Safety Notice: Electrical work can kill you and burn your house down. This calculator provides NEC-based sizing references, not a substitute for a licensed electrician. Many jurisdictions require permits and inspections for electrical work. When in doubt, hire a professional.
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Circuit Details
Check nameplate on your equipment, or total circuit load
Distance from panel/source to load (not round trip)
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Common Loads Reference15A/120V General lighting, outlets
20A/120V Kitchen, bath, garage outlets
30A/240V Electric dryer, small AC unit
40A/240V Electric range/oven
50A/240V Large range, welder, RV hookup
60A/240V Sub-panel, large HVAC
100A/240V Sub-panel feed, shop
200A/240V Main service entrance
Affiliate Disclosure: This calculator is free to use. Some product links are affiliate links. Product links and pricing are for the US market. Wire sizing calculations follow NEC standards; international users should verify local electrical codes.
How This Calculator Works
- Wire gauge: Selected per NEC Table 310.16 ampacity ratings for the wire type and temperature. For continuous loads, the wire is sized at 125% of the actual load per NEC 210.19(A)(1).
- Breaker size: Standard NEC sizes (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200). For continuous loads, breaker must be rated at 125% of load per NEC 210.20(A).
- Voltage drop: Calculated using VD = (2 x L x I x R) / 1000 for single phase, where L is one-way distance, I is current, and R is resistance per 1000ft from NEC Chapter 9 Table 8. NEC recommends max 3% for branch circuits, 5% total (feeder + branch).
- Conduit: Minimum size per NEC Chapter 9 Table 4 based on wire gauge, number of conductors, and insulation type.