The cables used in high voltage or low voltage application, it should be sized correctly otherwise there may be a fire risk. The cable sizing is properly done based on the voltage transmission through the wire.
The purpose of cable sizing is to determine how best to transport a given current, determined by voltage through the insulated wire. The calculation is done for each cable individually.
Why is cable sizing needed?
- 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.
Cable selection:
The first step of cable selection is to acquire the needs in the application. One should know the load type, type of source, phase of current transmission and it’s important factors.
And from the details of the application environment and load, we should fix the conductor material can be used.
The installed current rating: The installed current rating is calculated by multiplying the base current rating with each of the derating factors,
Ic = Ib * Kd
Ic - the installed current rating Ib - the base current rating (A) Kd - the product of all the derating factors.
Current condition for cabling:
When sizing cables for non-motor loads, the upstream protective device (fuse or circuit breaker) is typically selected to also protect the cable against damage from thermal overload.
Ip is the protective device rating (A), Ic is is the installed cable current rating (A).
Voltage drop for a cable:
For a three phase system:
For single phase:
Calculating Maximum Cable Length due to Voltage Drop:
It may be more convenient to calculate the maximum length of a cable for a particular conductor size given a maximum permissible voltage drop (e.g. 5% of the nominal voltage at full load) rather than the voltage drop itself.
The maximum cable length that will achieve this can be calculated by re-arranging the voltage drop equations and substituting the maximum permissible voltage drop:
For 3 phase:
For a single phase system:
Minimum Cable Size Due to Short Circuit Temperature Rise:
The minimum cable size due to short circuit temperature rise is typically calculated with an equation of the form:
Then k is:
Cable sizing calculator:
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Cable sizing calculator software:
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