Motor Starting Current Calculator

More generators are undersized by motor starting than by anything else. This works out your starting current for each starting method and tells you what generator capacity it needs.

How this calculation works

A motor at standstill looks almost like a short circuit. Until the rotor turns and generates back-EMF, current is limited only by winding resistance and reactance. That’s why starting current is a multiple of running current, and why the starting method matters so much.

Method	Starting current	Starting torque	Notes
Direct on line (DOL)	6–8×	100%	Simplest, hardest on the supply
Star-delta	2–2.5×	33%	Cheap, but a torque dip at transition
Soft starter	3–4×	Adjustable	Ramps voltage; good general compromise
VFD	1–1.5×	Full, controlled	Best for the generator, most expensive

Why this matters on generator supply. The mains grid is effectively infinite — a motor starting barely registers. A generator is not. When a large motor starts, the surge drags the alternator voltage down and the engine speed with it. If the dip is deep enough, undervoltage protection trips and the whole site drops.

The usual design limit is a transient voltage dip of no more than 15–20%. Beyond that, contactors drop out, VFDs fault, and any other motor running at the time may stall.

The practical consequence: on generator supply, a motor above roughly 30 kW is rarely started DOL. A soft starter or VFD on the largest motor is very often cheaper than the two extra generator frame sizes DOL starting would demand — and it reduces mechanical shock on the driven equipment too.

Worked example

A 55 kW three phase motor, 400 V, 0.85 power factor, 90% efficient.

Running current: (55 ÷ 0.90 × 1000) ÷ (400 × 1.732 × 0.85) = 104 A. Running kVA: about 72 kVA.

Starting demand by method:

Method	Multiple	Starting current	Starting kVA
DOL	6×	622 A	431 kVA
Star-delta	2.5×	259 A	180 kVA
Soft starter	3.5×	363 A	251 kVA
VFD	1.2×	124 A	86 kVA

If this motor is the only significant load, DOL starting needs a generator around 500 kVA. With a soft starter, around 300 kVA. With a VFD, 100 kVA covers it comfortably.

That’s a five-fold difference in generator size from one decision about how the motor starts. Over a twelve-month rental, the VFD pays for itself several times over.

Common mistakes

Sizing the generator on running current. Fine until you press start.

Assuming star-delta solves everything. It cuts starting current but also cuts starting torque to a third. A motor starting against high load may not accelerate, and the transition from star to delta produces its own current spike.

Forgetting the motor may be started under load. A pump starting against a closed valve and a pump starting against full head are different problems.

Adding every motor’s starting current together. Only the largest single start matters — provided you actually stagger the starts. If they genuinely start simultaneously, that’s a control problem worth fixing.

FAQ

How much current does a motor draw at startup? Six to eight times running current for direct-on-line starting, for a few seconds. Reduced voltage methods bring that down substantially.

What size generator do I need to start a 55 kW motor? Around 500 kVA direct on line, around 300 kVA with a soft starter, around 100 kVA with a VFD. The starting method matters more than the motor size.

Can I start a large motor on a small generator? Not directly. But with a VFD, a soft starter, or by staggering starts so only one motor starts at a time, you can often use a much smaller set than DOL starting would require.

What is an acceptable voltage dip when starting? Commonly specified as 15–20% maximum transient dip. Deeper than that and you risk dropping contactors and faulting other equipment on the same supply.