Free fault current and arc fault calculator (AIC, AFCI, GFCI)
Available Fault Current Calculation
E
transformer Z
Isca = ampere short-circuit current RMS symmetrical.
Subpanel Feeder Runs (Service Equipment to Panels)
Technical Calculation Method, 2023 NEC Rules & FAQsClick to expand ↓
Point-to-Point Method & Conductor Constants
Fault current starts at the utility transformer: full-load current divided by nameplate impedance %Z (adjusted 10% low for tolerance). Along the service entrance conductors, fault current diminishes according to the IEEE / Bussmann formula:
f = (2 × L × I) / (N × C × V) | Multiplier M = 1 / (1 + f)
The C-constant reflects the conductor resistance and reactance based on conduit material (PVC vs. Steel EMT) and conductor metal (Copper vs. Aluminum), calibrated at 25°C starting temperature.
2023 NEC Branch Circuit Protection Rules
Each circuit gets a location, read from its description or picked by hand. Its location, voltage and breaker size set the protection: AFCI for dwelling 120 V circuits of 20 A or less in living spaces (NEC 210.12), GFCI by location for circuits up to 50 A (NEC 210.8), and dual-function where both apply.
Who uses this engineering tool
- Electrical engineers & MEP drafters sizing service entrance equipment and subpanels
- Contractors verifying that utility available fault current does not exceed equipment AIC rating
- Electricians determining AFCI, GFCI, and dual-function breaker schedules for permit submittals
- Plan checkers verifying compliance with NEC 110.9 (Interrupting Rating) and 110.24 (Available Fault Current Marking)
Frequently Asked Questions
Which circuits need AFCI protection?
Under NEC 210.12(A) (2023), dwelling-unit 120 V branch circuits of 10, 15 and 20 A that supply outlets or devices in kitchens, family, dining and living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas and similar rooms. Bathrooms, garages and outdoor circuits are not on the list, though they need GFCI.
Which circuits need GFCI protection?
NEC 210.8(A) covers dwelling receptacles up to 50 A in bathrooms, garages and accessory buildings, outdoors, crawl spaces, basements, kitchens, laundry areas and near sinks, tubs and showers; 210.8(F) adds dwelling outdoor outlets up to 50 A, hard-wired ones included; 210.8(D) the dishwasher. Other buildings follow 210.8(B). On a 240 V circuit the tool flags GFCI where it feeds a receptacle.
What if a circuit needs both?
Kitchen and laundry circuits in a dwelling usually need both. That can be a dual-function AFCI/GFCI breaker, or an AFCI breaker feeding GFCI receptacles. The tool marks these AFCI + GFCI and counts them separately so the breaker order is right.
How is the available fault current calculated?
By the IEEE / Bussmann point-to-point method. At the utility transformer, full-load amps are calculated from kVA and voltage, then multiplied by 100 / %Z. The service conductors then reduce fault current by M = 1 / (1 + f), where f = 2LI / (N C V) and C is looked up from raceway type, conductor material, and wire gauge (#14 AWG to 750 kcmil).
What AIC rating does the panel need?
At least the available fault current at its line terminals (NEC 110.9), and the service equipment is marked with that figure (NEC 110.24). Residential breakers are commonly 10 kA, with 22 kA panels and breakers available; a series-rated combination listed for the fault current is the other route (NEC 240.86).
Can I use the circuits from my panel schedule?
Yes. If you have built a panel in the free panel schedule calculator in this browser, "From my panel schedule" brings its circuits over with the breakers it sized. "Send to my email" emails you the PDF, ready to add to your drawings.
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