Fault current calculator

The fault current calculator works out the available fault current (AFC) at every point from the source to the load, using the point-to-point method. This page covers which plans include it, how to build a calculation and how to get the PDF.

Which plans include it

• Commercial Pro includes unlimited fault current calculations, and lets you edit them after they are saved.
• Every other account, including free, Residential and Residential Pro, gets 2 free fault current calculations. You can view and download those 2, but editing a saved calculation needs Commercial Pro.

Archived calculations still count toward the 2 free ones. When you have used them, New asks you to upgrade. See pricing.

Create a calculation

1. In your dashboard, click Fault current calculations in the left menu.
2. Click New and enter the address.
3. Set up the source. Choose Direct entry if you have a known AFC, for example from the utility, and enter it in kA. Choose Calculate to work it out from the transformer's Transformer kVA and Impedance %Z.
4. Under Secondary voltage configuration, pick the service and the branch you are studying.
5. Click + Add a segment for each step in the run. A segment is a wire run followed by a point: Panel, Disconnect, Meter, Equipment, Transformer or Other.
6. For each wire run, enter the Length (ft), Conductor size, Material (Copper or Aluminium), Parallel runs and conduit type (Steel or Non-steel).
7. Check the AFC shown at each point, then click Create fault current calculation.

Download and share

Open the calculation from Fault current calculations. Use Download or Print to get the PDF, or Share to email a PDF copy or invite a teammate. Every report prints its method and assumptions.

Other ways in

• Without an account: try the free fault current calculator. You need a free account to unlock the final document.
• On the mobile app: go to Tools, then Fault current calculation.

Frequently asked questions

How does the calculator work?

It uses the point-to-point method. You model the system from the source (a known utility AFC, or a transformer's kVA and %Z) through wire runs, transformers and panels, and the calculator gives the available fault current at every point in the chain.

What assumptions does it make?

In Calculate mode it assumes an unlimited (infinite bus) utility source, which gives the worst-case value. Wire impedance comes from conductor C-values. Motor contribution is not included, and busway is not modeled yet.

Are there limits on transformer size?

Yes. Transformers of 10,000 kVA and above are not calculated. Above 5,000 kVA and below 10,000 kVA, the calculator gives a result with a warning to verify it against a full short-circuit study.

How is available fault current different from SCCR?

AFC is how much current the system can deliver into a bolted fault at a point. SCCR and a breaker's interrupting rating (AIC) are how much fault current the equipment can safely handle. The rating needs to be at least the AFC at that point (NEC 110.9 for interrupting ratings, 110.10 for short-circuit current ratings).

Can I delete a calculation?

Fault current calculations can be archived but not deleted.