Subpanel Size Calculator - NEC Feeder & Breaker Sizing
Calculate required subpanel amperage, feeder conductor gauge, breaker size, voltage drop, and panel spaces with this free subpanel size calculator.
Subpanel Size Calculator
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What Is Subpanel Size Calculator?
The subpanel size calculator determines the required electrical amperage, feeder breaker capacity, conductor wire gauge, voltage drop, and physical panel spaces needed for any subpanel. Whether adding a garage workshop, finishing a basement suite, installing level-2 EV charging, or powering an outbuilding, using a subpanel size calculator prevents electrical overheating, eliminates breaker trips, and ensures compliance with National Electrical Code standards.
- • Detached Garage & Workshop Power: Calculate feeder wire gauge and subpanel breaker size for table saws, air compressors, welders, and lighting.
- • Finished Basement & ADU Suites: Size feeder lines for auxiliary dwelling units containing electric cooking, water heating, and HVAC mini-splits.
- • EV Charging & Pool Equipment: Handle high-amperage continuous loads for EV chargers and pool pumps with 125% continuous duty derating.
- • Outbuildings & Barns: Calculate long-distance feeder voltage drop and upsize conductor gauge over runs exceeding 100 feet.
An electrical subpanel functions as an extension of your home’s main service panel. A single high-capacity 4-wire feeder line delivers power to a centralized subpanel, where branch circuit breakers distribute electricity safely.
Calculating subpanel size requires balancing thermal ampacity and voltage drop. Conductor ampacity must safely carry connected load, while voltage drop over distance should remain below 3% to protect motors and electronics.
When checking individual branch circuit wire gauges or specific equipment conductors, the Amp To Wire Size Calculator provides exact ampacity and distance tables.
How Subpanel Size Calculator Works
Subpanel sizing applies NEC Article 215 feeder calculations, Article 220 load demand factors, and Table 310.16 conductor ampacities.
- Continuous Load Watts: Power consumed for 3 hours or longer (e.g. EV chargers), requiring 125% sizing under NEC 215.2.
- Non-Continuous Watts: Lighting and intermittent appliance loads calculated at 100% connected capacity.
- System Voltage (V): Nominal feeder supply voltage (240V split-phase in standard residential installations).
- K (Resistivity Constant): Resistivity at 75°C: 12.9 ohms-cmil/ft for Copper and 21.2 ohms-cmil/ft for Aluminum per NEC Chapter 9 Table 8.
- Circular Mils (CM): Cross-sectional conductor area determining internal electrical resistance.
This subpanel size calculator matches feeder wires with appropriate overcurrent protection devices per National Electrical Code guidelines.
When feeder runs extend over long distances, electrical resistance causes voltage loss. Keeping voltage drop below 3% ensures maximum equipment performance.
Worked Example: Sizing a 60-Amp Detached Garage Subpanel
Load: 12,000W (40% continuous, 60% non-continuous), 240V, 50ft Aluminum feeder, 3% max drop.
1. Continuous = 4,800W, Non-Continuous = 7,200W.\n2. Design Amps = [(4,800 × 1.25) + 7,200] / 240 = 55.0A.\n3. Breaker Selection: Next standard rating is 60A.\n4. Base Wire: 4 AWG Aluminum rated 65A at 75°C per NEC Table 310.16.\n5. Voltage Drop: (2 × 21.2 × 60 × 50) / 41,740 = 3.05V (1.27% <= 3%).\n6. Ground: 8 AWG Aluminum per NEC Table 250.122.\n7. Conduit: 1-inch trade size for 4 conductors.
60A Breaker, 4 AWG Al Hots & Neutral, 8 AWG Al Ground, 1 inch Conduit, 1.27% Drop, 12 Spaces.
The 60A subpanel safely supports the 55A design load with low voltage loss and expansion room.
According to NFPA 70 National Electrical Code (NEC), subpanel feeder conductors must have an ampacity not less than the non-continuous load plus 125 percent of the continuous load, terminated at equipment rated for 75°C.
To verify protective overcurrent breaker sizing for specific branch appliances feeding from your subpanel, use our Circuit Breaker Size Calculator.
Key Concepts Explained
Understanding core electrical engineering principles ensures your subpanel installation remains safe and code-compliant.
NEC 4-Wire Feeder System (Separated Neutral & Ground)
Subpanels must keep neutral and equipment ground isolated on separate busbars. Floating the neutral prevents return currents from energizing metal panel enclosures, conduit, or equipment frames.
75°C Terminal Temperature Rating Rule
Standard breakers and panel lugs are rated for 75°C terminations. NEC 110.14(C) mandates sizing conductor ampacity from the 75°C column of Table 310.16 to match equipment terminal ratings.
Continuous vs Non-Continuous Load Derating
Under NEC Article 215.2, loads operating continuously for 3 hours or more (such as EV chargers and space heaters) require a 125% multiplier. Non-continuous loads are factored at 100%.
Aluminum vs Copper Feeder Economics
Compact-strand aluminum alloy is the industry standard for subpanel feeders, offering 50% to 70% material cost savings with excellent performance when properly sized and torqued.
When installing a subpanel in a detached building, NEC Article 250.32 mandates dedicated grounding electrodes connected directly to the subpanel ground bus, along with a main disconnecting switch.
For custom electrical runs involving three-phase commercial feeders or low-voltage DC setups, our specialized Voltage Drop Calculator offers comprehensive wire analysis.
How to Use This Calculator
Follow these straightforward steps in the subpanel size calculator to determine your feeder wire gauge, breaker rating, and raceway requirements.
- 1 Select Application or Enter Connected Watts: Choose a preset scenario (e.g. Garage, ADU, EV) or sum the connected wattage of planned lighting and appliances.
