For a 100-amp subpanel (the most common output of a residential sub panel load calculator for a garage or workshop), use 3 AWG copper THHN or 1 AWG aluminum XHHW wire on a 100A double-pole breaker. This baseline assumes standard installation conditions, but real-world jobsite variables frequently force an upsize.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit Type: PVC Schedule 40 or 80, containing a maximum of 3 current-carrying conductors (2 hots, 1 neutral; the equipment grounding conductor is not counted for ampacity derating).
The Baseline Assumptions for Your Load Calculation
Before pulling wire, you must understand how your sub panel load calculator arrived at its amperage figure. The National Electrical Code (NEC) Article 220 dictates two primary methods for residential load calculations: the Standard Method and the Optional Method. Most online calculators use a simplified version of the Standard Method, which requires you to separate your loads into two categories:
- Non-Continuous Loads: Equipment expected to run for less than 3 hours at a time (e.g., table saws, general lighting, receptacles). These are calculated at 100% of their nameplate rating.
- Continuous Loads: Equipment expected to run for 3 hours or more (e.g., EV chargers, HVAC compressors, baseboard heaters, commercial lighting). These must be multiplied by 125%.
If your calculator shows a total load of 75A, but 40A of that is a continuous EV charger load, your actual minimum circuit ampacity is 90A (40A × 1.25 + 35A). This pushes you directly into the 100A breaker and 3 AWG copper wire territory. Never feed raw, unmultiplied continuous loads into a wire sizing chart.
Sizing the Feeder: Ampacity, Voltage Drop, and Derating
Once your calculator provides the final adjusted amperage, you must select a wire that can safely carry that current without overheating, while also ensuring the voltage at the far end remains within acceptable limits.
Ampacity and the "Next Size Up" Rule
According to standard ampacity charts based on NEC Table 310.16, we look strictly at the 75°C column because almost all modern panelboard lugs and breakers are rated for 75°C terminations, regardless of the 90°C rating printed on the THHN wire jacket.
| Wire Size (AWG/kcmil) | Copper Ampacity | Aluminum Ampacity | Max Standard Breaker |
|---|---|---|---|
| 6 AWG | 65A | 50A | 60A / 50A |
| 4 AWG | 85A | 65A | 90A / 70A |
| 3 AWG | 100A | 75A | 100A |
| 2 AWG | 115A | 90A | 125A / 100A |
| 1 AWG | 130A | 100A | 100A (Al) |
Why 3 AWG and not one size smaller (4 AWG)? A 4 AWG copper wire is rated for 85A. You cannot protect an 85A wire with a 100A breaker. While NEC 240.4(B) allows the "next size up" rule (e.g., protecting an 85A wire with a 90A breaker), it does not allow you to protect an 85A wire with a 100A breaker. If your calculated load requires a 100A breaker, the wire's ampacity must meet or exceed 100A. Therefore, 3 AWG copper is the absolute minimum.
Voltage Drop: The Hidden Upsize Trigger
Ampacity only tells you if the wire will melt. Voltage drop tells you if your tools will actually run. The NEC recommends a maximum 3% voltage drop on feeders. Let's run a voltage drop check for our 100A subpanel using 3 AWG copper over a 120-foot run, assuming an 80A continuous load.
Formula: VD = (2 × K × I × L) / Circular Mils
VD = (2 × 12.9 × 80 × 120) / 26,240 = 9.4 Volts
A 9.4V drop on a 240V system is a 3.9% drop. This exceeds the 3% recommendation. To fix this, you must upsize to 2 AWG copper (which drops the VD to roughly 3.1%, still borderline) or 1 AWG copper (dropping it to a safe 2.4%).
- Under 50 feet: Stick to the baseline ampacity size (3 AWG Cu / 1 AWG Al).
- 50 to 100 feet: Upsize one step if continuous loads exceed 60A (2 AWG Cu / 1/0 Al).
- Over 100 feet: Upsize two steps, or run a dedicated 240V/120V calculation using an online voltage drop calculator to prevent motor burnout and dimming lights.
What Changes the Answer: Bundling and Aluminum
Bundling (Derating): If you pull more than three current-carrying conductors in the same conduit (for example, feeding two separate subpanels from the main via the same PVC pipe), NEC Table 310.15(C)(1) requires you to derate the ampacity. Four to six conductors require an 80% multiplier. Your 100A-rated 3 AWG copper drops to 80A, meaning you must upsize to 1 AWG copper just to maintain your 100A breaker rating.
Aluminum vs. Copper: Aluminum and copper are not interchangeable. Aluminum has higher resistance and expands/contracts more under heat. If you choose aluminum (which is significantly cheaper for large gauges), you must use 1 AWG for a 100A feed, and you must apply an anti-oxidant compound (like Noalox) to the terminations to prevent galvanic corrosion and high-resistance arcing over time.
When to Override the Calculator and Call an Engineer
Online sub panel load calculators are excellent for standard residential garages and workshops. However, they cannot account for site-specific electrical physics. You must consult a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ) in the following scenarios:
- High Available Fault Current: If your utility transformer is very close to your main panel, the available short-circuit current might exceed 10,000 Amps (10kAIC). Standard residential breakers are rated for 10kAIC. An engineer must calculate the exact fault current to ensure your subpanel breakers won't physically explode during a dead short, potentially requiring 22kAIC or 42kAIC rated breakers and specialized bracing.
- Utility Transformer Limits: Adding a 100A subpanel for a welder and an EV charger might push your total home load past the utility's transformer rating (often 25kVA or 50kVA for single-family homes). The utility may require a service upgrade before approving the subpanel.
- Local AHJ Amendments: Some municipalities have strict local codes that override the NEC baseline. For example, some jurisdictions require a minimum 100A underground feeder regardless of the calculated load, while others mandate rigid metal conduit (RMC) instead of PVC for physical protection under driveways.
Sub Panel Load Calculator FAQ
How does a sub panel load calculator handle continuous vs non-continuous loads?
A proper calculator requires you to input them separately. Non-continuous loads (like a drill press or standard receptacles) are added at 100% of their amperage. Continuous loads (running 3+ hours, like EV chargers, space heaters, or server racks) are multiplied by 1.25 (125%). The calculator sums these two adjusted figures to give you the minimum feeder ampacity required.
Can I use a 100A sub panel load calculator result with a 60A breaker?
Yes, but with a major caveat. If your calculated load is only 55A, you can install a 60A breaker and use 6 AWG copper wire. However, the subpanel itself must still be rated for at least the calculated load (100A in this scenario). You are simply "choking" the feed at the main panel breaker. This is common when planning for future expansion but currently only running light tooling.
Why does my sub panel load calculator show a different wire size for aluminum?
Aluminum has a lower conductivity per volume than copper (roughly 61% of copper's conductivity). To carry the exact same 100A current at the same 75°C temperature rating without overheating, an aluminum conductor must have a larger cross-sectional area. This is why a 100A copper feed requires 3 AWG, while a 100A aluminum feed requires 1 AWG.
Does the equipment grounding conductor count in load calculations?
No. The equipment grounding conductor (EGC) carries zero current under normal operating conditions; it only carries fault current during a short circuit. Therefore, it is entirely excluded from the sub panel load calculator's amperage math, and it is not counted as a "current-carrying conductor" when applying conduit fill derating factors. However, for a 100A feeder, the NEC still mandates a minimum 8 AWG copper or 6 AWG aluminum EGC to safely clear a fault.
