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Cable size calculation
Whether the differential protection is provided for the Feeder or for the Motor windings or both. In all cases usage of differential protection or not will not result increase/reduction in the size of the cable and/or the load current that have to pass through.
Please note the following:-
1. D.O.L the most easy / simple method of starting.
2. But, the required starting means which to be adopted totally depends on the fault level at the power source in conjunction with the local service provider regulation. No need to adopt start - delta, auto Transformer or soft starter for small Electrical Drives. These measures are usually included in the Authorities regulations to avoid dimming & severe Voltage drop in the system.
3. Cable size as well as the cables current carrying capacities of any Electrical load to be exercised as per Standard or via adequate relevant software, in brief it may be as follows:-
a. Calculate the Design current (load current) = KW/ (1.73 X 400 X p.f X eff.) = In
b. Apply all applicable correction Factors, say Ct.
c. Determine the protective device rating. = In if the intention for Overload protection and Greater than In if for O/C in general.
d. Select from the manufacturer Schedule for current carrying capacity in conjunction with the method of Installation the appropriate cable size, Say the current carrying capacity = Iz
e. Verify the voltage Drop as usual. If more than the permissible limit, go to the next cable size.
f. Verify that In < In1 < Iz & I2 >= 1.45 In1 (I2 Circuit Breaker operating Current)
g. Verify the thermal Constraints K²S² = I²t -------- (1)
where
K² = Material Factor (cable) S = Cross Section Area of Cable.
I² =Through Current
And t = time duration, then I²t is the Let Through Energy.
4. For DOL the cable size calculations initially to be as above, but you have to verify the Dynamic / Thermal constants as in 3.g above. Where I = Motor starting Current as (6In), t = duration of starting period. Then the required Cross section area can be determine from equation “1” above.
Based on the above there is no relation between Differential Protection and cable size.
Please note the following:-
1. D.O.L the most easy / simple method of starting.
2. But, the required starting means which to be adopted totally depends on the fault level at the power source in conjunction with the local service provider regulation. No need to adopt start - delta, auto Transformer or soft starter for small Electrical Drives. These measures are usually included in the Authorities regulations to avoid dimming & severe Voltage drop in the system.
3. Cable size as well as the cables current carrying capacities of any Electrical load to be exercised as per Standard or via adequate relevant software, in brief it may be as follows:-
a. Calculate the Design current (load current) = KW/ (1.73 X 400 X p.f X eff.) = In
b. Apply all applicable correction Factors, say Ct.
c. Determine the protective device rating. = In if the intention for Overload protection and Greater than In if for O/C in general.
d. Select from the manufacturer Schedule for current carrying capacity in conjunction with the method of Installation the appropriate cable size, Say the current carrying capacity = Iz
e. Verify the voltage Drop as usual. If more than the permissible limit, go to the next cable size.
f. Verify that In < In1 < Iz & I2 >= 1.45 In1 (I2 Circuit Breaker operating Current)
g. Verify the thermal Constraints K²S² = I²t -------- (1)
where
K² = Material Factor (cable) S = Cross Section Area of Cable.
I² =Through Current
And t = time duration, then I²t is the Let Through Energy.
4. For DOL the cable size calculations initially to be as above, but you have to verify the Dynamic / Thermal constants as in 3.g above. Where I = Motor starting Current as (6In), t = duration of starting period. Then the required Cross section area can be determine from equation “1” above.
Based on the above there is no relation between Differential Protection and cable size.
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