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Calculator 09 / UL 508A Supp. SB

Panel short-circuit current rating (SCCR)

Evaluate the panel SCCR component-by-component per UL 508A Supplement SB Method 1 and compare it with the available fault current at the panel terminals.

Panel inputs

Available fault current

IEEE 551-style transformer estimate — NOT an NEC table. Utility data wins; feeder impedance reduces fault current, so this transformer-terminal value is conservative.

Main OCPD

Prefilled from typicals (capped by class interrupting rating 200 kA); replace with the nameplate value. UL 248-8 · 0–600 A · 600 V.

Branch components

Branch 1

Often 100 kA with Class CC/J/RK1 fuses (marked combination).

Up to 10 rows. Per UL 508A SB4.2.1, an unmarked component is assumed 5 kA. Typical values are prefill guidance only — the nameplate ALWAYS governs.

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SCCR Report

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Inputs

System voltage480 V
Available fault current31.4 kA (estimate)
Main OCPDClass J, 400 A, 100 kA SCCR
BranchesBranch 1: 5 kA, starter

Results

Panel SCCR

5 kA at 480 V

Minimum of the main (100 kA) and all 1 branch effective ratings.

UL 508A SB4.2 (Method 1) · NEC 409.110

Available fault current

31.4 kA

Transformer-terminal estimate; feeder impedance reduces fault current.

IEEE 551 transformer method — not an NEC table

FAIL — panel SCCR is below available fault current.

Engineering disclaimer: This design aid is not a substitute for engineering judgment. Confirm final design with a licensed professional engineer and the authority having jurisdiction.

Panel SCCR

5 kA at 480 V

Minimum of the main (100 kA) and all 1 branch effective rating (per-branch: 1→5 kA).

UL 508A SB4.2 (Method 1) · NEC 409.110

Available fault current

31.4 kA

Transformer-terminal estimate: 1500 kVA, 480 V, 5.75%Z. Feeder impedance reduces fault current, so this value is conservative.

IEEE 551 transformer method — not an NEC table

FAIL — panel SCCR 5 kA < available fault current 31.4 kA. Limiting: Row 1 (5 kA). The panel rating must be raised or the available fault current reduced.

How this was calculated

  1. Available fault current at the panel terminals: transformer estimate 31.4 kA = (1500 kVA × 1000) ÷ (√3 × 480 V × 5.75%).
  2. Main OCPD: Class J 400 A fused disconnect, marked 100 kA (current-limiting).
  3. Branch 1 — starter: base 5 kA (marked 5 kA) → effective 5 kA.
  4. Panel SCCR = min(main 100 kA, branch 1 5 kA) = 5 kA at 480 V.
  5. Compare with available fault current: 5 kA < 31.4 kA → FAIL.
Raise the panel SCCR
  • Use a current-limiting main fuse (Class J / RK1 / T / CC / L) rated ≥ the available fault current — per UL 508A SB4.2.2 the panel SCCR then becomes that device's rating (verify component markings).
  • Use components with a marked combination rating that covers the fault (UL 508A SB4.2.1 + the manufacturer's combination table).
  • Replace low-rated components with higher-SCCR (marked) components.
  • Use listed series-rated breaker combinations (NEC 240.86).
  • Reduce the available fault current: larger transformer %Z, longer feeder, or better utility data — this transformer-terminal estimate is often conservative.
Footnotes
  • An unmarked component is assumed 5 kA per UL 508A SB4.2.1 — the standard's conservative assumption; confirm every nameplate.
  • Crediting an in-series current-limiting device with raising a branch rating reflects the permissive SB4.2.2 reading; the conservative reading credits only the device's own marked SCCR. Verify let-through / peak data with the manufacturer before relying on it.
  • If this panel is fed from another panel, enter the available fault current at THIS panel's terminals — the upstream panel's SCCR is not an impedance and does not reduce fault current unless the upstream device is current-limiting with verified data. Service and feeder connections use the same math.
  • Control-circuit components on the CPT secondary are excluded from the evaluation; the CPT and line-side components count.
Engineering disclaimer: These results are a design aid and must be reviewed by a licensed professional engineer and the authority having jurisdiction. Verify component nameplate SCCRs and the manufacturer's combination tables, confirm the available fault current with the utility or a short-circuit study, and follow all manufacturer installation instructions.

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