Voltage Drop Calculator

Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. It occurs due to the resistance (or impedance in AC circuits) of the conductors carrying the current. When current flows through a wire, the wire's resistance causes some of the electrical energy to be converted into heat, resulting in a voltage drop along the length of the wire.

For direct current (DC) circuits, voltage drop is calculated using resistance. The resistance of a wire depends on its material, cross-sectional area, and length. For alternating current (AC) circuits, voltage drop is calculated using impedance, which includes both resistance and reactance. Reactance accounts for the effects of inductance and capacitance in AC circuits.

The relationship between voltage drop, current, and resistance (or impedance) is governed by Ohm's Law:

Excessive voltage drop can cause several problems in electrical systems:

• Flickering Lights: Incandescent and some LED lights may flicker or appear dim when voltage drop is significant.
• Poor Heater Performance: Electric heaters will produce less heat when operating at reduced voltage, as power is proportional to voltage squared (P = V²/R).
• Motor Burnout: Motors operating at reduced voltage may draw excessive current to compensate, leading to overheating and potential failure.
• Reduced Equipment Lifespan: Electrical equipment operating outside its designed voltage range may experience premature failure.
• Energy Inefficiency: Voltage drop results in power loss (I²R losses), wasting energy and increasing operating costs.

For most applications, voltage drop should be kept to less than 5% of the source voltage. For critical circuits or sensitive equipment, even lower voltage drop (2-3%) may be required. The National Electrical Code (NEC) recommends that voltage drop not exceed 3% for branch circuits and 5% for the total circuit (feeder + branch).

The fundamental formula for voltage drop is based on Ohm's Law:

Resistance is often given as length-specific resistance (ohms per kilometer or per 1000 feet). Since wire is "round-tripped" (current flows from source to load and back), the total length is twice the one-way distance.

For single-phase or direct current circuits:

For three-phase circuits:

For AC circuits with power factor, the calculation becomes more complex as it must account for both resistance and reactance. The voltage drop formula for AC circuits with power factor is:

Ampacity (the maximum current a wire can safely carry) is affected by several factors:

• Wire Material: Copper has higher ampacity than aluminum for the same wire size.
• AC Alternation Speed: At higher frequencies, skin effect and proximity effect reduce effective ampacity.
• Temperature: Higher ambient temperatures reduce ampacity. Wire resistance increases with temperature, and insulation may degrade.
• Bundling: When multiple cables are bundled together, heat dissipation is reduced, requiring derating of ampacity.
• Installation Method: Free air, conduit, underground, or direct burial all affect heat dissipation and ampacity.

Bundling Cables: When cables are bundled, they generate heat collectively, and the heat cannot dissipate as easily. This requires derating the ampacity of each cable. The NEC provides specific derating factors based on the number of current-carrying conductors in a raceway or cable.

Cable Selection Principles: When selecting wire size, two main principles apply:

1. Carrying Current Without Overheating: The wire must be large enough to carry the expected current without exceeding its ampacity, preventing overheating and potential fire hazards.
2. Providing Sound Earthing: For safety, the wire must provide adequate grounding. In many jurisdictions, the grounding conductor must be sized according to the circuit breaker rating, not just the load current.