Voltage Drop at Startup vs Running: The Motor Circuit Calculation Engineers Forget

Every cable sizing procedure includes a voltage drop check. Calculate the drop at full load current, compare against the standard's limit (typically 3–5%), and move on. For lighting and heating circuits, this is sufficient. For motor circuits, it's dangerously incomplete.

Motor circuits experience starting currents of 6–8× rated current during direct-on-line (DOL) starting. A cable that shows 3.5% voltage drop at running current will show 21–28% voltage drop during startup. At this voltage, the motor may not develop enough torque to accelerate the load. Contactors on adjacent circuits may drop out. VFDs on the same bus may trip on undervoltage. LED lighting flickers. Sensitive electronic equipment resets.

This is not a hypothetical problem. At a copper-gold concentrator plant, we had a 200 kW SAG mill discharge pump that would intermittently fail to start during high-load conditions. The cable had been correctly sized for running current — 3.8% voltage drop at 350 A. But during DOL starting at 2,450 A, the voltage at the motor terminals dropped to 295 V (29% below nominal). At this voltage, the motor could barely develop 50% of its rated locked-rotor torque — insufficient to accelerate a loaded pump against static head.

The solution was either a larger cable (expensive, disruptive) or a soft starter (installed in 3 days). The calculation that would have prevented the problem takes 5 minutes.

Starting Current by Method

The starting current depends on the starting method:

The Startup Voltage Drop Calculation

The formula is identical to the running voltage drop formula, but with the starting current substituted for the running current:

Or, using the full impedance formula for more accurate results (essential for large cables):

Note that the power factor during starting (φ_start) is different from the running power factor. Motors draw starting current at a power factor of approximately 0.15–0.35 (highly inductive). This significantly increases the reactive voltage drop component.

Worked Example

Motor: 30 kW, 415 V, 3-phase, FLC = 56 A, DOL starting (7× FLC) Cable: 16 mm² 4-core XLPE/SWA, route length 80 m Cable data: R = 1.47 Ω/km, X = 0.082 Ω/km

Running voltage drop (PF = 0.85):

Result: 2.5% — well within the 5% limit.

Starting voltage drop (I_start = 7 × 56 = 392 A, PF_start = 0.25):

The starting voltage drop is 26.5 V — the motor terminal voltage during starting is 388.5 V, which is 93.6% of nominal. This is borderline acceptable.

But the actual situation is worse: the motor starting current also causes a voltage dip on the supply bus. If the upstream transformer has 5% impedance and the motor starting load is significant relative to transformer capacity, the bus voltage may dip by an additional 3–5% during starting. The motor now sees 88–90% voltage.

Motor Torque vs Voltage

Motor torque is proportional to the square of the applied voltage:

At 90% voltage: T = 0.9² × T_rated = 0.81 × T_rated (81% of rated torque) At 80% voltage: T = 0.8² × T_rated = 0.64 × T_rated (64% of rated torque) At 70% voltage: T = 0.7² × T_rated = 0.49 × T_rated (49% of rated torque)

If a motor needs 60% of its rated torque to start the load (common for pumps against static head), it needs at least 77% voltage to develop that torque. Below this voltage, the motor stalls.

What the Standards Say (And Don't Say)

No major cable sizing standard specifies a maximum allowable startup voltage drop. This is a significant gap:

• BS 7671: No requirement for startup voltage drop. Appendix 12 only addresses steady-state voltage drop.
• IEC 60364: No specific startup requirement. IEC 60364-5-52 addresses running conditions only.
• NEC: No mandatory startup voltage drop limit. The 3%/5% recommendations in NEC Informative Annex apply to running conditions.
• AS/NZS 3000: Clause 6.3 mentions "starting conditions" but doesn't specify a numerical limit.

IEEE 141, Section 3.5 — Voltage Considerations

The best guidance comes from IEEE 141 (IEEE Red Book), which recommends that voltage dips during motor starting should not exceed 15% at the motor terminals. Many engineering firms adopt this as a design standard.

The Decision Matrix

Practical Solutions

When startup voltage drop is excessive:

1. VFD (Variable Frequency Drive) — Eliminates the problem. Starting current = FLC. Best solution for variable-speed loads. Cost: $100–200/kW.

2. Soft starter — Reduces starting current to 2–4× FLC (adjustable). Good for pumps and fans where speed control is not needed. Cost: $30–60/kW.

3. Star-delta starting — Reduces starting current to 33% of DOL. Free (only contactors and timer needed). But creates a torque dip during changeover.

4. Increase cable size — Reduces R₁ and X₁ in the voltage drop equation. Works but expensive on long runs.

5. Reduce cable route length — Relocate the MCC closer to the motor. Often impractical.

6. Autotransformer starting — Reduces starting current and voltage in a controlled ratio. Good for very large motors. Expensive.

When to Calculate Startup Voltage Drop

Calculate startup voltage drop for any motor circuit where:

• Cable route length exceeds 50 m
• Motor is ≥ 15 kW with DOL starting
• Motor drives a high-inertia load (crusher, conveyor, large pump)
• The supply transformer is close to its capacity limit
• Other sensitive loads share the same bus (VFDs, UPS, control systems)

For circuits shorter than 50 m with motors under 15 kW, running voltage drop is almost always adequate as a proxy.

Related Resources

• King's Cross Fire: Motor Circuit Cable Sizing — Motor starting current in a real incident scenario
• Motor Starting: The 75kW Pump — How fuse and cable selection changes across standards
• Voltage Drop: The 100m Workshop Cable — Voltage drop limits that catch motor feeders on long runs
• View all worked examples →