Coordination studies and selective coordination: what NEC 700/701 actually requires

What selective coordination means

When a fault occurs in an electrical system, the protective device closest to the fault should operate to clear it, leaving the rest of the system energized. A fault on one branch circuit shouldn’t trip the main breaker or the service entrance breaker.

The failure mode this prevents: a single fault cascading into widespread loss of power because upstream devices tripped along with (or instead of) the device closest to the fault. For emergency and standby systems, this matters even more — the fault that knocks out the emergency lighting and life-safety branch is the fault that should be contained at its source.

NEC 700 and 701 requirements

The NEC requires selective coordination in specific applications:

• NEC 700.32 — Emergency systems (life safety branch in hospitals, egress lighting and exit signs, fire alarm) require selective coordination throughout. No exceptions.
• NEC 701.27 — Legally required standby systems require selective coordination throughout.
• NEC 708.54 — Critical operations power systems (COPS) require selective coordination.
• NEC 645.27 — Critical loads in IT equipment rooms have specific coordination requirements when selective coordination is part of the design intent.

"Optional standby" systems (not emergency, not legally required) don’t have a coordination requirement in code — but most facility owners want coordination anyway for operational reliability.

How protective devices coordinate

Coordination is achieved by comparing the time-current characteristics (TCC curves) of upstream and downstream devices. For coordination to exist, the downstream device must operate (trip and clear) faster than the upstream device for any possible fault current in the protected zone.

Coordination at high fault currents

At currents above several thousand amps (typical for faults near the source), most protective devices operate in their instantaneous region. Coordination here depends on careful selection of devices with different instantaneous characteristics. This is often the most challenging part of coordination studies.

Coordination at moderate fault currents

At currents in the hundreds to low thousands of amps (typical for faults farther from the source), devices operate on their long-time and short-time delay regions. Adjustable trip units and properly-set delays achieve coordination here.

Coordination at overload

At currents just above normal load (typical of equipment overload, not faults), thermal overload elements and long-time delays come into play. Coordination at overload is generally easier than at fault conditions.

What makes coordination hard in real systems

• Standard breakers have fixed characteristics. Molded-case circuit breakers without adjustable trip units have factory-set time-current curves. Achieving coordination between standard breakers is mostly limited to selecting different sizes with non-overlapping curves.
• Instantaneous regions overlap at high faults. A 600A breaker and a 200A breaker both operate instantaneously at 10,000A fault current. They cannot be coordinated at that fault level without specific design provisions (current-limiting fuses, fully-rated electronic trip units, or zone-selective interlocking).
• Series-rated combinations don’t coordinate. Series-rated breakers rely on the upstream device opening first when the downstream device’s interrupting rating is exceeded. They specifically do NOT coordinate — both devices trip. NEC 700.32 explicitly prohibits series rating in emergency systems for this reason.
• Generator vs utility fault levels differ. A breaker that coordinates on utility power may not coordinate when the system is on generator (lower available fault current). Coordination must be verified for both source conditions.
• Motor inrush and transformer inrush. Inrush currents cause apparent "faults" that protective devices must NOT respond to. Coordination must avoid nuisance trips on inrush while still clearing real faults.

Tools and techniques that achieve coordination

Electronic trip units with adjustable settings

Modern breakers (Eaton Magnum, Square D MasterPact, ABB Emax, Siemens WL) have electronic trip units with adjustable long-time, short-time, instantaneous, and ground fault settings. Proper setting achieves coordination across a wide fault current range.

Zone-selective interlocking (ZSI)

Trip units communicate with each other. A downstream device tells the upstream device "I see this fault, you wait." Allows shorter delays on upstream devices than would otherwise be needed for coordination, reducing both arc-flash energy and equipment damage.

Current-limiting fuses

Class L, J, or RK fuses with current-limiting characteristics. The let-through current is much less than the available fault current. Allows coordination at high fault levels that breakers alone can’t achieve.

Selective protective relaying

For MV systems and large LV systems, microprocessor protective relays (SEL, Schweitzer, GE Multilin, ABB) provide much finer coordination than breakers alone. Multiple protection elements (overcurrent, ground fault, differential) coordinated against upstream relays.

Differential protection on transformers and generators

87-series differential relaying responds only to faults inside the protected zone. Allows fast tripping for internal faults without coordinating against downstream feeder faults.

The coordination study deliverable

A proper coordination study produces:

• Up-to-date single-line diagram
• Short-circuit study results at every bus
• Time-current coordination curves for every protective device pair in the system
• Documentation of any non-coordinated combinations and the rationale
• Specific setting recommendations for adjustable devices
• Verification that coordination meets NEC 700/701 requirements where applicable
• Arc-flash incident energy calculations using the verified settings

The deliverable typically comes from a qualified electrical engineering firm (not the electrical contractor), using software like SKM PowerTools, ETAP, or EasyPower.

Common compliance failures

• Series rating in emergency systems. A specific NEC 700 violation, but still occasionally seen in practice.
• Coordination claimed without analysis. Specifications claim "selective coordination" but no study verifies it. AHJ inspection may catch this if rigorous; many don’t.
• Coordination valid for one source only. System coordinates on utility but loses coordination on generator. The fault during a power outage trips the main generator breaker.
• Coordination drifts after maintenance. Replacement breakers don’t match the original specifications. Settings changed during troubleshooting without restoration. Field modifications without re-study.
• Future loads not analyzed. Original study coordinates the original system. Future panel additions, transformer swaps, or breaker upgrades require re-analysis but don’t always get it.