Selective Coordination in Panelboards Calculator – IEEE, NEC

Selective coordination in panelboards ensures only the faulted section disconnects, maintaining system reliability. Calculating selective coordination requires precise analysis of protective device characteristics and standards.

This article explores the IEEE and NEC guidelines, formulas, tables, and practical examples for selective coordination in panelboards. Learn how to optimize protection schemes for safety and operational continuity.

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Calculate selective coordination for a 400A main breaker with 100A downstream breakers.
Determine coordination time between a 225A MCCB and a 150A fuse.
Evaluate selective coordination for a panelboard with multiple 60A circuit breakers.
Analyze coordination between a 600A feeder breaker and 200A branch breakers.

Comprehensive Tables for Selective Coordination in Panelboards

Table 1: Typical Time-Current Characteristics (TCC) for Molded Case Circuit Breakers (MCCBs)

Breaker Rating (A)	Instantaneous Trip (x In)	Long-Time Pickup (x In)	Long-Time Delay (s)	Short-Time Pickup (x In)	Short-Time Delay (s)
100	10–12	1.0	0.1–1.0	5.0	0.05–0.5
225	10–12	1.0	0.1–1.0	5.0	0.05–0.5
400	10–12	1.0	0.1–1.0	5.0	0.05–0.5
600	10–12	1.0	0.1–1.0	5.0	0.05–0.5

Table 2: Fuse Time-Current Characteristics (Typical Values)

Fuse Rating (A)	Melting I2t (A²s)	Clearing I2t (A²s)	Typical Clearing Time @ 10x In (s)	Typical Clearing Time @ 100x In (ms)
100	1.5 × 10⁶	2.0 × 10⁶	0.1	10
150	2.0 × 10⁶	2.5 × 10⁶	0.08	8
200	2.5 × 10⁶	3.0 × 10⁶	0.06	6
300	3.5 × 10⁶	4.0 × 10⁶	0.04	4

Table 3: Selective Coordination Time Margins Recommended by IEEE and NEC

Device Pair	Minimum Time Margin (s)	NEC Reference	IEEE Reference
Main Breaker – Branch Breaker	0.3	NEC 240.12	IEEE Std C37.2
Feeder Breaker – Branch Breaker	0.2	NEC 240.12	IEEE Std C37.2
Fuse – Breaker	0.1	NEC 240.12	IEEE Std C37.2
Breaker – Fuse	0.15	NEC 240.12	IEEE Std C37.2

Fundamental Formulas for Selective Coordination in Panelboards

Selective coordination requires understanding the time-current characteristics (TCC) of protective devices and ensuring their operating times do not overlap undesirably. The following formulas are essential for calculating coordination margins and verifying selective coordination compliance.

1. Time Margin Calculation

The time margin between two devices (upstream and downstream) is calculated as:

Time Margin (Δt) = tupstream – tdownstream

t_upstream: Operating time of the upstream protective device (seconds)
t_downstream: Operating time of the downstream protective device (seconds)

A positive Δt indicates proper selective coordination; the upstream device operates after the downstream device clears the fault.

2. Operating Time from TCC Curves

Operating time for a device at a given fault current (I_f) can be approximated using inverse time characteristics:

t = k / ((If / Ipickup)α – 1) + tdelay

t: Operating time (seconds)
k: Time multiplier constant (varies by device)
I_f: Fault current (Amperes)
I_pickup: Pickup current setting of the device (Amperes)
α: Exponent defining curve steepness (typically 0.02 to 0.1)
t_delay: Fixed time delay setting (seconds)

This formula models the inverse time behavior of breakers and fuses, critical for coordination analysis.

3. I2t Energy for Fuse Coordination

Fuses operate based on energy let-through, quantified by I²t. The melting and clearing I²t values are used to ensure coordination:

I²t = ∫ I(t)² dt

I(t): Current as a function of time (Amperes)
I²t: Energy let-through (Ampere squared seconds)

Selective coordination requires the upstream fuse’s I²t to be greater than the downstream fuse’s clearing I²t, preventing upstream operation before downstream clearing.

4. Coordination Verification Inequality

To verify selective coordination between two devices:

tupstream(If) ≥ tdownstream(If) + Δtmin

Δt_min: Minimum time margin per NEC or IEEE (seconds)

This ensures the upstream device operates only after the downstream device has cleared the fault plus the required margin.

