Plan the project as a feeder installation, not simply a panel replacement. The electrician must calculate the garage or shed load, confirm the main service has capacity, select a 60A or 100A feeder, choose an approved cable or conduit method, install required grounding and bonding components, and coordinate permits and inspection with the local authority having jurisdiction. A detached subpanel normally has an isolated neutral bar and a bonded equipment-ground bar, while the feeder's equipment grounding conductor connects back to the source panel. Ground rods may also be required at the detached building, but they do not replace the feeder equipment grounding conductor.
What determines the cost of a detached-building subpanel?
The largest cost variable is the path between the source panel and the detached garage or shed. A short, accessible run may require only a panel, feeder conductors, fittings, grounding components, labor, and permit work. A long run may also require excavation, conduit, weatherproof transitions, wall penetrations, restoration, and additional conductor length.
The electrician should begin with a load calculation for the building. Lighting, receptacles, a garage door opener, tools, compressors, welders, electric heaters, air-conditioning equipment, battery chargers, and future equipment all affect the feeder size. A 60A feeder may be appropriate for a lightly equipped workshop, while a 100A feeder may be needed for a larger load. Neither rating should be selected only because it is common or because the subpanel has a matching main breaker.
The existing home's service and main panel also need evaluation. A new feeder breaker must fit the source panel and the calculated service or feeder capacity. If the main service is already heavily loaded, installing a larger subpanel does not create more available electrical capacity.
60A versus 100A subpanel cost factors
A 60A installation generally costs less than a 100A installation when the distance, route, and wiring method are otherwise identical. The difference can involve conductor size, conduit size, feeder breaker, terminations, panel rating, and the labor needed to handle larger conductors. In some projects, however, trenching and access dominate the estimate, so the difference between feeder sizes may be smaller than the cost of opening and restoring the route.
| Item | 60A feeder considerations | 100A feeder considerations |
|---|---|---|
| Load | Often suited to lighter lighting, receptacle, and tool loads when the calculation supports it | Provides more capacity when the calculated load and source service support it |
| Conductors | Usually smaller than a comparable 100A feeder, but the required size depends on code conditions and installation details | Usually requires larger conductors and may require larger raceway or more difficult terminations |
| Panel | Can use a panel rated for the feeder and planned branch circuits | Should be selected with sufficient bus, breaker, and branch-circuit capacity for the planned use |
| Future capacity | May limit later additions such as electric heat or high-demand equipment | May provide more planning flexibility, but does not eliminate service-capacity limits |
| Cost impact | Often lower conductor and termination costs | Often higher material and labor costs, especially on long or difficult runs |
The panel's bus rating is not the same as the amount of power that can be used simultaneously. The feeder breaker, feeder conductors, source equipment, calculated load, and panel listing all matter. Ask for the load calculation and feeder design in the written estimate.
SER cable versus THHN wire in PVC conduit
SER, or service-entrance cable, contains insulated conductors and may include an equipment grounding conductor. Whether a particular SER cable is permitted depends on its listing, location, protection, moisture exposure, burial conditions, and the locally adopted electrical code. It is not automatically interchangeable with individual conductors in every part of a detached-building feeder run.
THHN or THWN conductors are individual wires pulled through conduit. For underground or damp locations, the conductor insulation and listing must be suitable for those conditions. Many conductors marked THWN or THWN-2 are used in wet locations, but the electrician must match the conductor marking, raceway, burial method, and installation to the applicable rules.
| Wiring method | Potential advantages | Cost and planning concerns |
|---|---|---|
| SER cable | Can simplify some accessible feeder runs and may reduce raceway work where the cable is listed and permitted | Requires careful review of physical protection, exposure, transitions, and location-specific limitations |
| THHN or THWN in PVC conduit | Useful for underground routes and locations where raceway provides the required protection | Requires conduit, fittings, pulling labor, correct conductor listings, and often more layout work |
| Mixed methods | May allow different wiring methods for indoor, exterior, and underground portions | Transitions must be properly protected, enclosed, supported, and terminated |
PVC conduit is nonmetallic, so it does not provide an equipment grounding path by itself. A grounding conductor is normally installed with the feeder conductors. The exact conductor count and size depend on the feeder arrangement, code rules, and the equipment involved.
If the route must pass through a concrete foundation or slab, the penetration method can affect the estimate. For difficult masonry or concrete access, concrete wall penetration costs may help explain why a seemingly short feeder route still requires significant labor.
How trenching and route access affect the estimate
Underground feeders usually require more than measuring the distance between buildings. The contractor may need to locate existing utilities, establish the route, excavate to the locally required depth, install suitable conduit and warning or protection components where required, backfill, and coordinate inspection before concealment. Requirements vary by jurisdiction and installation method, so the building department or electrical inspector should confirm the applicable rules.
Rock, tree roots, steep grades, landscaping, concrete walks, driveways, and limited equipment access can increase labor. A route under a paved surface may need boring or a different approved method. A route along an exposed wall may require physical protection and weather-rated fittings rather than simply fastening cable to the siding.
Get the estimate separated into feeder materials, trenching, boring or drilling, panel work, grounding, permit fees, inspection-related work, and surface restoration. This makes it easier to compare bids that use different routes or wiring methods.
