Voltage Drop Calculation: 3 Methods Compared [Worked Example]

Three different engineers can calculate voltage drop on the same circuit, using three different methods, and get three different answers. All three are technically valid — under specific conditions. The problem is knowing which method is appropriate for your circuit and understanding the error introduced when you use a simpler method than the situation demands.

This tutorial runs all three methods on a single circuit so you can see exactly where they agree, where they diverge, and why.

The Test Circuit

We will use the same circuit for all three methods:

• Load: 50 A, three-phase, 415 V, power factor 0.80
• Cable: 25 mm2 4-core XLPE copper
• Length: 50 metres
• Cable parameters at 75 degrees C: R = 0.889 ohm/km, X = 0.0803 ohm/km

These resistance and reactance values are representative for 25 mm2 copper conductors at operating temperature with typical spacing in a multicore cable.

Method 1: Simplified Resistance-Only

This is the method that many engineers learn first and some never move past. It treats the cable as a pure resistance, ignoring reactance entirely:

As a percentage: 3.85 / 415 x 100 = 0.93%

This method is fast and requires only the cable resistance. But it assumes the entire cable impedance is resistive and completely ignores the load power factor.

Method 2: mV/A/m Table Method

This is the most commonly used method in Australian and British practice. You look up a combined mV/A/m value from a table and multiply by current and length.

AS/NZS 3008.1.1, Table 42 — Three-phase voltage drop — mV/A/m

For 25 mm2 4-core XLPE copper, three-phase, the table provides separate resistive (r) and reactive (x) components:

• r = 1.54 mV/A/m (resistive component)
• x = 0.139 mV/A/m (reactive component)

The table also provides a combined value calculated at a specific reference power factor. However, for accurate results at our actual power factor of 0.80, we use the component values:

At PF 0.80: cos(phi) = 0.80, sin(phi) = 0.60

As a percentage: 3.29 / 415 x 100 = 0.79%

This is the method specified in AS/NZS 3008.1.1:2017, Clause 4.4. It correctly accounts for both resistance and reactance at the actual load power factor.

Method 3: Full Impedance Method

This is the fundamental formula from which the mV/A/m tables are derived. It uses the cable impedance components directly:

IEC 60364-5-52, Annex G — Voltage drop calculation

Using our cable parameters (R and X in ohm/m):

• R = 0.889 ohm/km = 0.000889 ohm/m
• X = 0.0803 ohm/km = 0.0000803 ohm/m

As a percentage: 3.29 / 415 x 100 = 0.79%

Methods 2 and 3 agree, as they should — they are mathematically equivalent. The mV/A/m table is simply a pre-calculated version of the full impedance formula.

Comparison of Results

For this 25 mm2 cable, Method 1 overestimates voltage drop by 17%. On this circuit, 17% is the difference between 0.79% and 0.93% — both well within limits. But the percentage error grows with cable size.

How the Error Scales with Cable Size

The simplified method's error depends on the X/R ratio of the cable and the load power factor. Let us run all three methods across a range of cable sizes for the same 50 A, PF 0.80, 50 m circuit:

Something interesting happens at 240 mm2: the simplified method actually underestimates voltage drop. At this cable size, reactance (X) is significant relative to resistance (R). The term X x sin(phi) contributes enough that ignoring it no longer produces a conservative result. This is the danger zone — the simplified method is not always conservative.

Acceptable Voltage Drop Limits by Standard

Each standard specifies different permissible voltage drop limits:

AS/NZS 3000, Clause 3.6.2 — Voltage drop BS 7671, Appendix 12 — Voltage drop NEC/NFPA 70, 210.19 Informational Note 4 — Voltage drop — branch circuits

Note that the NEC values in 210.19 and 215.2 are informational notes, not mandatory requirements. However, they reflect industry best practice and most AHJs (Authorities Having Jurisdiction) expect compliance.

The BS 7671 limits are also recommendations, not regulations. Appendix 12 notes that the values are "a guide" — but failing to meet them requires justification.

When to Use Which Method

Use Method 1 (Resistance-Only) When:

• Cable size is 16 mm2 or smaller
• Cable run is short (under 30 m)
• Quick mental estimate is sufficient
• Power factor is close to unity (resistive loads like heaters)

Use Method 2 (mV/A/m Tables) When:

• You are working to AS/NZS 3008 or BS 7671
• Cable size is up to 150 mm2
• You need a documented, standards-referenced result
• This is the standard method for most projects

Use Method 3 (Full Impedance) When:

• Cable size is 150 mm2 or larger
• Power factor is below 0.85
• Cable run exceeds 100 m
• You are working to IEC 60364-5-52 or need maximum accuracy
• The mV/A/m tables do not cover your specific cable type or installation

Practical Tips

Always check voltage drop at design current (Ib), not protective device rating (In). The cable operates at Ib under normal conditions. Voltage drop at In represents a transient condition that may not even occur.

Account for the entire voltage drop chain. A 2.5% drop from the transformer to the distribution board plus a 2.8% drop from the board to the load gives 5.3% total — over the AS/NZS 3000 limit even though each individual run looks acceptable in isolation.

Motor starting voltage drop is a separate calculation. The methods above apply to steady-state running current. Motor starting draws 5-8 times rated current for several seconds. The voltage drop during start must be checked separately to ensure the motor can develop adequate starting torque and other connected equipment is not affected.

Temperature affects resistance. The cable resistance values in the mV/A/m tables are at a specific conductor operating temperature (typically 75 degrees C for XLPE at full load). At light load, the conductor runs cooler, resistance is lower, and actual voltage drop is less than calculated. This provides an inherent safety margin in the calculation.

Three methods, one answer — when applied correctly. The mV/A/m method strikes the right balance of accuracy and convenience for the vast majority of circuits. Reserve the full impedance method for large cables and critical circuits where every tenth of a percent matters.