Wire Size by Distance: When Voltage Drop Changes the Answer
Quick Summary
- Longer runs increase resistance, which increases voltage drop.
- The breaker size can stay the same while the conductor still needs to grow.
- Wire-size decisions get more sensitive as load amps go up.
- Detached garages, driveway chargers, and backyard shops are common trouble spots.
Formula Symbols (Quick Guide)
- I = Current (amps)
- V = Voltage (volts)
- R = Resistance (ohms)
- P = Power (watts)
Need a beginner refresher? Start with Electricity Basics.
Why distance matters so much
Homeowners usually start wire planning with the breaker size. That makes sense, but it is only half of the job. The other half is the run length. Every foot of conductor adds resistance, and that resistance steals a little voltage before the power reaches the load.
On a short run, the lost voltage may be small enough that the common rule of thumb still works. On a long run, the exact same breaker and exact same load can need the next larger conductor because the far end of the circuit would otherwise see too much voltage drop.
This is why wire sizing questions come up so often around detached garages, EV chargers at the end of a driveway, backyard sheds, and shop circuits. The load is not unusual. The distance is.
The basic voltage-drop idea
A simple planning formula is:
V drop = (2 x I x L x R) / 1000
Where:
- I = load current in amps
- L = one-way run length in feet
- R = conductor resistance in ohms per 1000 ft
The "2" is there because current travels out and back through the circuit path. The longer the run, the more the resistance adds up. Thicker wire has lower resistance, which is why upsizing the conductor reduces voltage drop.
Worked examples
Example 1: 40A charger 75 ft from the panel
Use 8 AWG copper as a common starting point and assume about 0.764 ohms per 1000 ft. Plugging that into the formula gives:
V drop = (2 x 40 x 75 x 0.764) / 1000 = 4.58V
On a 240V circuit, that is about 1.91% drop, which is still a comfortable planning result for many installs.
Example 2: Same 40A load, but 160 ft from the panel
Keep the load the same and stretch the run:
V drop = (2 x 40 x 160 x 0.764) / 1000 = 9.78V
Now the drop is about 4.08% on a 240V circuit. That is the kind of result that often pushes a homeowner to move from 8 AWG copper to 6 AWG copper.
Quick reference table
| Load and run | Common starting point | Distance risk | Likely next move |
|---|---|---|---|
| 40A at 75 ft, 240V | 8 AWG copper | Usually low | Often stays put |
| 40A at 150 to 160 ft, 240V | 8 AWG copper | Noticeable | Often move to 6 AWG |
| 60A at 75 ft, 240V | 6 AWG copper | Usually modest | Often stays put |
| 60A at 150 ft, 240V | 6 AWG copper | High enough to check closely | Often move to 4 AWG |
Where homeowners feel this in real life
- Detached garages: tools, heaters, and chargers are farther from the panel than people think.
- Driveway EV chargers: long trench runs can turn a simple charger project into a wire-size project.
- Workshops and sheds: heavier loads plus longer distance add up fast.
- Outdoor equipment: long paths to pool, spa, gate, or yard equipment often need a second look.
None of those situations automatically require giant wire. They just deserve a real voltage-drop check instead of a single rule-of-thumb answer.
Use the calculators together
The Wire Size Calculator is the best starting point when you want a common conductor recommendation. If you want to test a specific resistance value or compare multiple conductor choices directly, use the Voltage Drop Calculator next.
When to verify with a licensed electrician
Ask for a professional review when the run is buried, outdoors, crossing to another structure, or feeding expensive equipment you do not want to starve for voltage. Long runs are exactly where real installation details matter most.
Related wire and circuit planning pages
FAQ
What is voltage drop in plain English?
It is the amount of voltage lost in the wire before power reaches the load. Longer, thinner conductors lose more voltage than shorter, thicker ones.
Does a longer run always mean a bigger wire?
Not always. It depends on the load, the voltage, and how long the run really is. But longer runs are exactly where upsizing becomes more common.
Why can the breaker stay the same while the wire gets bigger?
Because the breaker is set by the load and protection requirements, while the wire also has to deal with resistance and voltage drop over distance.
Is 3% voltage drop a hard code rule?
Many people use 3% as a practical planning target, but the exact standard depends on the application and your code or design criteria. Treat it as a helpful benchmark, not a universal law.
Do aluminum conductors make distance issues worse?
They can, because aluminum has higher resistance than copper for the same gauge. That usually means a larger aluminum conductor is needed to hit the same voltage-drop target.
Disclaimer: Results are informational estimates for learning and planning only. Always follow the applicable electrical code and consult a qualified licensed electrician for safety-critical work.