larger spaces, a 480Y/277 V panel for lighting and HVAC. Each panel is fed from the building's metered distribution section, and the panel schedule labels every circuit by area and load.

7. Office circuiting

Offices circuit by floor and zone: each floor's electrical room holds a 277 V lighting panel and one or more 208Y/120 V receptacle panels fed through a step-down transformer, all fed from a riser.

Panel strategy. Size each floor's receptacle panel for 20–25% spare capacity, since tenant fit-outs rarely match the base design. Many owners specify separate panels or meters for lighting, receptacles and HVAC so energy use can be tracked by end use, as energy codes increasingly require.

8. School circuiting

Schools circuit by wing and by space type. Each wing has lighting and receptacle panels; labs, shops, kitchens and gyms get their own panels or dedicated circuits with safety shutoffs.

Panel strategy. Locate panels by wing so classroom circuits stay short, and size feeders for growth in device charging and air conditioning, the two loads most often added to existing schools.

Sources

• Required branch circuits and GFCI/AFCI protection, Open Exam Prep (NEC 210.11, 210.12, 210.8, 210.52)
• Feeders and outside branch circuits, Articles 215 and 225, Open Exam Prep
• Commercial lighting and receptacle load, Open Exam Prep (2026 NEC Article 120)

Wire and breaker pairings, outlets per circuit, typical feeder sizes and building-type practice are from general industry knowledge, not these sources. Size actual circuits and feeders from a load calculation under the adopted code, and have a licensed engineer or electrician confirm them.