Every cable has resistance, so some voltage is lost along its length. On short runs it doesn't matter. On long runs to a garage, workshop, pump, backyard office or EV charger, it can mean dim lights, motors that struggle to start, chargers that throttle back, and an installation that fails inspection. Voltage drop is often what decides the cable size on these jobs.
Why it happens
Current flowing through a conductor's resistance loses voltage: V = I × R. The resistance grows with length and shrinks with conductor size. So a long run carrying a big current in a small cable loses the most. Copper has lower resistance than aluminum, so an aluminum conductor needs a bigger size for the same drop.
The formula
For single-phase: VD = 2 × K × I × L ÷ CM, where K is about 12.9 for copper and 21.2 for aluminum, I is the current in amps, L is the one-way length in feet, and CM is the conductor's area in circular mils. For three-phase, use √3 (1.732) instead of 2.
Divide the drop by the supply voltage and multiply by 100 to get the percentage.
Typical limits
The Canadian Electrical Code (Rule 8-102) limits voltage drop to 3% in a feeder or branch circuit and 5% in total, from the supply side of the service to the point of use. These are requirements, not just recommendations.
A worked example
A 30 A load on a 240 V single-phase circuit, 100 ft from the panel, in 10 AWG copper (10,380 circular mils): VD = 2 × 12.9 × 30 × 100 ÷ 10,380 = 7.46 V, or 3.1%. That's just over the 3% limit, so 8 AWG would bring it to about 2%.
Where it bites
- Outbuildings and backyard offices, often 20–50 yards from the house.
- EV chargers on long runs to a driveway or parking space.
- Pumps and motors, which draw high current when starting.
- Low-voltage lighting, where a few volts lost is a large percentage.
- Solar and battery systems, where losses cost energy every day.
Voltage drop is only one check
Cable sizing also depends on current-carrying capacity, installation method, grouping with other cables, ambient temperature, the protective device, and fault protection. Voltage drop often decides the size on long runs, but it's never the only check. Underground runs also need the right cable type, depth and protection.
This work must be designed and installed by a qualified electrician to the Canadian Electrical Code as adopted in your province, with a permit and inspection (in Ontario, by a Licensed Electrical Contractor, with notification to the Electrical Safety Authority). Use the calculator to sense-check a quote or to understand why a bigger cable is specified, not to replace the design.
Reducing voltage drop
- Use a larger conductor.
- Shorten the route where possible.
- Use a higher voltage or three-phase for big loads, where available.
- Split loads across separate circuits.
- For distant buildings, run a sub-main to a local subpanel instead of several long final circuits.
The voltage drop calculator gives the drop for any run and the smallest conductor that passes, and the Ohm's law calculator converts between watts, amps, volts and resistance.
Common questions
Is voltage drop the same as a loose connection?
No, although both cause lost voltage. Voltage drop is the normal, predictable loss along a cable. A loose or corroded connection adds extra resistance at one point, which can overheat and is a fire risk. If voltage is far lower than the calculation predicts, check the connections.
Does voltage drop waste energy?
Yes. The lost voltage becomes heat in the cable. On circuits that run for many hours, such as heaters, pumps and EV chargers, a bigger cable pays for itself in lower losses over its life.
How do I measure it on an existing circuit?
An electrician measures the voltage at the supply and at the load with the load running, or works it out from the circuit’s measured resistance. The difference is the drop.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.