Electrical Wire Calculator

How to Calculate Electrical Wire for Residential Circuits

Selecting the right wire gauge is critical for safety and code compliance. Undersized wire can overheat, trip breakers, or cause fires. The National Electrical Code (NEC) sets minimum wire sizes based on circuit amperage, and local codes may be stricter.

Wire Gauge by Amperage (NEC Table 310.16)

• 15-amp circuit: 14 AWG (most lighting circuits, general-use receptacles)
• 20-amp circuit: 12 AWG (kitchen, bathroom, garage receptacles)
• 30-amp circuit: 10 AWG (dryers, water heaters, large window AC)
• 40-amp circuit: 8 AWG (electric ranges, cooktops)
• 50-amp circuit: 6 AWG (ranges, EV chargers, sub-panels)

NM-B (Romex) vs THHN

• NM-B (Romex): Non-metallic sheathed cable. Contains 2 or 3 insulated conductors + bare ground in a plastic jacket. Used in dry residential locations. Cannot be used in conduit (wastes space), outdoors, or in commercial buildings.
• THHN: Individual insulated conductors pulled through conduit. Required in commercial, exposed, or wet locations. More labor-intensive but allows future wire replacement.

NEC Code Requirements

• Kitchen: Minimum two 20A small-appliance circuits (12 AWG)
• Bathroom: Dedicated 20A GFCI circuit
• Garage/outdoor: GFCI protection required
• Smoke/CO detectors: Interconnected on dedicated circuit
• AFCI protection: Required in bedrooms, living rooms, family rooms (2020 NEC)

Voltage Drop Considerations

For long wire runs, voltage drop becomes a concern. NEC recommends no more than 3% voltage drop for branch circuits and 5% total (feeder + branch). For runs over 100 feet at 120V, consider upsizing the wire gauge. Use the formula: Voltage Drop = (2 × Length × Current × Resistance per ft) / 1000.