NEC Wire Size Calculator

What does a typical NEC Wire Size Calculator (AWG) result look like?

A 20 amp, 120 volt copper branch circuit at 75°C with three conductors in EMT and a 100-foot one-way run needs 12 AWG for ampacity (25 A rated) but about 6.4% drop on 12 AWG. To stay under 3% drop you need 8 AWG copper. This tool reports 8 AWG as the final size so both ampacity and voltage drop pass.

Frequently asked questions

• What is the difference between ampacity and wire size?

  Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation temperature rating. Wire size (AWG) determines ampacity; larger diameter wire has higher ampacity. For example, 12 AWG copper wire has an ampacity of 20 amps at 60°C, 25 amps at 75°C, and 30 amps at 90°C, but the NEC requires derating based on termination temperature and installation conditions, so practical capacity is typically lower than table ampacity.

• Can I use aluminum wire instead of copper to save money?

  Yes, but aluminum requires larger wire sizes to match copper ampacity. 12 AWG copper (20-amp capacity at 60°C) is roughly equivalent to 10 AWG aluminum. Aluminum also requires anti-oxidant compound on connections and compatible terminals (rated AL or CU/AL) to prevent corrosion and loose connections that cause arcing and fire. Most residential circuits use copper for reliability and ease of installation; aluminum is reserved for large feeder cables where cost savings are substantial.

• How does voltage drop affect wire sizing?

  Voltage drop is the reduction in voltage along a wire due to resistance. NEC recommends limiting drop to 3 percent for branch circuits and 5 percent total for feeders and branches combined. Excessive voltage drop causes motors to overheat, lights to dim, and electronics to malfunction. Long wire runs or high-current loads increase voltage drop. Upsizing wire reduces resistance and drop; doubling wire diameter (e.g., 12 AWG to 10 AWG) cuts voltage drop roughly in half for the same load and length.

• What is the 80 percent rule for circuit breakers and wire sizing?

  NEC requires that continuous loads (operating three or more hours) not exceed 80 percent of the circuit breaker rating. A 20-amp breaker can carry 20 amps instantaneously but only 16 amps continuously. Wire must be sized for 125 percent of the continuous load (the inverse of 80 percent) to ensure it does not overheat. A 16-amp continuous load requires wire rated for at least 20 amps (typically 12 AWG copper at 60°C).

• How do I size wire for a multi-wire branch circuit?

  Multi-wire branch circuits share a neutral conductor between two hot conductors on opposite phases (in 120/240V systems). The neutral carries only the imbalance current, so it can be the same size as the hots if loads are balanced. However, NEC requires counting each hot as a current-carrying conductor for derating purposes. If two 12 AWG conductors run in the same conduit, ampacity may be derated by 80 percent, reducing capacity from 20 amps to 16 amps. Always verify derating factors when bundling conductors.

• Do I need to upsize the ground wire when I upsize the circuit conductors?

  Not always. NEC Table 250.122 specifies equipment grounding conductor size based on the overcurrent protection device rating, not the conductor size. If you upsize circuit conductors from 12 AWG to 10 AWG to reduce voltage drop but keep a 20-amp breaker, the grounding conductor can remain 12 AWG (the size required for a 20-amp breaker). However, if you upsize to accommodate a 30-amp breaker, the ground must be 10 AWG per the table.

For a full walkthrough of the NEC sizing process, see How to calculate wire size for NEC compliance.

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