ADU Electrical Requirements in Lake Arrowhead: What Your Panel and Subpanel Actually Need

Quick answer: Most California ADUs need a dedicated 100-125 amp subpanel fed from the main house's electrical service, and that main panel often needs to be upgraded to 200 amps to safely support both the primary residence and the new unit combined. California's Title 24 energy code also requires new ADUs to be "EV-ready," meaning conduit must be run from the subpanel to a future charging location even if you're not installing a charger right away. For Lake Arrowhead properties specifically, propane versus Southwest Gas availability adds another layer most standard ADU guides don't address.



Does Your Main Panel Need to Be Upgraded to Support an ADU?

Often, yes — and this is worth checking before you get far into ADU planning. A standard 100-amp main panel rarely has enough spare capacity to support both a full residence and a fully equipped ADU at the same time, which is why most ADU projects on properties with an older panel end up requiring an upgrade to 200 amps as part of the process. This is exactly the situation our panel upgrade guide covers in detail — a formal load calculation determines whether your existing panel can handle the combined demand or whether an upgrade needs to happen first.

What Size Subpanel Does a Lake Arrowhead ADU Actually Need?

Most ADUs are built around a dedicated 100 to 125-amp subpanel, with 125 amps being the more commonly recommended size since it leaves headroom for future additions like an EV charger or electric heat pump without requiring another upgrade down the line. The exact size ultimately depends on a proper load calculation covering your ADU's HVAC, kitchen appliances, and lighting circuits — smaller studio units may need less, but 125 amps has become the practical default recommendation across most California jurisdictions.

Why Are Most New ADUs Going All-Electric — and What Does That Mean If You're on Propane?

This is a genuinely relevant wrinkle for Lake Arrowhead specifically. Many new ADUs statewide are being designed all-electric — electric cooking, electric heat pump water heating, and electric space conditioning — largely because extending new gas service to a detached unit is expensive and energy code compliance is often simpler without it. For Lake Arrowhead properties, where Southwest Gas serves some areas while more remote properties rely on propane, going all-electric for your ADU sidesteps the fuel-availability question entirely — worth factoring into your planning conversation, especially if your main house already relies on propane delivery.

What Do Title 24's "EV-Ready" and "Solar-Ready" Requirements Mean for Your Wiring?

California's Title 24 energy code requires new ADUs to be electric-ready for EV charging, meaning conduit must run from the subpanel to a designated future charging location even if you're not installing a charger during initial construction — a requirement worth planning around now rather than trenching again later. Detached, newly constructed ADUs also generally need to be solar-battery-ready, typically requiring a minimum 60-amp breaker feeding an emergency subpanel, though attached units, garage conversions, and manufactured dwellings are often exempt from the solar requirement specifically. If you're planning to add an EV charger to your ADU setup, our EV charging guide and SCE Charge Ready Home rebate guide are both worth reading alongside this.

Shared Meter or Separate Submeter: Which Makes Sense for Your Property?

Many ADUs share a single utility meter with the main house, which keeps things simpler but means all electricity use gets billed together. A separate submeter lets you track ADU energy consumption independently — a genuinely useful option if you're planning to rent the unit out and want a clean way to separate utility costs, or if you simply want visibility into how much power the new unit is actually using. This is worth deciding early, since it affects how the electrical work is scoped from the start.

What Does San Bernardino County's ADU Electrical Permit Process Involve?

ADU electrical work requires a San Bernardino County electrical permit, generally reviewed alongside the broader building permit for the unit itself. Expect load calculations, a single-line electrical diagram, and GFCI/AFCI compliance documentation to be part of the submission — the same fundamentals covered in our guide to choosing a mountain electrician, applied at ADU scale.

What Does ADU Electrical Work Cost in Lake Arrowhead?

• Dedicated 125A subpanel installation: $1,500–$3,500 depending on distance from the main panel
• Main panel upgrade to 200A (if needed): $2,800–$4,800
• EV-ready conduit run for future charging: $300–$800 depending on distance
• Full ADU electrical scope (subpanel, circuits, fixtures, GFCI/AFCI compliance): quoted per project based on square footage and finish level

FAQ

Does building an ADU always require a main panel upgrade?
Not always, but it's common, especially with older panels under 150 amps. A load calculation determines whether your existing panel can support the combined main house and ADU demand.

What size subpanel does a typical ADU need?
Most ADUs use a 100 to 125-amp subpanel, with 125 amps being the more common recommendation to leave room for future additions like an EV charger.

Do I need to run electrical conduit for an EV charger even if I'm not installing one yet?
Yes, generally. California's Title 24 code requires new ADUs to be EV-ready, meaning conduit must be run to a future charging location during initial construction.

Should my ADU share a meter with my main house or have its own?
It depends on your plans. A shared meter is simpler, while a separate submeter gives you independent tracking, particularly useful if you plan to rent the unit.

Can my Lake Arrowhead ADU be all-electric if my main house uses propane?
Yes. Many new ADUs are built all-electric specifically to avoid the cost of extending gas service to a detached unit, which can be a practical choice for propane-served properties.

Planning an ADU in Lake Arrowhead, Blue Jay, or the surrounding area? Get your electrical scoped correctly from day one.

Call (909) 403-4740 for a free assessment.