Free Tool · Electrician
NEC Wire Size Recommender
Enter your load, distance, and system voltage — get the minimum conductor size that keeps voltage drop within your specified limit. Full table from 14 AWG to 500 MCM with go/no-go status per row.
NEC recommends ≤ 3% branch circuits, ≤ 5% combined
Enter load amps and one-way distance to see the wire size table.
Pro Insights
- NEC 215.2 feeder conductor sizing check for this load:See in app
- Conduit fill % at the recommended wire gauge:See in app
- Estimated material cost for this conductor run:See in app
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How NEC Conductor Sizing Works
Conductor sizing has two independent requirements under the NEC: ampacity (the wire's current-carrying capacity from NEC Table 310.12) and voltage drop (the percentage of voltage lost over the length of the run). A wire that satisfies ampacity may still need to be upsized if the run is long enough to cause excessive voltage drop.
NEC Table 310.12 provides allowable ampacities for conductors at 60°C, 75°C, and 90°C temperature ratings in conduit. Correction factors apply when ambient temperature exceeds 30°C (86°F), and adjustment factors apply when more than three current-carrying conductors share a conduit. Both factors reduce the effective ampacity and may require a larger conductor.
This tool calculates voltage drop across the full AWG-to-MCM range and highlights the minimum size that keeps drop within your specified limit. Use it alongside ampacity tables to confirm the selected conductor satisfies both NEC requirements.
Frequently Asked Questions
What is the minimum wire size for a 20-amp circuit?
NEC Table 310.12 allows 12 AWG copper (rated 20A at 60°C) for a 20-amp branch circuit in most residential and commercial applications. However, if the run is long, voltage drop may require upsizing to 10 AWG to stay within the 3% NEC recommendation.
What does AWG stand for, and how does it work?
AWG stands for American Wire Gauge. Counterintuitively, the larger the AWG number, the smaller the wire. 14 AWG is thinner than 10 AWG. Above 0 AWG, sizes go to 00 (2/0), 000 (3/0), 0000 (4/0), then switch to MCM (thousand circular mils) for 250 MCM and above.
When should I use aluminum instead of copper conductors?
Aluminum is commonly used for service entrance conductors, feeders, and large branch circuits where the cost savings are significant. Aluminum requires a conductor two sizes larger than copper for equivalent ampacity (e.g., 2 AWG aluminum ≈ 4 AWG copper). Aluminum terminations require anti-oxidant compound and connectors rated for aluminum.
What voltage drop percentage is acceptable?
The NEC recommends 3% maximum on branch circuits and 5% combined on feeders plus branch circuits. For sensitive electronics, motors, and LED drivers, many engineers design to 2% or less to prevent performance issues and premature equipment failure.
Also try: Voltage Drop Calculator
NEC-compliant voltage drop with pass/fail status for any circuit