ADU Electrical and HVAC in California: Panels, Load Calculations, Meters, Alarms, Heat Pumps, Solar and Battery Readiness

An ADU does not fail on square footage — it fails on amps. Whether the existing panel can carry a second kitchen, a heat pump, and a water heater is a calculation, not an opinion, and three different California rulebooks answer three different parts of the question.

Keep the rulebooks straight, because they share initials and get confused constantly. The California Electrical Code (Title 24, Part 3, based on the National Electrical Code) decides panel size, circuits, and protection. The California Energy Code (Title 24, Part 6) decides lighting, heating and cooling efficiency, ventilation, solar, and battery readiness. The California Residential Code (Part 2.5) decides smoke and carbon monoxide alarms. Edition matters: permits submitted before January 1, 2026 are generally reviewed under the 2022 codes (2020 NEC); permits submitted on or after that date fall under the 2025 codes (2023 NEC) — and several rules in this guide changed between those editions. The permit-submission date, not the construction date, is what counts, and your building department confirms which edition your project is under. This page is the Electrical and HVAC chapter of our California Accessory Dwelling Unit Handbook, and the companion to our plumbing and utilities guide.

Can Your Panel Carry an ADU?

Existing panel capacity (Topic 226)

No code assigns an ADU a panel size. Capacity is a comparison: the calculated load of the house plus the ADU, worked under Article 220 of the Electrical Code, against the rating of the service and panel you already have. A 100-amp service serving a small all-gas house may pass; a 200-amp service loaded with electric heat, a spa, and an EV charger may not. Pitfall: treating the number stamped on the main breaker as the answer, in either direction — the calculation, not the label, is the legal test.

Load calculations (Topic 227)

Article 220 offers more than one lawful method, and choosing the right one is often the whole game. The standard method (Part III) builds the load from general lighting, small-appliance and laundry circuits, appliance nameplates, and demand factors. The optional method (section 220.82) serves many single-family projects. For adding load to an existing dwelling, section 220.83 applies its own demand pattern (the first 8 kVA at 100%, the remainder at 40%), and section 220.87 allows documented maximum-demand data from the utility to prove real headroom. On 2025-code permits, section 220.70 also recognizes a listed energy management system that caps the calculated load at its set point. The load study belongs before the bid, not after rough-in — skipping it is one of the classic homeowner mistakes in this field.

Panel upgrades (Topic 228)

Here is a verified negative worth stating plainly: no California law requires a 200-amp panel, or any panel upgrade, just because you are adding an ADU. An upgrade is required only when the Article 220 calculation says the existing service cannot carry the combined load — or when the serving utility or a local checklist imposes its own service standard, which is a local rule to confirm at intake, not a state mandate. When an upgrade does happen, it pulls in its own modern requirements, including surge protection and an emergency disconnect under the current Electrical Code, and it is the moment to think about the reserved spaces the Energy Code asks for (future 240-volt use, battery readiness) so the new panel is not full on day one. Our cost guide carries panel work as its own line item for exactly this reason.

Subpanel placement (Topic 229)

The default architecture for a detached ADU is one feeder to one subpanel. Under section 225.30, a separate building is supplied by only one feeder or branch circuit unless a listed exception applies — you do not run three separate branch circuits in one trench. The detached building needs its own disconnect at a readily accessible location nearest the point of entrance (sections 225.31–225.32), its own grounding electrode system (section 250.32), and an equipment grounding conductor run with the feeder — with neutral and ground kept separate in the subpanel; they bond at one point only, back at the service. Placement rules apply to every panel: 3 feet of clear working space in front, no storage in that space (section 110.26), and overcurrent devices may not sit in clothes closets, in dwelling bathrooms, or over stair steps (section 240.24).

Meters, Services, and the Utility

Separate electric meters (Topic 230)

State ADU law sets no separate-meter requirement for electricity. Government Code section 66311.5 limits when a local agency may require a new or separate utility connection and what it may charge — conversions and JADUs are protected from a required new connection, and charges for other ADUs must be proportionate — but HCD itself notes it has no authority over gas and electric service, which sits with the utility and, for investor-owned utilities, the CPUC. Whether a second meter is available, required by the utility’s tariff, or worth its cost is therefore a utility question answered for your address, not a code question. If house and ADU share a meter and a tenant is billed for energy, Civil Code section 1940.9 requires written disclosure when a tenant’s meter also serves areas outside their unit. Decide the metering strategy at design: it drives the panel layout, the trenching, and the lease.

