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

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

The single most important decision on this project is whether a sub-panel is actually the right tool. Two scenarios drive the answer.

When a sub-panel is the right answer

A sub-panel makes sense when your main panel has spare capacity but you need branch-circuit space in a different location. Common Arlington scenarios:

• Finished basement. Adding a basement living space with general lighting, outlets, a small bath, and a kitchenette. A 60-amp sub-panel mounted in the basement keeps the new branch circuits close to where they're used and avoids running 8-12 home runs back to the main panel.
• Detached garage or workshop. A separate building needs its own panel. Code requires (NEC 225 / 250) a means of disconnect and a separate grounding electrode at the detached structure. A 100-amp sub-panel mounted in the garage is the clean answer.
• Accessory dwelling unit (ADU). Arlington County's 2023 ADU bylaw expansion has made ADU additions much more common. A 100-amp or 125-amp sub-panel feeding the ADU keeps the unit's load on its own panel, simplifies metering if the homeowner ever wants to separately submeter, and meets code requirements for the unit's disconnect.
• EV charger plus other heavy loads. If you're adding a Level 2 EV charger plus a heat pump conversion plus a hot tub, a 100-amp sub-panel can be a cleaner solution than threading three new home-run circuits through finished walls back to the main panel.
• Pool house or detached studio. Less common in Arlington than McLean, but the same code logic applies — a separate building gets its own panel.

When a main panel upgrade is the right answer instead

A sub-panel doesn't help if the main panel itself is the bottleneck. Main panel upgrade is the right move when:

• The main panel is at or near capacity per a load calculation against NEC Article 220.
• The main panel is an unsafe brand: FPE Stab-Lok, Zinsco / Sylvania, Pushmatic, or Federal Pioneer. A sub-panel hanging off an unsafe main panel is still unsafe.
• The main panel has no available breaker spaces for a feeder breaker.
• Your project will exceed the panel's rating even after relocating loads (e.g., adding an ADU plus EV charger plus heat pump on a 100-amp service almost certainly requires 200-amp service, not just a sub-panel).

If you're not sure, the answer is a load calculation. AJLE runs that calc as part of every quote. See our Arlington panel upgrade guide for the main-panel discussion.

Sizing the sub-panel

The sub-panel's main breaker (and feeder cable size) scale with the load you're feeding it.

• 60-amp sub-panel. Right for a finished basement with general lighting, outlets, and one or two dedicated circuits. Fed by 6 AWG copper or 4 AWG aluminum on a 60-amp breaker in the main.
• 100-amp sub-panel. Right for a basement with a kitchenette and bathroom, a small ADU, or a detached garage with workshop loads. Fed by 3 AWG copper or 1 AWG aluminum on a 100-amp breaker.
• 125-amp sub-panel. Right for a substantial ADU, a pool house, or a workshop with welders or major tools. Fed by 2 AWG copper or 1/0 aluminum on a 125-amp breaker.
• 200-amp sub-panel. Rare in residential. Right for a large detached structure that's effectively a second house — full-size ADU with central HVAC and an electric range, or a substantial pool-house / guest-house. Fed by 2/0 copper or 4/0 aluminum on a 200-amp breaker.

Always size up rather than down — adding capacity later is much more expensive than starting at the right size.

What a Sub-Panel Installation Costs in Arlington

A residential sub-panel install in Arlington typically runs $1,500 to $4,000. Larger 200-amp sub-panels for substantial ADUs can push to $4,500.

Cost factors:

• Sub-panel amperage. A 60-amp basement panel is the low end. 100-amp is the most common; 125-amp and 200-amp are progressively more.
• Feeder cable run length and routing. A short interior feeder from the main panel to a basement sub-panel is the cheapest scenario. A run from the main panel through finished walls to a detached garage is more involved.
• Underground vs. interior. A detached structure typically needs underground PVC conduit for the feeder. Trenching adds $400-$1,200 depending on length and ground conditions (Arlington's clay-and-shale soil makes hand digging slow on long runs).
• Main panel work. Adding a feeder breaker is straightforward if there's a slot. If the main panel is full, adding a tandem breaker arrangement or a small main upgrade adds $300-$700.
• Grounding electrode at the detached structure. Code (NEC 250) requires a separate grounding electrode at any detached building. A new ground rod and grounding electrode conductor adds $150-$300.
• Arlington County permit fee. Currently in the $75-$150 range (verify with the current Arlington County fee schedule). Included in the written quote, not added separately.

