Voltage Drop vs. Ampacity: Why Cables Fail in Long Runs

It is a scenario I have seen play out on construction sites more times than I care to count. A 75kW (100HP) motor is installed at a pump station 200 meters away from the main MCC. The electrical contractor opens the code book (NEC or IEC), looks up the Full Load Amps (approx 130A), and selects a 3/0 AWG (95mm²) copper cable. The table says it’s good for 200 Amps. It looks safe. It complies with code.

Then comes commissioning day. The operator hits "Start." The motor groans. The lights in the pump house dim significantly. The contactor chatters violently, and then... silence. The breaker trips, or worse, the motor windings overheat before it ever reaches full speed.

The engineer is baffled. "But the cable is rated for 200 Amps! The motor only draws 130 Amps!"

The problem isn't the Ampacity (the cable's ability to handle heat). The problem is Voltage Drop (the cable's resistance over distance). In this deep dive, we are going to look at the physics that code books often gloss over, and why designing for the "sticker rating" of a cable is a recipe for expensive failures.

The Case Study: The 200m Pump Station

Let’s analyze that failed pump station mathematically to see exactly what went wrong.

• System Voltage: 480V, 3-Phase
• Motor Load: 130A (Full Load)
• Distance: 200 meters (656 feet)
• Cable: 3/0 AWG Copper (Resistance approx 0.203 Ω/km)

1. Running State (Steady State)

In a steady run, the voltage drop calculation ($V_d = \sqrt{3} \times I \times R \times L$) shows a drop of roughly 9 Volts (1.8%). This is well within the standard NEC recommendation of 3% or 5%. So, why did it fail?

2. Starting State (The Killer)

When a motor starts Direct-On-Line (DOL), it pulls an Inrush Current of typically 6 times the full load amps.
Starting Current = 130A × 6 = 780 Amps.

Now, push that 780 Amps through 200 meters of cable.
The voltage drop spikes to over 54 Volts. That is an 11% drop instantly. The voltage at the motor terminals falls to roughly 426V.

Here is the physics trap: Torque is proportional to the square of the Voltage ($T \propto V^2$).

If your voltage drops to 89% (which is 426/480), your starting torque drops to $0.89^2$, which is 79%. If that pump needed 85% torque to overcome the static head of the water in the pipe, the motor physically cannot turn the shaft. It stalls. It draws Locked Rotor Current (LRA) continuously until it burns out or trips the protection.

Calculate Motor Starting Drop

AC Voltage Drop: It's Not Just Resistance

For small wires (lighting circuits), using DC resistance ($R$) is usually close enough. But for large feeder cables (1/0 AWG and larger), you cannot ignore Reactance ($X$).

As cable diameter increases, the "Skin Effect" pushes electrons to the outside of the conductor, slightly increasing resistance. More importantly, the magnetic fields between conductors create Inductive Reactance. The formula changes from $V=IR$ to:

V_drop = I × (R·cos(θ) + X·sin(θ))

Where $\theta$ is the angle of your Power Factor.
Here is the kicker: Motor starting power factor is terrible (usually 0.2 to 0.4 lagging). This means the $\sin(\theta)$ component is large. If you only calculated using Resistance ($R$), you underestimated the drop. The Reactance ($X$) of the cable combined with the low power factor of the starting motor creates a "perfect storm" for voltage collapse.

The Economic Reality: Copper vs. Aluminum

There is a stigma against Aluminum cable, largely from the 1970s when it was poorly used in residential branch wiring (outlets and switches). But in the world of industrial feeders, Aluminum is king—if you know how to use it.

Let's go back to our 200m pump station. To fix the voltage drop problem using Copper, we might need to upsizing from 3/0 AWG to 350 kcmil.
Cost of 350 kcmil Copper (200m): Expensive. Very expensive. Copper prices fluctuate wildly and are generally 3x-4x the price of Aluminum by weight.

The Aluminum Alternative:
Aluminum has about 61% the conductivity of Copper. This means you generally need to go up two sizes (e.g., from 3/0 Cu to 250 kcmil Al) to get the same ampacity.

However, when you are sizing for Voltage Drop, you are essentially buying conductive cross-section. Because Aluminum is so much cheaper and lighter, you can buy a massive Aluminum cable (say, 500 kcmil) for half the price of the smaller Copper cable.

Compare Cable Sizes Now

Temperature Correction: The Forgotten Variable

Another factor engineers miss is that resistance is not constant. Resistance rises with temperature.

Standard tables often list resistance at 20°C or 25°C. But a cable running at full load isn't at 25°C. It might be at 75°C or 90°C. The resistance of copper rises approximately 0.4% for every degree Celsius.

If your cable heats up to 75°C, its resistance increases by roughly 20% compared to room temperature. That 5% voltage drop you calculated? It just became 6%. In a marginal design, that extra heat could be the difference between a running plant and a nuisance trip. Always perform voltage drop calculations using the resistance at the operating temperature of the conductor.

Conclusion: Design for the Destination

Sizing a cable is not about ensuring the cable doesn't melt. That is the bare minimum requirement for not starting a fire. Good engineering is about ensuring the load at the end of the line receives the quality of power it was designed to consume.

When you see a long run on a drawing—anything over 50 meters (150 feet)—stop looking at the Ampacity table. Start calculating the Voltage Drop. Consider the inrush current. Look at the power factor. And seriously consider Aluminum feeders. Your project budget (and your motors) will thank you.