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Overcurrent Protection & Coordination

Fuses vs CBs · TCC curves · selective coordination · NEC 700.27

Every conductor in your system has an OCPD upstream. Picking the right one isn't just about ampacity — it's about what trips first when something faults. Selective coordination keeps a single fault from taking down half the building.

Fuses vs Circuit Breakers

Two technologies, both meeting NEC requirements, very different operating characteristics. Coordination strategies depend on which type you choose.

Property	Fuses	Circuit Breakers
Operation	Sacrificial — element melts on overcurrent	Reusable — mechanical contacts open
Speed (low fault)	Slower (thermal element)	Faster (thermal-magnetic)
Speed (high fault)	Faster (current-limiting fuses can clear in < 1/4 cycle)	Slower (must wait for half cycle minimum)
Coordination	Easier — fuse curves naturally cascade	Harder — requires careful selection or zone-selective interlocking
Replacement	Stock 3 fuses, replace blown ones	Reset, no inventory
Single-phase tripping	Single fuse blows on single-phase fault → motor singles-out	3-pole CB trips all 3 phases together
Cost (per device)	Lower for fuse + holder	Higher for breaker
Typical use	Industrial, MV, high-fault situations, motor branches	Commercial buildings, panelboards, lighting branches

Time-Current Curves (TCC) — How to Read Them

A TCC plots how long a device takes to trip vs the current flowing through it. Both axes are logarithmic — covers 6+ decades on a single chart. Reading a TCC is the foundation of every coordination study.

Two CBs. Branch (green) is to the LEFT of feeder (copper) at every current — coordinated.

Selective vs Cascading Coordination

Coordination type	Description	Pros	Cons	Where used
Selective	Downstream device opens FIRST for any fault current. Upstream remains closed.	Minimum disruption. Only the faulted branch loses power.	More expensive equipment. May require larger upstream breakers.	Hospitals (NEC 700.27), data centers, life-safety systems
Cascading	Upstream device may also trip on high faults. Downstream sometimes never opens.	Less expensive. Upstream protects downstream rated lower than fault current.	Larger sections lose power on fault. Some equipment may not get isolated.	Most commercial buildings (cost-driven)
Series-rated	UL-listed combination where downstream CB has lower interrupting rating than fault current.	Allows lower-rated downstream CBs in high-fault systems.	NEC 240.86: must use UL-listed combination. Many AHJs question this.	Sometimes residential service entrance (200A 22 kA breaker behind 100kA fault).
Zone-Selective Interlocking (ZSI)	Modern electronic CBs communicate. Downstream CB tells upstream "I see the fault, don't trip."	Selective coordination AT FULL FAULT levels. Best of both worlds.	Requires modern electronic CBs and signal wiring.	New construction in critical facilities; data center MV switchgear.

NEC 700.27 — Selective coordination required for life safety

For NEC Article 700 emergency systems (life safety), selective coordination is mandatory. A fault in one part of the emergency system cannot trip an upstream device serving other emergency loads. This is a code requirement, not optional. Hospitals, schools, places of assembly all touch this.

Worked Example 1 — Atlas DC1 Coordination Cascade

Example 01 · Atlas DC1 spineCoordinating UPS-A1 output → PDU-A1 input → RPP-A1-1 main → branch breaker

The chain (top to bottom)

Position	Device	Trip A	Why this rating
1 (UPS output)	2,000 A static-trip CB	2,000 A	Sized for full UPS-A1 output (1,500 A × 125% = 1,875 → round up to 2,000)
2 (PDU primary)	800 A LSIG (electronic) CB	800 A	PDU-A1 input current 602 A × 125% = 753 → round up to 800. Electronic trip allows instantaneous setting tuned for selectivity.
3 (RPP main)	400 A MCB	400 A	RPP bus rated 400 A (from §05 calc). 124 A demand × 125% = 155 → 400 A bus allows future growth.
4 (branch)	30 A 1-pole	30 A	Server rack: 24 A continuous × 125% = 30 A.

