Overview
Knowing a motor's power lets you estimate its operating current, which provides the basis for selecting the circuit breaker, contactor, and conductor size.
Miniature molded-case circuit breakers (MCBs)
Miniature molded-case circuit breakers are commonly classified by tripping characteristic: type D for motor/power protection and type C for lighting/general protection. Typical selection factors are:
- Type C: 1.5–2.0 times the motor rated current
- Type D: 1.25–1.5 times the motor rated current
- Main/distribution MCB: about 1.15 times the motor rated current
Practical selection rules
General guidance:
- Circuit breaker: choose an MCB rated at approximately 2.5 times the motor rated current, with its adjustable (setting) current set to about 1.5 times the motor rated current. This accommodates frequent starts while keeping short-circuit protection sensitive.
- Contactor: select an AC contactor with a rated current around 2.5 times the motor rated current to ensure reliable long-term operation under frequent starting. Also check auxiliary contact availability and matching.
- Conductors: choose conductor cross section based on the motor rated current and the conductor's permissible ampacity. For frequent starting, use a larger conductor; for infrequent starts, a smaller conductor may suffice. Also consider minimum installation diameter and mechanical strength.
Example: 7.5 kW three-phase induction motor
A 7.5 kW, 4-pole three-phase motor has a rated current of about 15 A.
- Circuit breaker: select an MCB rated at 1.5–2.5 times the rated current; following the above rule, use a breaker rated around 37.5 A to 37.5 A (practical choice often 40–50 A). Setting the trip at 1.5 times the motor current is common to allow frequent starts while maintaining short-circuit sensitivity.
- Contactor: select a contactor rated at about 2.5 times the motor current.
- Conductors: select based on ampacity. For frequent starting, use a relatively larger conductor; for this example a 4 mm2 copper conductor is a common choice.
Example: 15 kW motor
Estimated current: approximately 15 kW * 2 = 30 A.
- Circuit breaker: for type D, choose 1.25–1.5 times → roughly 38–45 A; a 50 A D-type or a 63 A C-type is often used depending on characteristic.
- Contactor: direct-on-line start, choose about 65 A; for star-delta reduced-voltage start, choose the main contactor at rated motor current (~32 A) and the star contactor one size smaller (~25 A).
- Conductors: for direct-on-line start use 10 mm2 copper; for star-delta reduced-voltage start, 4 mm2 copper may be sufficient.
Contactor selection: direct start vs reduced-voltage start
When selecting a contactor, determine whether the motor will be direct-on-line (DOL) or reduced-voltage started, and whether the load is heavy or light:
- Direct-on-line or heavy-load start: account for inrush and impact currents; typical selection is 2.0–2.5 times the motor rated current.
- Reduced-voltage start (e.g., star-delta): the main contactor can be rated at the motor rated current; the star contactor can be one size smaller.
Conductor selection
Conductor size is chosen from tables of safe ampacity or estimated from experience. Also consider minimum installation diameter and mechanical strength. A common rule of thumb used here is roughly 1 mm2 conductor area per 1 kW when using aluminum-core conductors; when converting to copper-core conductors, choose one size smaller due to higher conductivity of copper.
Table: Cable safe ampacity
Power calculation for three-phase induction motors
Three-phase induction motors are common. The basic formula for real input power of a balanced three-phase load is:
P = √3 × U × I × cosφ
This formula applies when the three-phase load is symmetrical. Variables:
- P — motor input real power
- U — line voltage supplied to the motor
- I — line current supplied to the motor
- cosφ — motor power factor
- φ — phase angle between phase voltage and phase current
Derivation
For a balanced three-phase load, total power equals three times the per-phase power: P = 3 × u × i × cosφ, where u and i are phase voltage and phase current.
For Y connection: U = √3 × u, I = i, so P = √3 × U × I × cosφ.
For Δ connection: U = u, I = √3 × i, so P = √3 × U × I × cosφ.
The same formula applies to any balanced three-phase circuit: P = √3 × U × I × cosφ.
Three-phase connection basics
Star (Y) connection: the three winding ends X, Y, Z are joined and the starts A, B, C are taken out. The common junction is the neutral point N.
Delta connection: the windings are connected end-to-end in sequence.
Definitions:
- Line terminals: A, B, C
- Neutral: neutral point N (not present in delta)
- Line voltage: voltage between line terminals
- Phase voltage: voltage across each winding
- Line current: current in the line conductors
- Phase current: current in each winding
