What wire gauge for a 30A, 40A, or 50A breaker?

At 75°C: 30A → 10 AWG, 40A → 8 AWG, 50A → 8 AWG, 60A → 6 AWG. Per NEC Table 310.16.

Breaker Size Calculator

What size circuit breaker do you need? Select wire gauge and circuit type — get NEC-compliant breaker sizing instantly.

Circuit specifications

Wire gauge (AWG)?

Wire temperature rating?

Circuit type?

Load current?

Recommended breaker size

25A breaker

Wire ampacity (75°C)25A

Max breaker for wire protection25A

Min breaker for 16A load20A

Load fits on this circuitYes — within NEC limits

NEC 240.4: Breaker must not exceed wire ampacity. NEC 210.19: Continuous loads limited to 80% of breaker/wire rating.

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How to Use This Calculator

Select wire gauge and temperature rating

Choose the AWG wire gauge for your circuit. This is the wire already installed or the wire you plan to use. Smaller AWG numbers mean larger, higher-capacity wire. For the temperature rating, use 75°C for most modern THWN-2 or USE-2 wire — this is the standard for residential wiring. The 90°C rating applies when THHN wire is used in conduit and terminations are also rated 90°C.

Select circuit type

Choose continuous load for any load that runs 3+ hours at a time — solar inverter output circuits, EV charger circuits, and motor loads fall into this category. NEC 210.19 requires continuous loads to not exceed 80% of the breaker and wire rating. This is the most commonly misunderstood rule in solar installations.

Enter your load current

Enter the actual current your load draws. For a solar inverter: output watts ÷ voltage. For a 7,600W inverter at 240V: 7,600 ÷ 240 = 31.7A. The calculator will tell you if the load fits and what the minimum breaker size is.

The Formula

Wire ampacity = NEC Table 310.16 value for gauge & temp rating Continuous load limit = Wire ampacity × 0.80 Breaker max = Wire ampacity (NEC 240.4 — protects wire) Continuous load breaker = Load amps × 1.25 (rounded to next standard size)

The NEC 80% rule works both ways: the load must not exceed 80% of the breaker rating, AND the breaker must not exceed the wire's ampacity. For a solar inverter on 10 AWG (30A wire): breaker max = 30A; inverter output must be ≤ 24A (80% of 30A). If your inverter outputs 28A, you need 8 AWG wire to use a 40A breaker.

Example

Solar inverter output circuit — 7.6 kW inverter at 240V

A 7.6 kW string inverter at 240V outputs a maximum of 31.7A. What wire and breaker are needed for the continuous-rated output circuit?

Load current31.7A (continuous)

NEC 125% derate31.7 × 1.25 = 39.6A

Minimum breaker40A (next standard size up)

Wire for 40A breaker8 AWG THWN-2 (50A at 75°C)

A 40A breaker with 8 AWG wire is the correct combination. Note: 10 AWG (30A) would be undersized because 30A × 80% = 24A maximum continuous, which is less than the 31.7A load. This is why solar inverter circuits are often larger than installers expect — the 125% rule for continuous loads requires upsizing from what you'd use for a non-continuous 32A load.

FAQ

What is the NEC 80% rule for circuit breakers? ▼

NEC 210.19(A)(1) states that branch circuit conductors must have an ampacity not less than the load served. For continuous loads (operating 3+ hours), this means the wire and breaker must be rated at 125% of the load (or conversely, the load must not exceed 80% of the rating). Solar inverter output circuits, EV chargers, and most commercial loads are continuous. Breakers listed for 100% continuous duty exist but are expensive — standard breakers assume 80% continuous loading.

Can I use a 20A breaker on 14 AWG wire for a solar circuit? ▼

No — 14 AWG wire is limited to a 15A breaker per NEC 240.4(D). A 20A breaker on 14 AWG would not protect the wire adequately and violates code. For solar circuits, 12 AWG minimum is typical (20A breaker, 16A continuous load limit), and most inverter output circuits require 10 AWG or larger.

How do I find the right breaker size for a solar inverter? ▼

Most inverter manufacturers specify the required breaker and wire in the installation manual. The standard formula: inverter max AC output amps × 1.25 = minimum breaker size (round up to next standard breaker). Example: 32A × 1.25 = 40A breaker with 8 AWG wire. Always check the specific inverter's manual — some have built-in OCPD requirements that may differ.

What wire gauge do I need for a 30A, 40A, or 50A breaker? ▼

Per NEC Table 310.16 at 75°C: 30A breaker → 10 AWG, 40A breaker → 8 AWG, 50A breaker → 8 AWG (which can handle 50A at 75°C), 60A breaker → 6 AWG. These are the minimum sizes; use the next larger gauge if you have long runs where voltage drop is a concern. Use our AC Wire Size Calculator to check voltage drop over distance.