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.

Load (Amps)

One-Way Distance (ft)

System Voltage

Phase

Conductor Material

Max Voltage Drop (%)

NEC recommends ≤ 3% branch circuits, ≤ 5% combined

Enter load amps and one-way distance to see the wire size table.

For exam practice and estimation only — not a substitute for engineered design, manufacturer data, current codes, or a licensed professional's judgment. Verify all values before relying on them.

How NEC Conductor Sizing Works

Conductor sizing has two independent requirements under the NEC: ampacity (the wire's current-carrying capacity from NEC Table 310.16) 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 310.16 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.

NEC Wire Size & Ampacity Chart (Table 310.16, 75°C)

Allowable ampacities for copper and aluminum conductors at the 75°C terminal rating most overcurrent devices use, from 14 AWG to 500 MCM. These are the same values the calculator above checks against.

Conductor Size	Circular Mils	Copper 75°C (A)	Aluminum 75°C (A)
14 AWG	4,110	15	—
12 AWG	6,530	20	—
10 AWG	10,380	30	—
8 AWG	16,510	40	30
6 AWG	26,240	55	40
4 AWG	41,740	70	55
3 AWG	52,620	85	65
2 AWG	66,360	95	75
1 AWG	83,690	110	85
1/0 AWG	105,600	125	100
2/0 AWG	133,100	145	115
3/0 AWG	167,800	165	130
4/0 AWG	211,600	195	150
250 MCM	250,000	215	170
300 MCM	300,000	240	195
350 MCM	350,000	260	210
400 MCM	400,000	280	225
500 MCM	500,000	320	260

Values are NEC Table 310.16, 75°C column, for not more than three current-carrying conductors in a raceway at 30°C (86°F) ambient. Apply temperature-correction and conductor-bundling adjustment factors for other conditions. The small-conductor rule (NEC 240.4(D)) caps overcurrent protection at 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper regardless of the table ampacity, and branch-circuit conductors must be sized to 125% of any continuous load (NEC 210.19).

Frequently Asked Questions

What is the minimum wire size for a 20-amp circuit?

NEC Table 310.16 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 size wire do I need for a 30-amp or 50-amp circuit?

For a 30-amp circuit (dryer, RV outlet), use 10 AWG copper. For a 50-amp circuit (range, EV charger, welder), use 6 AWG copper — 8 AWG is permitted only where the 75°C terminal rating applies and the load allows it. Always upsize for long runs: enter your distance above to check voltage drop.

What size wire do I need for a 100-amp service?

For a 100-amp residential service or feeder, NEC Table 310.12 permits 4 AWG copper or 2 AWG aluminum — this dwelling-services table allows conductors sized at 83% of the service rating. For a 100-amp non-dwelling feeder sized from Table 310.16, you need 3 AWG copper or 1 AWG aluminum at 75°C. Long runs may require upsizing for voltage drop.

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.