What Are the Electrical Panel Requirements for an ADU in California?

Electrical Load Calculations

San Diego requires electrical load calculations for most accessory dwelling unit (ADU) projects to confirm the electrical service panel can safely handle all appliances, lighting, HVAC, and electronics. These load requirements must be submitted with permit applications and reviewed during permitting and inspections to comply with building codes and the NEC 2020.

• City of San Diego: All non–“simple” electrical permits, including most ADU electrical requirements, require detailed load calculations per city guidelines. These must account for every electrical circuit in the ADU, from lighting and outlets to major appliances and HVAC systems.
• County of San Diego: A “Circuit Card and Load Summary” must be completed, following NEC Article 220 and local code for safe service and feeder sizing (form link). This ensures proper planning of service conductors, overhead power lines, or underground lines feeding the ADU.
• Licensed Professional Requirement: Only a California-licensed electrician or a qualified property owner may certify calculations. This step is critical for projects requiring service conduit changes, main breaker upgrades, or electric subpanel installations.
• SDG&E: For any service upgrade or utility connection, SDG&E requires a load letter and site plan from a licensed electrician. The plan must show utility meter placement, service conduit, and whether the service will use overhead lines or underground lines.

Most ADUs need 100–200 amps to operate efficiently, though smaller units may require as little as 60 amps (Better Place Design Build). Planning for future loads—such as electric vehicle chargers, smart home technology, or a solar power system—can help avoid costly electrical service upgrades later.

Include all major systems—like HVAC—when calculating your ADU’s electrical load to avoid undersized service panels.

Determining Panel Size

In San Diego and across California, most accessory dwelling units require 80–100 amps of power, but many contractors recommend installing a 125-amp sub-panel to avoid running the electrical service panel at full capacity and to allow for future upgrades.

• Most ADUs need 100–200 amps to operate efficiently, with smaller units sometimes requiring as little as 60 amps (Better Place Design Build). The amperage requirement depends on the size in square feet, appliance load, and whether the unit is all-electric.
• A 100-amp panel works for many projects, but high-demand or all-electric ADUs may require additional capacity—especially if adding electric vehicle chargers, smart meter systems, or solar power systems.
• Many builders install a 125-amp sub-panel for all-electric ADUs to provide headroom for future needs, such as more appliances or smart home technology (Reddit).
• Always consult a licensed electrician to confirm your ADU’s specific load requirements and ensure compliance with the California Electrical Code, NEC Table 310.12, and NEC 210.25. Proper electric planning helps avoid costly upgrades later and ensures safe integration with the main home’s panel.

Accurately sizing your panel from the start ensures your ADU can handle both current and future electrical loads.

Expert Recommendation

While San Diego’s building codes may allow a 100-amp electrical service panel for a standard accessory dwelling unit, most licensed electricians and ADU builders recommend installing a 125-amp sub-panel. This prevents the system from operating at full capacity, extends the panel’s life, and accommodates future additions like electric heating, air conditioning, electric vehicle chargers, or other high-demand appliances that may increase kWh consumption.

“An independent electric panel with a minimum 100-amp capacity must be installed.” – Jamar Power Systems

If an ADU is added to an existing main home’s panel, NEC Table 310.16 and conductor sizing rules may not apply when the service feeds more than one dwelling unit. In these cases, a licensed electrician should verify load requirements, capacity, and compliance with the California Electrical Code to ensure safe operation and compatibility with future upgrades such as SPAN Smart Panels, electricity management systems, or Energy Storage Systems.

Shared vs. Separate Electrical Panels

SDG&E guidelines show that a separate electric meter is standard for detached ADUs or units needing major new service.

• The SDG&E ADU Checklist directs applicants to request approval for new gas and electric services for all ADU types.
• The 2025 SDG&E Service Standards & Guide states: “An individual SDG&E service is generally required to each of the multi-unit buildings. Both residential and non-residential applications are subject to this requirement.”
• The SDG&E Builder Services page also confirms that approval of new gas and electric services is required for new construction, including ADUs.

