How to Check Cable Voltage Drop

What inputs does a voltage drop calculation need?

A calculation needs the system voltage, the number of phases, the running current, the conductor material and cross-section, the one-way route length, the power factor and the operating temperature. The cable datasheet supplies the conductor resistance and, where it is needed, the reactance. Tinned conductors have slightly higher resistance limits than plain copper, so confirm the conductor finish as well as the size. For more on conductor construction, see our guide to conductor classes and stranding.

Which formula should you use for DC, single-phase and three-phase circuits?

DC circuits depend mainly on loop resistance, because current flows out and back through two conductors. AC circuits add the effect of reactance, and the power factor sets how much each term contributes. The formulas differ by circuit type:

• DC: ΔU = 2 × I × L × R
• Single-phase AC: ΔU = 2 × I × L × (R cos φ + X sin φ)
• Three-phase AC: ΔU = √3 × I × L × (R cos φ + X sin φ)

In these formulas ΔU is the voltage drop in volts, I is the current in amperes, L is the one-way length, R is resistance per unit length, X is reactance per unit length, and cos φ and sin φ are from the power factor. Use the same length unit for R and X, and keep the units consistent throughout.

At unity power factor, which means a purely resistive load, the sin φ term is zero and reactance drops out. At lower power factor, reactance becomes a larger part of the result, so do not leave it out for inductive loads without checking the datasheet.

Worked example, resistance only. The IEC 60228 maximum DC resistance at 20 °C for 16 mm² plain copper class 1 or 2 conductor is 1.15 Ω/km. At unity power factor the three-phase formula reduces to ΔU = √3 × 1.15 × I × L volts, with L in kilometres. That is close to 2 V for every ampere carried over each kilometre. Because 1.15 Ω/km is a maximum at 20 °C, treat it as a starting point only: correct it to the operating temperature and use the datasheet value for the real design.

What voltage drop limits apply?

Limits come from the project specification, the equipment requirements and local code. The table compares the two widely cited rules. IEC 60364-5-52 Annex G is informative, and national rules may differ, so identify the load type and the rule that applies before fixing the cable size.

Percentages are the voltage drop limits given in each source. Final voltage-drop limits should follow the project specification, equipment requirements and local code.

How do motor starting and the operating case change the check?

Starting current may be several times the running current, so a feeder that gives acceptable drop at running load can leave a distant motor without enough voltage to accelerate. Check whether the terminal voltage stays sufficient for acceleration and for contactor and control operation. A feeder may satisfy running drop but fail to start that motor, while a PV string may meet thermal limits yet lose too much energy over its life. For the thermal side of the same design, see our guide to cable ampacity and derating.

Before calculating, state the operating scenario and the acceptance criterion, such as the minimum terminal voltage needed for starting or the maximum drop allowed in running. Then check the case with these points in mind:

• Use conductor resistance at the relevant temperature.
• Include power factor, and reactance for AC where required.
• Check both normal and transient conditions.

How can you reduce voltage drop?

Several design changes reduce voltage drop. Recheck the calculation after each change, because each option alters the result:

1. Increase conductor cross-section.
2. Reduce route length.
3. Raise distribution voltage where the design permits.
4. Use parallel conductors under applicable rules.
5. Improve power factor where appropriate.
6. Locate equipment or transformers closer to the load.

Larger cross-section has a large effect on resistance. The IEC 60228 maximum DC resistance at 20 °C for plain copper class 1 or 2 falls from 7.41 Ω/km at 2.5 mm² to 1.15 Ω/km at 16 mm² and 0.193 Ω/km at 95 mm².

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