Branch Circuits & Wiring Methods
Conductor sizing, ampacity derating, conduit types and bending rules, NMD90 cable, box fill calculations, and voltage drop.
- State the minimum conductor sizes for 15A and 20A branch circuits
- Explain conductor ampacity derating for conduit fill and ambient temperature
- Identify common Canadian wiring methods including NMD90, EMT, and TECK cable
- Apply conduit bending rules and box fill calculations per the CEC
- Calculate voltage drop and determine minimum burial depths for underground cables
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Conductor Sizing, Ampacity & Derating
Minimum Conductor Sizes
The CEC establishes minimum conductor sizes for branch circuits based on the overcurrent protection rating. The 2024 CE Code (Section 4) formally recognizes both AWG/kcmil and mm2 sizing - both systems are equally valid and you may encounter either on drawings and specifications.
According to the CEC, the minimum size conductor permitted for branch circuits is 14 AWG. A 14 AWG copper conductor is rated for a maximum of 15 amps of overcurrent protection. The overcurrent protection for a 15 amp branch circuit using 14 AWG copper conductors should not exceed 15 amps.
For a 20 amp branch circuit, the minimum conductor size is 12 AWG copper.
AWG to mm2 Conductor Size Equivalents
Section 4 of the 2024 CE Code formally recognizes mm2 sizing alongside AWG/kcmil. The mm2 cross-sectional area values are the metric equivalents used in specifications, drawings, and imported equipment documentation:
| AWG / kcmil | mm2 (approx.) | Common Use |
|---|---|---|
| 14 AWG | 2.5 mm2 | 15A branch circuits (residential) |
| 12 AWG | 4 mm2 | 20A branch circuits |
| 10 AWG | 6 mm2 | 30A branch circuits |
| 8 AWG | 10 mm2 | 40-50A circuits |
| 6 AWG | 16 mm2 | 60A circuits, EGC for 200A service |
| 4 AWG | 25 mm2 | 100A feeders |
| 2 AWG | 35 mm2 | 125A feeders |
| 1/0 AWG | 50 mm2 | 150A feeders |
| 3/0 AWG | 95 mm2 | 200A service entrance |
AWG vs mm2 on the Exam
The 2024 CE Code recognizes both sizing systems equally. A specification calling for 2.5 mm2 is the same as 14 AWG, and 4 mm2 equals 12 AWG. Memorize the key pairs: 14 AWG = 2.5 mm2, 12 AWG = 4 mm2, 10 AWG = 6 mm2. These three cover the vast majority of residential branch circuit work.
Ampacity
The ampacity of a conductor is defined as the maximum current it can carry continuously under the conditions of use without exceeding its temperature rating. Ampacity depends on:
- Conductor material (copper vs. aluminum)
- Conductor size (AWG or kcmil)
- Insulation temperature rating (60C, 75C, or 90C)
- Installation conditions (ambient temperature, conduit fill)
CEC Table 2 provides the ampacity values for conductors at a standard ambient temperature of 30C.
Derating
Derating conductor ampacity is required to account for heat buildup in conduit or high ambient temperatures. When multiple current-carrying conductors are installed in the same conduit or cable, they generate heat that cannot dissipate as easily. The CEC requires ampacity correction in two situations:
Conduit Fill Derating
4-6 conductors: 80% of table ampacity
7-24 conductors: 70% of table ampacity
25-42 conductors: 60% of table ampacity
Applies when more than 3 current-carrying conductors share a raceway
Ambient Temperature Correction
31-35C: Multiply by 0.94
36-40C: Multiply by 0.87
41-45C: Multiply by 0.79
Applies when ambient temperature exceeds 30C
The minimum branch circuit conductor is 14 AWG (2.5 mm2) (rated for 15A max). A 20A circuit requires 12 AWG (4 mm2) minimum. The 2024 CE Code formally recognizes mm2 sizing alongside AWG/kcmil. Conductor ampacity is the maximum continuous current a conductor can carry. Derating accounts for heat buildup from conduit fill or high ambient temperatures.