Placement rules: If the ADU is within 150 feet of the main home, the new meter must be installed next to the main panel. If more than 150 feet away, it can be installed separately.

“When there is more than one multi-unit building on a single premises, an individual SDG&E service is generally required to each of the multi-unit buildings.” – SDG&E Service Standards & Guide

Plan wiring routes and panel placement during framing to streamline inspections and prevent costly retrofits.

Compliance with California Regulations

Local Codes and Permits

In California, any electrical service panel upgrade—whether for your main home’s panel or an accessory dwelling unit—requires a building permit. The panel is considered part of the building’s permanent electrical wiring infrastructure and directly impacts safety. Permits ensure compliance with the California Electrical Code (CEC), local amendments, and proper installation of safety devices such as Ground Fault Circuit Interrupters and Arc Fault Circuit Interrupters.

“Upgrading your electrical panel requires a permit from your local government agency, as it is considered part of the building’s infrastructure and a safety issue. Depending on where you live, this could involve submitting plans and diagrams, meeting with inspectors, paying associated fees and always working with a professional electrician.” – HomeWiP

San Diego–Specific Requirements

City of San Diego: Panel upgrades require an Electrical Permit, which may be combined with a Building Permit if part of a larger remodel or ADU project. Projects that exceed “simple” electrical work require plan review, with load requirements and electrical calculations submitted through the city’s permitting portal (City of San Diego Electrical Permits).

County of San Diego: The Building Division mandates a Circuit Card and Load Summary to document each circuit’s anticipated load, ensuring the combined demand of the main house line and ADU complies with NEC Article 220 (San Diego County PDS Form PDS-184).

SDG&E Requirements: If a separate utility meter is installed for an ADU, property owners must submit a Builder Services application to SDG&E. This includes a site plan showing service conduit and panel locations, along with load calculations prepared by a licensed electrician (SDG&E Builder Services).

Permit Process Overview in San Diego

1. Consultation with a Licensed Electrician – To determine if a panel upgrade or 200 Amp Service is needed and to prepare compliant load calculations.
2. Permit Application – Submit through the City or County portal with plans, specs, and demand data.
3. Plan Review – Jurisdiction reviews electrical load, Feeder For ADU, and panel specifications.
4. Installation & Inspection – Work must be performed by a licensed C-10 Electrical Contractor or qualified homeowner.
5. Final Approval – SDG&E connects new service after sign-off by the jurisdiction and city inspector.

Tip: Not obtaining the proper permit can delay your ADU build, result in fines, or require redoing work to meet NEC compliance.

Always verify local zoning and permit requirements early—compliance issues can delay your ADU project and increase costs.

Components of an ADU Electrical System

Circuits and Circuit Breakers

The California Electrical Code (CEC) requires that high-demand areas in accessory dwelling units (ADUs), such as kitchens and bathrooms, have dedicated circuits and properly sized breakers to ensure safety, prevent overloads, and maintain compliance. Proper planning helps ensure the main home’s panel or ADU sub-panel can handle these loads without tripping breakers.

• Kitchens: Must have two or more 20-amp small appliance branch circuits, plus dedicated circuits for major appliances like refrigerators, dishwashers, microwaves, and ranges. All countertop outlets must have Ground Fault Circuit Interrupter (GFCI) protection, and lighting circuits must be separate from general outlets (AB 2533). Using quality Copper THHN/THWN-2 electrical wire ensures safe operation under high demand.
• Bathrooms: Require a dedicated 20-amp circuit serving no other room, with at least one outlet within 3 feet of the sink. All outlets must have GFCI protection (Toupin Construction). This setup reduces risk when moisture is present and complies with NEC safety standards.