Be skeptical of any quote significantly below $1,500 for a 100-amp sub-panel install — that price usually means the contractor is undersizing the feeder, skipping the grounding electrode at a detached structure, or not pulling a permit.

Arlington County Permits & Inspection

Every sub-panel installation in Arlington requires an electrical permit. This isn't optional — unpermitted electrical work surfaces during home sales and during insurance claims.

Permit authority. Arlington County's Department of Community Planning, Housing and Development handles residential electrical permits, with applications processed through permitarlington.com. (Verify current URL and authority name before publication.)

Who pulls the permit. AJ Long Electric pulls the permit as the licensed Master Electrician. The fee is included in the written quote.

Typical timeline. Same-day or next-day permit issuance. Inspection within 1-3 business days post-install. Total elapsed time from contract to closed permit is under two weeks for an interior basement sub-panel; add a few days for detached-structure work that includes 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 most common rejection reason on sub-panel installs). Grounding electrode at any detached structure. Proper labeling of all branch circuits at the new sub-panel. Arc-fault and ground-fault protection per the current code cycle on applicable circuits. (Arlington County is currently on NEC 2020 — verify the current adopted cycle before publication.)

What Happens on Install Day

A residential basement sub-panel install with a short feeder is a half-day job — typically 4-6 hours of on-site work. Detached-structure work with trenching is usually 1-2 days. Here's the typical flow:

• Morning arrival. The crew walks the main panel and the sub-panel install location, confirms the feeder routing matches the quote, and lays drop cloths through the work area.
• Brief power down. A 30-60 minute outage at the main while we install the new feeder breaker. Most of the day's work — pulling cable, mounting the sub-panel, landing branch circuits — doesn't require power off.
• Feeder install. EMT, NM cable, or PVC conduit (for underground) runs from the main panel to the sub-panel location. For detached structures, that includes trenching to 18-24 inches per code, laying the PVC, and pulling 4-conductor cable (two hots, neutral, ground — no neutral-ground bond at the sub-panel).
• Sub-panel mount and termination. The new sub-panel is mounted to the wall, the feeder lands on the main breaker, the grounding electrode conductor (for detached structures) is connected, and any new branch circuits are landed and labeled.
• Power on, test, document. Energize the feeder, verify voltage at the sub-panel, test every new branch circuit, and photograph the labeled panel for our records and yours.
• Cleanup and walk-through. Broom-clean the work area, walk you through what's where in the new panel, and explain the inspection schedule.

You don't need to be home for the install once we've confirmed access. For detached-structure work with trenching, expect some yard restoration — we backfill the trench but final landscaping (re-seeding, replacing pavers) is the homeowner's choice.

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

Arlington's mix of post-war ramblers, brick colonials, and a wave of recent ADU additions creates several distinct sub-panel scenarios:

• Basement-finishing wave on 1940s-60s brick ramblers (Lyon Park, Cherrydale, Aurora Hills, Westover, Bluemont): unfinished basements being converted to family rooms, home offices, in-law suites, and rental units. A 60- or 100-amp basement sub-panel is standard.
• Detached garage + workshop on larger lots (parts of Cherrydale, Bluemont, Donaldson Run): older homes with detached single-car garages being modernized for workshops, home gyms, or parking + EV charging. 100-amp sub-panel with PVC conduit feeder.
• Arlington County ADU additions (county-wide since the 2023 bylaw change): basement, attached, or detached ADUs being added to existing single-family homes. Typically 100-amp or 125-amp sub-panel feeding the ADU.
• 1990s-2000s townhouses (Pentagon City corridor, Shirlington): generally 200-amp main panels at construction. Sub-panel work here is usually about adding a basement or attic sub-panel for a renovation, not for a detached structure.