Utility interconnection (Topic 231)

Service work has two masters. Everything on the utility’s side of the service point follows the utility’s service standards (and CPUC General Orders for overhead construction); everything on your side follows the Electrical Code. A service upgrade, a relocated drop, or a new meter position is scheduled by the utility on the utility’s timeline — there is no statewide energization deadline we can quote you, and any project plan that assumes one is guessing. Solar and battery systems add a second interconnection: the export system interconnects under the utility’s Rule 21 process, which is separate from the building permit and should be filed early, in parallel, not after final inspection.

Overhead service clearance (Topic 232)

Two regimes split at the service point. The utility’s drop is built to CPUC General Order 95; the conductors you own are governed by Electrical Code section 230.24, whose familiar figures include 10 feet over pedestrian areas, 12 feet over residential driveways, and 18 feet over public streets, with a separate clearance rule for conductors passing over a roof — read the current section 230.24 text for your roof’s slope and voltage before you place a detached ADU under an existing drop. Pitfall: siting the ADU first and discovering the drop crosses the new roof at an illegal height; moving a service drop is utility-scheduled work.

Underground feeders (Topic 233)

Burial depth in the Electrical Code (Table 300.5) is stated as minimum cover — the earth above the cable or conduit — not trench depth; the trench itself is deeper by the conduit’s diameter and bedding. Cover requirements vary by wiring method and location (under a driveway is not the same as under a lawn), direct-burial cable needs more cover than conduit, and circuits can earn shallower cover with GFCI protection and limited ampacity in the residential column of the table. The utility’s lateral segment follows the utility’s own trenching standard, which is the controlling document for that piece. Photograph and dimension every trench before backfill — the as-built route is what the next project digs around.

Protection: AFCI and GFCI

AFCI protection (Topic 234)

Arc-fault circuit-interrupter protection (section 210.12) is required for 120-volt, 15- and 20-ampere branch circuits supplying outlets or devices in the dwelling areas the section lists — kitchens, family and living rooms, bedrooms, laundry areas, and the other named spaces. The list is the same in the 2020 and 2023 editions; what changed in 2023 is the section’s internal organization, so the citation on your plans should match your permit’s edition. In practice, most ADU circuits land on AFCI or dual-function breakers, which is one more reason the panel schedule is drawn at design, not bought at rough-in.

GFCI protection (Topic 235)

Ground-fault protection (section 210.8) guards the wet and work areas: bathrooms, kitchens, garages, laundry areas, outdoors, and the other listed locations. This topic carries the batch’s biggest edition split: on 2020-code permits the kitchen rule centered on countertop receptacles, while the 2023 code (section 210.8(A)(6)) reaches all kitchen receptacles, and section 210.8(D) adds a list of specific appliances — including ranges, wall ovens, counter-mounted cooking units, dryers, and microwaves. Which list applies to your ADU is decided by your permit’s edition, full stop. Pitfall: roughing a 2026 kitchen to the 2020 rulebook because a how-to page had not caught up.

The Kitchen, the Car, and the Range

Kitchen circuit planning (Topic 236)

Every dwelling kitchen — including a small ADU kitchen — gets at least two 20-ampere small-appliance branch circuits (section 210.11(C)(1)) serving the countertop and receptacle outlets, and those circuits serve no other outlets beyond the kitchen exceptions the code allows. Laundry gets its own 20-ampere circuit and bathrooms get theirs (section 210.11(C)(2)–(3)). There is no reduced “kitchenette” tier in the Electrical Code, and no code rule requiring a dedicated refrigerator circuit by name — the small-appliance circuits may serve it, and a separate circuit is a design choice. Island rules also split by edition: the 2023 code made island receptacles optional but, if none is installed, requires a provision for a future receptacle, and it prohibits the old below-counter side-mount position.

