mV/A/m Voltage Drop: Formula, Units and Worked Example

The short answer: 10 mV/A/m gives 5 V in this example

At 10 mV/A/m, 10 A and a 50 m one-way route, the estimated voltage drop is 5 V: 10 × 10 × 50 ÷ 1000 = 5 V. The general relationship is ΔV = (mV/A/m coefficient × current in A × one-way route length in m) / 1000.

The coefficient packages the conductor and circuit conditions specified by its source. Its table heading, construction, geometry, temperature and power-factor notes therefore matter. There is no universal mV/A/m value for every cable of the same area. Open the calculator with the 10 A, 50 m example when you have a traceable coefficient for the intended circuit.

Read the unit literally

The unit means millivolts of drop for each ampere of current and each metre of one-way route under the stated data conditions. Multiply the coefficient by amperes and metres, then divide by 1,000 to convert millivolts to volts.

The multiplication first gives 5,000 millivolts; dividing by 1,000 converts it to volts:

ΔV = 10 × 10 × 50 ÷ 1000 = 5.0 V

At a 230 V nominal supply, that is 5 ÷ 230 × 100 = 2.17%. A result of 5,000 V means the millivolt conversion was missed. A result of 10 V may indicate that a loop factor was added even though it was already included in the coefficient.

The table heading carries engineering meaning

An mV/A/m value is not a universal property of “a 4 mm² cable”. It may depend on conductor material, construction, circuit arrangement, phase system, temperature, installation assumptions and power factor. Some sources provide separate values or columns for different conditions.

Keep the table title, row, column, revision and explanatory notes with the value. If the source does not match the intended circuit geometry, choosing a number that looks close is not a traceable calculation.

Why loss is not available from the coefficient alone

Voltage drop is related to impedance and current. Real conductor loss depends on resistance specifically: in a balanced model, terms involving I²R are used. A combined mV/A/m coefficient may include reactive contribution and geometry without exposing resistance separately.

ElectricalDyev therefore reports loss as unavailable in this mode instead of displaying 0 W. Zero would be a numeric claim that no loss exists, which is generally false. Use cable-specific resistance data when a loss estimate is required.

Parallel runs and coefficient basis

For equal parallel runs, the ideal effective drop coefficient can be divided by the number of runs, or the current per run can be used with the per-run coefficient. Apply one approach only. Confirm the source coefficient is per run rather than already describing a grouped arrangement.

Equal current sharing is an engineering assumption that needs identical conductors, route and terminations. Record it with the result and check ampacity, grouping, fault sharing and installation requirements separately.

mV/A/m, mV/A and MVA are different units

The lower-case m in mV means milli: one thousandth of a volt. A value in mV/A/m is a drop coefficient per ampere per metre. A value in mV/A may already describe a complete length; it cannot be used as a per-metre value without knowing its definition. Upper-case MVA means megavolt-amperes, an apparent-power rating. These are not interchangeable spellings.

Do not multiply by another geometry factor already included by the source. This coefficient alone does not separate resistance from reactance, so it cannot establish conductor watt loss.

Use custom mV/A/m mode. Supply a traceable coefficient for the actual circuit geometry. The voltage-drop and conductor-data route compares this method with R/X and simplified resistivity.