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Electrical · New
Wire Size Calculator
Determine the correct wire gauge based on amperage, distance, voltage, and conductor material.
CircuitLoad + run
A
ft
RecommendedLIVE
Sample data
Example numbers for a 1–2 person shop. Edit any field to make it yours — it saves on this device.
Minimum wire size
8 AWG
Copper conductor
Voltage drop
2.59%
Drop (volts)
3.11V
Wire ampacity
40A
NEC ≤3%
Pass
Load20 A
Run length100 ft
System120V · 1Ø
Target max drop3%
NEC compliant
8 AWG copper holds voltage drop at 2.59%, within the NEC-recommended 3%.
Vdrop = amps × resistance × dist × k ÷ 1000
k = 2 (1Ø) or 1.732 (3Ø)
drop% = Vdrop ÷ system volts × 100
Field notes
How wire size is determined.
01
Ampacity
The wire must carry the load current without overheating. Per NEC 310.16 the conductor's rated ampacity has to meet or beat the maximum load.
02
Voltage drop
It also has to deliver usable voltage at distance. NEC recommends ≤3% on a branch circuit and ≤5% total (feeder + branch).
03
NEC rules that apply
- 210.19(A)(1) — branch conductors ≥ max load
- 215.2(A)(1) — feeders ≥ load supplied
- 310.16 — ampacity tables, 0–2000V
04
Copper vs. aluminum
Copper has ~61% of aluminum's resistance at the same gauge — more efficient, but pricier. On aluminum, expect to go up 1–2 sizes for the same run.
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