Garage Subpanel Calculator
Calculate feeder requirements for garage subpanels and detached building electrical service. Complete NEC Article 225 compliant calculations for safe, code-compliant installations.
Detached Building Requirements
- • Feeder disconnect required at detached building per NEC 225.31
- • Equipment grounding conductor required with ALL feeders per NEC 225.30
- • Grounding electrode system required if none exists at building
- • Maximum 6 disconnects per building location per NEC 225.33
- • Professional installation recommended - improper grounding can be deadly
Real-World Installation Examples
Standard 2-Car Detached Garage
100A subpanel, 75ft run, mixed loads
Load Breakdown: Lighting: 20A, Outlets: 40A, Garage Door: 15A, Welder: 50A
Calculation Details:
Total Connected Load: 125A Demand Calculation: 125A × 0.8 = 100A Feeder Size: 100A minimum Voltage Drop Check: 100A × 75ft × 2.6Ω/1000ft ÷ 240V = 8.1% (too high) Upsize to 2 AWG: 100A × 75ft × 1.6Ω/1000ft ÷ 240V = 5.0% Grounding: 6 AWG equipment ground required
Workshop with Heavy Equipment
200A subpanel, 100ft underground run
Load Breakdown: Table Saw: 30A, Dust Collector: 20A, Compressor: 40A, Welder: 60A, General: 50A
Calculation Details:
Total Load: 200A nameplate Demand Factor (industrial): 200A × 1.0 = 200A Feeder: 200A service Underground Run: USE-2 or RHW-2 required Wire Size: 3/0 AWG copper (200A rating) Voltage Drop: 200A × 100ft × 1.0Ω/1000ft ÷ 240V = 8.3% Upsize: 250 kcmil → 6.7% drop Grounding: 4 AWG equipment ground
Basic Garage with EV Charging
60A subpanel, EV charger + basic loads
Load Breakdown: EV Charger: 40A, Lighting: 15A, Outlets: 20A
Calculation Details:
EV Charger: 40A continuous load Basic Loads: 35A @ 75% demand = 26A Total Demand: 40A + 26A = 66A Feeder Size: 70A → use 80A standard Wire Size: 4 AWG copper (85A capacity) Grounding: 8 AWG equipment ground Voltage Drop: Acceptable for typical runs
Detached Shop - 3-Phase Service
100A 3-phase, metal fabrication shop
Load Breakdown: Welder: 75A, Mill: 40A, Lathe: 30A, General: 55A
Calculation Details:
3-Phase Load Analysis: Total Load: 200A per phase Demand Factor: 200A × 0.85 = 170A Balanced Load: 170A per phase Feeder: 175A → 200A service Wire: 3/0 AWG per phase + neutral Grounding: 4 AWG equipment ground Service Disconnect: 200A 3-pole
RV Garage with Multiple Services
Large garage, RV hookup + workshop
Load Breakdown: RV Service: 50A, Shop: 100A, Compressor: 30A, Welder: 60A
Calculation Details:
Multiple Services Analysis: RV Service: 50A dedicated (NEC 551) Shop Loads: 190A connected Shop Demand: 190A × 0.8 = 152A Main Feeder: 200A to handle both Separate RV disconnect required Feeder Wire: 3/0 AWG copper Multiple disconnect rule: 6 max per NEC 225.33
Barn/Agricultural Building
Livestock facility, 150A service
Load Breakdown: Ventilation: 60A, Lighting: 30A, Outlets: 40A, Water: 20A
Calculation Details:
