Circuit Breaker and Cable Sizing

Several standards such as BS 7671 or the National Electrical Code (NEC) specifies “Ampacity” for different materials. Clearly, Ampacity is the maximum current that a conductor of a specific size can carry under a set of conditions without exceeding a specific operating temperature – Thermoplastic (70°C) or Thermosetting (90°C). Ampacity is also known as current-carrying capacity.

When the Ampacity is exceeded, the operating temperature rises above 70°C (Maximum Temperature that Thermoplastic Insulation such as PVC can withstand), or 90°C (Maximum Temperature that Thermosetting Insulation such as XLPE can withstand), causing the insulation to distort. This is when a Circuit Breaker should serve its role to trip the circuit off the current before any insulation begins to distort or melt. Therefore, a Circuit Breaker shall be selected with a Nominal Breaking Capacity below the “Ampacity” of the Cable that it is protecting. This is known as Thermal or Overload protection. The selection of a Circuit Breaker shall also take other factors such as the tripping time delay for inrush current/overload protection, and short circuit protection into consideration.

If a mixture of Thermoplastic and Thermosetting Insulations is used, it may be usual that the layer of insulation closer to the conductor be the determining operating temperature for the cable. For example, the ampacity of an XLPE/PVC cable shall effectively refer to operating temperature at 90°C (Thermosetting), because the inner layer which would melt first, is a Thermosetting Insulation (XLPE). However, the actual maximum operating temperature of cables vary between models and manufacturers and shall be referred from a product datasheet.

In this Calculator, the following assumptions are made to reflect the most common configurations applicable in the industry. For other configurations, refer to the standards for different Ampacity Rating to use.

  1. Material of Cable = Copper

  2. Method of Installation = Conduit or Trunking

  3. Thermosetting or Thermoplastics Insulation

  4. Ambient Temperature = 30°C

Based on the above assumptions, a set of commonly used Cable Sizes and their corresponding Circuit Breaker Sizes (Nominal Breaking Capacity) can be summarized in the Tabulation below:

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The selection of Circuit Breakers (Amps) depends on the selection of Cable, which in turn depends on the Full Load Current Rating of connected load. More often, Full Load Amps are derived from the power rating of the equipment, and the total current across a circuit shall be the sum of power of all equipment connected to the circuit.

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This value of Current is then corrected with a desired safety factor and then matched to the closest larger value of Circuit Breaker and its corresponding Cable Size from the Tabulation above.

 

Example

Given:

Power = 20 kW

Voltage = 400 V

Phase = 3 Phase

Power factor = 0.85

 

Solution:

Current = 20kW/1.732/400/0.85 = 33.96 A

Min. Breaker size = 33.96 x 1.2 (Safety Factor) = 41 A

Type of Insulation (Conductor Op Temp.) = PVC (70°C)

Selected Breaker Size = 60 A (Closest Larger Match)

Selected Cable Size = 4 x 1C 25 mm2 PVC + 16 mm2 CPC

4 x 1C (4 Numbers of 1 Core) Cable specified are for the 3 Phases + Neutral (TPN), while 2 x 1C shall be specified for single phase circuits. Circuit Protective Conductors (CPC) can usually be half of the size of Conductor above 25mm2, and caps at 120 mm2 per conductor. The full Cable Naming can be, for example,

4 x 1 Core 25 mm2  PVC/PVC CABLE + 16 mm2 CPC IN G.I. TRUNKING

 

It shall be noted that other factors such as Capacitance and Inductances, Skin Effect, Voltage Drop and Impedances shall be taken into consideration, especially when sizing for cables with higher current-carrying capacities or over longer distances.

To Estimate Voltage Drop, go to Voltage Drop Calculator.

A downloadable Excel version of This Cable and Circuit Breaker Size Calculator is available below. Please be reminded again that this Calculator is based on the above-mentioned 4 assumptions/conditions.

Cable & Circuit Breaker Calculator (Excel)