Wire Size Calculator
Use 4 AWG copper.
- Smallest for ampacity
- 6 AWG (65 A)
- Smallest for voltage drop
- 4 AWG (2.3% = 5.6 V)
- Equipment ground (Table 250.122)
- 10 AWG Cu / 8 AWG Al
Enter the breaker size or load, the one-way distance, the voltage, the conductor material and the wiring method. The calculator checks three things and returns the largest size any of them requires:
- Ampacity, from NEC Table 310.16.
- Terminal temperature rating, from NEC 110.14(C), which decides which ampacity column you may use.
- Voltage drop, to keep the circuit at or under your target (3% by default).
Step 1: Ampacity (Table 310.16)
Every conductor has an allowable ampacity that depends on its size, material and insulation temperature rating (60°C, 75°C or 90°C). The breaker protecting the conductor generally may not exceed that ampacity.
Two rules trip people up:
- Small conductors (240.4(D)). Even though 12 AWG copper THHN is listed at 25 A (75°C) and 30 A (90°C), its breaker is limited to 20 A. 14 AWG copper is limited to 15 A and 10 AWG copper to 30 A.
- Next size up (240.4(B)). For conductors of 800 A or less, you may round up to the next standard breaker size when the conductor ampacity falls between two standard sizes, as long as the circuit is not a multi-outlet branch circuit supplying receptacles for cord-and-plug loads. This rule does not apply to the 240.4(D) small conductors.
Step 2: Terminal ratings decide the column (110.14(C))
A conductor is only as good as its weakest termination. For circuits rated 100 A or less, or conductors 14 through 1 AWG, you must use the 60°C column unless all equipment is listed and marked for 75°C. Most modern breakers and panelboards are rated 60/75°C, which allows the 75°C column when the conductor is rated 75°C or higher.
The wiring method matters too. NM-B cable ("Romex") and UF cable are limited to the 60°C column by 334.80 and 340.80, even though the individual conductors inside NM-B are 90°C rated. That is why a 50 A range circuit in NM-B needs 6 AWG copper, while 8 AWG THHN in conduit is enough when every termination is rated 75°C.
Step 3: Voltage drop
On long runs, the conductor that satisfies ampacity can still lose too much voltage. The calculator increases the size until the drop meets your target. See the voltage drop calculator for the formula and sizing tables.
Quick reference: minimum wire size by breaker
| Breaker | Copper, 60°C column (NM-B, UF) | Copper, 75°C (THHN in conduit) | Aluminum, 75°C | Equipment ground (Table 250.122) |
|---|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG | 14 AWG Cu / 12 AWG Al |
| 20 A | 12 AWG | 12 AWG | 10 AWG | 12 AWG Cu / 10 AWG Al |
| 30 A | 10 AWG | 10 AWG | 8 AWG | 10 AWG Cu / 8 AWG Al |
| 40 A | 8 AWG | 8 AWG | 8 AWG | 10 AWG Cu / 8 AWG Al |
| 50 A | 6 AWG | 8 AWG | 6 AWG | 10 AWG Cu / 8 AWG Al |
| 60 A | 4 AWG | 6 AWG | 4 AWG | 10 AWG Cu / 8 AWG Al |
| 70 A | 4 AWG | 4 AWG | 3 AWG | 8 AWG Cu / 6 AWG Al |
| 80 A | 3 AWG | 4 AWG | 2 AWG | 8 AWG Cu / 6 AWG Al |
| 90 A | 2 AWG | 3 AWG | 2 AWG | 8 AWG Cu / 6 AWG Al |
| 100 A | 1 AWG | 3 AWG | 1 AWG | 8 AWG Cu / 6 AWG Al |
| 125 A | 1/0 AWG | 1 AWG | 2/0 AWG | 6 AWG Cu / 4 AWG Al |
| 150 A | 3/0 AWG | 1/0 AWG | 3/0 AWG | 6 AWG Cu / 4 AWG Al |
| 200 A | 250 kcmil | 3/0 AWG | 250 kcmil | 6 AWG Cu / 4 AWG Al |
Smallest conductor whose Table 310.16 ampacity meets the breaker rating, before voltage drop, ambient or bundling corrections. The 75°C column applies only when every termination is rated 75°C.
For dwelling services and main feeders, Table 310.12 allows smaller conductors (the 83% rule). For example, a 200 A dwelling service may use 2/0 copper or 4/0 aluminum. See wire size for a 200 amp service.
