Wire size calculator

How the wire size is found

A conductor has to pass two separate checks. The first is ampacity: the current the conductor can carry continuously without exceeding its insulation temperature rating, taken from the column that matches the wiring method and the terminations. The second is voltage drop: the loss along the run, which grows with current and length and falls as the conductor gets larger. The calculator finds the smallest size that passes each check and recommends the larger of the two.

On short runs ampacity almost always governs. On long runs, or at 120 V, voltage drop takes over, and the recommended size can be two or three sizes above the ampacity minimum.

Breaker size or load current

With Breaker size, the conductor is sized to carry the full rating of the breaker, which is the usual question for a new circuit: which wire goes with a 30 A or a 50 A breaker. Voltage drop is then computed at the full breaker current, a conservative figure, since most circuits carry less.

With Load current, the calculator sizes the conductor for the load and picks the next standard breaker rating from NEC 240.6(A). A continuous load, one expected to run for 3 hours or more, is sized at 125% of its current under NEC 210.19(A) and 215.2(A); the voltage drop uses the actual load current.

Which ampacity column applies

NM-B cable (Romex is a common brand name) is limited to the 60 °C ampacity column by NEC 334.80, even though its individual conductors are rated 90 °C. The 90 °C value is used only as a starting point for derating.

THHN/THWN-2 conductors in conduit carry a 90 °C insulation rating, but the conductor can run no hotter than the lowest-rated part of the circuit, and terminations on breakers and equipment are normally rated 60 °C or 75 °C. NEC 110.14(C) sets the default: terminations on equipment for circuits rated 100 A or less, or marked for 14 AWG through 1 AWG conductors, use the 60 °C column unless the equipment is listed and identified for 75 °C; larger circuits use the 75 °C column. Most modern breakers and panels are marked 60/75 °C, which is why the conduit option uses the 75 °C column. Equipment marked 60 °C only calls for the NM-B column instead.

The small-conductor limits

NEC 240.4(D) caps the overcurrent device on the smallest conductors regardless of their ampacity: 14 AWG copper at 15 A, 12 AWG copper at 20 A and 10 AWG copper at 30 A, and 12 AWG and 10 AWG aluminum at 15 A and 25 A. At 75 °C, 14 AWG copper is listed at 20 A, but it still may not sit on a 20 A breaker. The calculator applies these limits before choosing the ampacity minimum and says when a smaller size was ruled out by them.

Voltage drop

The drop is VD = 2 × K × I × L ÷ CM for single phase and 1.732 × K × I × L ÷ CM for three phase, with L the one-way length in feet, CM the conductor area in circular mils and K the resistivity of the conductor at 75 °C (about 12.9 Ω·cmil/ft for stranded copper and 21.2 for aluminum). This is the same resistance-only method as the voltage drop calculator; reactance and power factor are ignored, which is reasonable for the conductor sizes used in buildings.

The 3% default comes from informational notes in NEC Articles 210 and 215, which suggest 3% on a branch circuit or a feeder and 5% in total. These notes are recommendations, not requirements, although some local codes and energy codes make them mandatory.

What the calculator does not cover

Ambient temperature correction (NEC 310.15(B)) and adjustment for more than three current-carrying conductors in a raceway or cable (310.15(C)(1)) reduce the allowable ampacity and can push the size up; the ampacity calculator applies both. Motor circuits follow Article 430, air-conditioning equipment uses the minimum circuit ampacity on its nameplate under Article 440, and the equipment grounding conductor is sized separately from Table 250.122. Parallel conductors and sizes above 1000 kcmil are outside the range.

The result is engineering arithmetic for planning. It does not approve an installation; the code edition and local amendments adopted by the authority having jurisdiction govern.

Worked example

A 30 A, 240 V single-phase circuit runs 100 ft one way in THHN copper in conduit, with 75 °C terminations and a 3% voltage drop target.

1. Breaker 30 A: conductor ampacity of at least 30 A
2. By ampacity: 10 AWG copper, 35 A in the 75 °C column (240.4(D) allows up to 30 A on this size)
3. Voltage drop: VD = 2 × K × I × L ÷ CM, K = 12.86 Ω·cmil/ft (copper, 75 °C, stranded)
4. By voltage drop: 8 AWG, 2 × 12.86 × 30 A × 100 ft ÷ 16,510 cmil = 4.68 V (1.95% of 240 V, limit 3%)
5. Recommended: the larger of the two, 8 AWG copper

Ampacity alone allows 10 AWG, but over 100 ft it loses more than 3%, so the run is upsized to 8 AWG.

Frequently asked questions