What's the difference between a 15A and 20A breaker?

A 15A breaker protects 14 AWG wire; a 20A breaker protects 12 AWG wire. The outlet/receptacle type also changes — 20A circuits use T-slot receptacles. Most general lighting circuits are 15A; kitchen and bathroom circuits are typically 20A.

Can I upgrade a breaker without changing the wire?

Only if the new breaker is rated for the existing wire gauge. Upgrading from 15A to 20A requires replacing 14 AWG wire with 12 AWG. Using a larger breaker with undersized wire is a serious fire hazard.

What is a tandem breaker?

A tandem (duplex) breaker fits two circuits in one breaker slot, useful when your panel is full. They're rated the same way as single breakers and follow the same NEC rules.

Quick answer: A 16-amp continuous load requires a 20-amp circuit breaker under the NEC 80% rule, with 12 AWG wire to support it safely.

Home & Homelab

Circuit Breaker Size Calculator

Find the right breaker size for any load. The NEC 80% continuous load rule is applied automatically.

Inputs

Enter load as

Load wattage

Circuit voltage

Continuous load?

Continuous = load runs for 3+ hours at a time (per NEC)

Existing wire gauge

Recommended breaker

20A

15.0A load · ×1.25 continuous factor

✓ 12 AWG is adequate (20A rated)

Standard breaker sizes

15A20A25A30A35A40A50A60A70A80A100A125A150A200A

NEC 210.20: Circuit breakers must be rated at 125% of continuous loads.

NEC 240.4: Wire must be protected at or below its ampacity rating.

How we calculate circuit breaker sizing

The NEC 80% continuous load rule (NEC 210.20) requires circuits serving continuous loads — loads energized for 3 or more hours — to be rated at 125% of the actual load. The calculator applies this factor, then rounds up to the next standard breaker size.

Breaker Size = Load (A) × 1.25 → Next Standard Size

After sizing the breaker, your wire gauge is checked against the NEC ampacity table to confirm compatibility. Using 14 AWG wire on a 20A breaker is a code violation and fire hazard.

Frequently Asked Questions

What's the difference between a 15A and 20A breaker?: A 15A breaker protects 14 AWG wire; a 20A breaker protects 12 AWG wire. The outlet/receptacle type also changes — 20A circuits use T-slot receptacles. Most general lighting circuits are 15A; kitchen and bathroom circuits are typically 20A.
Can I upgrade a breaker without changing the wire?: Only if the new breaker is rated for the existing wire gauge. Upgrading from 15A to 20A requires replacing 14 AWG wire with 12 AWG. Using a larger breaker with undersized wire is a serious fire hazard.
What is a tandem breaker?: A tandem (duplex) breaker fits two circuits in one breaker slot, useful when your panel is full. They're rated the same way as single breakers and follow the same NEC rules.

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Circuit Breaker Size Calculator

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Reviewed April 2026

Methodology and source note

PowerSizing calculators use shared formulas, documented assumptions, and current planning inputs that are summarized on the methodology page. Use these tools for first-pass planning, comparison, and sanity checks, then confirm local code, pricing, utility tariff, and installer specifics before you buy equipment.