Subpanel installation cost in San Antonio: garage, shop and detached building feeders

If you are pricing a subpanel installation, you likely have a garage that trips a breaker every time the compressor starts, a shop you want to run real tools in, or a detached building living on an extension cord. The subpanel installation cost depends less on the panel itself than on where it goes, how far the feeder runs and whether your main service can support it. Choose the wrong fix and you pay twice: once for a subpanel your service cannot carry, and again for the service upgrade you needed from the start.

This guide covers when a subpanel is the right answer, what changes when the building stands apart from the house, how trenching and grounding work for a detached garage or shop, and what the permit involves in San Antonio. Where our flat-rate price book has a number for the work, it appears below and is pulled live from the book.

A subpanel adds breaker space, not capacity

A subpanel is a smaller panel fed by a breaker in your main panel. It gives you more breaker spaces in the place where you need circuits, so a garage or shop can have its own circuits close by instead of long runs back to the main panel. What it does not do is add power to the house. Every amp the subpanel delivers still comes through your service, your meter and your main breaker.

That distinction decides most of these jobs. If your main panel is out of spaces but the service has headroom, a subpanel is usually the cleaner and cheaper answer. If the house already runs close to the limit of its service, adding a subpanel to feed a welder, a second refrigerator and a mini-split only moves the problem to a new box. The right answer then is a service upgrade, and a subpanel may come along with it.

A subpanel is usually the right call when:

• The main panel is full, but the service rating is adequate for the house and what you plan to add.
• You want circuits in a garage, shop or detached building far from the main panel.
• A remodel or addition puts a cluster of new circuits in one part of the house.
• You want one place to shut off a shop or outbuilding without walking back into the house.

A service upgrade is usually the better call when:

• The home has 100-amp or smaller service and you are adding large loads such as an EV charger, a heat pump or a shop with 240-volt equipment.
• A load calculation shows the existing service is already near its rating.
• The main panel is a brand with a known history of problems and should be replaced rather than extended.

If you are not sure which side of that line you are on, a written load calculation settles it. The bigger option is covered in our guide to the cost to upgrade an electrical panel to 200 amps, and the panels we replace rather than extend are covered in Federal Pacific and Zinsco panels in San Antonio.

What the subpanel installation cost includes

A subpanel job has four parts: the feeder breaker in the main panel, the feeder conductors, the subpanel with its breakers, and the circuits it serves. The cost to install a subpanel rises mostly with feeder length and the route it takes. A panel mounted on the far side of the garage wall from the main panel is a short, simple run. A panel at the other end of the house means a longer feeder through the attic, more material and more hours. A panel in a detached building adds a trench.

For the most common case, a 60-amp subpanel installed close to the main panel, our flat-rate book lists the Subpanel installation, 60A task, from $1,859. The higher tiers in the same task cover longer feeder runs across the house. You see the price and approve it before work starts, and the permit and inspection are part of the job. Starting price. Your tech confirms the final number on site before any work.

The details that move the number within a tier:

• Feeder size. A 100-amp feeder uses larger conductors and a larger breaker than a 60-amp feeder.
• Distance and route. Attic runs are faster than runs that have to cross finished ceilings or go around obstacles.
• Space in the main panel. If there is no open space for the feeder breaker, circuits have to be rearranged first.
• Wall finish. Surface-mounting in an unfinished garage is quicker than recessing a panel into finished drywall.
• Day-one circuits. Each new circuit you want fed from the subpanel is its own line on the estimate.

Call us with the distance from your main panel to where you want the subpanel and what you plan to run on it, and we will tell you over the phone which tier fits. Call or book a visit and the technician brings the exact number with him.

Garage subpanel: attached garages and home shops

An attached garage subpanel is the most common version of this job in San Antonio. The main panel often already sits in the garage or on the garage wall outside, which keeps the feeder short. The usual reasons are a shop bench that needs more than the one garage receptacle circuit, a 240-volt outlet for a compressor or welder, a freezer that should not share a circuit with power tools, or an EV charger the main panel has no room for.

Two rules shape a garage subpanel even when the building is attached. First, neutral and ground are kept separate in the subpanel. They are bonded together at the main panel and nowhere downstream. A subpanel with its neutral and ground tied together is one of the most common defects we find on work done without a permit, and it can put current on metal parts that should never carry it. Second, garage receptacles need GFCI protection, which can come from the breaker or from the first receptacle on the circuit.

If the subpanel is going in to feed a welder, a compressor or a dryer for the shop, the outlet itself is covered in our guide to the cost to install a 240V outlet. If you are adding a charger, read how EV charger panel capacity is checked before anything is installed.