- 2 Specify Continuous Load Percentage: Estimate what percentage of total wattage runs continuously for 3 hours or longer (e.g. 80-100% for EV charging).
- 3 Enter Feeder Run Distance & Material: Measure the one-way distance along the conduit path between main panel and subpanel, and select Aluminum or Copper.
- 4 Set Target Voltage Drop & Circuit Slots: Confirm 3% max allowable voltage drop and enter the number of branch circuits planned for the subpanel.
- 5 Review Sizing Summary & Conduit Requirements: Note recommended feeder breaker amperage, hot/neutral/ground wire gauges, conduit trade size, and panel spaces.
For a 150-foot detached workshop requiring 20,000 Watts at 240V with 50% continuous load, the subpanel size calculator factors a 93.75A design current, selects a 100A breaker, and upsizes copper conductors to 2 AWG (2.43% drop) with 1-1/4 inch conduit and 20 spaces.
Once you have selected your feeder wire gauge, verify conduit raceway trade dimensions using the Conduit Size Calculator.
Benefits of Using This Calculator
Using this precision sizing tool protects your property, optimizes material costs, and confirms code compliance.
- • Eliminate Nuisance Tripping & Overheating: Accurate continuous load derating ensures your feeder breaker never overheats during simultaneous appliance operation.
- • Prevent Voltage Drop Motor Burnout: Calculates circular mil requirements over distance, protecting compressor and saw motors from low-voltage damage.
- • Substantial Conductor Material Savings: Compares copper versus aluminum feeder configurations to find the most cost-effective wire and conduit package.
- • Verified National Electrical Code Compliance: Fully aligns with NEC Articles 215, 220, 250, and 310, giving homeowners and electricians confidence during inspections.
- • Future-Proof Electrical Expansion Capacity: Recommends physical panel spaces with a 25% spare buffer so future circuits can be added without replacing panels.
A properly sized subpanel reduces home maintenance costs by shortening branch circuit runs and maximizing voltage stability.
Ensure code compliance for multi-conductor conduit pulls by verifying cross-sectional jamming ratios with our Conduit Fill Calculator.
Factors That Affect Your Results
Several technical variables influence final feeder conductor size, conduit dimensions, and panel ratings.
Main Electrical Service Headroom
Your existing main panel capacity (100A, 150A, or 200A) determines the maximum subpanel feeder breaker that can be installed.
Ambient Temperature Derating Factors
Conduits routed through hot attics or rooftops require ampacity correction under NEC Table 310.15, potentially requiring larger wire.
Raceway Conductor Bundling Derating
When more than 3 current-carrying conductors share a conduit, NEC Table 310.15 requires derating base ampacities by 80% to 70%.
Conduit Fill & Pulling Constraints
NEC Chapter 9 Table 1 limits raceway fill to 40% for three or more conductors to prevent insulation damage during installation.
- • This calculator computes feeder sizing based on standard NEC demand factors; complex commercial facilities with harmonic distortion require specialized engineering analysis.
- • Subpanel installation must follow local municipal building codes, which may impose stricter grounding, disconnect, or trench depth requirements.
Before purchasing wire and hardware, always verify that your existing main electrical service has adequate surplus ampacity to support the new subpanel.
According to IEEE Standard 141 Electric Power Distribution, conductors for feeders sized to prevent a voltage drop exceeding 3 percent at the farthest outlet provide reasonable efficiency of operation.
Before installing a new subpanel, confirm that your existing main service panel has sufficient surplus capacity with the Electrical Load Calculator.
Frequently Asked Questions
Q: How do I determine what subpanel size (amperage) I need?
A: Sum the total connected wattage of all planned branch circuits, apply a 125% multiplier to continuous loads (running 3+ hours) and 100% to non-continuous loads, then divide by voltage (240V). Select the next standard breaker size: 60A is common for basic garages, 100A for workshops/ADUs, and 125A-200A for heavy EV and heating additions.
Q: Can a subpanel have a higher amp rating than the main panel or feeder breaker?
A: Yes. A subpanel busbar can have a higher rating (e.g. installing a 100A-rated subpanel fed by a 60A breaker in the main panel). The feeder conductors and subpanel are fully protected by the 60A supply breaker. The higher bus rating simply provides more physical circuit breaker slots.
Q: What feeder wire gauge and circuit breaker size are needed for a 60A or 100A subpanel?
A: For a 60A subpanel at 75°C, use 6 AWG Copper or 4 AWG Aluminum with a 60A feeder breaker. For a 100A subpanel, use 3 AWG Copper (or 1 AWG Cu for long runs) or 1 AWG Aluminum (or 1/0 AWG Al) with a 100A feeder breaker.
Q: Why must the neutral and ground buses be separated (isolated) in a subpanel?
A: In a subpanel, the neutral bus must remain floating (unbonded) and the green bonding screw removed. Neutral carries return current, while ground carries fault current only. Bonding them in a subpanel creates dangerous parallel ground return paths through metal conduit and equipment cabinets.
Q: How does distance affect the feeder wire size for a subpanel?
A: Wire resistance causes voltage drop over distance. On runs over 50 to 100 feet, voltage drop can exceed the recommended 3% limit. Stepping up conductor gauge increases circular mil cross-sectional area, reducing resistance and keeping delivered voltage within acceptable operating tolerances.
Q: How many breaker spaces or slots should I choose for my subpanel?
A: Choose a panel with at least 20% to 25% more spaces than your immediate circuit count. For a typical workshop with 8 planned circuits, choose a 12-space or 16-space panel. For an ADU or basement suite with 12 to 14 circuits, choose a 20-space to 24-space panel.