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Wire Size Calculator (NEC)

Sizing electrical wire isn't just about ampacity (the current the wire can carry) — you also need to keep voltage drop under 3% for any meaningful run, or your equipment runs slow, hot, and inefficient. A 20A circuit at 50 ft uses #12 wire (ampacity is plenty); the same 20A at 150 ft needs #10 wire to keep voltage drop in spec. This calculator runs both checks: NEC ampacity from Table 310.16 (75°C copper or aluminum) PLUS voltage drop limit at 3%, and returns the larger wire (smaller AWG number) of the two. Critical for any electrical install that involves long cable runs.

Minimum AWG

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Ampacity limit
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Voltage drop %
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Per NEC + 3% drop
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Two sizing checks: ampacity AND voltage drop

Ampacity: the maximum current a wire can carry without overheating, set by NEC Table 310.16. A 20A circuit needs #12 copper minimum; 30A needs #10; 40A needs #8; 50A needs #8 or #6.

Voltage drop: the volts lost in the wire as current flows. NEC recommends (not requires) 3% max on branch circuits, 5% total. Beyond that, motors run hot, electronics misbehave, lights dim. Voltage drop scales with: amperage, length (round-trip), and inverse wire size.

Formula: VD = (2 × K × L × I) / CM, where K = 12.9 (copper) or 21.2 (aluminum), L = one-way distance in feet, I = current in amps, CM = circular mils for the wire size.

Worked example: 20A circuit, 100 ft one-way, 120V, copper. Ampacity says #12 (rated 20A) is enough. Voltage drop check: with #12 (6,530 CM), drop = (2 × 12.9 × 100 × 20) / 6530 = 7.9V = 6.6% drop. Way over 3%. Upsize to #10 (10,380 CM): drop = 5.0V = 4.1%. Still over. #8 (16,510 CM): 3.1V = 2.6%. Use #8 for this run.

When voltage drop dominates over ampacity

For runs under ~50 ft, ampacity usually dominates and the standard wire size for the load works. For longer runs (over 100 ft) or higher loads (over 30A), voltage drop starts dominating and you need to upsize.

  • 50-ft, 20A: #12 (ampacity); voltage drop ~3%, just within spec.
  • 100-ft, 20A: #10 needed for voltage drop.
  • 150-ft, 20A: #8 needed for voltage drop.
  • 50-ft, 50A circuit (EV charger): #6 for ampacity; voltage drop check usually passes at #6.
  • 200-ft, 30A subpanel feed: #6 minimum, often #4 for voltage drop. Aluminum #2 for cost savings on long runs.

How to use this calculator

  1. Load in amps: the circuit's current draw.
  2. One-way distance: from panel to load, in feet.
  3. System voltage: 120V single-phase (typical receptacle), 240V (heavy appliances, EV chargers), 480V (commercial).
  4. Conductor: copper (typical) or aluminum (long runs to save cost).
  5. Output: minimum AWG sized for both ampacity AND 3% voltage drop, plus voltage drop at that gauge.
  6. Always confirm against NEC and local code — this is a planning tool, not a substitute for a licensed electrician.

Common scenarios

Garage outlet 75 ft from panel, 20A circuit, 120V. #12 ampacity-OK, voltage drop ~5% (over). Upsize to #10: voltage drop 3.1% — just under. Use #10 copper.

EV charger circuit, 50 ft, 50A continuous (62A breaker after 125% rule), 240V copper. #6 copper handles 65A ampacity and 1.6% voltage drop — well within spec. Standard install.

Detached shop subpanel 200 ft from house, 50A 240V copper. Ampacity demands #6 (65A rated). Voltage drop at #6: 2.8% — in spec but tight. Upsize to #4 for safety margin and future load growth.

FAQ

Why 3% voltage drop, not higher? +
NEC FPN 210.19(A)(1): 3% max on branch circuits, 2% on feeders, 5% total at the load. Above 5% total, equipment loses noticeable performance. Motors run hot and inefficient; electronics may malfunction; resistive loads (heaters, incandescents) run cooler/dimmer than rated.
Aluminum vs copper wire? +
Aluminum is about 60% the ampacity of copper for the same gauge — #4 aluminum ≈ #6 copper. Aluminum is dramatically cheaper for large gauges (4 AWG and bigger), so it's common in service entrance and subpanel feeds. Aluminum requires AL-CU rated termination devices, anti-oxidant compound (Noalox) at connections, and more careful torque. Never mix aluminum and copper on the same termination without proper rated connectors.
What's the difference between THHN, NM-B, and UF? +
THHN: thermoplastic high-heat insulated single conductors, used in conduit. NM-B (Romex, brand name): non-metallic sheathed cable with 2-3 insulated conductors + bare ground, for residential interior dry locations. UF: underground feeder, sheathed for direct burial. THHN in conduit underground is also common. Each has slightly different ampacity ratings due to insulation rating.
Do I need a different size wire for ground? +
Equipment grounding conductor (EGC) size is set by NEC Table 250.122 based on the circuit's overcurrent device. For 20A circuits: #12 EGC minimum (often same as hot/neutral). For 50A: #10 EGC. The ground doesn't need to match the hot/neutral size in most cases.
What does "75°C ampacity" mean? +
The temperature rating of the wire's insulation determines ampacity. 60°C (older fixtures): lower ampacity. 75°C (standard for most modern wire): the ampacity values this calculator uses. 90°C (THHN, premium): even higher ampacity, but devices it terminates to must also be 90°C rated, which is rare for residential.
Can I add wires in parallel to handle more amps? +
Yes — NEC Table 310.10(H) allows paralleling wires #1/0 and larger to share load. Common for 200A+ service entrances where a single conductor would be impractically large. Each parallel set must be exactly the same length and routed the same way.
What about derating for ambient temp and bundled wires? +
NEC Table 310.15(B) requires derating ampacity for high ambient temps (above 86°F / 30°C) and for bundles of 4+ current-carrying conductors. Hot attic install: derate by 20-30%. 8-conductor bundle in conduit: derate by 30%. Critical for proper sizing in real installations.
Should I always upsize for future loads? +
For panel feeds and service entrances: yes, plan for 25-50% headroom. Long-run wire is expensive to replace; copper futures aren't going down. For branch circuits to specific loads (single outlets, lights): size for actual load, since adding more outlets to a circuit increases load.