Wire Size for 100 Amp Sub Panel 50 Feet Away: Copper & Aluminum Sizing Guide

For a 100 amp sub panel 50 feet away, use 3 AWG copper or 1 AWG aluminum THHN/THWN-2 conductors, protected by a 100A double-pole breaker. This assumes standard 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the conduit.

The Ampacity Matrix: Sizing Copper and Aluminum Feeders

When sizing feeders, electricians must reference NEC Table 310.16. The table below isolates the exact rows relevant to a 100A feeder, comparing the 75°C and 90°C columns alongside the mandatory Equipment Grounding Conductor (EGC) sizes per NEC Table 250.122.

Wire Size (AWG/kcmil)	Material	75°C Ampacity (Termination Limit)	90°C Ampacity (Derating Base)	Required Copper EGC Size
3 AWG	Copper	100A (Meets 100A Load)	110A	8 AWG
4 AWG	Copper	85A (Fails 100A Load)	95A	8 AWG
1 AWG	Aluminum	100A (Meets 100A Load)	115A	6 AWG
1/0 AWG	Aluminum	120A (Exceeds 100A Load)	135A	6 AWG

Source reference: Ampacities derived from NEC Table 310.16 guidelines for standard residential raceway installations.

Why 3 AWG Copper and Not 4 AWG? (The Termination Rule)

A common mistake on the jobsite is looking at the 90°C column of Table 310.16, seeing that 4 AWG copper is rated for 95A, and assuming it is close enough to 100A, or attempting to use the 90°C rating for the breaker termination. This is a direct violation of NEC 110.14(C).

NEC 110.14(C) mandates that the ampacity of a conductor must be selected based on the lowest temperature rating of any connected device, termination, or conductor. Almost all residential breakers, panel lugs, and disconnect switches are tested and listed for a maximum of 75°C. Therefore, even if your THHN wire has a 90°C insulation rating, you must use the 75°C column to determine the base ampacity at the termination points.

In the 75°C column, 4 AWG copper is only rated for 85A. To legally and safely carry a 100A continuous or non-continuous load to the subpanel lugs, you must step up to 3 AWG copper, which lands exactly at 100A in the 75°C column. (Note: The 90°C column is only permitted to be used as a starting point for applying ambient temperature or bundling derating factors, provided the final derated ampacity still meets the 75°C termination requirement).

Voltage Drop Reality Check at 50 Feet

While ampacity dictates the minimum wire size to prevent melting and fire, voltage drop dictates the efficiency of the circuit. NEC Chapter 2, Article 310 (Informational Note) recommends a maximum voltage drop of 3% for feeders.

Let us run the single-phase voltage drop formula for 50 feet using 3 AWG copper:

At 1.02%, the voltage drop is well under the 3% recommended threshold. Because 50 feet is a relatively short run, there is zero electrical justification to upsize to 2 AWG or 1 AWG copper for voltage drop mitigation. 3 AWG copper is both code-compliant and electrically efficient for this specific distance.

What Changes the Answer? (Derating, Distance, and Load Types)

The 3 AWG copper / 1 AWG aluminum answer holds true only under the base assumptions listed at the top of this guide. Here is the decision tree for when you must deviate from this baseline:

Scenario / Condition	Why It Changes the Math	New Required Wire Size (Copper)
Bundling (4+ Current-Carrying Conductors)	NEC Table 310.15(C)(1) requires an 80% derating factor. Neutral counts as a CCC on circuits with high non-linear harmonic loads.	2 AWG (110A @ 90°C × 0.80 = 88A. 3 AWG fails; 2 AWG yields 104A derated, satisfying the 75°C 100A termination limit).
High Ambient Temperature (e.g., Attic at 50°C/122°F)	NEC Table 310.15(B)(1) requires ambient correction factors. At 50°C, the THHN 90°C correction factor is 0.82.	2 AWG (110A @ 90°C × 0.82 = 90.2A. 3 AWG fails; 2 AWG required to maintain 100A post-correction).
Distance Extended to 150 Feet	Voltage drop scales linearly with distance. At 150 ft, 3 AWG yields a 3.06% drop, violating the 3% feeder recommendation.	2 AWG (Lowers the drop to ~2.4% at full 100A load).
Using Aluminum Instead of Copper	Aluminum has lower conductivity per volume and higher thermal expansion rates at lugs.	1 AWG Aluminum (Minimum). Many electricians prefer 1/0 AWG Aluminum to reduce voltage drop slightly and provide a stiffer, more reliable lug connection.

The Continuous Load Trap (NEC 210.20)

If your subpanel will supply continuous loads (defined by the NEC as loads expected to run for 3 hours or more, such as EV chargers, server racks, or commercial HVAC), you must apply the 125% rule. If the subpanel has 80A of continuous load, the breaker and feeder must be sized at 125% of 80A, which equals 100A. However, if the subpanel has a full 100A of continuous load, the feeder and breaker must be sized at 125A. This would immediately force an upgrade to 1 AWG Copper or 1/0 AWG Aluminum.

When the AHJ or an Engineer Must Confirm

While this guide provides NEC-style guidance for standard residential and light-commercial installations, local Authority Having Jurisdiction (AHJ) interpretations and utility company limitations always hold final authority. You must pull a permit and consult your local inspector or a licensed Professional Engineer (PE) in the following scenarios:

1. Utility Service Limits: If your main home service is only 100A or 150A, adding a 100A subpanel feeder might violate the utility's load calculation requirements. The AHJ will require a whole-house NEC Article 220 load calculation to prove the main service can handle the new subpanel without triggering a service upgrade.
2. Conduit Fill and Jam Ratios: Pulling four 3 AWG wires plus an 8 AWG ground through long sweeps of 1-inch PVC can exceed jam ratios or physical pull tension limits. An engineer may need to specify a larger conduit or a pull box.
3. Grounding Electrode Systems: A subpanel requires its own grounding electrode system (e.g., two ground rods or a ufer ground) if it is in a detached structure (NEC 250.32). The AHJ must inspect the bonding jumper removal and the neutral/ground separation at the subpanel.