BuildMeter guide · Energy & Electrical
Voltage Drop Planning Guide
Understand the inputs behind a voltage-drop estimate—current, conductor material, size, length, phase and system voltage—without replacing electrical design or code review.
Direct answer
What to know first
Voltage drop increases with current, circuit length and conductor resistance. A calculator can compare planning scenarios, but conductor ampacity, temperature, terminals, installation method, fault protection and applicable electrical rules must also be checked by a qualified person.
Open Voltage Drop CalculatorWhat voltage drop represents
Current flowing through conductor resistance produces a reduction in voltage between the source and the load. The estimate depends on conductor resistance, current and the complete current path. Longer runs, higher current and smaller conductors generally increase the calculated drop.
| Input | Why it matters | Common error |
|---|---|---|
| System voltage | Converts lost volts into a percentage of supply voltage. | Using line-to-line voltage for a line-to-neutral load or the reverse. |
| Load current | Voltage loss is proportional to current in a simple resistance model. | Using breaker rating when the design current is different. |
| One-way route length | Calculators account for return path or phase configuration from this value. | Entering straight-line distance instead of actual conductor routing. |
| Conductor material | Copper and aluminum have different resistance. | Leaving the default material unchanged. |
| Conductor size | Larger cross-section generally lowers resistance. | Confusing AWG, kcmil and metric area. |
| Single-phase, three-phase or DC | The circuit formula and path differ. | Applying one formula to every system. |
| Temperature and installation | Resistance and allowable ampacity can change. | Treating a room-temperature estimate as a complete design. |
Basic planning formulas
For a simple two-conductor DC or single-phase resistance model, voltage drop can be expressed as current × total circuit resistance. Percentage drop = lost volts ÷ source voltage × 100. Three-phase calculations commonly use a √3 relationship with one-way length and conductor impedance. Real AC systems can also involve reactance and power factor.
Worked comparison
Assume a 120 V load and a calculated circuit drop of 3.6 V. The percentage is 3.6 ÷ 120 × 100 = 3%. The same 3.6 V loss on a 240 V system is 1.5%. This illustrates why both volts lost and percentage drop are useful.
| Lost voltage | 120 V system | 208 V system | 240 V system |
|---|---|---|---|
| 2 V | 1.67% | 0.96% | 0.83% |
| 3.6 V | 3.00% | 1.73% | 1.50% |
| 5 V | 4.17% | 2.40% | 2.08% |
Voltage drop is not conductor approval
A conductor that produces an acceptable planning drop may still be unsuitable because of ampacity, temperature rating, bundling, terminal limitations, fault current, overcurrent protection, insulation type or installation method. Conversely, a code-compliant conductor may need to be increased for equipment performance on a long run.
Loads that deserve extra attention
- Motors, compressors and pumps with starting current.
- Long outdoor feeders and detached buildings.
- Low-voltage lighting where a small lost voltage is a large percentage.
- EV charging and other sustained loads.
- Inverters, generators and equipment with manufacturer voltage limits.
- Circuits with uncertain route length or multiple connection points.
Common mistakes
- Entering total loop length into a calculator that already doubles one-way distance.
- Using rated power without converting it to the correct current and phase.
- Ignoring aluminum versus copper.
- Choosing conductor size solely from voltage drop.
- Assuming a planning percentage is a universal legal limit in every jurisdiction.
Use the calculators
Compare current, length, material and conductor size in the Voltage Drop Calculator. Use the Electrical Load Calculator for preliminary load totals and the Generator Size Calculator for generator capacity scenarios.
Sources and electrical safety
- Southwire voltage drop calculator
- Southwire voltage drop fundamentals
- NIST SI units — electric current, volt and ohm
- OSHA construction electrical flexible-cord safety
Electrical work can cause shock, fire or equipment damage. Calculators do not replace a licensed electrician, equipment instructions, inspection or the electrical rules applicable to the project.