How do I read a wire ampacity chart?

Reading a wire ampacity chart is straightforward once you understand the three key variables: wire gauge, insulation type, and installation conditions. Ampacity simply means the maximum current (in amps) a wire can safely carry without overheating — and getting this right is critical for fire safety.

Understanding Wire Gauge (AWG)

The American Wire Gauge (AWG) system works backwards from what you'd expect — the smaller the number, the thicker the wire and the more current it can carry. The most common gauges you'll encounter in residential work are:

• 14 AWG — rated for 15A circuits (lighting, general outlets)
• 12 AWG — rated for 20A circuits (kitchen counters, bathrooms)
• 10 AWG — rated for 30A circuits (dryers, water heaters)
• 8 AWG — rated for 40A circuits (ranges, EV chargers)
• 6 AWG — rated for 55A circuits (large appliances, subpanels)

These are the baseline numbers you'll see in the Canadian Electrical Code (CEC) Table 2, which is the governing reference for New Brunswick electrical work.

The Two Big Modifiers: Insulation and Conduit Fill

Raw ampacity numbers get adjusted based on real-world conditions. The two biggest factors are temperature rating and how many conductors share a conduit or cable bundle.

Most modern residential wire uses 90°C-rated insulation (RW90 or THHN), but it's typically derated to 60°C or 75°C for termination points at breakers and devices. This is why your 12 AWG wire might be rated 30A at the conductor but is limited to 20A at the breaker — the weakest point in the connection determines the rating.

Conduit fill (bundling) is the other major factor. The CEC requires derating when more than three current-carrying conductors share a conduit. For example, with 4–6 conductors, you apply a 0.80 derating factor. With 7–9 conductors, it drops to 0.70. This matters a lot for panel wiring and multi-circuit conduit runs.

How to Actually Use the Chart

Start with your circuit's breaker size, then confirm the wire gauge matches. Cross-reference the insulation type column, then check whether any derating factors apply (bundling, high ambient temperature, long runs). For most NB homes, you're working with standard residential conditions, so the base table values apply without much adjustment.

One NB-specific note: For EV charger installations, the CEC requires the circuit to be rated at 125% of the continuous load — so a 32A charger needs a 40A circuit with 8 AWG wire minimum. This is a common area where undersized wiring causes problems.

If you're trying to size wire for a specific project, confirm your calculations with a licensed electrician before purchasing materials. TSANB requirements apply to all new circuits and wiring work in New Brunswick — most projects require a permit and inspection. When in doubt, contact TSANB directly at 1-800-999-0813 or visit tsanb.ca.

Need help finding a licensed electrician to assess your wiring project? New Brunswick Electrical can match you with a local pro for free.