📈 Voltage Drop

Voltage Drop Calculator

Calculate voltage drop in HVAC circuits from wire gauge, run length, and load current. Checked against the CEC 3% branch circuit guideline.

⚡ Circuit Configuration

🖌 Conductor and Load

Wire Gauge

Load Current

Circuit Voltage

One-Way Run Length

📈 Voltage Drop Results

📈 Voltage Drop Percentage

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Enter values and calculate

Calculation Breakdown

Full Calculation Table

Value	Basis	Result

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How to Use the Voltage Drop Calculator

Select Phase Configuration

Single-phase and three-phase circuits use different voltage drop multipliers, so this choice affects the result directly.

Enter Wire Gauge, Load Current, Voltage, and Run Length

Use the actual installed or planned wire gauge, the load's current draw, and the one-way distance from panel to equipment.

Calculate

The calculator shows the voltage drop in both volts and percentage, and flags whether it stays within the CEC's 3% branch circuit guideline.

Why Voltage Drop Matters for HVAC Circuits

Every length of wire has some electrical resistance, and that resistance causes a small loss of voltage between the panel and the equipment it feeds. This loss, called voltage drop, grows with both the current flowing through the wire and the distance it travels. A circuit that looks perfectly sized by ampacity alone can still deliver noticeably less voltage to the equipment than the panel is supplying if the run is long enough.

The CEC Voltage Drop Guideline

CEC Rule 8-102 recommends keeping voltage drop to 3% on a branch circuit and 5% total when a feeder is included in the same circuit path. These aren't hard code violations in the same way an undersized breaker would be, but they represent good design practice that protects equipment performance and longevity. Going significantly over these guidelines on an HVAC circuit is a common, avoidable mistake, especially on long runs to rooftop or remote equipment.

What Happens When Voltage Drop Is Too High

A motor or compressor running on a circuit with excessive voltage drop has to draw more current to produce the same output, since power output depends on both voltage and current. That extra current generates extra heat in the motor windings, which accelerates insulation breakdown and shortens the equipment's service life. In severe cases, undervoltage can also cause a compressor to struggle on startup, trip thermal overload protection more frequently, or fail prematurely.

Single-Phase vs. Three-Phase Calculations

This calculator applies a multiplier of 2 for single-phase circuits, accounting for both the outgoing and return conductor carrying the load current. Three-phase circuits use a multiplier of the square root of 3, about 1.732, instead, since the current splits differently across three conductors rather than two. This is the same underlying principle covered in the three phase calculator, and it's why an otherwise identical three-phase circuit typically shows less voltage drop than its single-phase counterpart.

When Voltage Drop Forces a Bigger Wire

If this calculator shows a voltage drop above your target limit, increasing the wire size is the most direct fix, since a larger conductor has lower resistance per unit length. The wire size calculator handles this forward direction directly: enter your circuit's MCA, run length, and voltage drop limit, and it works out the minimum wire size that satisfies both ampacity and voltage drop together.

Next Steps

If voltage drop comes back too high, check the wire size calculator for the next size up, and the conduit fill calculator to confirm that larger conductor still fits the existing conduit run.

Frequently Asked Questions

Excessive voltage drop forces a motor to draw more current to deliver the same output torque, which generates extra heat in the windings. Over time, this added heat stress shortens motor and compressor life, can trip overload protection more often, and in severe cases contributes to premature compressor failure. Keeping voltage drop within the CEC guideline protects the equipment as much as it protects energy efficiency.

Yes. Single-phase voltage drop calculations use a multiplier of 2 to account for both the outgoing and return conductor carrying the full current. Three-phase calculations use a multiplier of the square root of 3, about 1.732, instead, since the current splits across three conductors rather than two. This means a three-phase circuit will generally show a lower voltage drop than an equivalent single-phase circuit carrying the same current over the same distance.

The 3% branch circuit and 5% total figures in CEC Rule 8-102 are stated as recommendations rather than mandatory limits in most jurisdictions, but they reflect well-established good design practice. Some equipment manufacturers set their own stricter voltage drop requirements in their installation instructions, which CEC Rule 2-100 then makes mandatory to follow, so always check the specific equipment documentation too.

Use the actual cable path length, including any vertical risers, conduit bends, and routing around obstacles, not a straight-line distance between the panel and the equipment. Real-world wire runs are almost always longer than a direct line on a floor plan, and underestimating run length will underestimate voltage drop too.