Breaker Size & Load Calculation Basics (NEC Article 220)

Last updated: June 15, 2026

A circuit breaker is not chosen in isolation. Its size is determined by the electrical load it must protect, and the wire feeding that load must be large enough to carry the current safely. This guide explains how load calculations drive breaker selection, why breaker and wire must be paired correctly, and the NEC rules that govern this relationship.

How Load Calculations Feed Breaker Sizing

NEC Article 220 provides two methods to calculate the electrical load a service, feeder, or branch circuit must carry: the Standard Method (Part III) and the Optional Method (Part IV). Both yield a total demand load in volt-amperes (VA) or amperes (A).

For example, a residential kitchen circuit serving a 240 V electric range might have a demand load of 8,000 VA. Dividing by the system voltage (8,000 VA ÷ 240 V) yields a design current of approximately 33 A.

The breaker must be sized to protect the wire from damage if that current is exceeded. Per NEC Article 240 (Overcurrent Protection), the breaker rating cannot be greater than the ampacity of the wire it protects — but conversely, the breaker protects the wire by limiting current to a safe level.

The 125% Continuous Load Rule

A critical load-calculation rule affects both breaker and wire sizing. If a load is expected to run continuously for 3 or more hours (NEC Article 100 definition), both the wire and the breaker must be sized at 125% of the continuous-load current.

Example: A 24 A continuous load requires a wire and breaker rated for at least 24 A × 1.25 = 30 A.

EV charger circuits always satisfy the 3-hour test because charging sessions frequently exceed that duration. Lighting, HVAC systems, and commercial equipment commonly do as well.

Standard Breaker Sizes (NEC 240.6)

Breakers come in standard sizes only. Per NEC 240.6(A), the allowable ratings are:

Breaker Size (A)	Common Use Cases
15	Lighting, general receptacles (branch circuits only)
20	Kitchen countertop, small appliance branch circuits, baths
30	Dryer, stove feeders; 10 AWG copper main feeders
40	Heavy equipment, some HVAC units, service upgrades
50	RV pedestals, large stoves, service feeders
60, 70, 80, 100	Service panels, large commercial feeders
125–600	Industrial services and feeders

Breaker–Wire Pairing: The Core Rule

The breaker and wire must always be sized so that:

The wire ampacity equals or exceeds the breaker rating. The breaker, when it trips, prevents the wire from heating up and failing.
The breaker rating does not exceed the wire's ampacity. Per NEC 240.4(D), absolute maximum rules apply: 14 AWG copper cannot exceed 15 A, 12 AWG cannot exceed 20 A, and 10 AWG cannot exceed 30 A, regardless of the wire's higher thermal capacity.

In practice: a 15 A breaker protects a branch circuit wired in 14 AWG copper; a 20 A breaker protects 12 AWG; a 30 A breaker protects 10 AWG or larger.

When the Load Does Not Match a Standard Breaker Size

If your load calculation yields a current that does not fall exactly on a standard breaker size, NEC 240.4(B) allows use of the next standard size up — with one exception: the absolute maximum-protection rule (above) always applies.

Example: A load calculation yields 32 A. The next standard breaker size up is 35 A, and you would use 35 A provided the wire ampacity supports it (likely 8 AWG or larger). But you cannot use a 35 A breaker on 10 AWG, even if the load is 32 A, because NEC 240.4(D) limits 10 AWG to 30 A maximum.

Overview of NEC Article 220: Load Calculation Methods

NEC Article 220 defines two calculation approaches. In both, the goal is to determine the worst-case (peak) demand the circuit or service will see, so that the breaker and wire can be sized correctly.

Standard Method (Part III)

Lists loads by category (lighting, kitchen, laundry, heating, cooling) and applies demand factors from NEC tables. For example, residential general lighting uses the first 3,000 VA at 100% and the remainder at 35%. Small-appliance circuits add 1,500 VA each (minimum two). Use when you need accuracy for complex or mixed-use buildings.

Optional Method (Part IV, Section 220.82)

Simpler: sum all loads at nameplate rating, then apply a single demand factor — 100% of the first 10 kVA, then 40% of the remainder. More conservative (results in slightly larger breaker and wire) but easier to calculate. Commonly used for residential services.

Load Calculations vs. Ampacity Lookup

Do not confuse them:

Load calculation (NEC Article 220) determines the design current the circuit must supply. It accounts for demand factors, usage patterns, and the 125% continuous rule.
Ampacity lookup (NEC Table 310.16 or wire tables) determines the wire size needed to carry that design current safely. It accounts for insulation type, conductor material, and ambient temperature.

Once you have the design current from Article 220, you use it to select the wire size and the breaker rating together, ensuring both constraints are satisfied.

Why This Matters: Two Real Examples

Undersized breaker: A 40 A load on a 30 A breaker will nuisance-trip under normal operation, indicating you miscalculated the load or chose the wrong circuit.

Oversized breaker on undersized wire: A 20 A breaker on 14 AWG violates NEC 240.4(D). If a fault drives current higher, the wire can overheat and fail before the breaker trips, creating a fire hazard.

This Tool Can Help

Once your load calculation is done and you have a design current, use the Wire Size Calculator to determine the minimum wire size. Select your breaker size to match the wire ampacity (per the rules above), and double-check the voltage drop to ensure it stays acceptable for your run length.

When to Consult a Licensed Electrician

Load calculations and service sizing involve permanent infrastructure and safety-critical decisions. Mistakes can lead to nuisance outages, code violations, or fire risk. If your load calculation is complex, involves industrial equipment, motors, or a service upgrade, engage a licensed electrician or electrical engineer. Verify your results against your local authority having jurisdiction (AHJ) requirements, which may differ from the NEC baseline.