Detailed Real-World Examples of Selective Coordination Calculations

Example 1: Coordination Between a 400A Main Breaker and 100A Branch Breakers

A facility uses a 400A molded case circuit breaker (MCCB) as the main panelboard breaker, feeding multiple 100A branch breakers. The goal is to verify selective coordination for a fault current of 5000A.

Main breaker pickup current (I_{pickup_main}): 400A
Branch breaker pickup current (I_{pickup_branch}): 100A
Time multiplier constant (k): 0.5 for both devices
Exponent (α): 0.02 for both devices
Fixed delay (t_delay): 0.1s for main, 0.05s for branch
Minimum time margin (Δt_min): 0.3s (per NEC 240.12)

Step 1: Calculate operating time for branch breaker

tbranch = 0.5 / ((5000 / 100)0.02 – 1) + 0.05

Calculate the ratio:

5000 / 100 = 50

Calculate 50^0.02:

≈ 1.1487

Calculate denominator:

1.1487 – 1 = 0.1487

Calculate time:

tbranch = 0.5 / 0.1487 + 0.05 ≈ 3.36 + 0.05 = 3.41 seconds

Step 2: Calculate operating time for main breaker

tmain = 0.5 / ((5000 / 400)0.02 – 1) + 0.1

Calculate the ratio:

5000 / 400 = 12.5

Calculate 12.5^0.02:

≈ 1.056

Calculate denominator:

1.056 – 1 = 0.056

Calculate time:

tmain = 0.5 / 0.056 + 0.1 ≈ 8.93 + 0.1 = 9.03 seconds

Step 3: Calculate time margin

Δt = tmain – tbranch = 9.03 – 3.41 = 5.62 seconds

Since 5.62s > 0.3s (minimum margin), selective coordination is confirmed.

Example 2: Coordination Between a 225A MCCB and a 150A Fuse

Consider a feeder protected by a 225A MCCB upstream of a 150A fuse protecting a branch circuit. The fault current is 3000A. Verify selective coordination.

MCCB pickup current: 225A
Fuse melting I²t: 2.5 × 10⁶ A²s
Fuse clearing I²t: 3.0 × 10⁶ A²s
Fault current: 3000A
Minimum time margin: 0.2s

Step 1: Calculate MCCB operating time (using inverse time formula with k=0.5, α=0.02, t_delay=0.1s)

tMCCB = 0.5 / ((3000 / 225)0.02 – 1) + 0.1

Calculate ratio:

3000 / 225 = 13.33

Calculate 13.33^0.02:

≈ 1.059

Calculate denominator:

1.059 – 1 = 0.059

Calculate time:

tMCCB = 0.5 / 0.059 + 0.1 ≈ 8.47 + 0.1 = 8.57 seconds

Step 2: Calculate fuse clearing time

From Table 2, typical clearing time at 10x In (1500A) is 0.08s. At 3000A (20x In), clearing time is faster, approximately 0.04s.

Step 3: Verify selective coordination

tMCCB ≥ tFuse + Δtmin → 8.57s ≥ 0.04s + 0.2s = 0.24s

Since 8.57s > 0.24s, selective coordination is achieved.

Additional Technical Considerations for Selective Coordination

Device Characteristic Curves: Always refer to manufacturer-specific TCC curves for precise coordination analysis.
Short-Circuit Current Levels: Accurate fault current calculations are critical; use IEEE 399 or NEC 110.9 guidelines.
Coordination with Ground Fault Protection: Ground fault devices may require separate coordination analysis.
Adjustable Settings: Many MCCBs allow time dial and instantaneous trip adjustments to optimize coordination.
NEC 240.12 Compliance: NEC mandates selective coordination for emergency systems and critical loads.
Use of Coordination Software: Tools like SKM PowerTools or ETAP can automate complex coordination studies.

Selective coordination is a cornerstone of electrical safety and reliability, ensuring minimal disruption during faults. Proper calculation and verification using IEEE and NEC standards protect equipment and personnel effectively.

For further reading, consult the IEEE Standard C37.2 and the National Electrical Code (NEC).