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.

The transition, and the spike nobody expects

A star–delta starter has a gap in the middle. The star contactor opens, and for 30 to 100 milliseconds the motor is disconnected entirely before the delta contactor closes. That is open transition, and it is what most panels are built with because it needs three contactors and a timer and nothing else.

The motor keeps spinning through that gap, and it keeps generating. Its residual voltage decays slowly, and by the time delta closes, that voltage can be substantially out of phase with the supply it is being reconnected to. The result is a current transient that routinely exceeds the direct-on-line inrush the starter was installed to avoid — the one spike in the whole sequence larger than the problem being solved.

It is survivable on most loads and it is why star–delta has a reputation for being hard on couplings and gearboxes. Where it is not acceptable — long shafts, brittle drives, anything with backlash — the answer is closed transition, which adds three resistors and a fourth contactor so the motor is never fully disconnected. Or a soft starter, which removes the transition entirely and is now close enough in price that new panels rarely justify the alternative.

Starts per hour is a rating, not a preference

Every start dumps energy into the rotor. At six times full load current the heating is roughly thirty-six times normal, and the rotor cage is the part that absorbs it. Motor nameplates and datasheets state a permitted number of starts per hour for exactly this reason, commonly 6 to 10 for a standard cage motor and fewer for a large or high-inertia drive.

Exceed it and nothing trips. The overload relay is protecting the stator winding against a sustained overcurrent, and a rapid sequence of short starts does not look like one — the relay cools between attempts while the rotor does not. Repeated starting is the failure mode that most often destroys a motor whose protection was set perfectly correctly.

If the process needs more starts than the motor allows, the fix is a drive rather than a bigger starter. A VFD accelerates on a controlled ramp at perhaps 1.5 times full load current, so the thermal cost per start falls by more than an order of magnitude and the limit stops mattering.