Recent Arlington sub-panel projects

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

• Cherrydale 1952 colonial — basement finish. Existing 200-amp main panel; homeowner converting unfinished basement to family room + half-bath + small home office. 100-amp sub-panel mounted in the basement, 6 new branch circuits for lighting, outlets, and a dedicated bathroom GFCI circuit. One-day install.
• Lyon Park 1942 colonial — detached garage workshop. Existing 100-amp main panel needed an upgrade first to 200-amp before the sub-panel could be safely fed. 100-amp sub-panel in the detached garage with a 60-foot underground PVC feeder run, a new ground rod at the garage, and three workshop branch circuits. Two-day install across panel upgrade + sub-panel.
• Westover 1954 rambler — basement ADU. Homeowner adding a basement ADU under Arlington County's expanded bylaw. 125-amp sub-panel feeding the ADU's lighting, outlets, mini-split HVAC, kitchenette circuits, and laundry. Coordinated permit with the broader basement-finish permit.

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. Here's what to verify and what to avoid.

• Virginia Master Electrician license. Verify on dpor.virginia.gov. Sub-panel work requires a Master, not a Journeyman.
• Bonded and insured. Ask for a current certificate of insurance. Reputable contractors hand it over without hesitation.
• Performs a load calculation. A reputable installer runs a load calc against NEC Article 220 to confirm the main panel can support the new sub-panel feeder. Skip this step and you risk overloading the main; the sub-panel passes inspection but the main breaker trips repeatedly under real-world load.
• Doesn't bond neutral and ground at the sub-panel. This is the single most common sub-panel failure: an installer who's used to wiring main panels (where neutral and ground ARE bonded) carries that habit to a sub-panel (where they MUST NOT be bonded). Ask the contractor specifically if they're 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 garage is failing inspection before they leave.
• Pulls permits. Don't accept "we don't need a permit for a sub-panel." It's wrong, and unpermitted work surfaces at home sale.
• Itemized written quote. Should specify the sub-panel make and model, amperage, feeder cable size and type, conduit type, breaker count, permit fee, and any trenching or grounding electrode work.

Avoid: cash-only deals, refusal to pull permits, no proof of insurance, lowball quotes that don't separate the panel cost from the trenching cost, and contractors who can't explain why the neutral-ground bond rule is different at a sub-panel.

Frequently Asked Questions

How much does sub-panel installation cost in Arlington?

$1,500 to $4,000 for most residential sub-panels in Arlington. Larger 200-amp sub-panels for substantial ADUs can push to $4,500. Variance comes from amperage, feeder run length and routing, whether trenching is needed for a detached structure, and the Arlington County permit fee.

Do I need a sub-panel or a full panel upgrade?

A sub-panel is right when the main panel has spare capacity but you need branch-circuit space in a different location (basement, garage, ADU). A main panel upgrade is right when the main itself is at capacity or is an unsafe brand (FPE, Zinsco, Pushmatic). The decision comes down to a load calculation under NEC Article 220 — see our Arlington panel upgrade guide for the main-panel side.

What size sub-panel do I need?

60-amp for a finished basement with general lighting and outlets. 100-amp for a basement with a kitchenette, a small ADU, or a detached garage with workshop loads. 125-amp for a substantial ADU or a pool house. 200-amp only for a full-size ADU or guest house with central HVAC and major appliances.

Do I need an Arlington County permit for a sub-panel?

Yes. Every sub-panel installation in Arlington requires an electrical permit. AJ Long Electric pulls the permit as the licensed contractor; the fee is included in the written quote. Permit issuance is same-day to next-business-day; inspection follows within 1-3 business days post-install.

Can a sub-panel feed an EV charger?

Yes — and a 100-amp sub-panel is a common solution for adding an EV charger plus other heavy loads (heat pump, ADU electric range, workshop equipment) without upsizing the main service. The decision depends on the main panel's spare capacity and the total load you're adding. Read our Arlington EV charger guide for the EV-specific load math.

How long does a sub-panel install take?

A basement sub-panel with a short feeder is a 4-6 hour job. A detached-garage sub-panel with conduit and trenching is typically 1-2 days. The Arlington County inspection follows within 1-3 business days. Total elapsed time from contract to closed permit is usually under two weeks.