⚡ Voltage Drop in Canada: Rule 8-102, Explained
The formula, worked examples — and the big Canadian difference: here, the limit is the law. 🇺🇸 US version
The Canadian difference, up front
In the US, the famous 3%/5% voltage-drop figures are an official recommendation. In Canada they are CEC Rule 8-102 — an enforceable "shall": maximum 3% on a branch circuit, 3% on the feeder (including the consumer's service), and 5% combined from the service to the farthest point. An inspector can fail an over-limit run. If you remember one cross-border fact about voltage drop, make it this one.
The formula
For DC and single-phase AC: voltage drop = 2 × K × amps × one-way feet ÷ circular mils (three-phase uses √3 instead of 2). K is the material constant — 12.9 for copper, 21.2 for aluminum. Circular mils is the wire's cross-section (#12 = 6,530; #6 = 26,240). Measure distance one way; the 2 handles the round trip. The CEC's own reference method (Appendix D, Table D3) is published in metric, but it's the same physics — same answers, different units.
What amps do you plug in? Canada has an official answer
Use the connected load if you know it; otherwise 80% of the breaker rating (2024 CEC). This settles the argument that splits every calculator forum. Computing at 100% of the breaker — as our conservative tables do — is always acceptable and leaves margin.
Three worked examples
1. Garage receptacle: #12 copper, 20 A, 24 m (80 ft), 120 V → 6.3 V = 5.3%. Over the mandatory limit — upsize to #10 (3.3%) or shorten the run.
2. Hot tub feeder: #6 copper, 40 A load, 46 m (150 ft), 240 V → 5.9 V = 2.5%. Passes 8-102 with room.
3. 12-volt trolling motor (DC): #14 copper, 8 A, 4.6 m (15 ft), 12 V → 0.75 V = 6.3%. Same formula, brutal percentage — low-voltage wiring on boats, trailers, and solar runs far fatter than its amps suggest. Our calculator's DC mode exists for exactly this.
The dwelling shortcut: Table 68
For ordinary 120 V lighting and receptacle circuits up to 20 A in homes, the CEC provides precomputed maximum lengths (Rule 8-102(3), Table 68) — and where the service-to-panel run is under 20 m, the branch circuit may consume the full 5% by itself. Dedicated circuits (kitchen, fridge, heating) are excluded from the shortcut and use the standard calculation.
How to fix excessive voltage drop
In order of usual practicality: go up a wire size (each AWG step cuts drop ~21%), use 240 V instead of 120 V where the load allows (half the amps, double the tolerable volts — a 4× improvement), shorten the route, or for very long rural runs, step up with a transformer. Canadian commercial sites have an extra lever Americans lack: the 347/600 V system, where the same watts produce far less percentage drop.
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Voltage Drop calculator · Max Wire Length · Canadian sub-panel guide · How far can you run each gauge in Canada?