Plan for what you will add in the next few years, not only what you own today. Upsizing a feeder later means pulling new wire. Sizing it now for the compressor you plan to buy costs less than doing the job twice.

Detached garage electrical: what changes when the building stands alone

Detached garage electrical work follows the same idea as an attached subpanel, with three added requirements: a disconnect at the building, a four-wire feeder, and a grounding electrode at the building.

The disconnect means anyone working in the building can shut off all of its power without going back to the house. A subpanel with a main breaker usually serves as that disconnect.

The four-wire feeder carries two hot conductors, a neutral and a separate equipment ground. Many older detached garages in San Antonio were fed with three wires, with the neutral doing double duty as the ground. That arrangement is not allowed for new feeders, and when we extend or replace an old three-wire feed we bring it up to four wires.

The grounding electrode is one or more ground rods driven at the detached building and connected to its subpanel. In plain terms, the rods give lightning and other outside surges a path into the earth at that building. The equipment ground wire running back to the house does a different job: it gives a fault a low-resistance path so the breaker trips if a hot wire touches a metal case or box. A detached building needs both, and neutral and ground stay separated in its subpanel exactly as they do in an attached one.

A small building may not need a subpanel at all. A shed with a light and a couple of receptacles can often be served by a single circuit with a disconnecting means at the building. That is the option to consider when the building will never become a shop.

For a shed or small detached building that needs basic power, our book lists the Detached garage or shed power task, from $3,149. Higher tiers in the same task feed a full subpanel for a shop, sized for 240-volt tools or a future EV charger. The number is approved before any digging starts. Starting price. Your tech confirms the final number on site before any work.

Trenching to a detached garage or shop

The trench is often the biggest variable in the subpanel installation cost for a detached building. Length matters, and so does what is in the way: a driveway or walkway to bore under, caliche or rock, tree roots, irrigation lines, and gas or water lines.

• Before anyone digs, buried utility lines are located through Texas 811. Private lines the utilities do not mark, such as irrigation or a line to a pool or well pump, are worth pointing out to us on the visit.
• Required burial depth depends on the wiring method. Conductors in rigid conduit can be buried shallower than direct-burial cable, which is one reason conduit is often the better choice on a long run.
• We prefer conduit for detached building feeders, so the wire can be replaced or upsized later without digging the yard up again.
• Under a driveway, boring is usually less disruptive than cutting and patching concrete. The estimate states which method is planned.
• Backfill and compaction are part of the job. Replacing sod or plantings usually is not, and the estimate says so in writing.

Long runs also bring voltage drop into play. A feeder that is fine at 40 feet may need a larger conductor at 150 feet to keep tools running at full power. That is a calculation, not a guess, and it is part of why a detached building is quoted after a site visit rather than over the phone.

On acreage west and south of the city, where a shop may sit well back from the house, these runs get long. Our guide to rural acreage electrical in Castroville and Lytle covers what changes on bigger properties.

Replacing an old or failing subpanel

Some calls are about an existing subpanel rather than a new one. The common reasons are a subpanel from an obsolete brand, corrosion in a garage or outbuilding panel, a panel with no open spaces left, a three-wire feed to a detached building, or neutral and ground bonded where they should not be. A replacement keeps the feeder if it is sound and sized correctly, and corrects grounding and bonding as part of the work.

For a straight swap of an existing subpanel, our flat-rate book lists the Subpanel replacement task, from $1,869. If the feeder itself is undersized or damaged, the estimate says so before work starts, and that change is priced and approved on its own line. Starting price. Your tech confirms the final number on site before any work.

The permit and inspection for a subpanel in San Antonio

Adding or replacing a subpanel is permitted work. Inside San Antonio the electrical permit is issued by the Development Services Department and is pulled by a licensed electrical contractor. Wattsmith Electric is licensed by TDLR under TECL 41806. We pull the permit, schedule the inspection and meet the inspector, so you do not have to take a day off to wait for either. Outside city limits, the city or county you live in sets the process, and we handle that across our service areas. More detail is in our guide to electrical permits in San Antonio.

A permit matters beyond the inspection itself. When you sell, an unpermitted subpanel in the garage is exactly the kind of item a buyer's inspector writes up on the TREC inspection report, and insurers ask about it. Permitted work has a record you can point to.

How to get a firm price on your subpanel

• Tell us what the subpanel is for and the loads you plan to add over the next few years.
• Send a photo of the main panel with the door open and the label visible, plus the rough distance to the new location.
• We confirm on site whether the service has the headroom. If it does not, we say so before quoting a subpanel.
• You get a flat price from the book, approved before any work starts, with the permit included.

You can see the published flat-rate book on our pricing page, and what goes into this work on our panel upgrades and subpanels page. When you are ready, call us to schedule and we will set the visit.