Sub-Panel Installation in McLean, VA: Cost, Permits & What to Expect

Sub-Panel vs. 400A Main Upgrade — Which Do You Need?

McLean's high-end housing stock means many sub-panel inquiries come with a parallel question: should we just upgrade the main to 400-amp instead? Here's how to think about it.

When a sub-panel is the right answer

A sub-panel makes sense when the main panel has spare capacity but the new load is concentrated in a separate location. Common McLean scenarios:

• Pool house / pool equipment. Pool pumps, heaters, salt cells, and pool-house lighting / outlets / mini-splits cluster a 50-80 amp continuous load. A 100-amp pool-house sub-panel is the standard answer; mounted in the pool house mechanical room, fed from the main via underground PVC.
• Detached guest house / studio / au pair suite. McLean has a meaningful population of detached secondary structures. NEC 225 / 250 requires a separate disconnect and grounding electrode at any detached structure, which a sub-panel naturally provides. Typically 100-amp or 125-amp.
• Wine cellar / temperature-controlled storage. Wine refrigeration, dedicated HVAC, and lighting on a 60-amp basement sub-panel keeps the heavy loads grouped and easy to service.
• Multi-EV households. Two or three Level 2 chargers fed from a 100-amp or 125-amp sub-panel with smart load management. Common in Tesla / Rivian / Porsche Taycan households.
• Workshop, gym, or basement bar. A finished basement bar with a wine fridge, kegerator, dishwasher, and beverage cooler can hit 40-amp continuous load — a 60-amp sub-panel mounted nearby keeps the new circuits close to the load.
• Finished basement family room or in-law suite. Same logic as Arlington — 60-amp or 100-amp basement sub-panel for general lighting, outlets, dedicated bath circuits, and a kitchenette.

When a 400A main upgrade is the right answer instead

A sub-panel doesn't help if the household total load exceeds what a 200-amp main can support. The 400-amp upgrade is the right move when:

• The combined load — pool + multiple EVs + heat pumps + guest house + generator — exceeds 200-amp capacity per the load calc.
• The main panel itself is at or near capacity already, before adding the new load.
• The main panel is FPE Stab-Lok, Zinsco, Pushmatic, or Federal Pioneer — replace the main first regardless.
• Adding a whole-home automatic transfer switch for a Generac / Kohler generator alongside the new load.

Many McLean estates end up doing both: 400-amp main upgrade with a 200-amp sub-panel feeding the pool house. See our McLean panel upgrade guide for the main-panel side.

Sizing the sub-panel

Match the sub-panel main breaker (and feeder cable) to the load you're feeding it.

• 60-amp. Wine cellar, basement bar, finished basement with general lighting and outlets only. Fed by 6 AWG copper or 4 AWG aluminum.
• 100-amp. Most McLean pool houses, modest guest houses, multi-EV charging arrays, basement with a kitchenette plus dedicated circuits. Fed by 3 AWG copper or 1 AWG aluminum.
• 125-amp. Larger pool houses with full kitchens, larger guest houses, workshops with heavy equipment. Fed by 2 AWG copper or 1/0 aluminum.
• 200-amp. Full-size detached guest house with central HVAC, electric range, electric dryer, and laundry — effectively a second house. Fed by 2/0 copper or 4/0 aluminum.

McLean estates sometimes pair a 400-amp main with a 200-amp sub-panel feeding a guest house plus a 100-amp sub-panel feeding the pool house — a clean, code-compliant arrangement that keeps each load type on its own panel.

What a Sub-Panel Installation Costs in McLean

McLean sub-panel installs typically run $1,800 to $5,500. Larger 200-amp sub-panels for full guest houses can reach $6,000.

Cost factors:

• Sub-panel amperage. 60-amp basement is the low end. 100-amp is the McLean default; 125-amp and 200-amp scale up materially.
• Feeder run length. A short interior feeder is cheap. A 50-100 foot underground PVC feeder to a pool house adds substantially — both for cable cost (heavier gauge required for voltage drop on long runs) and for trenching labor.
• Trenching across landscaped property. McLean lots are larger and frequently heavily landscaped. Hand-trenching across mature plantings, irrigation lines, and stone hardscape adds $500-$2,000 depending on conditions. We protect what we can; replanting / hardscape repair is usually the homeowner's separate landscaping contractor.
• Underground PVC vs. interior conduit. Detached structure feeders go underground. Interior runs use NM cable, EMT, or MC cable.
• Main panel work. Adding a feeder breaker is straightforward when slots are available. Full main panels in McLean often need either tandem breaker rearrangement or a small main upgrade alongside.
• Grounding electrode at detached structure. NEC 250 requires it; new ground rod plus grounding electrode conductor adds $200-$400.
• Fairfax County permit fee. $90-$200 typical (verify with the current Fairfax County fee schedule). Included in the written quote.