Ground rods and detached-building grounding
A detached building may require a grounding electrode system, commonly involving one or more ground rods, depending on the installation and the locally adopted code. The ground rod system is connected to the building's grounding and bonding arrangement. It is not a substitute for the equipment grounding conductor run with the feeder back to the source panel.
Ground rods may add material, driving, testing or verification, bonding, and labor costs. Soil conditions can affect installation. If a rod cannot be driven normally, the acceptable alternative and any additional work should be confirmed by the electrician and local inspector rather than assumed.
Do not treat a ground rod as a way to clear a feeder fault. A fault-clearing path normally depends on the feeder equipment grounding conductor and proper bonding back to the source. The grounding electrode system serves a different purpose.
Neutral and ground isolation at the detached subpanel
At a typical detached-building subpanel, the neutral bar is isolated from the metal enclosure and equipment-grounding bar. The feeder neutral terminates on the isolated neutral bar, while the feeder equipment grounding conductor terminates on the grounding bar or approved grounding terminal. Branch-circuit neutral conductors remain separate from equipment grounding conductors.
The main bonding connection belongs at the service disconnecting means or at the location permitted by the applicable system design. Installing the bonding screw or strap in a downstream detached-building subpanel can create objectionable current on metal parts and grounding paths. The exact arrangement must be verified against the locally adopted code, the existing service configuration, and the manufacturer's panel instructions.
A panel sold as a main-breaker panel may include a factory-installed bonding screw or strap. That does not mean the neutral should automatically be bonded in a detached-building feeder panel. The installer must configure the panel for its actual role and preserve the listed separation between neutral and equipment grounding conductors where required.
Main breaker feeds and panel selection
A detached-building panel can be supplied by a feeder breaker in the main service equipment or another approved source panel. The feeder breaker protects the feeder conductors. A main breaker in the detached panel may provide a local disconnecting means or other operational convenience, but it does not increase the feeder's ampacity.
Panel selection should account for the calculated load, available spaces, breaker compatibility, environment, enclosure rating, and manufacturer's instructions. Outdoor panels need an enclosure and fittings suitable for the location. A panel in a garage or shed may also need protection from physical damage depending on its placement and the surrounding activity.
Ask the electrician to identify which device is the feeder overcurrent protection and which device serves as the building disconnect. These are related but not always the same device. The written plan should show the feeder breaker, conductor sizes, raceway or cable type, panel rating, grounding arrangement, and planned branch circuits.
How to compare estimates accurately
- Describe every planned load, including equipment that may be added later.
- Ask for a documented load calculation and confirmation that the existing service has capacity.
- Request the proposed feeder rating, conductor type and size, wiring method, route length, and protection method.
- Confirm whether trenching, concrete work, boring, backfill, landscaping, and surface restoration are included.
- Ask how the neutral will be isolated from the equipment-grounding bar at the detached panel.
- Confirm the proposed grounding electrode work and whether ground rods are included.
- Verify who will obtain the electrical permit, schedule inspection, and correct inspection issues.
- Compare panel brand and model, breaker compatibility, enclosure rating, warranty, and allowances for future circuits.
Permit and inspection requirements vary by municipality and project scope. Before work begins, verify the requirements with the local authority having jurisdiction. The electrician should also follow the panel and conductor manufacturer's installation instructions. Local amendments may differ from general model-code practice.
Common cost mistakes to avoid
- Choosing 100A simply because the panel is labeled 100A, without a load calculation.
- Assuming a ground rod replaces the feeder equipment grounding conductor.
- Using indoor-rated cable or fittings in an outdoor or underground location.
- Comparing bids without checking whether trenching, permits, restoration, and grounding are included.
- Bonding the neutral and ground together in a downstream detached-building subpanel without confirming that the system design requires it.
- Ignoring future loads such as electric vehicle charging, heat, welding, compressors, or high-powered tools.
The most reliable estimate is based on a site visit, a load calculation, a clearly drawn route, and an itemized scope. A low initial bid can become expensive if it excludes excavation, approved transitions, inspection corrections, or restoration.
FAQs
Is a 60A subpanel enough for a detached garage?
It may be enough for lighting, receptacles, a door opener, and moderate tools when the calculated load supports it. Electric heat, welders, compressors, vehicle charging, and other large loads can require more capacity. Have the feeder sized from the actual and planned loads rather than from the panel label.
Can I use SER cable to feed a detached shed?
Possibly, but the cable must be listed and permitted for each part of the route. Outdoor, underground, wet, exposed, and physically vulnerable locations may require a different wiring method or additional protection. The local electrical rules and the cable manufacturer's markings control the acceptable use.
Does a detached garage subpanel always need a ground rod?
A detached building may require a grounding electrode system, but the exact requirement depends on the installation and locally adopted code. Ground rods, where required, do not replace the feeder equipment grounding conductor. Confirm the design with the electrician and local authority having jurisdiction.
Should the neutral and ground be connected in the detached-building panel?
In a typical downstream detached-building subpanel, the neutral is isolated from the equipment-grounding bar and enclosure, while the equipment grounding conductor connects to the grounding bar. The final configuration must follow the adopted code, system design, and panel manufacturer's instructions.