Wire Size Calculator
Smallest listed AWG that stays under your drop cap — drop math only, not ampacity. Click any i for detail.
Run
Voltage
i
Source voltage
Nominal circuit volts. |
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|---|---|
Load (amps)
i
Current
Amps on the circuit. |
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One-way length (ft)
i
Length
One-way feet; single-phase already includes the return. |
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Max drop (%)
i
Cap
Common branch-circuit sketch is 3%. |
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Metal
i
Conductor metal
Aluminum needs larger AWG for the same drop. |
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Phase
i
Phase
Single- vs three-phase factor. |
How to use this calculator
- Enter volts, amps, and one-way length.
- Set a max % drop.
- Pick copper or aluminum and phase.
- Read the smallest listed AWG that meets the cap.
Results explained
Walks AWG 14 through 4/0 using the same VD formula as the voltage-drop page. It does not check NEC ampacity tables.
Quick reference: AWG vs drop
Long 120 V runs need fatter wire even when the breaker size looks fine.
| Item | Detail |
|---|---|
| 3% of 120 V | 3.6 V |
| 3% of 240 V | 7.2 V |
| Copper vs Al | Aluminum ≈ 1.6× the CM for similar drop |
| Ampacity | Use NEC tables — not this page |
Always take the more conservative of drop math and ampacity.
How the estimate is built
Choose smallest AWG with F×K×I×L/CM ÷ V ≤ max%.
Example scenario
120 V, 15 A, 80 ft, copper, 3% often wants 10 AWG rather than 14.
FAQ
Why ignore ampacity?
So the job of this page stays one thing: drop. Your electrician still sizes for heat.
12 V DC?
Put 12 in voltage. Drop percentages get brutal on long low-voltage runs.
Exact volts lost?
Use the voltage drop calculator with a chosen AWG.