EV charging coordination (Topic 237)

Three separate layers get quoted online as one “EV rule,” so take them separately. Load: an EV charger is usually the largest discretionary load in the project, counted at full value — on 2025-code permits, section 220.57 takes the load at 7,200 volt-amperes or the equipment nameplate, whichever is larger, on its own branch circuit as a continuous load. Listed load management (section 625.42) or an energy management system is the lawful alternative to buying a service upgrade for a worst case. Building standard: CALGreen (Title 24, Part 11, section 4.106.4) makes new one- and two-family dwellings EV Capable — raceway, panel capacity, and a reserved, labeled space — but its text excepts ADUs and JADUs without additional parking facilities, which describes most ADUs; where an ADU does add parking, or a panel is being replaced anyway, confirm the current CALGreen requirements with the building department. Energy Code: verified negative — the 2025 Energy Code and its ADU FAQs impose no EV requirement on ADUs at all. No layer requires an ADU to have a charger.

Electric cooking (Topic 238)

Two myths die here. First, the Electrical Code sets no minimum ampere rating for a range circuit: the circuit follows the appliance nameplate and manufacturer instructions, while the famous 8 kW figure is a demand value Table 220.55 uses when sizing the service — reading it as the circuit size inverts the table. Second, California has not banned gas cooking in ADUs. What the Energy Code does require, in a newly constructed building that installs a gas cooktop or range, is electric readiness under section 150.0(u): a dedicated 240-volt, 50-ampere-minimum circuit within 3 feet, a blank cover labeled “240V Ready,” and a reserved double-pole breaker space marked “For Future 240V Use.” Attached ADUs and conversions are additions, and the electric-ready requirements do not apply to additions. Some cities go further by local reach code — that is checked per city, at permit. And for a JADU, HCD is explicit: 220-volt circuits are not required for an efficiency kitchen, and a city may not use the efficiency-kitchen definition to dictate fuel or appliance types.

Alarms: The Life-Safety Pair

Electric water heating — the circuit side (Topic 239)

The water-heater types and Energy Code options live in our plumbing guide; the electrical half is this: a storage water heater is a continuous load, and its branch-circuit conductors and breaker are sized at not less than 125% of the nameplate rating (section 422.13) — a sizing step that happens once, at the circuit, and is not applied a second time inside the Article 220 service calculation. If a new detached ADU installs a gas water heater, section 150.0(n) electric-ready duties attach (a circuit and a reserved heat-pump space within reach of the appliance). Tankless electric units are nameplate-driven, multi-circuit appliances that can dominate a load calculation — there is no publishable amp table for them, only the nameplate of the unit you actually choose.

Smoke alarms (Topic 240)

California Residential Code section R314.3 places smoke alarms in each bedroom, outside each separate sleeping area, and on every story including basements and habitable attics. In new construction they are hardwired with battery backup and interconnected, so one alarm sounds them all (section R314.4 pattern). Battery-powered alarms sold in California have carried 10-year sealed batteries since 2015 under the State Fire Marshal framework (Health and Safety Code sections 13113.7–13113.8 and 13114, with the listing rules in Title 19). Verified negative: California sets no statewide rule choosing ionization versus photoelectric sensors for an ADU — buy listed alarms, place them per section R314, and follow the manufacturer’s instructions for kitchen-adjacent locations.

Carbon monoxide alarms (Topic 241)

CO alarms are conditional, and the condition is the point. Under Health and Safety Code section 17926, the duty attaches to dwellings with a fuel-burning appliance, a fireplace, or an attached garage. Where the trigger exists, Residential Code section R315.3 places alarms outside each separate sleeping area, on every story, and in a bedroom that itself contains a fuel-burning appliance; section 17926.1 puts maintenance duties on the owner and, in rentals, on the landlord. An all-electric detached ADU with no attached garage and no fuel appliance is not triggered — if the house has gas appliances or an attached garage, its own alarms are a house question, handled on the house.