Worked example: 50 A circuit for a range, 60 feet
- NM-B cable: 60°C column. 8 AWG copper is rated 40 A at 60°C, too small. 6 AWG copper is rated 55 A, which works. Answer: 6/3 NM-B with ground.
- THHN in conduit, 75°C terminations: 8 AWG copper is rated 50 A at 75°C, which works.
- Voltage drop at 60 feet, 240 V, running 40 A (a typical range rarely runs at the full 50 A): with 8 AWG copper, VD = (2 × 12.9 × 40 × 60) ÷ 16,510 = 3.75 V, or 1.6%. Well under 3%.
Worked example: 48 A EV charger (continuous load), 120 feet
An EV charger is a continuous load, so the breaker and the conductor are sized at 125% of the charger's rating: 48 A × 1.25 = 60 A.
- THHN in conduit, 75°C terminations: 6 AWG copper is rated 65 A, which covers 60 A.
- NM-B cable: 6 AWG copper is only 55 A in the 60°C column, short of the 60 A the conductor itself must carry, so you move to 4 AWG copper (70 A).
- Voltage drop at 120 feet, 240 V, 48 A with 6 AWG copper: VD = (2 × 12.9 × 48 × 120) ÷ 26,240 = 5.66 V, or 2.4%. Under 3%, so 6 AWG THHN stands. The equipment ground for a 60 A breaker is 10 AWG copper (Table 250.122).
Worked example: 20 A circuit to a shed, 100 feet
Ampacity says 12 AWG copper. At a 16 A load (80% of the breaker) on 120 V, voltage drop tells a different story:
| Copper size | Volts dropped | Percent drop |
|---|---|---|
| 12 AWG | 6.32 V | 5.3% |
| 10 AWG | 3.98 V | 3.3% |
| 8 AWG | 2.50 V | 2.1% |
8 AWG is the first size under 3%. Two consequences people miss: the 20 A breaker and the receptacles may not accept 8 AWG, so plan a pigtail splice at each end; and 250.122(B) requires the wire-type equipment grounding conductor to be increased in proportion to the circular-mil increase. 8 AWG has 2.53 times the area of 12 AWG, so a 12 AWG ground becomes 8 AWG too.
Adjustments the calculator also applies
- More than three current-carrying conductors in a raceway or cable: multiply ampacity by 80% (4–6), 70% (7–9) or 50% (10–20), per 310.15(C)(1).
- High ambient temperature (attics, rooftops): correction factors in Table 310.15(B)(1). Rooftop raceways exposed to sunlight need special attention (310.15(B)(2)).
- Continuous loads (3 hours or more, such as EV chargers and heaters): size the conductor for 125% of the continuous load (210.19(A)(1)).
Common mistakes inspectors flag
- Reading the 90°C column for the final answer. Use it only as the starting point for derating; the result is capped by the termination rating.
- Upsizing the hots for voltage drop but not the ground (250.122(B)).
- Using the 83% rule (Table 310.12) on a subpanel feeder or any feeder that does not carry the entire dwelling load.
- Forgetting bundling. Several NM-B cables run tightly together through insulated or fire-stopped holes can trigger adjustment factors (334.80).
- Aluminum on small branch circuits. Aluminum is common and economical for feeders (a 100 A aluminum feeder at 75°C needs 1 AWG), but branch-circuit devices must be marked for it.
Frequently asked questions
What size wire do I need for a 30 amp breaker?
10 AWG copper in almost all residential cases, including 10/2 or 10/3 NM-B. Aluminum needs 8 AWG. See 30 amp wire size.
What size wire for a 50 amp breaker?
6 AWG copper in NM-B cable, or 8 AWG copper THHN in conduit when every termination is rated 75°C. See 50 amp wire size.
Can I use 90°C ampacity for sizing?
Only for derating and correction calculations. After adjustments, the final ampacity may not exceed the value allowed by the lowest-rated termination, usually 60°C or 75°C.
Does distance change the wire size?
Not for ampacity, but it does for voltage drop. Past roughly 50 to 100 feet on 120 V branch circuits, voltage drop often pushes you one or two sizes up; the calculator checks both and returns the larger.
Is bigger wire always better?
Larger wire reduces voltage drop and heat, but it costs more and may not fit the terminals on the breaker or device. Check the lug range printed on the equipment.
Related
Pricing a job with dozens of circuits? SparkQuote prices every circuit in your takeoff and flags long runs that need upsizing. See the electrical estimator.
Last updated: October 6, 2026. Based on the 2023 NEC. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.