Agricultural Load Calculation: Total Connected: 150A Demand Factor (ag loads): varies by type Continuous Loads: Ventilation 60A × 1.25 = 75A Other loads: 90A × 0.8 = 72A Total Demand: 75A + 72A = 147A → 150A Feeder: 1/0 AWG copper Special: GFCI for wet locations Grounding: Enhanced for livestock
NEC Article 225 - Detached Building Requirements
Feeder Disconnect
NEC 225.31Disconnect required at building
Must disconnect all ungrounded conductors, rated for feeder load, lockable
Critical SafetyService Entrance
NEC 225.32Location of disconnect
Outside building or inside nearest point of entrance, readily accessible
Code ComplianceMaximum Disconnects
NEC 225.336 disconnect rule
Maximum 6 disconnects per building, each clearly marked
Safety & CodeGrouping of Disconnects
NEC 225.34Grouped together
All disconnects must be grouped, exception for large buildings
Emergency AccessGrounding Requirements
NEC 225.30Equipment grounding
Equipment grounding conductor required with feeder, sized per Table 250.122
Electrical SafetyOverhead Clearances
NEC 225.18Minimum clearances
10ft min over driveways, 12ft over residential, 18ft over public areas
Physical SafetyUnderground Installation
NEC 225.27Underground feeders
24" burial depth, conduit required in most cases, warning tape recommended
Installation StandardFeeder Sizing
NEC 225.39Minimum size
Sized for calculated load, 60A minimum for dwellings over 80A main service
Load CapacityFeeder Sizing Quick Reference
| Service Size | Feeder Wire | Equipment Ground | Conduit Size | Typical Application |
|---|---|---|---|---|
| 60A | 6 AWG Cu / 4 AWG Al | 10 AWG | 1.25" | Small garage, basic loads |
| 100A | 3 AWG Cu / 1 AWG Al | 8 AWG | 1.5" | Standard garage, moderate loads |
| 125A | 1 AWG Cu / 2/0 AWG Al | 8 AWG | 2" | Large garage with EV charging |
| 150A | 1/0 AWG Cu / 3/0 AWG Al | 6 AWG | 2" | Workshop with heavy equipment |
| 200A | 3/0 AWG Cu / 250 kcmil Al | 6 AWG | 2.5" | Professional shop, welding |
Voltage Drop Reference Table
Maximum current capacity for 5% voltage drop at 100ft run, 240V single-phase
| Wire Size | Resistance | Ampacity Rating | Max Current (100ft) |
|---|---|---|---|
| 10 AWG Cu | 1.24 Ω/1000ft | 30A | 24A (5% drop) |
| 8 AWG Cu | 0.78 Ω/1000ft | 40A | 39A (5% drop) |
| 6 AWG Cu | 0.49 Ω/1000ft | 55A | 62A (5% drop) |
| 4 AWG Cu | 0.31 Ω/1000ft | 70A | 98A (5% drop) |
| 3 AWG Cu | 0.25 Ω/1000ft | 85A | 122A (5% drop) |
| 2 AWG Cu | 0.20 Ω/1000ft | 95A | 152A (5% drop) |
| 1 AWG Cu | 0.15 Ω/1000ft | 110A | 203A (5% drop) |
| 1/0 AWG Cu | 0.12 Ω/1000ft | 125A | 253A (5% drop) |
Installation Cost Breakdown
Materials
$800-$3,500Wire size, conduit, panel, disconnect
Trenching (100ft)
$600-$1,200Depth, soil type, obstacles
Conduit Installation
$400-$800Underground vs overhead, length
Panel Installation
$300-$600Size, complexity, grounding
Inspection Fees
$150-$300Local jurisdiction requirements
Total Project
$2,250-$6,400Complete installation with permits
Note: Costs vary significantly by location, accessibility, and local labor rates. Always obtain multiple quotes from licensed electricians for accurate pricing.