Be skeptical of any 100-amp sub-panel quote significantly below $1,800 in McLean — that price usually means the installer is undersizing the feeder for the run length, skipping the grounding electrode at a detached structure, or not pulling a permit.

Fairfax County Permits & Inspection

McLean is unincorporated Fairfax County. All electrical permits run through Fairfax County's Land Development Services.

Permit authority. Fairfax County Land Development Services handles residential electrical permits, with applications processed through the county's online permit portal at fairfaxcounty.gov/landdevelopment. (Verify current portal URL before publication.)

Who pulls the permit. AJ Long Electric pulls the permit as the licensed Master Electrician.

Typical timeline. Same-day to 2 business day permit issuance. Inspection within 2-5 business days post-install. Total elapsed time from contract to closed permit is usually under three weeks for interior sub-panels; add a few days for detached-structure work with trenching.

What the inspector checks. Feeder cable size matches the sub-panel main breaker per NEC tables. Proper torque on terminations. Neutral and ground bonded only at the main panel — never bonded again at the sub-panel. This is the single most common rejection reason on sub-panel installs across all jurisdictions. Grounding electrode at any detached structure (NEC 250). Proper labeling of all branch circuits at the new sub-panel.

HOA considerations. Salona Village, Langley Forest, and parts of Chesterbrook have active HOAs that may require notification or design review for any exterior conduit runs visible from neighboring properties. AJLE handles HOA notification when needed.

What Happens on Install Day

A McLean basement sub-panel with a short feeder is a half-day to full-day job. A pool-house or detached-structure sub-panel with trenching is typically 1-2 days. Larger 200-amp guest-house installations with extensive branch-circuit work can run 2-3 days.

• Morning arrival and walkthrough. The crew confirms feeder routing, trench path (for detached work), grounding electrode location, and any landscaping concerns.
• Brief power down at the main. 30-60 minute outage to install the feeder breaker. Most of the day's work — pulling cable, trenching, mounting the sub-panel, landing branch circuits — doesn't require power off.
• Trenching (detached only). Hand-trench or mini-excavator depending on access. 18-24 inches deep per code; we protect mature plantings where possible and notify before any unavoidable disturbance.
• Conduit and feeder pull. PVC conduit underground; EMT or NM cable on interior runs. Voltage-drop calculations confirm cable size for the actual run length.
• Sub-panel mount and termination. Sub-panel mounts to the wall; feeder lands on the main breaker; grounding electrode conductor connects (detached only); branch circuits land and label.
• Power on, test, document. Energize the feeder, verify voltage, test every branch circuit, photograph the labeled panel.
• Cleanup. Trench backfill (compacted in lifts), broom-clean work area inside, walk-through of new panel directory.

For pool house and detached work: we coordinate with your pool contractor, landscape designer, and HOA where applicable. The electrical sub-panel install usually needs to dovetail with the pool/structure contractor's schedule — we plan around their pour and frame dates.

What McLean's Housing Stock Means for Sub-Panel Work

McLean's mix of 1950s-70s mid-century homes, 1990s-2000s custom builds, and 2010s-2020s tear-down-and-rebuilds creates distinct sub-panel install profiles:

• 1950s-70s mid-century ramblers + colonials (Westmoreland Hills, Chesterbrook, Franklin Park): typically 100A or 200A original main panels. Sub-panel work is most often basement-finishing or detached-garage-to-workshop conversions. Frequently paired with a main panel upgrade if the original panel is FPE / Zinsco.
• 1990s-2000s custom builds (Langley Forest, Salona Village, Old Dominion ridge): generally 200A original mains with spare capacity. Sub-panel work here is the classic "pool house add" or "guest house add" — clean install with a fresh feeder run from the main.
• 2010s-2020s new construction tear-downs (across McLean): typically 400A original service. Sub-panels for newly-built pool houses, finished basements, or workshops are routine and usually planned at construction.
• Townhomes and high-rise condos (Tysons-McLean corridor): unit-level sub-panel work for finished basement-equivalent storage spaces or den conversions; usually 60-amp.

