Breaker Size Guide: How to Size Circuit Breakers for Any Load

How Circuit Breakers Are Sized

A circuit breaker serves two functions: it protects the wiring from overheating, and it provides an overcurrent protection point for the circuit. Sizing a breaker means finding the amperage that:

1. Is at least 125% of the continuous load (loads running 3+ hours)
2. Does not exceed the wire's ampacity (the maximum current the wire can safely carry)
3. Matches a standard breaker size (15, 20, 30, 40, 50, 60, 70, 80, 100A, etc.)

Basic formula: Breaker size = Total load (A) × 1.25 for continuous loads, rounded up to the next standard breaker size.

Standard Household Breaker Sizes and Uses

The 80% Continuous Load Rule in Practice

For circuits where the load runs continuously for 3 hours or more (HVAC, commercial lighting, server equipment), the breaker can only be loaded to 80% of its rating:

• 20A breaker: max continuous load = 16A
• 30A breaker: max continuous load = 24A
• 40A breaker: max continuous load = 32A

Example: A mini split draws 15A continuously. You need a breaker rated at: 15 ÷ 0.80 = 18.75A → round up to the next standard size: 20A breaker with 12 AWG wire.

Motor Circuit Sizing: The 250% Rule

NEC 430.52 allows motor circuits to be sized larger than normal because motors draw several times their rated current on startup (inrush current). The breaker is sized at up to 250% of the motor's full-load current (FLC):

Motor breaker size = Motor FLC × 2.5, rounded up

Example: A 1.5 HP pool pump motor with an FLC of 8A: - Breaker: 8 × 2.5 = 20A (exact match — use 20A) - Wire: sized to 125% of FLC = 8 × 1.25 = 10A → use 14 AWG (15A ampacity, adequate for 10A design current)

AFCI and GFCI Requirements (NEC 2023)

Simply sizing the breaker by amperage isn't enough — the type of breaker matters:

GFCI protection (Ground Fault Circuit Interrupter) is required for circuits in: - Bathrooms - Kitchens (within 6 feet of a sink) - Garages - Outdoors - Crawl spaces - Unfinished basements - Near swimming pools and hot tubs

AFCI protection (Arc Fault Circuit Interrupter) is required for circuits serving: - Bedrooms, living rooms, dining rooms, family rooms - Hallways and closets - All 15A and 20A, 120V branch circuits in dwelling units (NEC 2023)

Combination AFCI-GFCI breakers are available and can satisfy both requirements in a single device.

How the Calculator Works

The calculator takes your load (amps or watts), voltage, and whether the load is continuous or intermittent, and applies the 125% continuous load rule to output the minimum breaker size. It also shows the required wire gauge for that breaker size.

For motors, use the motor's FLC from its nameplate or the NEC Table 430.250 (three-phase motors) or Table 430.248 (single-phase), then apply the 250% motor rule for breaker selection.

Common Wiring Mistakes

Matching breaker to load, not to wire. The breaker must protect the wire's ampacity. If you need 25A for a load, use 10 AWG wire (rated for 30A) and a 30A breaker — not 12 AWG with a 30A breaker.

Installing a 20A breaker without upgrading the wire. Many homeowners upgrade breakers without changing the wiring. This removes the wire's overcurrent protection — a significant fire hazard.

Not adding AFCI/GFCI where required. NEC 2023 significantly expanded AFCI requirements. Check current code for your jurisdiction when adding any new circuit.