What Size Wire Do I Need Calculator
Pick wire by two numbers: the amps it must carry and how far it runs — distance causes voltage drop. This calculator checks both against NEC ampacity tables at once, and includes a one-tap wire size calculator for 100 amp service.
Long runs often need a bigger gauge than ampacity alone requires — voltage drop is the silent upsize.
Planning estimate only — not wiring advice. Undersized wire starts fires. A licensed electrician must confirm sizing, and local code governs.
How wire sizing works
Two checks decide wire size. Ampacity is the current a gauge can carry without overheating — 12 AWG copper is good for 20 amps, 10 AWG for 30, and so on up the table. Voltage drop is the second check: pushing current through copper or aluminum resistance loses voltage along the way, and the NEC recommends staying under 3% per run so motors start and lights stay bright.
Distance is what surprises people. A 20-amp load 200 feet out on 12 AWG copper drops over 6% — double the recommended limit — even though 12 AWG passes the ampacity check easily. That run needs 8 AWG. The calculator runs both checks and picks the smallest gauge that passes both.
VD = 2 × K × I × L ÷ CM (keep under 3%)
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Keep exploring: wire the backup power too with the What Size Generator Do I Need Calculator, or plan the lighting circuits with the How Many Recessed Lights Do I Need Calculator.
What Size Wire Do I Need Calculator FAQ
What gauge wire do I need for 100 amp service?
For a 100-amp residential service entrance, use 2 AWG copper or 1/0 AWG aluminum (NEC 310.12). For a 100-amp subpanel feeder, 3 AWG copper or 1 AWG aluminum is typical. Use the 100-amp service mode above for the exact recommendation.
Copper vs aluminum wire — which should I use?
Copper carries more current per size, resists corrosion, and is easier to terminate — but costs 2–3x more. Aluminum is fine for large feeders when terminated with antioxidant compound and AL-rated lugs, which is why utilities use it. Never mix the two metals at one terminal without a rated connector.
What is voltage drop and why does the 3% rule matter?
Voltage drop is the voltage lost pushing current through wire resistance — it grows with distance and load. The NEC recommends keeping branch-circuit and feeder drop under 3% each (5% total) so motors start reliably and lights do not dim. Long runs often need a bigger gauge than ampacity alone requires.
Can I use 14 gauge wire for outlets?
Only on a 15-amp circuit — 14 AWG copper is limited to 15 amps. Most living-room and bedroom receptacle circuits are 20-amp, which requires 12 AWG. Kitchens, bathrooms, and laundry need dedicated 20-amp circuits by code.
What size wire do I need for 50 amps?
6 AWG copper (or 4 AWG aluminum) at 75°C terminations. That covers 50-amp RV outlets, ranges, and hot tubs — though hot tubs often need 60 amps, which steps up to 4 AWG copper.
Does wire distance really matter that much?
Yes. A 20-amp load at 200 feet on 12 AWG copper drops about 6.4% — double the recommended limit — even though 12 AWG is fine on ampacity. That run needs 8 AWG. Enter your distance in the calculator to see both checks at once.
When must I call an electrician instead of DIY?
For anything beyond replacing a like-for-like switch or outlet: new circuits, service upgrades, aluminum wiring, and anything in the panel. Permits and inspections are required for new work in most areas, and mistakes start fires.