Code Reference: The CEC 210.11(C) mandates dedicated branch circuits for kitchens, bathrooms, and laundry in all dwellings, including ADUs. Circuit breakers must be sized correctly based on wire gauge and the load demand for each circuit, whether using Master metered or individually metered electrical setups.

Key Point: Correct circuit planning and breaker sizing help protect the ADU’s electrical wiring from overload and ensure compatibility with future systems like transfer switches or standby generators without costly rewiring.

Use dedicated circuits for high-demand appliances to prevent overloads and ensure your ADU passes final inspections.

Main Service Panel Upgrades

1. Standard Overhead Panel Upgrades ($1,500–$5,500) Community reports and licensed electrician estimates show that upgrading an overhead service panel in San Diego typically costs between $3,000 and $5,500, depending on service size, location, and labor rates. For example, a homeowner reported paying $3,200 for a new main home’s panel connected to SDG&E (not including permits) (Reddit), while a North County contractor lists $3,500–$5,500 for similar upgrades. Proper installation ensures compliance with grounding electrode system requirements and safe routing of electrical wire around high voltage power lines.
2. Separate Electric Meter Installation (Up to $15,000) SDG&E does not post standard pricing for individually metered ADUs, but community feedback shows that basic residential meter preparation (outside of ADU projects) can cost $1,500–$2,000 for simple installs. In these cases, the electrician handles utility connections and wiring, while SDG&E completes the hookup (Tesla Motors Club). ADU projects often cost more because they may require service conductor upgrades, panel replacement, or coordination with outbuilding feeders.
3. Underground Service Upgrades ($20,000–$40,000+) Undergrounding electrical service is usually the most expensive option, especially if trenching or work in the public right-of-way is required. Homeowners in San Diego have reported costs exceeding $30,000–$40,000 for complex installations. SDG&E’s 2025 Unit Cost Guide lists:

• Primary trench & conduit installation: $1,357/ft
• New secondary meter hardware: $9,200
• New primary meter hardware: $25,300

These projects often involve routing service conduit underground, managing water meter or sewer line clearances, and ensuring code compliance with NEC standards.

Wiring and Fixture Placement

The California Electrical Code (CEC) sets strict requirements for wiring in accessory dwelling units (ADUs) to ensure safety, manage electrical loads, and prevent hazards. These standards apply whether the ADU is connected to the main home’s panel or has its own dedicated service.

Dedicated Appliance Circuits

Major appliances—including refrigerators, microwaves, dishwashers, washing machines, and dryers—must be on dedicated circuits with their own breakers to prevent overloads. The code specifies that each fixed appliance must have its own circuit, not shared with other devices (Safeway Electric & Solar). Proper wiring methods, such as using interconnection bonding termination blocks where needed, ensure both safety and compliance.

Outlet Placement & Safety Regulations

• Outlets must be no more than 6 feet from any point along the wall in habitable rooms, and every wall over 2 feet wide must have an outlet.
• Kitchens require countertop outlets at least every 4 feet, with any countertop over 12 inches wide having at least one outlet.
• Bathrooms must have at least one outlet within 3 feet of the sink.
• All kitchen, bathroom, garage, laundry, outdoor, and unfinished basement outlets must be protected by Ground Fault Circuit Interrupters (GFCIs).
• All general living spaces require Arc Fault Circuit Interrupter (AFCI) protection, and tamper-resistant outlets are required in all new and remodeled homes.

Appropriately Sized Breakers

Breakers must match the wire gauge and expected load for each circuit. For example, laundry equipment and bathroom receptacles require dedicated 20-amp circuits, while electric dryers and ranges often require higher-rated breakers and 240V dedicated circuits (EEVblog).

General Wiring Standards

All wiring, equipment, and installation must follow the California Electrical Code, which is based on the National Electrical Code (NEC) with additional state and local amendments for improved safety and energy efficiency. This includes ensuring clearances from plumbing systems like a pump system, backflow prevention device, or septic tank, as well as coordinating with the zoning office when layouts or floor plans affect electrical placement.

