A conductor must be large enough to carry its load without overheating, protected by a breaker or fuse that matches it, and, on longer runs, large enough that the voltage at the far end is still high enough for the equipment to work properly. The first two requirements set the minimum size; voltage drop often pushes it up. Getting this right is basic to safe design and one of the most common reasons wiring has to be redone.
Owners rarely see conductor sizes, but they feel the results. Undersized wiring shows up as warm cables, tripping breakers, dim or flickering lights, motors that run hot and EV chargers that deliver less than expected. Oversized wiring wastes money in copper or aluminum and conduit. The design job is to find the right size for each run.
Ampacity: how much current a conductor can carry
Every conductor has an ampacity, the current it can carry continuously without its insulation getting too hot. The Canadian Electrical Code publishes ampacity tables, and the value depends on several things:
- Size. Larger conductors have less resistance and carry more current.
- Material. Copper carries more current than aluminum of the same size. Aluminum is lighter and cheaper, and is common for large feeders, but needs connectors and installation practices rated for it.
- Insulation temperature rating. Insulation rated for higher temperatures allows more current, but the rating of the terminals at each end can limit how much of that can be used.
- Installation. A cable in free air sheds heat better than one in a crowded conduit, a cable tray full of other cables, or buried in insulation.
- Ambient temperature. Conductors in hot spaces, such as boiler rooms or rooftops in summer, can carry less current.
When conditions are worse than the table assumes, such as many current-carrying conductors in one raceway or a high ambient temperature, the ampacity is reduced by correction factors. This is called derating, and it is easy to forget when a contractor adds circuits to an existing conduit.
Matching the conductor to its protection
The breaker or fuse at the start of a circuit protects the conductor from overheating. Its rating must not exceed what the conductor can safely carry, with a few specific exceptions in the code, for example for motor circuits. Continuous loads, those expected to run for three hours or more, such as lighting, heating and EV charging, generally cannot use the full rating of a standard breaker; the code limits them to a percentage of the rating unless the equipment is marked for full continuous duty. Check the current edition for the details that apply to your circuit.
Conductors must also survive a fault until the breaker clears it, and the bonding conductor that runs with each circuit must be large enough to carry fault current back to the source so the breaker trips quickly.
Voltage drop
Every conductor has resistance, so current flowing through it loses some voltage along the way. The longer the run and the higher the current, the bigger the drop. Smaller conductors have more resistance, so they drop more voltage over the same distance. Voltage drop adds up from the source through the feeder and the branch circuit to the load.
The Canadian Electrical Code limits voltage drop. In recent editions the limits are broadly about 3 percent in a feeder or branch circuit and about 5 percent in total from the supply side of the service to the point of use, but check the current edition and your province's amendments before relying on these figures.
Why it matters
- Motors draw more current at low voltage, run hotter and wear out sooner.
- Heaters and EV chargers deliver less power than their rating.
- Electronic equipment and LED drivers may flicker, reset or fail early.
- Energy is lost as heat in the conductors, which shows up on the power bill.
Ways to reduce it
- Use a larger conductor for long runs, even when ampacity alone would allow a smaller one.
- Move the panel or transformer closer to the loads, keeping long feeders at a higher voltage.
- Split a heavily loaded circuit into two.
- Use a higher distribution voltage, such as 600 V for long feeders, and step down near the loads.
A worked example
Take a typical case: a building owner wants to add EV chargers in a far corner of a surface parking lot, a long way from the electrical room. The ampacity check, based on the chargers' continuous load, suggests one conductor size. The voltage drop calculation over the full length of the run shows that size would lose too much voltage, so the designer goes up one or two sizes. Because voltage drop is proportional to length, a run twice as long would need roughly twice the conductor area for the same drop.
The designer then compares that with an alternative: running a smaller feeder at a higher voltage to a small transformer and panel near the parking, and short branch circuits from there to each charger. The comparison takes in conductor and conduit cost, trenching, the transformer and panel, and space for future chargers. On long runs with many loads at the far end, the second option often wins.
Common mistakes
| Mistake | What happens | How to avoid it |
|---|---|---|
| Ignoring derating when circuits are added to a full conduit | Conductors run hotter than their rating | Recalculate ampacity when the fill changes |
| Sizing long runs on ampacity alone | Low voltage at the load | Check voltage drop on every long feeder and branch |
| Upsizing a breaker to stop nuisance trips | The conductor is no longer protected | Find the cause of the trips; never upsize without checking the wire |
| Mixing copper and aluminum without rated connectors | Loose, overheating connections | Use connectors and terminals rated for the material |
| Undersized neutral with many electronic loads | Neutral overheating from harmonic current | Allow for harmonics in buildings with heavy electronic loads |
Never upsize a breaker to fix tripping. A breaker that trips repeatedly is usually telling you the circuit is overloaded or faulty. Replacing it with a larger one leaves the wire unprotected and is a fire risk. Have a licensed electrician find the cause.
How ehZee can help
We size feeders and branch circuits, check voltage drop on long runs and compare layouts as part of our electrical design work. Related reading: receptacle and circuit planning and power quality and harmonics.
Common questions
Is aluminum wiring safe?
Aluminum feeders installed with rated connectors and correct torque are common and reliable in modern buildings. Problems with older aluminum branch wiring in houses came mainly from connections and devices not suited to it. See our guide to knob-and-tube and aluminum wiring.
Who calculates conductor sizes?
On engineered projects, the electrical engineer sizes feeders and major circuits and shows them on the drawings. On smaller jobs, the licensed electrical contractor sizes them under permit. In both cases the installation is inspected under the electrical permit.
Does voltage drop matter on short circuits?
Less so. On short branch circuits in a typical office, ampacity usually governs. Voltage drop becomes the deciding factor on long feeders, long branch circuits to outbuildings or parking, and circuits with heavy continuous loads.