Mike Holt Voltage Drop Calculator: Formula and Limits

A Mike Holt voltage drop calculator is a tool for estimating how much voltage a circuit loses along its conductors, using details such as circuit length, current, conductor size, and material. The result is an estimate to compare with the voltage supplied; it is not, by itself, a wire-sizing recommendation or proof that an installation complies with electrical rules.

Formula

This calculator uses the basic resistance-based, single-phase voltage-drop equation for a two-wire circuit:

V_drop = 2 × I × L × R ÷ 1000

• V_drop is the estimated voltage lost in volts (V).
• I is load current in amperes (A).
• L is one-way conductor length in feet (ft).
• R is conductor resistance in ohms per 1,000 feet (Ω/1,000 ft).

The factor 2 accounts for the outgoing and return conductors. The calculator then finds the percentage drop as 100 × V_drop ÷ V_source, and estimated load voltage as V_source − V_drop. Source voltage is in volts. The resistance input is a value you must obtain for the conductor and conditions being evaluated; it is not automatically selected by the calculation.

Worked Example

Using the calculator’s default values—120 V source, 20 A current, 100 ft one-way length, and 1.24 Ω/1,000 ft resistance—the round-trip length is represented by the factor 2:

1. Voltage drop = 2 × 20 A × 100 ft × 1.24 Ω/1,000 ft ÷ 1,000 = 4.96 V.
2. Percentage drop = 100 × 4.96 V ÷ 120 V = 4.13% (rounded to two decimal places).
3. Estimated voltage at the load = 120 V − 4.96 V = 115.04 V.

This is a mathematical example, not a judgment that the selected conductor is suitable. The resistance value is illustrative; use an appropriate value for the actual conductor and calculation method.

What People Mean by the Mike Holt Calculator

People searching for a “Mike Holt voltage drop calculator” are usually looking for an online calculator or reference associated with Mike Holt’s electrical education resources. Tools and pages can change over time, so check the current source directly for its available inputs, assumptions, and guidance. The label alone does not tell you whether a tool models alternating-current reactance, a three-phase circuit, conductor temperature, or a particular code edition.

The calculator above provides a transparent, simplified calculation rather than claiming to reproduce any particular branded tool. It makes its inputs and formula explicit so you can see what is being assumed. For a result that must support installation decisions, consult the applicable electrical code, manufacturer data, and a qualified electrical professional.

How to Use a Voltage-Drop Calculator

• Enter source voltage. Use the voltage at the circuit’s source, such as a nominal 120 V supply, and keep it in volts.
• Enter load current. Use the current relevant to the circuit condition being assessed. Do not substitute a device’s power rating for current without a suitable conversion and assumptions.
• Enter one-way length. Measure the distance from the source to the load, not the combined length of both conductors. The formula includes the return path.
• Enter resistance. Obtain a resistance value appropriate to conductor material, size, and temperature from reliable conductor data or a qualified source.
• Review all three outputs. Voltage drop gives the lost volts, percentage drop expresses that loss relative to source voltage, and load voltage estimates what remains.

For metric measurements, convert one-way length to metres and resistance to ohms per kilometre, then use a formula written for those units. Do not enter metres into a feet-based expression or Ω/km as though it were Ω/1,000 ft. One kilometre is about 3,280.84 ft, so the resistance figures are not numerically interchangeable.

What the Result Can—and Cannot—Tell You

Voltage drop is the difference between the voltage at the source and the voltage at the load while current is flowing. It depends on current, conductor impedance, and circuit length. In the simplified equation, doubling current, length, or resistance doubles the calculated drop, all else equal. A longer circuit can therefore have a meaningful voltage loss even when its source voltage is unchanged.

The estimate does not determine whether a conductor, protective device, or installation is safe or code-compliant. Those decisions also involve factors such as conductor ampacity, insulation rating, installation method, ambient temperature, terminals, fault protection, and the governing rules. A low percentage result cannot establish that a wire is correctly sized.

Actual AC circuits can require more detailed calculations. Alternating current interacts with conductor reactance as well as resistance; the significance depends on such factors as conductor arrangement, circuit configuration, and power factor. Three-phase systems also use a different equation and assumptions from the single-phase two-wire formula here. Use a suitable method for the system rather than applying the simple equation indiscriminately.

Common Mistakes and Useful Comparisons

• Doubling the length twice: This calculator expects one-way length and already includes a factor of 2. If you enter round-trip length, the result is overstated.
• Mixing units: Feet, metres, Ω/1,000 ft, and Ω/km must match the formula. Unit mismatch can produce a result that looks plausible but is wrong.
• Using an arbitrary resistance: Resistance varies with conductor size and material, and changes with temperature. A generic number may not represent the circuit.
• Confusing volts with percent: A 4 V drop is an absolute loss; its percentage depends on source voltage. The same loss is a different fraction on a 120 V supply than on a 240 V supply.
• Rounding too early: Keep precision through the calculation and round the displayed result at the end. This calculator displays two decimal places.

For perspective, a percentage is often easier to compare across different source voltages, while the voltage figure shows the actual number of volts lost. Neither figure is an automatic pass/fail test. Any applicable recommendation or limit must come from the relevant code, design specification, or equipment documentation.

Frequently Asked Questions

Is this the official Mike Holt voltage drop calculator?
This page does not claim to be an official Mike Holt tool. It explains a basic voltage-drop calculation. Check Mike Holt’s current resources directly if you need a particular calculator or its stated assumptions.

Do I enter the one-way or round-trip distance?
Enter the one-way distance from source to load. The formula multiplies by 2 to account for the outgoing and return conductors.

Does voltage drop depend on wire size?
Yes. Wire size affects conductor resistance, which is why the resistance input must correspond to the conductor being assessed. This calculator does not choose a wire size for you.

Can I use this formula for a three-phase circuit?
Not as written. This is a simplified single-phase, two-wire formula. Three-phase calculations use different relationships and may need additional system details.

Does a low voltage-drop result mean the circuit is safe?
No. Voltage drop is only one calculation. It does not verify conductor ampacity, overcurrent protection, installation conditions, code compliance, or overall safety.

Why might an actual measurement differ?
Real operating current, conductor temperature, connections, supply variation, and AC impedance can differ from the calculator’s assumptions. A qualified professional can assess a circuit using its actual conditions and an appropriate method.

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in v | Source voltage | V | 120 | 0
in i | Load current | A | 20 | 0
in l | One-way circuit length | ft | 100 | 0
in r | Conductor resistance | ohm/1000 ft | 1.24 | 0
out drop | Voltage drop | V | 2 * i * l * r / 1000 | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out loadv | Estimated load voltage | V | v - drop | 2