What is available fault current?

Available fault current is the maximum current that can flow during a short circuit at a given point in an electrical system. It depends on transformer size, impedance, and conductor impedance. NEC requires breakers to have interrupting ratings equal to or greater than available fault current.

How do you calculate fault current at the transformer secondary?

Three-phase: I_fault = kVA x 1000 / (1.732 x V_LL x Z%). Single-phase: I_fault = kVA x 1000 / (V_LN x Z%). The transformer impedance percentage is typically 2-7% depending on size and type.

What breaker interrupting rating do I need?

Standard interrupting ratings are 10 kA (residential), 22 kA (commercial), 65 kA (industrial), and 100 kA (high-fault locations). Always select a breaker with a rating equal to or greater than the available fault current at its location.

Fault Current Calculator

Calculate available short circuit current at transformer secondary and at the end of a conductor run. Determine required breaker interrupting rating.

Transformer kVA

Phase

Primary Voltage (V)

Secondary Voltage (V)

Impedance (%)

Optional: Conductor Run

Conductor Length (ft)

Conductor AWG

Step-by-Step Calculation

How to Calculate Fault Current

Determine available short circuit current for breaker selection.

Enter Transformer Data

Input kVA, primary and secondary voltages, and impedance. Default 4% is typical for dry-type transformers.

Add Conductor Info

Optionally add conductor length and AWG to account for voltage drop and impedance. Longer runs reduce fault current.

Get Breaker Rating

Compare fault current to standard interrupting ratings: 10 kA, 22 kA, 65 kA, or 100 kA. Color-coded results show safe vs exceed.

Standard Breaker Interrupting Ratings

Common interrupting ratings for molded case circuit breakers.

Rating	Typical Application	Notes
10 kA	Residential panels	Standard for most homes
22 kA	Commercial / Light industrial	Common for small businesses
42 kA	Industrial panels	Intermediate rating
65 kA	Heavy industrial	High fault capacity
100 kA	High fault locations	Near large transformers
200 kA	Utility / Switchgear	Extremely high fault capacity

Frequently Asked Questions

Common questions about fault current calculations and breaker selection.

Available fault current (AFC) is the maximum current that can flow at a given point during a bolted short circuit. It depends on transformer kVA, impedance, secondary voltage, and conductor impedance. NEC requires all overcurrent devices to be rated for the available fault current at their location.

Three-phase: I_fault = kVA x 1000 / (1.732 x V_LL x Z%). Single-phase: I_fault = kVA x 1000 / (V_LN x Z%). For example, a 75 kVA transformer with 4% impedance at 208V three-phase yields: 75,000 / (1.732 x 208 x 0.04) = 5,207 A or ~5.2 kA.

Select the next standard interrupting rating above your calculated fault current. Standard ratings: 10 kA (residential), 22 kA (commercial), 42 kA, 65 kA (industrial), 100 kA (high fault). Always verify the label on the breaker matches or exceeds the available fault current.