NEC 240.87: Arc Energy Reduction Requirements for Circuit Breakers
Arc flash hazards are one of the most serious risks in commercial and industrial electrical systems. NEC 240.87 exists to reduce arc energy exposure during maintenance and operation of large circuit breakers—protecting electricians, inspectors, and facilities from severe injury or fatal incidents.
For contractors working on 800A and larger breakers, this section is no longer optional—it is a core inspection requirement.
👉 If you’re new to the NEC framework, start with our NEC Overview: What It Is and Why It Matters.
👉 For distribution fundamentals, review Services and Feeders Explained before diving into breaker-level protection rules.
What NEC 240.87 Requires
NEC 240.87(A) requires arc energy reduction for circuit breakers rated 800 amps or higher when installed in systems operating at less than 1000 volts.
This applies to:
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Main service breakers
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Feeder breakers
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Large distribution breakers in switchboards and switchgear
The goal is simple: limit the duration and intensity of an arc flash event.
Why Arc Energy Reduction Is Required
Large breakers can sustain an arc fault for long periods before clearing, which dramatically increases:
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Arc flash incident energy
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Burn severity
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Equipment destruction
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Downtime and liability
NEC 240.87 requires methods that reduce clearing time during maintenance conditions—when electricians are most at risk.
Approved Arc Energy Reduction Methods
NEC 240.87 allows several methods. Contractors must ensure at least one approved method is implemented and functional.
1. Zone-Selective Interlocking (ZSI)
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Allows upstream and downstream breakers to communicate
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Reduces trip time during faults within the protected zone
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Common in switchgear lineups
2. Differential Relaying
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Uses current comparison to detect faults
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Extremely fast clearing times
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Often used in critical facilities
3. Energy-Reducing Maintenance Switching (ERMS)
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Maintenance mode switch temporarily lowers trip settings
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Must be clearly labeled and accessible
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One of the most commonly inspected features
4. Instantaneous Trip Setting
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Reduces clearing time during high fault currents
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Must comply with NEC and manufacturer limitations
Coordination Still Matters
Arc energy reduction does not override selective coordination requirements.
Contractors must coordinate:
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Service ground-fault protection (see NEC 230.95)
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Feeder ground-fault protection (see NEC 215.10)
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Breaker trip curves and time delays
Poor coordination is one of the top reasons arc energy reduction systems fail inspection.
How NEC 240.87 Ties into Other NEC Articles
Inspectors expect arc energy reduction to align with the entire distribution system:
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NEC 230.71 – Service Disconnecting Means
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NEC 230.62 – Barriers in Service Equipment
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NEC 240.21(B) – Feeder Tap Rules
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NEC 215.2 – Feeder Conductor Sizing
Arc energy reduction without proper system design still creates hazards.
Common Contractor Mistakes
❌ Installing breakers without arc energy reduction methods
❌ Maintenance switches not labeled or accessible
❌ Forgetting to reset ERMS after maintenance
❌ No documentation of arc reduction method
❌ Confusing arc flash labeling with arc energy reduction compliance
These mistakes almost always lead to inspection delays or rework.
Labeling Is Critical for NEC 240.87
NEC 240.87 works hand-in-hand with NEC 110.16 (arc flash warnings).
Inspectors expect:
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Clear identification of arc energy reduction method
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Maintenance switch labels
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Arc flash hazard labels with current data
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Pro Tips for Contractors
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Assume any breaker ≥800A will trigger NEC 240.87 requirements.
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Coordinate arc energy reduction early during design—not after install.
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Label maintenance switches clearly and permanently.
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Train facility staff on ERMS operation.
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Keep arc flash studies and settings available for inspectors.
Conclusion
NEC 240.87 dramatically improves electrical safety by reducing arc flash energy on large breakers. For contractors, compliance requires more than hardware—it demands coordination, labeling, documentation, and proper commissioning.
To understand how this fits into the broader NEC framework, review:
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