Lighting and Appliance Circuits

Lighting efficiency (Topic 242)

Energy Code section 150.0(k) applies to ADUs with no ADU exception, per the CEC’s own ADU FAQs: installed luminaires must be high-efficacy, which in practice means products certified under Joint Appendix JA8 (listed in the CEC’s certified directories) rather than a wattage guess at the supply house. Controls are part of compliance — the right switches, dimmers, and vacancy sensing where the section requires them — and they are verified, not assumed, on the compliance forms.

Recessed lighting (Topic 243)

Recessed luminaires in an insulated ceiling must be listed for insulation contact and certified airtight (tested under ASTM E283 at less than 2.0 cfm at 75 Pascals), sealed to the ceiling with a gasket or caulk, fitted with JA8 light sources, and built so the light source and driver can be serviced from below without disturbing the ceiling — and section 150.0(k) bars screw-base sockets in this application. Pitfall: a can that is IC-rated but not airtight passes the framing look and fails the energy inspection; the two listings are different lines on the box.

Dedicated appliance circuits (Topic 244)

Only section 210.11(C) names mandatory dedicated circuits as a list: the two kitchen small-appliance circuits, the laundry circuit, the bathroom circuit, and the garage circuit. Everything else follows from the appliance itself — nameplate rating, the rule that a fastened-in-place appliance may not exceed 50% of a branch circuit that also serves lighting or receptacles (section 210.23(A)(2)), and the manufacturer’s instructions, which the code adopts (section 110.3(B)). “Code requires a dedicated circuit for X” is true for a shorter list than the internet claims — and the panel schedule is where the real list for your ADU gets written.

Heating, Cooling, and Air

Ductless mini-splits (Topic 245)

Mini-splits are expressly allowed for ADUs in the CEC’s guidance, and they fit the ADU pattern: no ducts, zoned by room, heat pump heating and cooling in one system. Two limits matter. The ADU gets its own system — the house’s system may not simply be extended to serve the ADU under the Mechanical Code framework the CEC applies. And verification still applies: refrigerant charge is verified in all climate zones, while duct-leakage testing is scoped to ducted systems — a truly ductless install has no ducts to test, which is a scoping fact, not an exemption to claim on a ducted design.

Heat pump sizing (Topic 246)

Sizing is a calculation with a paper trail. Section 150.0(h) requires the heating and cooling loads to be determined under ASHRAE, SMACNA, or ACCA Manual J methods, at the code’s indoor design temperatures (68°F heating, 75°F cooling), using the design temperatures in Joint Appendix JA2 for your locality — then equipment is selected to meet those loads. Note what the section does not name: Manual S percentage bands and every square-feet-per-ton rule of thumb are sizing folklore, not California code, and a system sized by folklore is the one that short-cycles or cannot keep up in a heat wave. Federal minimums set the efficiency floor (split-system heat pumps at 14.3 SEER2 and 7.5 HSPF2 under the current DOE standards), and the 2025 Energy Code’s prescriptive path makes the heat pump the baseline system in all 16 climate zones for a newly constructed detached ADU (section 150.1(c)6) — conversions and attached ADUs follow the addition rules in section 150.2(a), covered in our plumbing guide’s energy discussion.

Ventilation standards (Topic 247)

Every ADU needs designed whole-dwelling ventilation under section 150.0(o), which adopts ASHRAE Standard 62.2-2022. The required rate is Equation 150.0-B: 0.03 cfm per square foot of floor area plus 7.5 cfm per person, counting bedrooms plus one — a 600-square-foot one-bedroom ADU therefore needs about 33 cfm of continuous-equivalent ventilation before the system’s own adjustment factors. Openable windows do not substitute for the mechanical system, and neither does running a central fan on a schedule the code does not credit. Local exhaust is a separate, additional requirement, next topic.

Bath exhaust fans (Topic 248)

A bathroom with a tub or shower needs mechanical exhaust ducted to the outdoors: 50 cfm intermittent or 20 cfm continuous under the 62.2 framework, with an ENERGY STAR fan and a humidity control the occupant can reach and use (Residential Code and section 150.0(o) framework, with CALGreen section 4.506.1). Kitchen exhaust is the bigger sibling — 100 cfm intermittent or its continuous equivalent. A fan that vents into the attic, a soffit cavity, or nowhere is a failed inspection and a moisture problem poured into the framing; the duct run is part of the design, not a field improvisation.

