DOL Motor Start: Empirical Formula, Voltage Drop & IEC 60947 Guide

When a three-phase induction motor is started direct-on-line at full rated voltage, the inrush current can spike to 6-8 times the motor nameplate full-load current. This surge does more than dim the lights -- it stresses stator windings, disrupts sensitive equipment sharing the same bus, and in severe cases can violate grid connection agreements. Since our original April 2024 article on the empirical formula for judging direct start, the engineering landscape has shifted. IEC 60947-4-1 coordination requirements have tightened across industrial markets, and a March 2026 IEEE paper delivered the first closed-form analytical expression for DOL transient starting current. This Engineering Note brings the formula into the 2026 compliance era with IEC-anchored limits, a cross-reference table, and practical selection criteria drawn from field experience.

The Empirical Formula, Re-Anchored for 2026

The core empirical relationship remains deceptively simple. What changed is the context around it -- specifically, which numbers you plug in and what you do with the result.

Where does I_start come from? For a standard squirrel-cage induction motor, locked-rotor current is typically 6 to 8 times the nameplate full-load current (I_FLA). Small motors under 7.5 kW can approach 8x; medium-frame machines between 10 and 50 HP settle around 6-7x; large motors above 50 HP, especially those built to NEMA Design B or IEC IE3 efficiency classes, often fall in the 5-6.5x range. If the motor nameplate lists a locked-rotor code letter (NEMA MG-1), use the corresponding kVA/HP range to derive a more accurate I_start -- this is far better than assuming 6x blindly.

I_sc, the short-circuit current at the motor busbar, is what most quick checks get wrong. You cannot use the infinite-bus assumption unless the motor is served directly from a utility transformer with negligible upstream impedance. In practice, cable length, transformer kVA rating, and percent impedance (%Z) all reduce I_sc at the motor terminals. A 500 kVA transformer with 5% impedance feeding a motor through 100 meters of cable may deliver an I_sc of only 6-8 kA at the motor -- far lower than the 20+ kA you would estimate at the transformer secondary. Undershooting I_sc overestimates the voltage drop, which can cause you to specify a soft starter or VFD unnecessarily.

What IEC 60947-4-1 Actually Requires in 2026

IEC 60947-4-1 covers contactors and motor-starters -- the hardware that actually closes the circuit for a DOL start. The standard defines two coordination types that every engineer specifying a DOL starter must understand:

The practical implication for DOL starting: Type 2 coordination means your contactor and circuit-breaker combination must come from a manufacturer-tested pair. You cannot independently pick a contactor by AC-3 rating and a breaker by frame size and assume they will coordinate. This is a common pitfall in brownfield upgrades where an old Type 1 starter is replaced piecemeal. If your facility maintains Mitsubishi or Siemens motor control gear, verify the tested combination in the manufacturer's coordination tables before placing a purchase order.

Voltage Drop: From Quick Desk Check to Code-Compliant Design

Once you have calculated the voltage drop percentage, the next question is: what number triggers a design change? The empirical thresholds below have been harmonized with IEC 61000-3-3 (voltage fluctuation and flicker limits) and common utility interconnection requirements as of mid-2026.

Practical kW limits for DOL on 400 V LV systems cluster around 75-90 kW, above which the voltage dip typically exceeds 10% unless the supply transformer is oversized. On 690 V systems, DOL may be viable up to 200-250 kW. These are empirical guidelines -- always run the formula with your actual upstream impedance. For critical processes where even a 5% dip causes nuisance alarms in Allen-Bradley PowerFlex or other VFD-driven auxiliaries on the same bus, target the sub-5% band.

Starting Method Cross-Reference (2026 Data)

When the empirical formula says DOL is marginal, here is how the alternatives compare. Use this table alongside the voltage-drop calculation to select the right method.

Use our PLC Analog Calculator to convert 4-20 mA motor current signals to engineering units when monitoring starting current through a PLC analog input module. For VFD-based starting solutions across all major brands, reach out with your motor nameplate data.

Practical Field Notes: What the Formula Does Not Tell You

After applying the empirical formula across dozens of motor control retrofit projects, we have observed three patterns that textbooks rarely mention:

Aging contactors change the equation. A DOL contactor that has interrupted thousands of starts develops pitted contacts. Contact resistance creeps up, and the voltage drop across the contacts during start becomes non-negligible. In a 2025 retrofit at a Southeast Asian food processing line, we measured an additional 2.3% voltage drop across a 12-year-old contactor that the empirical formula did not predict. The motor still started, but the PLC analog input module on the same bus reported erratic 4-20 mA readings during every start cycle. Replacing the contactor eliminated the issue without changing the motor or the feeder.

Parallel loads on the same bus matter more than the formula suggests. A 5% voltage dip at the motor busbar is acceptable per IEC 61000-3-3. But if that bus also feeds a CNC controller, a vision inspection system, or a communications gateway, the 5% dip may drop the control power supply below its hold-up threshold. The empirical formula only tells you about the motor bus -- map your auxiliary loads before signing off on a DOL start.

The 2026 IEEE transient model changes the game for large motors. The March 2026 IEEE Access paper "Analytical Expression for the No-Load Starting Current of an Induction Machine Under Direct-Online Conditions" provides a closed-form expression for starting current versus time that accounts for motor electrical parameters (stator resistance, rotor resistance, leakage inductance, magnetizing inductance). For motors above 90 kW, the paper shows that the peak inrush at t = 0+ can exceed the conventional 6-8x estimate by 15-20% for high-efficiency IE4 designs with low rotor resistance. If you are sizing protection for a new IE4 motor on DOL, consult this model rather than relying on the classic empirical ratio.

When DOL Is Still the Right Answer

Despite the availability of soft starters and VFDs at lower price points than a decade ago, DOL remains the correct choice in several scenarios:

• Small auxiliary motors under 5.5 kW -- the cost of a soft starter often exceeds the motor cost. DOL with a Type 2 coordinated contactor-breaker pair is the economical, reliable choice.
• Fire pump and life-safety motor circuits -- many codes (NFPA 20, local fire regulations) explicitly require DOL starting to eliminate failure modes in reduced-voltage electronics.
• Motors served by a dedicated transformer with ample kVA headroom -- if I_sc at the motor busbar is high enough to keep voltage dip under 5%, adding a soft starter adds complexity without benefit.
• Harsh environments -- outdoor, high-vibration, or high-ambient-temperature installations where VFD/soft-starter electronics have a reduced service life.

Related on KOEED

• KOEED Blog Archive -- Engineering Notes, How-to guides, and case studies for industrial automation professionals
• Allen-Bradley Motor Control -- Contactors, overload relays, PowerFlex drives, and SMC soft starters
• Siemens SIMATIC & SINAMICS -- SIRIUS motor starters, SINAMICS VFDs, and industrial controls
• PLC Analog Calculator -- Free online tool for 4-20 mA and 0-10V signal conversion
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