Wire Size Calculator
Match the conductor to both amperage and run length; longer runs often need a larger wire even when the breaker size stays the same
Use this calculator to get a common residential wire-size starting point before pricing cable or checking a longer run. It combines a basic ampacity check with a simplified voltage-drop target. For the breaker side of the decision, use the breaker size calculator. For a deeper look at distance, see Wire Size by Distance.
Recommended starting point: 8 AWG copper
Estimated voltage drop: 1.91% (4.58 V)
Ampacity and run length both support 8 AWG copper for this example.
How this wire size calculator works
Wire sizing is not just about the breaker number stamped on the handle. You need a conductor that can carry the load without overheating, and you need to keep voltage drop reasonable so the equipment at the far end still sees healthy voltage.
That is why a short 40A run in copper may stay at 8 AWG, while the same 40A load across a detached garage or backyard charger run may need to move up to 6 AWG or 4 AWG. The breaker stays the same, but distance changes the answer.
This calculator gives a homeowner-friendly starting point for common 120V, 208V, and 240V runs. It is most useful for EV chargers, subpanel feeders, shop circuits, and other longer dedicated runs where voltage drop can quietly change the wire choice.
- Load current in amps
- One-way run length in feet
- System voltage
- Copper vs aluminum conductor material
- Target voltage-drop limit, usually 3% for tighter planning or 5% for a looser early check
Start with the ampacity minimum, then let run length tell you whether you need to upsize.
How to convert
Step 1: Start with the load amperage
Use the breaker or calculated load current as the starting point. Heavier loads need thicker conductors even on short runs.
Load current sets the minimum starting conductor size
Step 2: Check voltage drop on the run
Longer runs increase resistance, and more resistance means more lost voltage before power reaches the load.
Voltage drop = (2 x amps x one-way length x conductor resistance) / 1000
Step 3: Choose the first size that passes both checks
The recommendation is the first conductor that satisfies the starting ampacity range and stays within the selected drop target.
Drop % = voltage drop / system voltage x 100
Worked examples
Question: What wire size for a 40A, 240V run that is 75 ft one way in copper with a 3% drop target?
Solution: 8 AWG copper is the first common starting point. Estimated drop: about 4.58V, or 1.91%.
Question: What wire size for a 60A, 240V run that is 150 ft one way in copper with a 3% drop target?
Solution: 6 AWG copper is the ampacity starting point, but the drop is about 3.68%, so upsizing to 4 AWG brings the estimate down to about 2.31%.
Question: What wire size for a 50A, 240V run that is 120 ft one way in aluminum with a 3% drop target?
Solution: 4 AWG aluminum is the first common starting point and keeps estimated drop near 2.5%.
Common mistakes and notes
- One-way run length means panel to load. The calculator doubles it internally to account for the full circuit path.
- Copper usually reaches the same goal with a smaller gauge than aluminum, but aluminum can still be a practical feeder choice.
- Voltage drop is not the same as ampacity. A wire can be large enough for current yet still need upsizing on a long run.
- If the result lands near a boundary, verify with the applicable electrical code and a licensed electrician before buying wire.
Assumptions
- Single-phase residential planning estimate for copper or aluminum conductors.
- Uses common household starting-point ampacity ranges, not a full code table or installation-specific derating method.
- Voltage drop is estimated with typical conductor resistance values and a simple round-trip formula.
- Final conductor size still depends on insulation, terminations, ambient temperature, conduit fill, and local code.
Worked example
Example: 40A over 75 ft at 240V in copper stays at 8 AWG because the estimated drop is about 1.9%, comfortably below a 3% target.
FAQ
Is this a code-approved wire sizing tool?
No. It is a planning tool that combines common residential starting points with a simplified voltage-drop check. Final conductor sizing still has to be verified against the applicable code, terminal ratings, temperature limits, and local inspection requirements.
Why does distance change wire size if the breaker stays the same?
Distance adds resistance. More resistance means more voltage lost along the run, so the equipment at the far end sees lower voltage. Upsizing the wire lowers resistance and helps keep performance and heat under control.
Should I choose copper or aluminum?
Copper is smaller for the same job and is common on branch circuits and shorter runs. Aluminum is often used on feeders because it can lower cost, but it usually needs a larger gauge and listed terminations made for aluminum conductors.
What voltage-drop target should I use?
Many homeowners use 3% as a tighter branch-circuit planning goal and 5% as a looser early estimate. The right target depends on the equipment, the installation, and your code or design standard.
Does this replace the voltage drop calculator?
Not really. This tool gives a starting wire recommendation. The dedicated voltage drop calculator is still the better tool when you want to test a specific conductor resistance, run length, and load in more detail.
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⚠️ Sanity Check Only
This tool provides informational estimates only. It is not professional engineering advice. Electrical work is dangerous and governed by strict local codes.
Before you start:
- Verify these results with a licensed electrician.
- Cross-reference with the latest Electrical Code (NEC/CEC).
- Never work on live circuits.