What Wire Size for 21.24 Amps at 50 Feet?

10 AWG copper is a typical pick for 21.24A at 50 feet on 120V under a 3% drop target. It balances NEC branch-circuit ampacity and voltage drop over distance. Drop with 10 AWG at these inputs: 2.63V (2.19%). Real-world sizing also depends on insulation temperature rating, cable type, and install conditions.

21.24A at 50ft · 120V single-phase / DC · 3% drop target
10 AWG copper
On a 240V circuit (copper)10 AWG
Voltage drop (120V, copper)2.63V (2.19%)

No aluminum row: every aluminum size in our reference table sits past the 3% drop target at 50 feet on 120V, or the amperage is below the 30A residential threshold where aluminum is not a typical pick. On a higher source voltage, a shorter run, or a looser drop target, aluminum is still the standard feeder material at higher amperages.

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

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

10 AWG branch-circuit OCP (30A under NEC 240.4(D)) ≥ 21.24A ✓

The conductor needs to carry at least 21.24A 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), 10 AWG sits at a branch-circuit OCP of 30A because the NEC 240.4(D) small-conductor rule caps it below the 35A 75°C ampacity table value. That is not a universal number: NM-B cable (Romex) follows the 60°C column in residential use per NEC 334.80 (10 AWG NM-B = 30A), 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 × 50 × 21.24 × 1.24) ÷ (1000 × 120) × 100 = 2.19%

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 12 AWG (one size thinner) at these inputs gives a voltage drop of 4.21V (3.5% on 120V), and its branch-circuit OCP cap under typical conditions is 20A.

Limiting factor here: branch-circuit ampacity. 12 AWG has a branch-circuit OCP cap of 20A under the typical 75°C-termination assumptions used here, which is below the 21.24A load. For this load it shouldn't be used without reassessing against the actual termination temperature, cable type, ambient conditions, and any 240.4(D) or 240.4(B) provisions.

What If You Go One Size Larger?

Using 8 AWG (one size thicker) would reduce voltage drop to 1.65V (1.38% on 120V). More expensive wire but better performance and more headroom for future load increases.

Wattage at This Amperage

21.24A at 120V delivers 2,548.8 watts (DC / resistive load). See conversion.

Related Calculations

trending_down V-drop: 10 AWG, 21.24A, 50ft electric_bolt 21.24A to watts at 120V functions Wire sizing formula

Frequently Asked Questions

21.24A at 50ft on 120V is commonly served by 10 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.
Yes, but you may need thicker wire. At 100ft on 120V, check the wire size calculator. You may need to go up one or two gauges.
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.
Voltage drop scales linearly with distance: doubling the one-way run length doubles the drop in volts. At 21.24A on 120V, a 50ft run is often served by 10 AWG to land under the 3% drop target, a run half that length can sometimes use one gauge thinner, and a run double that length usually needs one or two gauges thicker. Ampacity is set by the conductor itself (Table 310.16 at the applicable termination temperature), so the binding constraint is ampacity on short runs and voltage drop on long runs.
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.
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.
person Written by Sean Day verified Reviewed by WireResult update Last updated Apr 11, 2026