High-amperage feeders: engineered correctly.
Feeders carrying 100A+ have specific requirements: properly sized conductors, voltage drop calculation, appropriate conduit, engineered terminations.
By Steven Lockhart · Reviewed by Alex Towery, C-10 #989290
Updated 2026-06-16
High-amperage feeder install scope: conductor sizing per NEC table 310.16 with voltage drop adjustment per NEC 215.2(A)(3) recommendation, conduit sized for conductor count and bending radius, terminations rated for conductor size and material, supports per NEC 300.11 and code. Marin commercial/industrial feeder runs $50-$200 per linear foot depending on ampacity and routing complexity. Critical for service entrance feeders, MDP-to-sub-panel runs, and large equipment circuits.
01What makes high-amperage feeders specialized.
Feeders carrying 100A+ are physical objects of substantial size and weight. 200A copper conductors are roughly 3/0 AWG (about 0.5 inch diameter per conductor); 400A copper is 600 kcmil (about 0.9 inch); 600A copper is dual 350 kcmil parallel sets. These conductors require large conduit (2-4 inch typical), substantial support hardware (bending radius requirements per NEC 358.24 and similar), and engineered terminations (mechanical or compression lugs rated for the conductor size).
Voltage drop matters at high amperage. A 200A circuit at 5% voltage drop loses 12V on 240V service — significant. NEC recommends 3% maximum for branch circuits and 5% maximum for feeder + branch combined. We perform voltage drop calculation for any feeder run over 100 feet at any amperage, and any feeder over 50 feet at 200A+. Long runs sometimes require upsizing conductors beyond minimum NEC ampacity to manage drop.
02Sizing methodology.
Ampacity sizing
NEC table 310.16 for 75°C terminations (most modern equipment) or 60°C (older or specific applications). 200A feeder: 3/0 copper or 250 kcmil aluminum. 400A: 600 kcmil copper or 1,000 kcmil aluminum. Material choice (copper vs aluminum) affects cost significantly and has install considerations (aluminum requires anti-oxidant compound and specific termination procedures).
Voltage drop adjustment
Calculate voltage drop at full load over conductor length. If drop exceeds 3% (branch) or 5% (feeder + branch combined per NEC recommendation), upsize conductors to manage drop. Common adjustment: 200A feeder over 150 feet often needs 250 kcmil instead of 3/0 to manage drop.
Conduit sizing
NEC Chapter 9 tables determine conduit size from conductor count and gauge. 200A 3-wire feeder fits in 2-inch conduit; 400A 3-wire fits in 3-inch. Larger conduits ease pulling and provide future expansion capacity.
03Install practices for reliability.
Termination quality is critical at high amperage. Loose terminations heat up under load, eventually causing connection failure (with potential for fire). We use torque wrenches for all bolted terminations and follow manufacturer torque specifications. Compression lugs require proper crimper and die selection — improperly crimped terminations are unreliable. Thermographic verification after install confirms termination quality.
Conductor pulling for high-amperage feeders requires planning. Long runs may need pull boxes at intermediate points (NEC 314.28 prescribes minimum sizing). Lubricant required for all medium and large conductor pulls. Pull tension calculations sometimes required for long runs with large conductors to verify the conductors won't be damaged during pull.
Common questions.
Copper or aluminum for high-amperage feeders?
Both work; trade-offs differ. Copper: smaller conductor, easier termination, higher cost. Aluminum: larger conductor for same ampacity, requires anti-oxidant and specific termination procedure, lower cost (often 30-40% less material cost for large feeders). For Marin service entrance feeders, both materials are common; we recommend based on cost, equipment compatibility, and contractor familiarity. Modern aluminum (AA-8000 series) is reliable when properly installed.
How do you handle 400A+ feeders?
Often parallel conductor sets. 400A feeder can be single 600 kcmil per phase (large, hard to handle) or parallel 4/0 (two 4/0 per phase, easier handling, sometimes more economical). NEC 310.10(H) allows parallel conductors with specific requirements (matched length, same termination, same material). We design per project for handling and economic factors.
What about underground feeders?
Direct-burial cables (USE-2, URD) or conductors in buried conduit work for underground feeders. Direct burial saves conduit cost but limits future modifications. Buried conduit (typically PVC schedule 40 or 80) allows future conductor changes. Burial depth per NEC 300.5 — typically 18-24 inches for residential, deeper for commercial under traffic. Marin's clay soils require attention to backfill and protection from sharp rocks.
Can you upgrade feeders without full panel replacement?
Sometimes. Feeder upgrade typically requires upgrading or replacing the panel at both ends (source panel breaker rating, destination panel main breaker if applicable). If panels can accept higher rating, feeder upgrade alone is possible. Often the feeder is being upgraded specifically because panel capacity is being increased — both happen together. We assess specific configuration during planning.
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.