Introduction #

When this guide fits: You must confirm selective coordination (upstream/downstream OCPD) and assembly SCCR on a panel or MCC before procurement—not after breakers trip the wrong device during a fault.

When it is not suitable: Time-current curve plotting, arc-flash labels, or formal coordination studies—those require manufacturer data and licensed engineering deliverables.

Pair this overview with Short-Circuit Planning, Fuse vs Breaker Sizing, Breaker Size Calculator, and the Electrical Calculator hub.

Coordination vs SCCR — two different gates #

Gate Question Typical evidence
Interrupting rating Can the device open this fault? Breaker/fuse kAIC ≥ available Isc
SCCR (assembly) Is the panel/MCC listing valid with chosen devices? UL 508A / 891 label, manufacturer SCCR table
Selective coordination Does only the nearest upstream device clear the fault? TCC overlap study, fuse ratios, or code exception

Passing load × margin on the Breaker Size Calculator does not prove coordination or SCCR.

Selective coordination (planning vocabulary) #

Goal: A fault on a branch clears at the branch device while the feeder remains closed (within study limits).

Common patterns:

  • Fuse upstream / fuse downstream — manufacturer ratio tables (class-dependent).
  • Breaker upstream / breaker downstream — TCC curves with minimum gap per code edition and jurisdiction.
  • Breaker + current-limiting fuse — let-through energy reduces stress on downstream gear.

Motor feeders: starting curves and overload relay must be in the same study—see Motor Starting & Protection.

SCCR planning checklist #

Step Action
1 Collect available Isc at service and major buses
2 List every device in the chain (breaker, fuse, contactor, VFD)
3 Record lowest interrupting rating in the path
4 Compare to panel SCCR mark—replacing one breaker can lower assembly SCCR
5 Verify cable withstand with device let-through—Cable Size
6 Document study owner (EE of record)

Worked planning example (qualitative) #

Given: 480 V MCC bucket with 22 kA available; branch motor starter with 18 kAIC breaker; upstream MCC main 65 kAIC MCCB.

Planning questions:

  • Does 18 kAIC ≥ 22 kA? No — branch device underrated unless current-limiting protection upstream reduces let-through.
  • Does the MCC SCCR label remain valid with this branch breaker catalog number?
  • If branch is upgraded to 22 kAIC, does coordination with the 65 kA main still hold for a bolted fault at the starter terminals?

Resolve with curves—not amp screening alone.

Relationship to other protection topics #

Common mistakes #

  1. Mixing device classes (MCB inside assembly not listed for SCCR mark).
  2. Using steady-state amps as proof of kAIC adequacy.
  3. Assuming main breaker rating equals branch fault duty.
  4. Skipping let-through when sizing downstream cable for faults.

Next steps #

  1. Screen load amps: Factory Load → kVA to Amps.
  2. OCPD amp frame: Breaker Size or Fuse vs Breaker.
  3. Conductor mm²: Cable Size → Voltage Drop.
  4. Facility context: Power Calculator hub.

FAQ #

What is selective coordination?

It means a fault is cleared by the protective device closest to the fault while upstream devices remain closed (within study assumptions), reducing unnecessary outages.

What is SCCR on a panel label?

Short-circuit current rating of the assembly as tested/listing. Individual breakers may have higher kAIC than the panel is allowed to use with that combination of parts.

Can I coordinate fuses and breakers in the same panel?

Sometimes, with manufacturer guidance and tested combinations. Document the specific catalog numbers—generic amp ratings are not enough.

Does CalcPanel provide coordination curves?

No. This page is a planning checklist only. Curves and SCCR proofs require project-specific engineering tools and data.

How does this relate to transformer %Z?

Transformer impedance affects available fault current on secondary buses—coordinate with Transformer guides and fault planning, not ampacity screening alone.