Strategic wiring and outlet placement improve safety, meet code, and enhance daily functionality in your ADU.

Lighting and Power Considerations

The California Electrical Code (CEC) requires that accessory dwelling unit electrical systems be designed so the main home’s panel or ADU subpanel is never overloaded and does not operate at full capacity during normal use. Residential load calculations must verify that the electrical service can handle the additional demand; if the calculated load approaches or exceeds capacity, a panel upgrade is mandatory (City of Coachella).

Key compliance points:

• Load calculations must confirm the minimum rating of the service panel for the ADU’s added loads, as required by CEC 220.40 (City of Temple City).
• If the calculated demand exceeds the existing panel’s rating, an upgrade is required before final inspection.
• Planned electrical loads should never exceed 80% of the panel’s continuous capacity per NEC 210.20(A) and the California Electrical Code.
• High-demand appliances like HVAC systems, laundry equipment, and generator controllers require dedicated circuits, while lighting and smaller appliances use general-purpose circuits.
• Circuits must be distributed across the panel to prevent overloads, and breakers must match both wire gauge and demand.

Bottom line: Accurate load calculations and balanced circuit distribution are essential for preventing overloads, meeting code compliance, and ensuring long-term electrical safety—whether the ADU connects to the main house line or is part of a New Avenue ADU build that also incorporates solar incentive programs for energy efficiency.

Proper circuit planning and balanced power distribution prevent overloads and extend the life of your ADU’s electrical system.

Future-Proofing Your ADU

Smart Home Integration

Installing a dedicated sub-panel for your ADU makes it easier to add WiFi-enabled power monitors, sub-meters, and smart load management systems without needing to rewire the main home’s panel. This setup allows homeowners to:

• Track real-time energy usage for the ADU.
• Automate lighting, HVAC, and appliance operation for efficiency.
• Prepare for future upgrades, such as solar integration or battery storage, with minimal disruption.

Many California electricians recommend sub-panels for ADUs to simplify load tracking and ensure compliance if separate metering is later required.

Plan for future upgrades—integrating smart home technology now saves time and money while boosting your ADU’s long-term value.

EV Charger Provisions

California’s growing push for electric vehicles means pre-wiring for EV chargers during ADU construction can save thousands in retrofit costs later. The California Electrical Code allows EV charging circuits to be installed as part of the initial build, even if the charger itself isn’t added right away.

Benefits of early EV charger planning:

• Lower cost — avoids trenching or panel rewiring after construction.
• Future compliance — many California cities are adopting EV-readiness requirements for new residential builds.
• Convenience — ensures the ADU can support Level 2 charging (240V, 40–50A circuit) without panel overloading.

The California Green Building Standards Code (CALGreen) recommends EV-ready outlets in new residential projects, making pre-installation in ADUs a smart long-term move (California Energy Commission).

Hiring a Professional Electrician

Working with a licensed California electrician—preferably one experienced in San Diego ADU projects—is essential for ensuring code compliance, accurate load calculations, and smooth permitting with the City or County. ADU electrical work involves complex coordination with local building departments and SDG&E, making local experience especially valuable.

“Experienced electricians can offer guidance on when to upgrade an outdated electrical panel, as older setups typically do not support the increased demand of an all-electric ADU.” – Better Place Design Build

Key selection tips:

• Verify licensing via the California Contractors State License Board.
• Ask about past ADU projects and familiarity with San Diego’s permitting process.
• Ensure they can provide detailed load calculations and manage service upgrade applications with SDG&E.
• Request references from recent clients with similar scope.

Why it matters: Mistakes in load calculation, panel sizing, or SDG&E coordination can lead to costly delays, failed inspections, and unsafe electrical systems. A seasoned ADU electrician helps avoid these pitfalls while ensuring your project is future-proof and fully compliant.

Always work with a licensed electrician experienced in local ADU codes to ensure compliance and avoid costly project delays.