Sub-panels: distributing power efficiently.
Industrial and commercial sub-panels feed specific load groups (zones, departments, equipment clusters) from a main distribution panel. Proper sizing, location, and configuration matter.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Sub-panel install scope: feeder from main panel or upstream distribution panel sized for sub-panel capacity, sub-panel selection (amperage rating, bus configuration, breaker count needed), location (electrical room or distributed location near loads), grounding and bonding (sub-panel uses isolated neutral and grounded equipment ground unless service-side configuration). Marin install $2,500-$12,000 typical per sub-panel depending on capacity and feeder length.
01Why sub-panels make sense.
Sub-panels reduce feeder copper costs in larger buildings by locating panels close to loads — short branch circuits from sub-panel to loads vs long branch circuits from main panel across the building. They also enable zone-based control (sub-panel breakers can be turned off for entire zones during maintenance), simplify panel directory management (panels organized by area or function rather than one giant main panel), and provide expansion headroom (sub-panel has spare breaker slots for future loads without affecting main panel).
Common industrial use: main distribution panel near service entrance feeds sub-panels in each major department or production area. Each sub-panel handles its local loads (lighting, receptacles, motors). Wiring routes from sub-panel to loads are short and economical. Maintenance work in one area can de-energize that sub-panel without affecting other operations.
02Sub-panel configurations.
Main lug sub-panel
Feeder lands directly on bus lugs; no main breaker in sub-panel. Upstream feeder breaker provides protection. Lower cost. Most common for small to medium sub-panels.
Main breaker sub-panel
Sub-panel has its own main breaker matching or below feeder ampacity. Provides local shutoff for entire sub-panel. Required when sub-panel is in separate building or specific NEC configurations.
Sizing
Typical commercial/industrial sub-panel sizes: 60A, 100A, 200A, 400A, 600A. Larger building installations use 200-400A sub-panels per zone. Sizing based on load study of fed circuits.
03Install considerations.
Location: sub-panels typically install in dedicated electrical rooms or wall-mounted in service corridors with proper working clearance per NEC 110.26 (3 feet of clear depth, code-prescribed width and height). Industrial installations sometimes use NEMA 12 (oil-tight) or NEMA 4 (watertight) enclosures for harsh environments. Marin shops and warehouses use NEMA 1 (indoor general use) for typical electrical rooms.
Feeder routing: from main panel to sub-panel, conduit and conductors sized for sub-panel ampacity. Long feeder runs (200+ feet) may require larger conductors than minimum NEC ampacity to manage voltage drop — typically size for 2% maximum drop at full load. We perform voltage drop calculation for any feeder over 100 feet.
Common questions.
How big a sub-panel should I install?
Size for current loads plus reasonable expansion headroom. Common practice: identify current load (load study), add 25-50% expansion margin, round up to standard ampacity (60, 100, 200, 400, 600A). Installing oversized sub-panel marginally increases install cost but provides expansion capacity without re-doing feeder/panel later. Undersizing forces premature upgrade or constrains future operation.
Can I add a sub-panel to an existing main panel that's nearly full?
Yes, often. Adding a sub-panel can convert a full main panel from 'no spare slots' to 'one new double-pole breaker feeds entire sub-panel' — freeing up slots indirectly. The sub-panel handles all the new loads. This is often a more economical path than replacing the main panel when main panel capacity (amperage) is adequate but slot count is exhausted.
What about coordination with existing branch circuits?
During sub-panel install, we typically migrate selected branch circuits from main panel to sub-panel (the loads being served by the new sub-panel). Migration requires de-energizing affected circuits temporarily. For active commercial operations, we schedule migration for low-impact times (early morning, evening, weekends) and minimize downtime by sequencing work.
Do sub-panels need separate inspections?
Yes — sub-panel installation is permitted electrical work and requires AHJ inspection. Typical inspection focus: panel installation per code (working clearance, mounting, support), grounding and bonding (sub-panels use separate neutral and equipment ground unless main bonding jumper required), feeder ampacity and protection, panel directory accuracy. Inspection scheduled 1-2 weeks after install completion.
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.