Wire Sizing and FLA and LRA Motor Ratings

FLA in the USA. Don't know where you are.

Thank you for the info.

For sizing wire for the motor will be FLA + other factors like, the distance, Ambient temperature factors, fault level and your motor starting amps.

Sizing for the main breakers will be slightly different.

Sizing for the cable. (Example 1)

For example,

if you have a motor rated at 1KW with voltage at 415V and PF at 0.85 and your motor FLA is 1.636 Amp, then,

The cable size will be (as per in the cable data sheet) will be 1 mm2 but in generally, we will use 2.5 mm2. ( I don't know why we always use 2.5 mm2 cables)

Sizing for the Breaker (MCB)

Since your motor FLA is 1.636 Amp. The MCB will be 6 Amp MCB. (I think the 6 Amp MCB look better than the 2 Amp MCB)

Sizing for the cable (Example 2)

For example,

If you have a motor rated at 185KW with voltage at 415V and PF at 0.85 and your motor FLA is 302.83 Amp, then,

The cable size will be (as per in the cable data sheet) will be 185mm2. But in generally, we will use 240 mm2 by giving some allowance of 16% extra capacity for the starting motor current and the faults level of the motor. (in case the motor short circuited, at lease the cable still able to withstand the faults current until the Main MCCB trip Off.)

Sizing for the Breaker (MCB)

Since your motor FLA is 302.83 Amp. The MCB will be 600 Amp MCCB. Why, don't forget your motor starting current. (this is for motor with soft starter system)

Full load current. The locked rotor current should only be there for as long as the circuit protective device takes to operate.

I believe FLA criteria will take care!

For conductors 1.23 of FLA and for breakers, it will depend on the Code Letter for use of the LRA to determine breaker. Likewise, consider type of controller for use of LRA.

In the US your wire sizing is, according to the NEC, 125% of FLA. The starter, breakers aren't sufficient for motor duty, must have 25% headroom but can be adjusted to match the FLA of the motor.

When sizing the power supply wires, consider the full load current (FLA, full load amperes, In, rated current, nominal current) x 1.25 to account for additional heat generated during the starting process. This is valid for motors with continuous duty. Feeders for motors with heavier duty factor ( frequent starts, reversing, etc) have to be larger, considering the higher starting current occuring more often over the same period of time.

Since the starting process duration is also dependent on the voltage at the motor terminals, the wire size selection should also consider voltage drop during the starting period, especially for excessive feeder lengths.

The LRA (locked rotor amperes, short circuit current, locked rotor current) is dependent on the design type of the motor and is not to be considered in sizing the feeder under normal circumstances. If it would, the feeders would be way oversized.

Sizing the overcurrent protection (thermal-magnetic breaker) is (for general use motors) with breaker trip setting between 300% and 150% of the full load current ( the higher % for lower ratings; this is not intentional but due to the fact that the breakers are manufactured with standardized trip values ( as an example for the US) minimum rating of a general use circuit breaker is 15A and the current for a 1 HP, 208V, 3-phase motor is 4.8A, resulting 312% , for 2 HP (7.8A) it would be 192%, for 75 HP (221A) with a breaker rated 300A it would be 135%) Setting a too low value leads to nuisance tripping during start, a too high value defies the role of a protection device.

There are procedures that cover this matter in national electrical standards, such as British Standard 7671, for example.