Coordination check

Test fault at branch (rack PDU): Available fault ~12 kA at 240V branch.
30A CB clears in < 0.01 s (instantaneous region). 400A MCB instantaneous pickup at 5× = 2,000 A → no trip on 240V fault current. ✓ Branch isolates only.
Test fault at RPP main: Available fault ~18 kA at 415V.
400A MCB clears in 0.1-1 s depending on settings. 800A LSIG must wait at least 0.3 s before tripping. Coordinated if settings tuned. ✓
Test fault at PDU input: Available fault ~25 kA at 480V.
800A LSIG clears in 0.2-0.5 s. 2000A static trip set for short delay 0.5 s. Coordinated. ✓

Result: Full selective coordination achieved. A fault anywhere isolates only the affected branch.

Worked Example 2 — Hospital Life Safety Coordination (NEC 700.27)

Example 02 · Alternate scaleHospital essential electrical system — life safety branch must be selectively coordinated per NEC 700.27

System: 600 A generator → 400 A ATS → 225 A panelboard MCB → 20 A branch breakers (egress lighting, exit signs, fire alarm).
Coordination requirement: NEC 700.27 — for any fault current available, the OCPD closest to the fault must clear before any upstream OCPD operates. This is at ALL fault levels, not just bolted faults.
Method: Use fuses in the chain (their curves naturally cascade), or use ZSI-equipped electronic CBs.
Documentation: Submit a coordination study showing TCC plots with NO overlap at any fault level. AHJ reviews.

Why fuses won this market

Hospitals adopted fuses heavily after NEC 700.27 because their curves naturally coordinate without engineering analysis. A 100 A fuse won't blow before a 60 A fuse upstream — physics. Modern ZSI breakers achieve the same result electronically but require system design and commissioning.

TCC Plot — Real Coordinated Cascade (Atlas DC1)

This is the TCC plot for Atlas DC1's UPS → PDU → RPP → branch coordination. Each curve shows trip time vs current. Curves to the LEFT trip first.

Each curve to the LEFT of upstream curves at all fault levels. Selectivity = no overlaps. ✓

Drill — Quick Self-Check

Work each problem mentally; reveal to check. Goal: reflex, not deliberation.

Drill 1 · TCC reading

Two breakers on a TCC: A is to the LEFT of B at 5 kA. Which trips first at 5 kA fault?

Drill 2 · Selective coordination

A fault at branch level. Which breaker should open?

Drill 3 · NEC 700.27

When is selective coordination MANDATORY?

Drill 4 · Maintenance switch

Adjustable instantaneous trip set lower during energized work — what's it for?

Drill 5 · Atlas DC1 chain

Atlas DC1 fault at RPP. With 30A → 400A → 800A → 2000A chain, which opens?

If You See THIS, Think THAT

If you see…	Think / use…
"Coordination study"	TCC plot showing every protective device. Verify no upstream curve overlaps a downstream curve at any current.
"Selective coordination required"	NEC 700.27 — life safety. Mandatory. Only branch closest to fault opens.
"Cascade" or "non-selective"	Multiple devices may trip on a fault. Lower cost, more disruption.
"Series-rated combination"	NEC 240.86 — UL-listed combination only. Verify with manufacturer documentation.
"ZSI" or "Zone-Selective Interlocking"	Electronic CBs that communicate. Modern approach to selective coordination.
"LSI" or "LSIG" trip unit	Long-time, Short-time, Instantaneous (+ Ground for G). Adjustable trip settings on electronic CBs.
Inverse-time CB curve	Standard thermal-magnetic. Slower at low current, fast at high current.
"Current-limiting fuse"	Special fuse that opens in less than ¼ cycle. Limits let-through energy. Used where fault currents very high.
"Maintenance switch" on a breaker	Reduces instantaneous setting during maintenance. Lowers arc flash incident energy. (See §18.)