Wire sizing guide

Wire Size for a 100 Foot Run

At 100 feet, voltage drop — not just ampacity — controls your wire size. The right gauge depends on your amperage and source voltage. Use the table below for quick reference or run your exact scenario through the calculator.

Calculate voltage drop for your run View ampacity table
The core issue

Why 100 feet changes the sizing equation

Wire sizing has two constraints: ampacity (can the wire carry the current without overheating?) and voltage drop (does enough voltage reach the load for it to operate correctly?). For short runs, ampacity is usually the only constraint. For 100-foot runs, voltage drop almost always forces you to use larger wire than ampacity alone requires.

The physics is straightforward: every foot of conductor has resistance. At 100 feet one-way, the current travels 200 feet round-trip. The voltage lost across that resistance is Vd = 2 × I × R × L / 1000, where I is current, R is resistance per 1000 ft, and L is one-way length. Double the distance, double the drop.

The NEC recommends keeping branch circuit voltage drop under 3%, and total (feeder + branch) under 5%. At 120V, 3% is just 3.6 volts. At 240V, 3% is 7.2 volts — which is why higher voltage circuits are much more forgiving on long runs.

120V vs. 240V

How source voltage affects wire sizing

The same absolute voltage loss represents a very different percentage depending on source voltage:

  • At 120V: a 4V drop is 3.3% — already over the recommended 3% limit. At 100 feet with any meaningful load, you almost always need to upsize beyond the minimum ampacity wire.
  • At 240V: a 4V drop is only 1.7% — well within limits. The same wire that fails at 120V may work perfectly at 240V.

This is why, whenever practical, long runs benefit from 240V circuits. A 240V, 20 amp circuit can use the same 12 AWG wire that would fail voltage drop limits at 120V over 100 feet.

Quick reference

Wire size for 100 ft run — copper, under 3% drop

Load (amps) At 120V At 240V Ampacity minimum
15A 10 AWG 14 AWG 14 AWG
20A 8 AWG 12 AWG 12 AWG
30A 6 AWG 10 AWG 10 AWG
40A 4 AWG 8 AWG 8 AWG
50A 4 AWG 6 AWG 6 AWG
60A 3 AWG 6 AWG 6 AWG
100A 1/0 AWG 2 AWG 1 AWG

Values assume copper conductors, 75°C insulation, standard conditions. Aluminum requires 1–2 sizes larger. Always verify with a voltage drop calculator for your exact parameters.

Common scenarios

Typical 100-foot installations

  • Detached garage sub-panel (60A, 240V): 6 AWG copper or 4 AWG aluminum meets both ampacity and voltage drop. A very common residential feeder run.
  • Outdoor lighting circuit (15A, 120V): needs 10 AWG copper — two full sizes above the 14 AWG ampacity minimum. This catches many people off guard.
  • Workshop 20A circuit (120V): needs 8 AWG copper. 12 AWG would drop about 5% — enough to cause tools to run hot or underperform.
  • Well pump (20A, 240V): 12 AWG copper works — the 240V source keeps the drop at roughly 2.5%, under the 3% target.
  • Barn or outbuilding sub-panel (100A, 240V): 2 AWG copper or 1/0 aluminum for ampacity, but voltage drop at 100 ft pushes toward 1/0 copper or 2/0 aluminum.
Cost perspective

Upsizing is cheaper than problems

Voltage drop on long runs causes real problems: motors overheat, LED drivers flicker, compressors fail to start, and sensitive electronics malfunction. The cost of upsizing wire one gauge at 100 feet is typically $50–150 in additional material — far less than a callback, equipment damage, or failed inspection.

As a practical rule for 100+ foot runs: calculate first, then buy wire. The few minutes spent running numbers save hours of rework. Use the voltage drop calculator to check your exact amperage, voltage, material, and run length.

Run your voltage drop calculation
Assumptions

Limitations of this guide

  • Wire sizes assume copper conductors at 75°C unless noted. Aluminum requires 1–2 gauge sizes larger for the same ampacity.
  • Voltage drop estimates use the standard two-conductor loop model: Vd = 2 × I × R × L / 1000.
  • The 3% voltage drop target is an NEC recommendation, not a hard code requirement. Some loads tolerate more; some sensitive equipment needs less.
  • Conduit fill, ambient temperature derating, and continuous load factors may require further upsizing beyond what is shown here.
  • For aluminum conductors, use AL-rated terminals and anti-oxidant compound per manufacturer instructions.
FAQ

100-foot run wire sizing questions

What size wire for a 100 foot run?

It depends on amperage and voltage. At 100 feet, voltage drop usually forces wire one or two sizes larger than ampacity alone requires. Use the quick reference table above or run your exact scenario through the calculator.

Why does run length matter for wire sizing?

Every foot of wire has resistance. Longer runs mean more total resistance and more voltage lost before reaching the load. At 100 feet, the round-trip distance is 200 feet, which doubles the voltage drop compared to a 50-foot run.

Is 100 feet considered a long run for electrical wire?

Yes. At 100 feet, most standard wire sizes exceed the 3% voltage drop recommendation at full load on 120V circuits. It is long enough that voltage drop should always be calculated.

Does voltage matter for wire sizing on long runs?

Yes. A 240V circuit can run roughly twice as far as a 120V circuit before hitting the 3% drop threshold with the same wire. Higher voltage means the same absolute loss is a smaller percentage.