Solar and Batteries

Solar PV requirements (Topic 249)

Classification decides everything, and the CEC’s ADU FAQs draw the lines in words:

Where PV is required, the system is sized as the smaller of two methods: the solar access roof area (SARA) capacity — SARA times 18 watts per square foot on steep roofs, 14 on low-slope roofs — or the Equation 150.1-C result for your climate zone. An existing array on the house does not count toward the ADU’s requirement, but new modules may be placed anywhere on the lot that satisfies the rules. Five exceptions can reduce or eliminate the requirement, including a SARA under 80 contiguous square feet and a calculated system under 1.8 kWdc; a qualifying JA12 battery (7.5 kWh or larger) earns a 25% reduction in the PV size — a reduction, not an exemption. There is no separate section 110.10 solar-ready duty for ADUs.

Battery storage compatibility (Topic 250)

Lead with the verified negative, because it corrects the most common claim in this space: batteries are not required on a new ADU. What section 150.0(s) requires, where it applies to a newly constructed building, is battery readiness — the electrical provisions (a dedicated circuit path, reserved panel capacity and space) that let a battery be added later without reworking the service. An actual energy storage system is a choice, and installing one brings its own rulebook: 2025 Residential Code section R330 (renumbered from R328), UL 9540 listing, and location and capacity limits — 20 kWh per unit, with the corroborated aggregate caps of 40 kWh inside the dwelling and 80 kWh in an attached garage; confirm the full location table in the current code before placing a system anywhere else. Size the readiness into the panel at design (Topic 228) and the future battery becomes an appliance install instead of a second panel project.

The Order We Run These Checks

1. Load study first: Article 220 calculation for house + ADU on the permit edition that will actually govern, before design is priced.
2. Metering and service strategy: utility conversation on meters, service size, and drop or lateral route — on the utility’s timeline, not yours.
3. Panel schedule as one document: ADU circuits, AFCI/GFCI protection, EV and electric-ready reserved spaces, and battery readiness drawn together.
4. Energy forms matched to classification: detached-new versus attached versus conversion decides PV, electric-ready, and the prescriptive HVAC path.
5. Alarms and ventilation on the plan, not at final: R314/R315 locations, 62.2 rate math, and exhaust duct routes.

Run in that order and the electrical scope stops being the change order. When you want the load study run on your own property, our ADU Feasibility Report covers utility capacity before design — or start with a free consultation and bring us the address.

Sources (verified November 2026): California Electrical Code (Title 24, Part 3; 2022 edition based on the 2020 NEC and 2025 edition based on the 2023 NEC) Articles 110, 210, 215, 220, 225, 230, 240, 250, 300, 408, 422, and 625; California Energy Code (Title 24, Part 6, 2025) sections 100.1, 110.10, 150.0(h), 150.0(k), 150.0(n), 150.0(o), 150.0(s), 150.0(u), 150.1(c)6, 150.1(c)8, 150.1(c)14, and 150.2(a), with the CEC’s 2025 Energy Code ADU FAQs and Single-Family Solar PV FAQs; ASHRAE Standard 62.2-2022 (adopted by section 150.0(o)), Equation 150.0-B; California Residential Code sections R314, R315, and R330; Health and Safety Code sections 17926, 17926.1, and 13113.7–13113.8 and 13114; Civil Code section 1940.9; Government Code section 66311.5; CALGreen (Title 24, Part 11) section 4.106.4 (2022 text, as noted in the EV section); CPUC General Order 95 (utility-side overhead construction); HCD Accessory Dwelling Unit Handbook, March 2026 edition (utility-authority and JADU efficiency-kitchen passages). Where this guide describes design or installation practice, it is labeled as practice in the text; utility-specific requirements are confirmed with the serving utility, and edition-specific figures are confirmed against the code edition governing the permit. This page is part of the California Accessory Dwelling Unit Handbook on this site (Electrical and HVAC, Questions 451–500). Related: Plumbing & Utilities · ADU FAQs · ADU Permit Process · Property Eligibility · ADU Feasibility Report