Grounding System Requirements
Equipment Grounding Conductor
Requirement: Required with all feeders per NEC 225.30
Sizing: Table 250.122 - based on feeder OCPD rating
Installation: Run with feeder conductors in same raceway
Connection: Bonded to main panel ground and building ground rod
Grounding Electrode
Requirement: Required if no electrode exists at building
Sizing:
Installation:
Connection:
Bonding Requirements
Requirement: Metal parts connected to electrical system
Sizing:
Installation:
Connection:
Common Installation Mistakes
Undersized Equipment Ground
Consequence: Code violation, safety hazard
Correction: Size per Table 250.122 based on OCPD rating, not wire size
Repair Cost: Rewiring required - $500-$1,500
No Disconnect at Building
Consequence: Major code violation, failed inspection
Correction: Install listed disconnect switch at building entrance
Repair Cost: Disconnect + installation - $400-$800
Improper Burial Depth
Consequence: Safety hazard, code violation
Correction: Dig up and reinstall at proper depth (24" minimum)
Repair Cost: Re-excavation + labor - $800-$1,600
Multiple Neutrals in Panel
Consequence: Code violation, potential fire hazard
Correction: Separate neutral and ground bars in subpanel
Repair Cost: Panel modification - $200-$400
No Ground Rod at Building
Consequence: Incomplete grounding system
Correction: Install grounding electrode system per NEC 250
Repair Cost: Ground rod + connection - $150-$300
Voltage Drop Not Calculated
Consequence: Poor performance, equipment damage
Correction: Calculate drop, upsize wire if necessary
Repair Cost: Wire upgrade - $300-$1,200
Regional Code Variations
California
Requirements: Title 24 energy compliance, seismic considerations
Additions: EV-ready requirements, solar-ready provisions
Authority: CEC (California Electrical Code)
Florida
Requirements: Hurricane resistance, concrete encasement
Additions: Special flood zone requirements, corrosion protection
Authority: Local amendments to NEC
New York
Requirements: Frost depth considerations (4ft+), city permits
Additions: NYC has specific underground requirements
Authority: NYS Electrical Code + local amendments
Texas
Requirements: Heat considerations, expansive soil protection
Additions: Some areas require concrete encasement
Authority: NEC with local modifications
Minnesota
Requirements: Deep frost protection, heating considerations
Additions: Cold weather installation practices
Authority: Minnesota Electrical Code
Maintenance Schedule
Visual inspection of disconnect and panel
Check for damage, corrosion, loose connections
Professional RequiredGFCI testing (if present)
Test and reset all GFCI devices
Professional RequiredElectrical connection inspection
Check panel connections, torque specifications
Professional RequiredGround system testing
Test grounding electrode resistance
Professional RequiredComplete system inspection
Professional inspection of entire feeder system
Professional RequiredFrequently Asked Questions
What size feeder do I need for a 100 amp garage subpanel?▼
For a 100A subpanel, you need 3 AWG copper or 1 AWG aluminum feeder conductors, sized for 100A continuous load. The equipment grounding conductor must be 8 AWG copper minimum per NEC Table 250.122. Always check voltage drop calculations for your specific run length.
Can I run a subpanel feeder overhead vs underground?▼
Yes, both overhead and underground feeders are allowed. Overhead requires minimum clearances per NEC 225.18 (10ft over driveways, 12ft residential). Underground requires 24" burial depth with proper conduit. Underground is often preferred for aesthetics and weather protection.
Do I need a separate ground rod for my detached garage?▼
If the building has no existing grounding electrode (ground rod, water pipe, etc.), then yes, you must install a grounding electrode system per NEC 250.32. This is in addition to the equipment grounding conductor run with the feeder.
What is the maximum distance for a garage subpanel feeder?▼
There is no NEC distance limit, but voltage drop becomes the limiting factor. For 100A loads, practical limits are typically 150-200 feet before wire upsizing becomes necessary to maintain acceptable voltage drop (3-5% maximum recommended).
Can I use the same feeder for garage and workshop loads?▼
Yes, you can size one feeder for combined loads using demand factors. Calculate the total demand load, apply appropriate demand factors per NEC Article 220, and size the feeder accordingly. A single disconnect can serve multiple buildings if properly sized.
What permits are required for garage subpanel installation?▼
Most jurisdictions require electrical permits for subpanel installation. The work typically requires inspection of the feeder installation, grounding system, and panel connections. Check with your local building department for specific requirements and fees.
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