Electrical reference chart
Short Circuit Current Chart
Use this short circuit current chart after the calculator result to document the source data, transformer impedance, conductor path, evaluated equipment point, and rating comparison before approving equipment duty.
Quick reference table
Short-circuit current is a system worksheet, not a fixed chart value. Use the calculator with utility source data, transformer kVA, percent impedance, voltage, conductor length, motor contribution, and equipment location, then verify interrupting ratings, SCCR, series ratings, labels, adopted NEC requirements, and AHJ review.
Fault current worksheet inputs
| Input | Record | Why it matters |
|---|---|---|
| Utility source | Available fault current, transformer ownership, or source impedance note | Can dominate service equipment duty |
| Transformer data | kVA, voltage, phase, percent impedance, and nameplate source | Starts the transformer-based estimate |
| Downstream path | Conductor length, size, material, raceway, and equipment point | Adds impedance and changes current at each location |
| Motor contribution | Large motors, generators, or rotating load near the bus | Can increase available current for some checks |
| Equipment rating | Breaker AIC, fuse interrupting rating, panel SCCR, series rating notes | Must be suitable for the evaluated current |
After the short-circuit calculator result
| Result condition | Field follow-up | Why it matters |
|---|---|---|
| Current exceeds equipment rating | Select equipment with suitable rating or redesign source/path | Underrated equipment cannot be accepted from load current alone |
| Series rating relied on | Document tested combination and label requirements | Series ratings are not generic substitutions |
| Downstream current much lower | Keep conductor path and evaluated point attached to result | A service value should not be copied to every panel |
| Transformer data estimated | Replace assumptions with nameplate or utility/manufacturer data | Small impedance changes can move fault-current results |
How to use this chart
Collect source data
Record utility fault current when available, transformer kVA, voltage, phase, and percent impedance before estimating current.
Define the evaluation point
Short-circuit current changes downstream, so identify whether the worksheet point is service equipment, panelboard, switchboard, MCC, or load terminal.
Compare equipment ratings
Use the calculator result with breaker AIC, fuse interrupting rating, panel SCCR, equipment labels, series rating documentation, and AHJ expectations.
Formula basis
Approximate transformer secondary fault current = transformer full-load current / per-unit impedance, before utility source, conductor impedance, and motor contribution are added.
- Full-load current depends on transformer kVA, voltage, and phase.
- Per-unit impedance is transformer percent impedance divided by 100.
- Available fault current changes with utility source strength, conductor impedance, equipment location, and motor contribution.
Worked examples
Transformer-based fault estimate
A transformer impedance calculation can screen secondary fault current, but service equipment still needs utility data, transformer nameplate confirmation, and equipment rating verification.
Panel downstream from long feeder
A downstream panel may see lower available current than the service because feeder impedance reduces the fault current, so the worksheet must name the evaluated panel location.
Related calculators
Short Circuit Current Calculator
Estimate available fault current from transformer impedance, feeder impedance, or known source impedance.
Transformer Impedance Calculator
Calculate transformer percent impedance (%Z), X/R ratio, short circuit current, and impedance components. Essential for short circuit studies and protective device coordination.
Circuit Breaker Sizing Calculator
Calculate standard breaker sizes for general loads, continuous loads, and simplified motor branch-circuit reviews
Related charts
Transformer Impedance Chart
Plan transformer impedance checks for available fault current, voltage regulation, primary/secondary current, equipment rating, and coordination context.
Breaker Size Chart
Plan breaker size from load category, continuous duty, conductor protection, interrupting rating, equipment listing, and calculator result notes.
Fuse Size Chart
Plan fuse size from load current, load type, fuse class, conductor protection, interrupting rating, coordination needs, and equipment instructions.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.