Commercial amperage sizing: match the operation.
Standard Marin commercial service sizes range 400A to 1200A residential-class, larger for industrial. Choosing the right size balances current capacity, future expansion, and avoidable cost.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
Common Marin commercial service sizes (residential-class): 400A for small commercial (under 5,000 sq ft retail/office), 600A for medium commercial (5-10k sq ft, light restaurants, small medical), 800A for larger commercial (10-20k sq ft, full-service restaurants, multi-tenant), 1200A for substantial commercial (20k+ sq ft, large kitchens, multi-tenant with significant load). Voltage class drives capacity: 208Y/120 at 800A = 288 kVA total; 480Y/277 at 800A = 665 kVA total. Sizing decision should anticipate 5-10 year operational evolution.
01How current Marin commercial sizes line up with operations.
400A 208Y/120: small office (under 5,000 sq ft), boutique retail, small medical/dental office, casual food service (sandwich shop, coffee shop). Typical equipment load 40-80 kW peak. Service entrance cost: $12,000-$22,000 installed Marin. Service classification: Bx-class PG&E rate schedules typical.
600A 208Y/120: medium office (5-10k sq ft), larger retail with refrigeration, small restaurants with full kitchen, salon/spa, medical with imaging equipment. Typical equipment load 60-130 kW peak. Service entrance cost: $18,000-$32,000 installed Marin. 800A and 1200A scale up proportionally — typical equipment loads in 100-280 kW range, service costs $24,000-$48,000+.
02Sizing factors beyond peak load.
Continuous-duty derating
NEC requires service sized for continuous loads (3+ hours) at 125% of actual load. Pushes sizing up for businesses with consistent operating profiles (restaurants, retail with refrigeration always on, etc.). Diversity factors offset some derating for variable loads.
Future expansion margin
Sizing should anticipate 5-10 year load growth: equipment additions, electrification, EV charging, expansion. Adding 20-30% margin to current calculated need typically right-sizes for medium-term horizon. Upsizing later is expensive; modest upfront margin is cheap insurance.
PG&E rate schedule implications
Service size partly determines available PG&E commercial rate schedules. Larger services may qualify for time-of-use rates with better demand charges. Net effect on operating cost varies; consult with PG&E commercial rep during planning if rate optimization is priority.
03Why oversizing isn't free.
Oversizing commercial service adds upfront cost (larger conductor, larger main breaker, larger transformer, larger panel) without operational benefit until the additional capacity is actually used. 800A service vs 600A service for an operation that only ever needs 600A costs an additional $4,000-$8,000 with zero ongoing value. Modest oversizing (20-30% above calculated need) is appropriate insurance against growth; large oversizing (50%+ above need) is wasteful.
Right-sizing requires honest discussion of actual operational plans. Some business operators overstate future expansion plans during planning ("we might double in 5 years"); some understate them ("we're stable, no changes planned"). Both lead to mis-sizing. We push for honest 5-10 year operational forecasts with appropriate uncertainty discounts during planning conversations.
Common questions.
How do I know which size I need?
Load study (separate cluster) is the rigorous answer for actual current need. Add 5-10 year growth projection with realistic estimates. Round up to next standard service size. Most Marin small/medium commercial lands at 400A or 600A. Larger restaurants, full medical practices, and substantial retail land at 800A. Industrial-leaning or large multi-tenant land at 1200A+. We help with sizing decisions during quote.
Can I upgrade later if I undersized?
Yes, but it's expensive — service upgrade scope from 400A to 800A typically runs $20,000-$45,000 depending on existing infrastructure. Avoiding this future cost via modest upfront upsizing (paying $4,000-$8,000 more to go from 400A to 600A initially) is usually favorable economics if 5-10 year horizon supports the larger size. We discuss upgrade vs upsize tradeoffs during quote.
Does PG&E charge more for larger services?
Yes — larger services have higher monthly service charges and different rate schedule options. The cost difference is meaningful for very large services (1200A+ typically commercial rate schedules with significant demand charges) but modest for smaller commercial services. Operating cost analysis should factor service size into total cost of ownership.
What if I'm not sure about future plans?
Honest answer: most businesses don't know precisely what they'll need in 5-10 years. Practical approach: size for current need with 20-30% margin, ensure panel and infrastructure can accommodate future upgrade without complete replacement. Modular approach: install 600A service with panel that could accept 800A upgrade if business grows. Less upfront cost; less expensive upgrade if growth materializes.
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.