Wire Size and Ampacity in Canada: How to Read CEC Tables 1 to 4

"Which wire for how many amps?" is one of the most common Canadian code questions, and the answer is rarely a single number. The Canadian Electrical Code (CEC) gives conductor ampacities in Tables 1 to 4, then limits them by the temperature rating of the terminations, by correction factors, by continuous loading and by the breaker rules. This guide shows how to read the tables and how the checks fit together, with three worked examples.

Rules and tables below follow CSA C22.1:24, the 26th edition of the CEC, Part I, which Ontario adopts with amendments in the 29th-edition Ontario Electrical Safety Code. Check the edition your authority having jurisdiction (AHJ) enforces before you rely on a rule number or a table value.

The Short Answer

For copper conductors in a raceway or cable (Table 2), the common branch-circuit sizes work out as follows. The breaker column applies the small-conductor limits of Rule 14-104 2); the sections below explain why the 60 °C and 75 °C columns matter more than the 90 °C one.

Tables 1 to 4: Which One Applies

Rule 4-004 sets the ampacity of conductors and points to four tables. Pick the table by conductor material and by how the conductors are installed:

Each table has 60, 75 and 90 °C columns (plus higher-temperature columns for special insulations), matching the temperature rating of the conductor insulation. Most building wiring is in raceways or cables, so Table 2 (copper) and Table 4 (aluminum) do most of the work. This guide uses copper throughout; for aluminum conductors, use Table 4.

Reading Table 2

Find the conductor size in the left column and read across to the column for the temperature that applies. The values below are the copper ampacities for not more than three conductors in a raceway or cable.

Values in amperes, copper, not more than 3 conductors in a raceway or cable. For #14, #12 and #10, the overcurrent device is limited by Rule 14-104 2). Correction factors (below) reduce these values where they apply.

Why the 90 °C Column Usually Doesn't Apply (Rule 4-006)

A conductor is only as good as the terminal it lands on. Rule 4-006 ties the ampacity to the temperature rating of the equipment terminations:

• Marked equipment (Rule 4-006 1)): where equipment is marked with a maximum conductor termination temperature, the conductor is sized from the matching column.
• Unmarked, 100 A or less (Rule 4-006 2)): equipment rated 100 A or less, or marked for No. 1 AWG and smaller conductors, is treated as 60 °C.
• Unmarked, above 100 A: equipment rated more than 100 A, or marked for conductors larger than No. 1 AWG, is treated as 75 °C.
• Where it applies: the limit covers the first 1.2 m of conductor from the termination.

The key consequence: a 90 °C conductor, such as T90 or RW90, is usually limited to the 60 °C or 75 °C column at the breaker. The 90 °C rating still matters for the conductor itself beyond the first 1.2 m (for example where the run passes through a hot space), but within 1.2 m of the termination Rule 4-006 1) sizes the conductor from the termination-temperature column, with the Rule 4-004 correction factors applied to that column.

From the load to the wire: the checks in order

A design sequence for copper branch circuits and feeders, not a substitute for the code text.

Electrical points

1. 1

   Load current: start from the equipment rating or the calculated load, in amperes.

2. 2

   Continuous? A load is continuous unless it does not persist for more than 1 h in any 2 h (≤ 225 A) or 3 h in any 6 h (> 225 A). On a device marked for 80 %, the continuous load is ≤ 80 % of the conductor ampacity (70 % for single conductors in free air). Rule 8-104 3), 5), 6)

3. 3

   Table 2 column: use the column of the termination temperature: marked equipment as marked; unmarked 100 A or less at 60 °C; above 100 A at 75 °C, over the first 1.2 m. Rules 4-004, 4-006 1), 2)

4. 4

   Corrections: ambient temperature and more than 3 current-carrying conductors reduce the ampacity (6 conductors: 0.80). Tables 5A, 5C

5. 5

   Breaker: generally not above the conductor ampacity (the next Table 13 rating only under the conditions of Rule 14-104 1) a), up to 800 A); copper #14, #12 and #10 limited to 15 A, 20 A and 30 A. Rule 14-104 1), 2)

6. 6

   Voltage drop: 3 % in a feeder or branch circuit, 5 % in total; on long runs it sets the size. Rule 8-102

• Design sequence
• Column set by the termination temperature
• Step (Electrical points)

Figure 1. From the load to the wire: the checks in order

Correction Factors

Rule 4-004 also requires correction factors when the installation differs from the table's basis. The two that come up most often:

• Ambient temperature (Table 5A): conductors in a hot space carry less current.
• More than 3 current-carrying conductors (Table 5C): grouped conductors heat each other. For example, 6 conductors in a raceway take a factor of 0.80.

