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The proper sizing of an electrical (load bearing) cable is important to ensure that the cable can: * Operate continuously under full load without being damaged * Withstand the worst short circuits currents flowing through the cable * Provide the load with a suitable voltage (and avoid excessive voltage drops) * (optional) Ensure operation of protective devices during an earth fault
When to do the calculation?
This calculation can be done individually for each power cable that needs to be sized, or alternatively, it can be used to produce cable sizing waterfall charts for groups of cables with similar characteristics (e.g. cables installed on ladder feeding induction motors).
General Methodology
All cable sizing methods more or less follow the same basic six step process:
1) Gathering data about the cable, its installation conditions, the load that it will carry, etc
2) Determine the minimum cable size based on continuous current carrying capacity
3) Determine the minimum cable size based on voltage drop considerations
4) Determine the minimum cable size based on short circuit temperature rise
5) Determine the minimum cable size based on earth fault loop …show more content…
This voltage is commonly represented by applying some factor to the nominal voltage. A conservative value of = 0.8 can be used so that:
Where Vn is the nominal phase to earth voltage (V)
Worked Example
In this example, we will size a cable for a 415V, 30kW three-phase motor from the MCC to the field.
Step 1: Data Gathering
The following data was collected for the cable to be sized: * Cable type: Cu/PVC/GSWB/PVC, 3C+E, 0.6/1kV * Operating temperature: 75C * Cable installation: above ground on cable ladder bunched together with 3 other cables on a single layer and at 30C ambient temperature * Cable run: 90m (including tails) * Motor load: 30kW, 415V three phase, full load current = 61A, power factor = 0.85 * Protection: aM fuse of rating = 80A, max prospective fault I2t = 90 A2s , 5s melt time = 550A
Step 2: Cable Selection Based on Current
When to do the calculation?
This calculation can be done individually for each power cable that needs to be sized, or alternatively, it can be used to produce cable sizing waterfall charts for groups of cables with similar characteristics (e.g. cables installed on ladder feeding induction motors).
General Methodology
All cable sizing methods more or less follow the same basic six step process:
1) Gathering data about the cable, its installation conditions, the load that it will carry, etc
2) Determine the minimum cable size based on continuous current carrying capacity
3) Determine the minimum cable size based on voltage drop considerations
4) Determine the minimum cable size based on short circuit temperature rise
5) Determine the minimum cable size based on earth fault loop …show more content…
This voltage is commonly represented by applying some factor to the nominal voltage. A conservative value of = 0.8 can be used so that:
Where Vn is the nominal phase to earth voltage (V)
Worked Example
In this example, we will size a cable for a 415V, 30kW three-phase motor from the MCC to the field.
Step 1: Data Gathering
The following data was collected for the cable to be sized: * Cable type: Cu/PVC/GSWB/PVC, 3C+E, 0.6/1kV * Operating temperature: 75C * Cable installation: above ground on cable ladder bunched together with 3 other cables on a single layer and at 30C ambient temperature * Cable run: 90m (including tails) * Motor load: 30kW, 415V three phase, full load current = 61A, power factor = 0.85 * Protection: aM fuse of rating = 80A, max prospective fault I2t = 90 A2s , 5s melt time = 550A
Step 2: Cable Selection Based on Current