Motor Starter & Protection Sizing

Start with the right current

A motor nameplate gives shaft output. The supply has to deliver more than that, because the motor is not perfectly efficient and does not draw current in phase with the voltage:

I = kW × 1000 / (√3 × V × pf × η)

If the nameplate prints a full load current, use that instead — it is measured, not estimated. The calculation is for when you have a rating and no nameplate in front of you, which on a quotation is most of the time.

One conversion trap: 1 HP is 0.746 kW, so a 20 HP motor is 15 kW, and a “15 HP” and a “15 kW” motor are a third apart in current. Get this wrong and every device downstream is undersized.

Why star–delta contactors are smaller than you expect

In delta running, each winding carries the line current divided by √3. Both the main and the delta contactor sit in series with the windings, not the line, so both are chosen for 0.58 × full load current. The star contactor only carries the star-connected starting current — about a third of full load — so it is smaller again.

That is the real economy of star–delta: three small contactors often cost less than one large one, and the inrush drops to a third. The price is that starting torque also drops to a third. A conveyor starting under load or a loaded compressor will not accelerate, will sit drawing locked rotor current, and will trip. Star–delta is for fans, pumps and unloaded compressors.

The other star–delta trap is the open transition: during the changeover the motor is briefly disconnected, and it reconnects at whatever phase angle it has drifted to. That transient can exceed the DOL inrush. Use closed transition, or a soft starter, on anything with real inertia.

Setting the overload relay

Set it at the motor full load current, not at the contactor rating and not at the cable rating. The relay exists to protect the motor winding from slow thermal overload — a jammed impeller, a failed bearing, a lost phase. Anything higher and the winding cooks before the relay notices.

• Continuous duty, standard motor: set at 1.0 × FLC.
• Motor with a marked service factor: up to 1.15 × FLC.
• Star–delta with the relay in the delta legs: set at 0.58 × FLC, because that is what flows through it. Relays wired in the incoming line are set at full FLC.

Class 10 relays trip in 10 seconds at 6 × setting and suit most loads. Motors with long run-ups — big fans, centrifuges — need class 20 or 30, or the relay will trip on every healthy start.

Short circuit protection is a separate job

The overload relay cannot break a short circuit and the breaker cannot see a 20 % overload in time. You need both. A motor protection circuit breaker (MPCB) packages them: an adjustable thermal element set to FLC, and a fixed magnetic element around 13 × FLC that lets the starting inrush through untouched.

If you use a plain MCB or MCCB instead, choose a C or D curve. A B-curve device trips magnetically at 3–5 × rating and will not survive a DOL start.

Cable at 125 %

Motor circuits are continuous duty, so the cable is sized at 1.25 × full load current before any derating for ambient, grouping or installation method. Feed that figure into the cable sizing calculator along with the run length — on long runs volt drop, not current, usually picks the size, and a motor started at low voltage draws more current, not less.