Rule 4-006 1) bases the conductor size on the ampacity in the termination-temperature column, "with all relevant correction factors being applied as required by Rule 4-004": the factor applies to the termination-column value. A published IAEI example shows how much this can matter: No. 3 copper RW90 has 115 A in the 90 °C column of Table 2, but with 75 °C terminations it is assigned 100 A. With six conductors in the raceway, the 0.80 factor applies to that 100 A, giving 80 A, so the conductor moves up to No. 1 copper (130 A × 0.80 = 104 A).

Continuous Loads (Rule 8-104)

A load is continuous unless, in normal operation, it does not persist for more than 1 h in any 2 h period (loads of 225 A or less) or 3 h in any 6 h period (loads above 225 A). Continuous loads are limited by the marking of the overcurrent device:

Most standard breakers carry the 80 % rating. Where the device is marked for 80 %, a continuous load is held to 80 % of the conductor ampacity, and the breaker is sized at least at the continuous load divided by 0.8.

Choosing the Breaker (Rule 14-104)

Rule 14-104 sets the rating of the overcurrent device protecting a conductor. In general, the rating does not exceed the conductor's ampacity. Rule 14-104 1) a) allows the next higher Table 13 rating only where a device equal to the ampacity is not available, the calculated load meets Section 8, and the rating is 800 A or less.

Rule 14-104 2) adds a cap for small conductors: copper #14 is limited to 15 A, #12 to 20 A and #10 to 30 A. This applies even though Table 2 gives #14 copper 20 A in the 75 °C column. The same subrule also caps aluminum: No. 12 aluminum at 15 A and No. 10 aluminum at 25 A.

Motors, transformers and HVAC equipment follow their own rules. See the motor protection guide (MCA and MOP) and the transformer overcurrent protection guide for those.

Voltage Drop Comes Next

Ampacity is only the first check. A conductor that is large enough for the current can still be too small for the distance: on long runs, the voltage drop decides the size. The voltage drop guide explains the Rule 8-102 limits of 3 % in a feeder or branch circuit and 5 % in total, with the formulas.

Three Worked Examples

These examples use only the Table 2 values above. Real designs also apply correction factors, voltage drop and the equipment's own instructions.

Example 1: a 20 A, 120 V kitchen circuit

#12 copper. Table 2 gives #12 copper 20 A in the 60 °C column, the column a 20 A breaker (100 A or less, unmarked) uses under Rule 4-006, and Rule 14-104 2) allows #12 copper on a 20 A device.

Example 2: a Level 2 EV charger, 32 A continuous

A 32 A charger at 240 V (7.7 kW) is a continuous load: under Rule 86-302, the load of a branch circuit supplying EVSE is considered continuous for Rule 8-104. On a breaker marked for 80 %, the breaker is at least 32 ÷ 0.8 = 40 A. #8 copper has 40 A in the 60 °C column and 50 A in the 75 °C column, so it is at least 40 A whichever column the terminations allow. This matches the EV-ready wiring guide: a 40 A breaker with #8 copper for a 32 A load.

Example 3: a 200 A service

3/0 copper at 75 °C. Equipment rated above 100 A is treated as 75 °C under Rule 4-006, and Table 2 gives 3/0 copper 200 A in that column. The panel upgrade guide uses the same conductor for a 200 A service.

Common Mistakes

• Sizing from the 90 °C column at the breaker. Rule 4-006 usually limits the conductor to the 60 °C or 75 °C column at the terminal.
• Forgetting the small-conductor caps. #14, #12 and #10 copper are limited to 15 A, 20 A and 30 A devices by Rule 14-104 2).
• Ignoring grouping and ambient. More than three current-carrying conductors (Table 5C) or a hot space (Table 5A) reduce the ampacity.
• Treating a continuous load like a short one. On a device marked for 80 %, a continuous load is limited to 80 % of the conductor ampacity (Rule 8-104).
• Stopping at ampacity. Long runs are sized for voltage drop (Rule 8-102).
• Using copper values for aluminum. Aluminum conductors are sized from Tables 3 and 4.

Frequently Asked Questions

Sources

1. Ontario Electrical Safety Code, 29th edition (2024), CSA C22.1:24 with Ontario amendmentsRules 4-004, 4-006, 8-104, 14-104 and 86-302; Tables 2, 5A, 5C and 13.
2. IAEI Magazine — Application of Rule 4-006 of the Canadian Electrical CodeTermination temperature, the 100 A threshold, the first 1.2 m, and the No. 3 copper example with Table 5C.
3. Guide to the Canadian Electrical Code, 26th Edition — A Road Map: Section 8Continuous loads and Rule 8-104 limits.
4. Guide to the Canadian Electrical Code, 26th Edition — A Road Map: Section 14Protection and control; Rule 14-104 and Table 13.