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Calculation No.: Calc-001
Revision: A
Description:
Created By: Engineer Name
Project: Project Name
Project No.: P001
Client: Client Name

CB Overload Protection Coordination

Overload protection coordination per AS/NZS 3008.1 Clause 2.5.3.

1. Input Parameters

Circuit breaker and cable parameters for coordination check:
Parameter Symbol Value Notes
Load Current I_B 45 A Design current
CB Frame Size I_N 63 A Nominal rated current
Thermal Setting k_th 1 Multiplier of frame size
Cable Size A 16 mm² PVC insulation
Parallel Cables n 1 Per phase
Derating Factor k_d 0.87 Installation conditions

2.1 Effective CB Current (I_N)

Effective nominal current from frame size and thermal setting:

Formula: $$ \begin{aligned} I_N^{eff} &= I_N^{frame} \times k_{thermal} \\ &= 63 \times 1 \\ &= 63 \; \text{A} \end{aligned} $$

2.2 Cable Current Capacity (I_z)

Derated cable capacity including parallel cables:

Formula: $$ \begin{aligned} I_z &= I_{base} \times k_{derating} \times n_{parallel} \\ &= 72 \times 0.87 \times 1 \\ &= 62.6 \; \text{A} \end{aligned} $$

2.3 Conventional Operating Current (I_2)

For circuit breakers, I_2 = 1.45 × I_N:

Formula: $$ \begin{aligned} I_2 &= 1.45 \times I_N^{eff} \\ &= 1.45 \times 63 \\ &= 91.3 \; \text{A} \end{aligned} $$

3.1 Condition 2.1 Check

The load current must not exceed the CB setting, and the CB setting must not exceed the cable capacity: Result: FAIL

Formula: $$ \begin{aligned} I_B \leq I_N \leq I_z \\ \quad \Rightarrow \quad \\ 45 \leq 63 \leq 62.6 \\ \quad \text{FAIL} \end{aligned} $$

3.2 Condition 2.2 Check

The conventional operating current must not exceed 1.45 times the cable capacity: Result: FAIL

Formula: $$ \begin{aligned} I_2 \leq 1.45 \times I_z \\ \quad \Rightarrow \quad \\ 91.3 \leq 90.8 \\ \quad \text{FAIL} \end{aligned} $$

4. MCB Curve Recommendation

Load Type: general Recommended Curve: C (Instantaneous trip: 5-10× IN) Trip Range: 315 - 630 A General loads (lighting, socket outlets, small motors)

5. Summary

Condition 2.1 (IB <= IN <= Iz): FAIL Condition 2.2 (I2 <= 1.45 × Iz): FAIL OVERALL: FAIL - Coordination Failed Recommendations: - Reduce circuit breaker setting or increase cable size. - The circuit breaker operating current exceeds cable protection limit. Consider a larger cable or smaller breaker.

Approval:

This calculation has been reviewed and approved by:
Created By
Engineer Name
Date: ..........................
Checked
By: .............................
Date: ..........................
Approved
By: .............................
Date: ..........................
Client Approved
By: .............................
Date: ..........................

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