Voltage Drop Calculator Online Free
Voltage Drop Calculator
Calculate Voltage Drop
Calculates based on the resistance and reactance data from the National Electrical Code (NEC).
Calculation Results
What is Voltage Drop?
When electrical current moves through a wire, it is pushed by electrical potential (voltage) and it needs to surpass a certain level of contrary pressure caused by the wire. The voltage drop is the amount of electrical potential (voltage) loss caused by the contrary pressure of the wire.
If the current is alternating, such contrary pressure is called impedance. Impedance is a vector, or two-dimensional quantity, consisting of resistance and reactance (reaction of a built-up electric field to a change of current). If the current is direct, the contrary pressure is called resistance.
Why Voltage Drop Matters
Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. It is recommended that the voltage drop should be less than 5% under a fully loaded condition. This can be achieved by selecting the right wire, and by taking care in the use of extension cords and similar devices.
Four Major Causes of Voltage Drop
1Wire Material
The choice of material used for the wire. Silver, copper, gold, and aluminum are among the metals with the best electrical conductivity. Copper and aluminum are the most common materials used for wires due to their relatively low price compared with silver and gold. Copper is a better conductor than aluminum and will have less voltage drop than aluminum for a given length and wire size.
2Wire Size
Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. In American wire gauge, every 6-gauge decrease doubles the wire diameter, and every 3-gauge decrease doubles the wire cross sectional area. In the Metric Gauge scale, the gauge is 10 times the diameter in millimeters.
3Wire Length
Shorter wires will have less voltage drop than longer wires for the same wire size. Voltage drop becomes important when the length of a run of wire or cable becomes very long. Usually this is not a problem in circuits within a house, but may become an issue when running wire to an outbuilding, well pump, etc.
4Current Load (Ampacity)
The amount of current being carried can affect voltage drop levels; an increase in current through a wire results in an increased voltage drop. Current carrying capacity is often referred to as ampacity, which is the maximum number of electrons that can be pushed at one time – the word ampacity is short for ampere capacity.
Ampacity and Cable Selection
Factors Affecting Ampacity
The ampacity of a wire depends on several factors:
- •The basic material from which the wire is made (copper vs. aluminum)
- •For AC current, the speed of alternation can affect ampacity
- •The temperature in which the wire is used
- •Cable bundling - when cables are brought together, the total heat they generate affects ampacity
Cable Selection Principles
Cable selection is guided by two main principles:
The cable should be able to carry the current load imposed on it without overheating. It should be able to do this in the most extreme conditions of temperature it will encounter during its working life.
It should offer sufficiently sound earthing to (i) limit the voltage to which people are exposed to a safe level and (ii) allow the fault current to trip the fuse in a short time.
Cable Bundling Rules
There are strict rules about bundling cables which must be followed. When cables are brought together in bundles, they generate collective heat that reduces ampacity and increases voltage drop. Proper spacing and derating factors must be applied according to electrical codes.
Voltage Drop Calculation Formulas
Ohm's Law - Basic Formula
Vdrop = I × R
Where:
- I: the current through the wire, measured in amperes
- R: the resistance of the wires, measured in ohms
Single-Phase / DC Circuit
Vdrop = 2 × I × R × L
The factor of 2 accounts for the round-trip path of the current (both supply and return conductors).
Three-Phase Circuit
Vdrop = √3 × I × R × L
The factor of √3 (≈ 1.732) accounts for the three-phase configuration and balanced loads.