Recent McLean sub-panel projects

(Anonymized; details to be confirmed against AJLE project records before publication.)

• Salona Village 2002 custom — pool house addition. Existing 200-amp main with adequate spare capacity. New 100-amp sub-panel in the pool house mechanical room, fed via 80-foot underground PVC across landscaped property. Six branch circuits: pool pump, pool heater, salt cell, mini-split, lighting, and outlets. Two-day install (day one trench + feeder, day two sub-panel + branch circuits).
• Langley Forest 1998 custom — detached guest house. Combined with a main panel upgrade to 400A; the new 125-amp sub-panel feeds the detached guest house's lighting, outlets, kitchenette circuits, mini-split HVAC, and laundry. 60-foot underground PVC run with a separate ground rod at the guest house. Three-day install across panel upgrade + sub-panel.
• Chesterbrook 1965 colonial — basement wine cellar. 60-amp sub-panel mounted in the basement mechanical area, feeding wine refrigeration units, dedicated HVAC zone, and recessed lighting. Half-day install; clean code-compliant work that closed permit in under two weeks.

What to Look for in an Electrician

Sub-panel work has a couple of specific failure modes that distinguish a careful installer from a quick one. Especially important on McLean estate-scale work where the dollar amounts justify the additional scrutiny.

• Virginia Master Electrician license. Verify on dpor.virginia.gov.
• Bonded and insured. Higher coverage limits matter on estate-scale work; ask for the policy limit on the certificate of insurance.
• Performs a load calculation. Confirms the main panel can support the new sub-panel feeder. Skip this and you risk overloading the main; the sub-panel passes inspection but the main breaker trips repeatedly.
• Doesn't bond neutral and ground at the sub-panel. The single most common sub-panel failure mode. Ask the contractor specifically: "are you separating neutral and ground at the sub-panel?" The right answer is yes.
• Installs a separate grounding electrode at detached structures. NEC 250 requires it. An installer who skips the ground rod at a detached pool house or guest house is failing inspection.
• Pulls permits. Don't accept "we don't need a permit for a sub-panel." Wrong; unpermitted work surfaces at home sale.
• Voltage-drop calculation for long runs. McLean's larger lots often require feeder runs of 75-150 feet. Cable size must increase for voltage drop on these runs — a #1 aluminum feeder fine for 30 feet may need to be #1/0 or #2/0 for a 100-foot run. Ask if they're sizing for voltage drop.

Avoid: contractors who skip the load calc, refuse to pull permits, can't explain the neutral-ground-bond rule difference between main and sub-panel, or don't size for voltage drop on long McLean feeder runs.

Frequently Asked Questions

How much does sub-panel installation cost in McLean?

$1,800 to $5,500 typical. Larger 200-amp sub-panels for full guest houses can reach $6,000. Variance comes from amperage, feeder run length, trenching across landscaped property, and whether the main panel needs minor work to accept the new feeder breaker.

Sub-panel or 400A main upgrade?

A sub-panel is right when the main has spare capacity and the load is concentrated in a separate location (pool house, guest house, basement). A 400A main upgrade is right when the household total exceeds 200A capacity. Many McLean estates do both — see our McLean panel upgrade guide.

What size sub-panel does a McLean pool house need?

A 100-amp sub-panel is the standard answer for a typical pool house with pool pump, heater, salt cell, mini-split HVAC, lighting, kitchenette, and outlets. Larger pool houses with full kitchens and central HVAC scale up to 125-amp or 200-amp.

Do I need a Fairfax County permit?

Yes. Every sub-panel installation in McLean (Fairfax County jurisdiction) requires a Fairfax County electrical permit. AJ Long Electric pulls the permit; fee included in the written quote.

How long does the install take?

Interior basement sub-panel: half-day to full day. Pool house or detached structure with trenching: 1-2 days. Larger 200-amp guest house: 2-3 days.

Can a sub-panel feed multiple EV chargers?

Yes — a 100-amp or 125-amp sub-panel is a clean solution for multi-EV McLean households. Two Level 2 chargers with smart load management on a 100-amp sub-panel; three or more on 125-amp+. See our McLean EV charger guide for the EV-specific load math.