What Wire Size for 171.57 Amps at 125 Feet?

3/0 AWG copper is a typical pick for 171.57A at 125 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 3/0 AWG at these inputs: 3.29V (2.74%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.

171.57A at 125ft · 120V single-phase / DC · 3% drop target

3/0 AWG copper

Aluminum option4/0 AWG

On a 240V circuit (copper)2/0 AWG

Voltage drop (120V, copper)3.29V (2.74%)

check_circle Within the 3% branch and 5% feeder+branch total drop targets

Amps

Feet

Gauge (Cu / Al)

3/0 AWG Cu / 4/0 AWG Al

Assumes a 120V source on a single-phase / DC circuit and a 3% voltage-drop target. Each material is picked independently against the same target, so the copper and aluminum results are two separate recommendations, not an ampacity equivalence. Switch to three-phase L-L →

How Wire Size Is Determined

Step 1: NEC Branch-Circuit Ampacity

3/0 AWG branch-circuit OCP (200A) ≥ 171.57A ✓

The conductor needs to carry at least 171.57A without going past its temperature rating, and the OCP protecting it needs to respect the NEC branch-circuit cap. Under the typical assumptions used in this table (copper, 75°C termination, no bundling or ambient derates), 3/0 AWG sits at a branch-circuit OCP of 200A. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (3/0 AWG NM-B = 165A), bundling more than three current-carrying conductors requires a 310.15(C)(1) adjustment, ambient temperatures above 30°C require a 310.15(B) correction, and 60°C terminations on typical residential equipment can pull the usable value lower still. Use the nameplate and local code for the actual install value.

Step 2: Voltage Drop Check

%VD = (2 × L × I × R) ÷ (1000 × V) × 100 (single-phase / DC; round-trip factor of 2)

(2 × 125 × 171.57 × 0.0766) ÷ (1000 × 120) × 100 = 2.74%

NEC 210.19(A) Informational Note 4 recommends ≤ 3% for branch circuits and ≤ 5% for feeder + branch total as performance targets, not hard code requirements. This run sits within the 3% target used for this calculation.

Practical Information

What If You Go One Size Smaller?

Using 2/0 AWG (one size thinner) at these inputs gives a voltage drop of 4.15V (3.46% on 120V), and its branch-circuit OCP cap under typical conditions is 175A.

Limiting factor here: voltage drop, not ampacity. 2/0 AWG is still above the 171.57A load at its 175A branch-circuit OCP cap, so the conductor temperature margin is fine for this run. What pushes it off this page's pick is the 3.46% drop sitting past the 3% target, which is a performance recommendation (NEC 210.19(A) Informational Note 4), not a code requirement. On shorter runs or at higher source voltage the same gauge would often clear the drop target too.

What If You Go One Size Larger?

Using 4/0 AWG (one size thicker) would reduce voltage drop to 2.61V (2.17% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

171.57A at 120V delivers 20,588.4 watts (DC / resistive load). See conversion.

Related Calculations

Frequently Asked Questions

What wire size for 171.57 amps at 125 feet on 120V?expand_more

171.57A at 125ft on 120V is commonly served by 3/0 AWG copper to land under the 3% voltage-drop target, under the typical 75°C-termination assumptions used in this table. Actual install sizing also depends on conductor material, insulation and termination temperature rating, cable type, ambient and bundling conditions, and local code.

How much does 3/0 AWG wire cost?expand_more

Copper wire pricing tracks the LME copper spot price and varies with insulation type, cable assembly (THHN, NM-B, MC, SE, USE), and quantity. Check current pricing with a local electrical supply house or distributor catalog; commodity-driven numbers inlined on a calculator page age quickly.

Can I run 171.57A on a longer distance?expand_more

Yes, but you may need thicker wire. At 250ft on 120V, check the wire size calculator. You may need to go up one or two gauges.

What if I use a thinner wire than recommended?expand_more

It depends on which factor the thinner gauge violates. If its branch-circuit ampacity is still at or above the load, the limiting factor is usually voltage drop (a performance recommendation per NEC 210.19(A) Informational Note 4, not a hard code requirement) and the symptom is dimming lights, motor startup issues, or wasted energy as I²R losses. If the thinner gauge is actually below the load's ampacity ceiling at the relevant termination temperature, that is a conductor-heating / code compliance issue, and the wire should not be used for that load. A calculator page cannot tell you which category applies to your install: verify against the conductor type, termination temperature, and install conditions.

What is the NEC voltage drop recommendation?expand_more

NEC 210.19(A) Informational Note 4 recommends ≤3% for branch circuits and ≤5% total (feeder + branch). These are performance recommendations, not hard code requirements.

This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.