Wire Size for 100 Amp Service 150 Feet: Exact AWG & Voltage Drop Guide

The correct wire size for a 100 amp service 150 feet away is the smallest conductor that simultaneously satisfies the breaker's thermal ampacity limit and keeps voltage drop under 3% at maximum load. For a standard 240V split-phase residential feeder, you need 2 AWG copper or 1/0 AWG aluminum. Using this exact size prevents your 240V tools and appliances from dropping to 225V under heavy load, which protects motor windings from overheating and stops nuisance breaker tripping. Many DIYers confuse ampacity (the wire's ability to handle heat) with voltage drop (the energy lost over distance), mistakenly pulling 3 AWG copper because it matches the 100A breaker, only to end up with a 3.1% voltage drop that starves heavy inductive loads at the far end of the run.

The Two Rules: Ampacity vs. Voltage Drop

Sizing a long-distance feeder requires satisfying two separate National Electrical Code (NEC) requirements. If you fail either one, the installation is unsafe or non-functional.

1. Ampacity (NEC Article 310.16): This is the thermal limit of the wire. A 100-amp breaker requires a wire rated for at least 100 amps in the 75°C column (the standard rating for most modern panel lugs). According to the ampacity tables, 3 AWG copper (100A) or 1 AWG aluminum (100A) is the absolute minimum to prevent the wire from melting or starting a fire inside the conduit.

2. Voltage Drop (NEC Article 215.2 FPN): While the NEC treats voltage drop as a "Fine Print Note" (recommendation) rather than a strict mandate for most residential feeders, it strongly advises keeping feeder drop under 3%. Over 150 feet, electrical resistance bleeds voltage. If you feed a 100A subpanel with the minimum 3 AWG copper, the resistance of the wire will consume over 3% of your 240V before it reaches the destination.

Worked Numeric Example: Calculating the Drop

To prove why we upsize from the standard ampacity minimum, let's run the exact voltage drop formula: VD = (2 × K × I × D) / CM.

• K = Conductor resistivity (12.9 for copper, 21.2 for aluminum)
• I = Current in amps (100A for worst-case full load)
• D = One-way distance in feet (150 ft)
• CM = Circular mils of the wire cross-section

Let's test the standard ampacity minimums against the 3% limit (which is 7.2V on a 240V system):

As the math shows, 3 AWG copper and 1 AWG aluminum both fail the 3% threshold at a full 100A load over 150 feet. You must step up one size to 2 AWG copper or 1/0 AWG aluminum to deliver clean, stable power to the subpanel. For more on standard calculations, refer to the Southwire Voltage Drop Calculator or the NFPA 70 (NEC) standard documentation.

Where You Meet This in Practice

You will typically encounter the 100-amp, 150-foot scenario in three specific residential and light-commercial builds:

1. Detached Garage with an EV Charger: A 50A Level 2 EV charger combined with a 40A welder and standard lighting easily pushes a 100A subpanel to its continuous limits. Voltage drop here is critical; EV chargers will often throw fault codes and abort charging sessions if input voltage sags below 220V.
2. Backyard Workshops and Barns: Running power 150 feet to a pole barn for a 5HP air compressor. Inductive motor loads draw massive inrush current (LRA). If the feeder wire is undersized, the voltage dip during motor startup will cause the compressor to stall or trip the branch breaker.
3. Secondary Dwelling Units (ADUs): Feeding a detached guest house or studio. These require a full 100A service to handle HVAC, electric ranges, and dryers simultaneously without dimming the lights when the microwave turns on.

Decision Path: Picking Your Exact Conductor

Use this decision tree to lock in your final material and gauge based on your specific jobsite constraints.

The Default Pick: If you have no strict conduit size limits and want the most cost-effective, code-compliant installation for a 150-foot run, buy 1/0 AWG XHHW-2 Aluminum for your hots and neutral, and pair it with a properly sized ground.

Installation & Termination Gotchas

Sizing the wire is only half the battle. Long-distance 100A feeders frequently fail at the termination points due to three common installer errors:

1. The Proportional Grounding Rule (NEC 250.122(B))

For a 100A breaker, the standard Equipment Grounding Conductor (EGC) is 8 AWG copper. However, because you upsized your ungrounded conductors from 3 AWG to 2 AWG (or 1 AWG to 1/0 Al) to mitigate voltage drop, NEC 250.122(B) requires you to upsize the ground wire proportionally. For a 2 AWG copper feeder, you must pull a 6 AWG copper ground. For a 1/0 AWG aluminum feeder, pull a 4 AWG aluminum ground. Inspectors will fail the job if you use the standard 8 AWG ground with upsized feeders.

2. Aluminum Oxidation and Torque

Aluminum wire expands and contracts more than copper under thermal load, which can loosen lugs over time and cause arcing. You must apply an anti-oxidant compound (like Noalox or Penetrox) to the stripped aluminum conductors before terminating. Furthermore, use a calibrated digital torque screwdriver to tighten the panel lugs to the manufacturer's exact specification (typically between 40 and 50 in-lbs for 1/0 AWG). Hand-tightening is a fire hazard.

3. Detached Structure Grounding Electrode

If this 150-foot feeder goes to a detached garage or barn, the equipment grounding conductor (EGC) run with the wires is not enough. Per NEC 250.32, you must also install a local Grounding Electrode System (GES) at the subpanel—typically two 5/8-inch copper ground rods driven 6 feet apart and bonded to the subpanel's ground bar with 6 AWG bare copper. The neutral and ground bars in the subpanel must remain strictly isolated.