The correct wire size for a 100 amp sub panel is 3 AWG copper or 1 AWG aluminum, protected by a 100A 2-pole breaker. This assumes standard 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a single conduit.
The Core Sizing Assumptions (And Why They Matter)
Wire sizing is never a single universal number; it is the result of specific environmental and material variables. The 3 AWG copper / 1 AWG aluminum answer relies on a strict set of baseline assumptions. If your installation deviates from these, your required wire gauge will change.
- Conductor Material: Copper (Cu) or Aluminum (Al)
- Insulation Type: THHN/THWN-2 (rated for 90°C in wet/dry locations)
- Termination Temperature: 75°C (Standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Single raceway with a maximum of 3 current-carrying conductors (2 hots, 1 neutral)
- System Voltage: 240V/120V single-phase split system
The most common point of failure for DIYers is ignoring the termination temperature rule. According to NEC 110.14(C), even if your THHN wire is rated for 90°C, the ampacity must be based on the 75°C column of NEC Table 310.16 because standard panel lugs and breaker terminals are only rated to 75°C. You can use the 90°C column for derating purposes, but the final baseline ampacity cannot exceed the 75°C limit.
Ampacity, Voltage Drop, and the 150-Foot Rule
Why 3 AWG and not one size smaller? A 4 AWG copper wire only provides 85 amps of capacity in the 75°C column. Because the National Electrical Code (NEC) requires the conductor ampacity to be equal to or greater than the overcurrent protective device (the 100A breaker), 4 AWG is a code violation. 3 AWG copper lands exactly at 100A in the 75°C column.
| Wire Size (AWG) | Material | 75°C Column (Termination Limit) | 90°C Column (Derating Baseline) |
|---|---|---|---|
| 4 AWG | Copper | 85A (Fails 100A requirement) | 95A |
| 3 AWG | Copper | 100A (Passes) | 110A |
| 2 AWG | Copper | 115A | 130A |
| 2 AWG | Aluminum | 90A (Fails 100A requirement) | 105A |
| 1 AWG | Aluminum | 100A (Passes) | 120A |
The Voltage Drop Check at 150 Feet
Ampacity keeps the wire from melting; voltage drop keeps your equipment from burning out. While the NEC treats voltage drop as an informational recommendation rather than a strict mandate for most residential feeders (see NEC 210.19(A) Informational Note No. 4), best practice dictates a maximum 3% drop for feeders.
For a 240V system, a 3% drop equals 7.2 volts. Using standard voltage drop formulas (VD = 2 × K × I × D / CM) for 3 AWG copper (Circular Mil area = 52,620) carrying a full 100A load:
- At 100 feet, the drop is roughly 4.9V (2.0%). Safe.
- At 146 feet, the drop hits exactly 7.2V (3.0%). This is the maximum distance for 3 AWG.
- At 150 feet, the drop exceeds 3%. You must upsize to 2 AWG copper to maintain acceptable voltage regulation at the subpanel.
What Changes the Answer? (Derating and Material Swaps)
Real-world jobsites rarely match textbook assumptions. Use this decision tree to determine if your specific installation requires upsizing the feeder conductors.
| Condition | Impact on Sizing | Required Action |
|---|---|---|
| Aluminum Conductors | Lower conductivity requires larger gauge. Prone to oxidation and thermal creep at lugs. | Use 1 AWG Aluminum. Apply Noalox anti-oxidant paste and torque lugs to manufacturer specs (usually 40-50 in-lbs). |
| Conduit Bundling (4-6 wires) | NEC 310.15(C)(1) requires an 80% derating factor for 4-6 current-carrying conductors. | 3 AWG Cu (110A @ 90°C × 0.80 = 88A) fails. Upsize to 1 AWG Copper (130A @ 90°C × 0.80 = 104A). |
| High Ambient Temp (e.g., Attic) | Temperatures above 30°C (86°F) reduce the wire's ability to shed heat. | Apply Table 310.15(B)(1) correction factors. At 40°C (104°F), multiply 90°C ampacity by 0.91. |
| Continuous Loads (>3 Hours) | NEC 210.20(A) requires conductors and breakers to be sized at 125% of continuous loads. | If the subpanel feeds a continuous 80A load (like EV charging), 80A × 1.25 = 100A. 3 AWG Cu is fine. If continuous load is 100A, upsize to 125A feeder. |
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline, the local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer must sign off on your design under the following conditions:
- Utility Transformer Limitations: If your main service is only 150A, adding a 100A subpanel alongside existing loads might trigger a utility demand calculation. The utility may require a service upgrade before approving the tap.
- Feeder Taps (NEC 240.21): If you are tapping a 200A main breaker to feed a 100A subpanel using smaller wire (the 10-foot or 25-foot tap rule), the exact routing, physical protection, and termination points must be verified by the inspector to ensure the smaller wire is adequately protected from short circuits.
- Multi-Family or Commercial Dwellings: If this subpanel feeds a detached ADU (Accessory Dwelling Unit) or commercial space, local zoning and fire codes often require stamped engineering drawings for the feeder and grounding electrode system.
Frequently Asked Questions
Can I use 4 AWG wire for a 100 amp sub panel?
No. While 4 AWG copper wire has an ampacity of 95A in the 90°C column, NEC 110.14(C) mandates that you use the 75°C column for termination sizing unless the equipment is explicitly rated otherwise. In the 75°C column, 4 AWG copper is only rated for 85 amps. Installing it on a 100A breaker is a direct code violation and a fire hazard. You must use a minimum of 3 AWG copper.
What size ground wire do I need for a 100 amp sub panel?
According to NEC Table 250.122, the minimum Equipment Grounding Conductor (EGC) for a 100A feeder is 8 AWG copper or 6 AWG aluminum. Crucially, at a subpanel, the ground busbar and the neutral busbar must be kept strictly isolated. Do not install the green bonding screw or jumper strap that comes pre-installed in the subpanel; that bond only belongs at the main service disconnect.
Does a 100 amp sub panel mean I can pull 100 amps continuously?
No. The NEC defines a continuous load as one where the maximum current is expected to persist for three hours or more (like HVAC, EV chargers, or server racks). For continuous loads, you must apply a 125% safety multiplier. Therefore, a 100A breaker and 3 AWG wire can only safely support an 80A continuous load. If your calculated continuous load exceeds 80A, you must upsize the breaker to 125A and the wire to 1 AWG copper.