Formula Variables
- I: Load current in amperes (A)
- R: Length-specific resistance in ohms per kilometer (Ω/km) or ohms per 1000 feet (Ω/1000ft)
- L: One-way length of the wire run
Typical AWG Wire Sizes Reference
American Wire Gauge (AWG) is a wire gauge system used predominantly in North America for the diameters of round, solid, non-ferrous, electrically conducting wire. Below are common wire sizes and their specifications:
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/km) |
|---|---|---|---|
| 0000 (4/0) | 11.684 | 107 | 0.1608 |
| 000 (3/0) | 10.404 | 85.0 | 0.2028 |
| 00 (2/0) | 9.266 | 67.4 | 0.2557 |
| 0 (1/0) | 8.252 | 53.5 | 0.3224 |
| 1 | 7.348 | 42.4 | 0.4066 |
| 2 | 6.544 | 33.6 | 0.5127 |
| 3 | 5.827 | 26.7 | 0.6465 |
| 4 | 5.189 | 21.2 | 0.8152 |
| 6 | 4.115 | 13.3 | 1.296 |
| 8 | 3.264 | 8.37 | 2.061 |
| 10 | 2.588 | 5.26 | 3.277 |
| 12 | 2.053 | 3.31 | 5.211 |
| 14 | 1.628 | 2.08 | 8.286 |
| 16 | 1.291 | 1.31 | 13.17 |
| 18 | 1.024 | 0.823 | 20.95 |
| 20 | 0.812 | 0.518 | 33.31 |
| 22 | 0.644 | 0.326 | 52.96 |
| 24 | 0.511 | 0.205 | 84.22 |
Note: In American wire gauge, every 6-gauge decrease doubles the wire diameter, and every 3-gauge decrease doubles the wire cross-sectional area. Smaller gauge numbers indicate thicker wires with lower resistance and higher current carrying capacity.
Wire Installation Best Practices and Safety Considerations
Understanding Electrical Code Requirements
The National Electrical Code (NEC) and local building codes establish minimum safety standards for electrical installations. These codes specify maximum allowable voltage drop percentages for different types of circuits. Branch circuits should not exceed 3% voltage drop, while feeder and branch circuit combined should stay below 5% total voltage drop. These requirements ensure that electrical equipment operates efficiently and safely without overheating or performance degradation.
When planning electrical installations, always consult the latest version of the NEC and your local electrical code. Requirements can vary based on jurisdiction, and professional electricians must stay updated on code changes. Proper voltage drop calculations are not just recommendations—they're often legal requirements for passing electrical inspections and ensuring insurance coverage in case of electrical failures.
Environmental Factors Affecting Wire Performance
Temperature has a significant impact on wire resistance and ampacity. As temperature increases, wire resistance increases, leading to greater voltage drop and reduced current-carrying capacity. Wires installed in hot attics, near heating equipment, or in direct sunlight must be derated accordingly.
Ambient temperature ratings typically assume 30°C (86°F) conditions. For every 10°C increase above this baseline, wire ampacity can drop by 10-15%. Similarly, moisture, corrosive environments, and physical stress can degrade wire insulation and performance over time, necessitating more conservative wire sizing and regular inspection schedules.
Long-Distance Wire Runs and Voltage Compensation
For extremely long wire runs—such as those to detached garages, workshops, or agricultural buildings—voltage drop becomes the primary design consideration. In these cases, simply meeting ampacity requirements isn't enough; you must upsize the wire significantly to maintain acceptable voltage levels.
Professional electricians often use a rule of thumb: for runs over 100 feet, increase the wire size by at least one gauge. For runs over 200 feet, consider two or more gauge increases. Additionally, using higher supply voltages (240V instead of 120V) can dramatically reduce voltage drop, as the same power delivery requires only half the current at double the voltage.
Economic Considerations in Wire Selection
While larger wire sizes cost more upfront, they can provide significant long-term savings through reduced energy waste. The power loss in a conductor equals I²R (current squared times resistance), meaning that even small reductions in resistance can yield substantial energy savings in high-current applications.
For commercial and industrial installations running 24/7, the cost of electricity lost to wire resistance over the system's lifetime can exceed the initial wire cost many times over. Performing a lifecycle cost analysis that includes energy costs often justifies investing in larger conductors than minimum code requirements would dictate.
Common Voltage Drop Mistakes to Avoid
One frequent error is forgetting to account for the round-trip distance of current flow. Voltage drop occurs in both the supply and return conductors, so calculations must use twice the one-way distance. Another common mistake is using the nameplate current instead of the actual operating current, which can lead to undersized wires.
Additionally, many DIY installers overlook the cumulative effect of multiple voltage drops in a system. If a subpanel already experiences 2% voltage drop from the main panel, circuits fed from that subpanel have only 3% remaining budget to stay within the 5% total limit. Proper planning requires analyzing the entire electrical distribution system, not just individual circuit runs.