Wire resistance is the opposition a conductor presents to current flow. Voltage drop is the voltage lost across that resistance while the circuit is carrying load current.
The calculator produces two numbers:
- Voltage drop in volts
- Voltage drop as a percentage of system voltage
Those results are used during branch-circuit and feeder design to compare conductor options, review long runs, and identify whether a proposed circuit remains within an entered voltage-drop limit. A conductor can have adequate ampacity while still producing an unacceptable voltage drop over a long distance, especially on low-voltage DC circuits, motor loads, and heavily loaded feeders.
Circuit Resistance and Load Current
Voltage drop rises when any of these values increases:
- Load current
- Total circuit resistance
- Conductor length
The calculator applies the basic electrical relationship:
\(\displaystyle V_{\text{drop}} = I \times R_{\text{total}}\)
Where:
- \(V_{\text{drop}}\) is voltage drop in volts
- (I) is load current in amperes
- \(R_{\text{total}}\) is total circuit resistance in ohms
Total circuit resistance represents the resistance of the complete current path used for the calculation. For a given current, a higher-resistance conductor produces a larger voltage drop. For a given conductor resistance, a higher load current produces a larger voltage drop.
The percentage result is calculated from the system voltage:
\(\displaystyle \text{Voltage Drop \%} = \frac{V_{\text{drop}}}{V_{\text{system}}} \times 100\)
A 3 V drop has a very different effect on a 12 V circuit than on a 480 V circuit. Expressing the result as a percentage shows the impact relative to the circuit’s available voltage.
Voltage-Drop Calculation Example
Assume two conductor options serve the same load over the same circuit length. Keep the load current and length unchanged, then compare the total circuit resistance for each option.
| Input or result | Conductor Option A | Conductor Option B |
|---|---|---|
| Load current | 20 A | 20 A |
| Total circuit resistance | 0.20 Ω | 0.10 Ω |
| System voltage | 120 V | 120 V |
| Voltage drop | 4 V | 2 V |
| Voltage drop percentage | 3.33% | 1.67% |
For Option A:
\(\displaystyle 20 \text{ A} \times 0.20 \ \Omega = 4 \text{ V}\)
\(\displaystyle \frac{4 \text{ V}}{120 \text{ V}} \times 100 = 3.33\%\)
For Option B:
\(\displaystyle 20 \text{ A} \times 0.10 \ \Omega = 2 \text{ V}\)
\(\displaystyle \frac{2 \text{ V}}{120 \text{ V}} \times 100 = 1.67\%\)
Option B has half the total circuit resistance, so it produces half the voltage drop at the same current. If the entered voltage-drop limit is 3%, Option B remains below that limit while Option A exceeds it.
This comparison is useful when deciding whether a lower-resistance AWG or kcmil conductor is justified for a long branch circuit or feeder. It can also help identify when a circuit layout should be reconsidered before changing conductor size.
Conductor Resistance and Wire Size
Conductor resistance depends on conductor size, material, temperature, and length. In general, larger conductors have lower resistance per unit length than smaller conductors.
For example, moving from a smaller AWG conductor to a larger conductor can reduce total circuit resistance and therefore reduce voltage drop. The voltage-drop calculation does not directly establish ampacity, but it helps compare conductors that are otherwise being evaluated for an installation.
Use the resistance value that represents the actual circuit path being reviewed. The result should be interpreted alongside conductor ampacity, insulation temperature rating, terminal rating, current-carrying conductors, correction factors, adjustment factors, and installation conditions.
A conductor selected solely to reduce voltage drop may still require a separate ampacity and termination review. Conversely, a conductor selected for ampacity may need to be increased if its circuit resistance causes excessive voltage drop over the installed length.
Voltage Drop in Circuit Layout
Voltage-drop review is most useful when the conductor run is long, the load current is substantial, or the system voltage is relatively low.
Common applications include:
- Long branch circuits serving remote equipment
- Feeders to detached buildings, panels, and distribution equipment
- Low-voltage DC equipment and battery-connected loads
- Motor circuits where terminal voltage affects starting and operating performance
- Controls, lighting, and electronic loads that may be sensitive to reduced supply voltage
- Raceway routes where a longer path changes the conductor-length assumption
Conduit routing affects the length used in the resistance calculation. A direct distance on a plan is not necessarily the installed conductor length. Account for the actual route through raceways, pull points, elevation changes, equipment connections, and other layout conditions that add conductor length.
Raceway fill and bending layout affect whether conductors can be installed as planned, but they do not change the calculator’s resistance formula. Verify the physical raceway design separately from the voltage-drop calculation.
Use the Calculators
Use the following tools to calculate voltage drop, determine a conductor resistance value, or compare conductor-size options:
A practical sequence is to calculate or obtain the conductor resistance, enter the load current and system voltage, then compare the voltage-drop percentage against the entered limit. If the result is too high, reduce total circuit resistance by revising conductor size, shortening the route where feasible, or changing the circuit arrangement.
Field Verification
The voltage-drop result is a resistance-and-current calculation. It does not select a conductor or replace the required review of ampacity, terminations, insulation temperature rating, conductor material, installation method, raceway conditions, overcurrent protection, equipment requirements, or adopted-code requirements.
Confirm the final conductor selection against the applicable electrical requirements and the AHJ’s adopted code. Use actual field lengths and the intended load condition when reviewing the completed circuit.