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The metal's resistance constant: 12.9 for copper, 21.2 for aluminum. Aluminum drops more voltage per foot — another reason it sizes up.
THE WIRE GUIDE — CHICAGO
How Far Can You Run Electrical Wire? Voltage Drop, Explained Simply
Wire has resistance. Push current through a long run and some of your voltage never arrives. Here's how far is too far — and when the answer is a bigger wire.
Part of The Wire Guide — every wire topic, explained plainly.
THE CONCEPT
What voltage drop actually is
Every foot of wire resists current a little. That resistance eats a small amount of voltage along the way — this is voltage drop. On a short run it's nothing. On a long run to a garage, a shed, or a barn, it adds up: the appliance at the far end gets 112 volts instead of 120, and it shows. Lights dim when the saw starts. Motors run hot and weak. Heaters heat slowly.
Three things control it: how much current (more amps = more drop), how far (more feet = more drop), and how big the wire is (bigger wire = less drop). You can't change the current or the distance — so the lever you pull is wire size.
The 3% / 5% guidelines
The NEC's informational notes suggest keeping voltage drop to 3% on a branch circuit and 5% total from the service to the farthest outlet (feeder plus branch combined). On a 120V circuit, 3% is 3.6 volts. These are guidelines, not hard code — an inspector won't fail you for 3.5% — but they're the industry's line for "equipment runs the way it was designed to."
THE MATH
One formula, four numbers
If you want the exact number for your run, here it is:
Voltage drop = (2 × K × amps × distance) ÷ circular mils
Distance
One-way distance in feet — panel to load. The "2 ×" in the formula already accounts for the round trip out and back.
Circular mils
The wire's cross-section: 14 AWG = 4,110 · 12 AWG = 6,530 · 10 AWG = 10,380 · 8 AWG = 16,510. Bigger number, less drop.
Don't want to do the arithmetic? Use the table below — it's the same formula, pre-computed for the circuits people actually run.
THE DISTANCE TABLE
How far each circuit goes at 120V, 3% drop
Maximum one-way distance before voltage drop passes 3% — figured at the 80% continuous load of each circuit, copper wire:
| Circuit | Wire (copper) | Load used | Max distance ≈ |
|---|---|---|---|
| 15A | 14 AWG | 12A | 48 ft |
| 20A | 12 AWG | 16A | 57 ft |
| 20A | 10 AWG | 16A | 90 ft |
| 30A | 10 AWG | 24A | 60 ft |
| 30A | 8 AWG | 24A | 96 ft |
Two things worth knowing: 240V circuits get roughly double the distance — the same current at twice the voltage means the drop is half the percentage. And upsizing one gauge buys a big jump: a 20-amp circuit at 16 amps goes from 57 feet on 12 AWG to 90 feet on 10 AWG.
THE DIAGRAM
Distance, drawn spatially — with the math on the plate
A 100-foot run to a garage, worked two ways:
WORKED EXAMPLE
The 100-foot garage run, step by step
Say you're running a 20-amp circuit 100 feet out to a detached garage, and the steady load is 16 amps — the 80% continuous max for that circuit. Sizing the circuit itself is one question; this is the second one.
On 12 AWG copper: (2 × 12.9 × 16 × 100) ÷ 6,530 = 6.3 volts dropped — that's 5.3% of 120V. Past the 3% guideline, past even the 5% total. The tools at the far end will feel it.
On 10 AWG copper: (2 × 12.9 × 16 × 100) ÷ 10,380 = 4.0 volts — 3.3%. Just over the 3% line, well inside the 5% total. That's the wire this run wants.
The pattern: when the table says your run is near or past the limit, go up one size and recheck. The code sets minimums — nothing stops you from pulling bigger wire, and the cost difference on one run is small compared to doing the job twice.
WHEN TO UPSIZE
Symptoms that the run is too long for the wire
Lights dim on startup
A saw or compressor kicking on visibly dims the lights — the motor's inrush current is dragging the voltage down through too much wire resistance.
Motors run hot and weak
Motors are the canary: low voltage makes them draw more current to do the same work, which makes them run hot. Chronic undervoltage shortens motor life.
Heat is slow, charge is slow
Heaters and chargers just underperform — a heater putting out noticeably less heat than its rating is the classic long-run symptom.
The safety line: this page teaches the concept — it doesn't replace your local code or a licensed electrician. Long runs, subpanels, and detached buildings have grounding and disconnect rules on top of the voltage-drop math, and the actual work belongs to someone licensed.
Upsized the run and now you've got the smaller roll left over? We buy it — anywhere in the country. Fast, free quote.
FREQUENTLY ASKED QUESTIONS
Voltage drop questions, answered.
What is an acceptable voltage drop?
The NEC's informational notes suggest 3% maximum on a branch circuit and 5% total from the service to the farthest outlet. They're guidelines rather than hard code, but they're the industry standard for equipment running as designed.
How far can you run 12 AWG wire on a 20-amp circuit?
About 57 feet one-way at 16 amps (the 80% continuous load) before voltage drop passes 3% at 120V. Past that, step up to 10 AWG — which reaches about 90 feet.
Does voltage drop matter on 240V circuits?
Less. The same current at twice the voltage means the drop is half the percentage — so 240V runs can go roughly twice as far on the same wire before hitting the 3% guideline.
Is aluminum wire worse for voltage drop?
Yes, per foot — aluminum's resistance constant (21.2) is higher than copper's (12.9). That's one of the reasons aluminum installations size up a gauge.
Can voltage drop damage equipment?
It can shorten motor life — motors draw more current at low voltage and run hot. Electronics with wide input ranges usually tolerate it, but motors and heaters suffer first.
Do I measure one-way or round-trip distance?
One-way, panel to load. The standard formula's "2 ×" already accounts for the current traveling out on one conductor and back on the other.
IRONCREST KNOWS WIRE
I know wire. That's how I know what it's worth.
Gauge, distance, load — that's the math we run on every roll that comes through the door. Know your wire, and you know what it's worth.