Sub-panels: when they solve the problem, when they don't.
A sub-panel adds capacity for a specific location or load category — but it's not a substitute for upgrading an overloaded main panel. Knowing when each is right saves $3,000-$5,000 on the wrong scope of work.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Sub-panels are dedicated breaker panels installed away from the main service panel — useful for garages, ADUs, workshops, pool equipment, and similar dedicated locations. They draw from the main panel via a feeder breaker, so they don't add total capacity to the home. The right scope when you need slot capacity or run-distance to a remote location; the wrong scope when the main panel itself is the bottleneck.
01What a sub-panel actually does (and doesn't).
A sub-panel is a secondary breaker panel that gets its power from a feeder breaker in the main service panel. It distributes power locally — so circuits for a garage workshop, ADU, or pool equipment can be managed nearby instead of running every individual circuit back to the main panel. The sub-panel doesn't add total electrical capacity to the home; it just relocates and organizes part of what the main panel is already delivering.
This distinction matters because customers often ask for a sub-panel when what they actually need is a main panel upgrade. A 100A main panel that's full and overloaded won't be helped by adding a sub-panel — the sub-panel is fed from the same main panel that's already at capacity. The sub-panel only adds value when the main panel HAS available capacity but doesn't have slots, or when you need power distributed to a remote location.
02Three common scenarios where sub-panels fit.
Detached garage or workshop
Garage with multiple circuits (lighting, outlets, EV charger, shop equipment) is easier to manage with a sub-panel in the garage than with individual circuit runs back to the main house panel. Cost typically $1,800-$3,400 installed including feeder, sub-panel, and 4-6 breakers.
ADU or in-law unit
ADUs almost always get a dedicated sub-panel — separates ADU electrical from main house, makes future metering possible, allows ADU-specific load management. Cost typically $2,400-$4,800 including 100A or 125A sub-panel and ADU branch circuits.
Main panel slot exhaustion
Main panel has capacity (passes load calc) but is physically full of breakers. Sub-panel adds slot space without service-side work. Cost typically $1,800-$3,200 installed. Most common scenario in Marin: 200A panel that's been added-to multiple times until no slots remain.
03When sub-panel isn't the right answer.
Two scenarios commonly attract sub-panel proposals when they shouldn't: an overloaded main panel where the actual demand exceeds capacity (sub-panel doesn't help — main panel needs upgrade), and a service-side capacity issue where adding loads requires more amperage from PG&E (service upgrade is the answer, not sub-panel). We run a load calc before quoting any sub-panel work to make sure we're solving the right problem.
The sub-panel-vs-main-upgrade decision usually comes down to load calc results. If the existing service amperage is adequate for current and planned future loads (with appropriate margin), a sub-panel can solve slot exhaustion or distribution problems. If existing service amperage is inadequate or marginal, a main panel upgrade is the right answer — and sometimes it makes sense to do both (upgrade main AND add a sub-panel for the ADU or garage) in a single project.
Common questions.
What size sub-panel do I need?
Common residential sub-panel sizes: 60A (small detached structures, light shop use), 100A (typical garage with EV charger, larger workshop), 125A (ADU with normal residential loads), 200A (large ADU or workshop with major equipment). Sizing is driven by load calc — adding up the equipment and circuits the sub-panel will feed, applying NEC demand factors, and matching to a sub-panel rating with margin. For most garage and ADU applications in Marin, 100A is the typical answer.
How much does sub-panel installation cost?
For Marin in 2026: $1,800-$3,400 for typical garage or interior sub-panel installation (100A panel, 30-50 foot feeder run from main, 4-8 breakers). $2,400-$4,800 for ADU sub-panel work (longer feeder run, more circuits, 125A typical). $1,600-$2,800 for slot-exhaustion sub-panels installed near the main panel (short feeder run). All-in including panel, feeder cable, breakers, permit, and inspection.
Can I install a sub-panel in my basement or crawl space?
Sometimes yes, sometimes no — depends on the space. NEC requires sub-panels be in 'accessible' locations with specified working clearances (typically 30 inches wide, 36 inches deep, full headroom). Basements with adequate clearance and ventilation usually work. Crawl spaces typically don't — most are too low for working clearance and have moisture concerns. We assess feasibility during the quote visit; sometimes a slight relocation makes a non-compliant space work.
Do sub-panels need their own permit?
Yes — sub-panel installations require electrical permit from the AHJ, same as main panel work. Permit fee is typically lower for sub-panels than main panels ($180-$320 range in most Marin AHJs vs $240-$540 for main panels) because the inspection scope is narrower. Inspection happens at completion to verify proper bonding (sub-panel must have separated neutral and ground), feeder breaker rating, conductor sizing, and labeling.
How do I know if I need a sub-panel or a main panel upgrade?
Quick decision logic: if your main panel passes the NEC load calc with current and planned loads but is out of slots, sub-panel is right. If the load calc fails (current demand exceeds panel rating with new loads), main panel upgrade is right. If you have a remote location (garage, ADU) needing significant power distribution, sub-panel makes sense regardless. We do the load calc as part of the quote — usually clear within 20-30 minutes which is the correct scope of work.
How do you handle pricing — flat fee or hourly?
Both, depending on scope. Routine service calls and predictable scope (panel replacement, EV charger install, transfer switch install, fixture replacement) are flat-fee quoted up front. Diagnostic visits and emergency scope are hourly because we don't know what we'll find. Either way, you get a written quote before work starts on anything significant, and we don't proceed beyond the quote without your sign-off.
How quickly can you schedule?
Depends on scope and season. Emergency dispatch: same-day across Marin County (1-3 hours business hours, 1-4 hours after-hours). Same-day standard service: 3-6 hours from call when capacity allows; otherwise next business day. Scheduled service work: 1-3 weeks typical. Larger projects requiring permits and PG&E coordination: 4-12 weeks depending on scope. We're upfront about timeline at the quote stage.
What warranty do you offer on work?
Our standard residential warranty: 2 years on labor, manufacturer warranty on parts (varies by product — typically 1-10 years). Major installs (panels, generators, EV chargers) often have longer manufacturer warranties (5-10 years). If there's an issue with our work in the warranty period, we come back and fix it at no cost. We also stand behind work past warranty — repeat customers and referrals matter more than a transactional fix.
Are you licensed and insured?
Yes. California State License Board C-10 Electrical Contractor #989290, in good standing. Carry general liability insurance and workers' compensation insurance as required by California law. Bondable on projects requiring it. Happy to provide proof of insurance to clients or property managers on request. License lookup: cslb.ca.gov.
Talk to a licensed electrician in Marin.
Family-run, C-10 #989290, 20+ years in the trade. Honest scoping, fair pricing, work that lasts.