Sizing a Subpanel for a Workshop: Load Inventory, Feeder Calculation, and Breaker Selection

Sizing a subpanel for a workshop or detached garage is not a matter of picking a round number and hoping it works. The NEC requires a load calculation that accounts for every circuit you plan to install, applies the correct demand factors, and then sizes the feeder conductors and overcurrent protection to match. Get it wrong, and you either end up with a subpanel that trips under normal use or one that costs hundreds more than necessary.

This guide walks through the complete process for sizing a subpanel for a workshop or garage—from building a load inventory through feeder conductor selection with voltage drop verification.

Step 1: Build the Load Inventory

Before you can size anything, you need to know what the subpanel will feed. List every circuit with its load in VA (volt-amperes) or amps. For a typical detached workshop, the inventory looks something like this:

Step 2: Apply NEC Demand Factors

You do not size the subpanel for the full connected load because not everything runs simultaneously. The NEC provides demand factors that reduce the calculated load to reflect realistic usage patterns.

For a workshop subpanel that is part of a residential service, you can apply the NEC 220.82/120.82 optional method for the overall service calculation, but the feeder to the subpanel is typically calculated using standard Article 220 demand factors:

• General lighting and receptacles: first 3,000 VA at 100%, remainder at 35% per NEC Table 220.42
• Fixed appliances (4 or more): 75% demand factor per NEC 220.53
• Motor loads: largest motor at 125% per NEC 430.24
• EV charger: continuous load, so the conductor must be sized at 125% of the load current per NEC 210.20(A)

Step 3: Select the Subpanel Size

The calculated demand of 129.4A means you need at least a 150A subpanel (next standard size up). A 100A panel would be undersized. A 200A panel provides room for future expansion but requires larger (and more expensive) feeder conductors.

Consider these factors when choosing between 150A and 200A:

• Future loads: if you anticipate adding a second EV charger, battery storage, or additional power tools, size up to 200A now—running a new feeder later costs far more than oversizing the initial installation
• Main panel capacity: verify the main panel has enough spare capacity to feed the subpanel. A 200A main service feeding a 150A subpanel with a full house load may require a service load calculation to confirm capacity.
• Circuit spaces: a 150A panel typically has 30–42 spaces; a 200A panel has 40–42 spaces. For a workshop with 8–12 circuits, either provides adequate room.

Step 4: Size the Feeder Conductors

The feeder from the main panel to the subpanel must be sized for the calculated load. For a 150A subpanel with 129.4A calculated demand:

For a cost-effective installation, many contractors choose aluminum for subpanel feeders of this size. A 3/0 aluminum feeder costs significantly less per foot than 1/0 copper while providing equivalent ampacity.

Step 5: Check Voltage Drop on the Feeder

This is the step that separates a code-minimum installation from a reliable one. For a detached workshop, the feeder run is often 75–150 feet or more. NEC 215.2(A) Informational Note recommends keeping feeder voltage drop at or below 3%, with the total (feeder plus branch circuit) at or below 5%.

Step 6: Size the Overcurrent Protection

The feeder breaker in the main panel must not exceed the ampacity of the feeder conductors. For 3/0 aluminum at 155A (75°C column), the next standard breaker size down that does not exceed the conductor ampacity is 150A. Per NEC 240.4(B), you may use the next standard size up only when the conductor ampacity does not match a standard breaker rating—155A is not a standard size, so a 150A breaker works.

Step 7: Four-Wire Feeder and Grounding

A subpanel in a detached structure requires a four-wire feeder: two hots, a neutral, and an equipment grounding conductor (EGC). Per NEC 250.32(B), the neutral and ground must be kept separate at the subpanel—no neutral-ground bond in the sub. Remove the bonding screw or strap in the subpanel.

Size the EGC per NEC Table 250.122 based on the feeder overcurrent device:

For a detached structure, NEC 250.32(A) also requires a grounding electrode system at the workshop (typically a ground rod or concrete-encased electrode). This is in addition to the EGC in the feeder.

Quick-Reference: Workshop Subpanel Sizing Summary

Permit Checklist

Before pulling wire, most AHJs require a permit application with:

• Load calculation worksheet showing all circuits and demand factors applied
• Feeder conductor size, type, and installation method
• Subpanel rating and number of spaces
• Grounding electrode system for the detached structure
• Conductor sizing justification referencing NEC table numbers

An inspector will verify that the installed feeder matches the permitted load calculation. Showing up with a 100A subpanel when the load calculation demands 150A means pulling wire twice.