Motor Conductor Sizing Guide for Red Seal Exam (2024 CEC Updated!)

Alright, so you’ve got a motor and you want to know what size wires (conductors) to use. Don’t panic — you just need to follow the right steps and look at the right tables.

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Step 1: Pick Your Temperature Column

The conductor sizing tables (Tables 1-4) have columns for different temperatures

60°C, 75°C, 90°C, 110°C, 125°C, 200°C

Yeah, it’s a lot of temperatures to pick from — but don’t just pick your favorite number. It depends on where the wires are going in the motor setup.

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Step 2: Know What You’re Sizing For

There are two parts we usually care about:

• Branch Circuit Conductor → Use Rule 4-006
• Motor Supply Conductor → Use Rule 28-104 (this rule says: go with the 75°C column)

⚠️ except if it’s a Class A Motor. If it is, and you’re using a 90°C conductor—you use the 90°C column.

So yes, it’s totally possible you’ll end up with two different wire sizes for the same motor depending on which part you’re wiring.

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Step 3: What Kind of Duty Are We Talkin’?

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Option 1: Continuous Duty

• Use Rule 28-106(1)
• Take your FLA (Full Load Amps)
• Multiply it by 125%
• Go to the temperature column in Tables 1-4, and if your number isn’t there? Go UP to the next size.

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Option 2: Non-Continuous Duty

• Use Rule 28-106(2)
• Go peek at Table 27
• Pick your duty: Short-time, Intermittent, Periodic, Varying... all with options like 5 min, 15 min, etc.
• Each one has a multiplier.

⚠️ Heads-up: “Continuous Duty” and “Continuous Rating” are NOT the same thing. Don’t mix ‘em up!

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Example Time: Continuous Duty

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Question: What size Teck 90 wire do I need for a 240V, 3-phase, 30 HP Squirrel Cage Motor (SCIM)?

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Let’s break it down:

• Copper? (default if not mentioned)
• Conduit? (assumed if not specified)
• Table to use? Table 2
• Type of duty? Continuous — use the 125% multiplier

From Table 44 → FLA = 80A
80 × 125% = 100A

Branch Circuit Wire:

• No equipment temp given = follow Rule 4-006
• 100A or less? Use the 60°C column
• You’ll need #1 AWG

Motor Supply Wire:

• Rule 28-104 = 75°C column
• You’ll need #3 AWG

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Example Time: Non-Continuous Duty

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Question: Same motor (240V, 3ø, 30HP SCIM), but now it’s got a 5-minute intermittent duty rating. What now?

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• Table 44 FLA = 80A
• Table 27 multiplier = 85%
• 80 × 0.85 = 68A

→ Rule 28-104 → Table 2 75°C column → #4 AWG

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And Now For Something Completely Different

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As with all things code, there is a special motor out there where the rules go wonky.

That would be the Wound Rotor Motor (WRM)!

The Wound Rotor Motor is special because it’s essentially a Squirrel Cage Motor, with the added bonus of having another component added to it.

A Secondary current is used to control a resistor bank!

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You do not calculate a secondary current, it is given to you.

The secondary current is used for:

• Controller Supply 28-112 (1)(a) & (b)
• Resistor Bank Supply 28-112 (2) and Table 28

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Controller Supply

Basically, you use the secondary current and if it is continuous duty, use the 125% multiplier. If it is non-continuous duty — Table 27. And you follow all the same rules as the “regular” part of the motor. Rule 4-006 applies!

⚠️ But! And hold on, this is a little confusing…

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Resistor Bank Supply

This is based off of the Secondary current, and whatever it says on Table 28.

Table 28 has options like: Heavy Starting Duty, Light Duty, Continuous Duty, etc.

Except THIS Continuous Duty has a multiplier of 110%.

Rule 4-006 applies here too.

Also, because this is a RESISTOR BANK, it is considered a “Resistive Device”. Which is covered in Section 26. Rule 26-552 (a).

⚠️ Even if you use the 60°C, 75°C or 90°C columns to calculate your wire size, you HAVE TO USE A 90°C rated cable. Because resistive stuff is hot.

Yes, that is confusing. We are sorry.

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Let’s do some examples:

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Question

We have a 208V 3ø 20HP Wound Rotor Motor with a secondary current of 90A. What is the size of Teck90 Aluminum conductor required for the Controller Supply and Resistor Supply?

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This does not mention what type of duty it is, so we assume Continuous.

It’s asking for Controller Supply, which makes this a Secondary Circuit, so we use the Secondary Current.

Controller Supply:

• 28-112 (a) says a multiplier of 125%
• 90A × 125% = 112.5A
• It's aluminum cable, and since they didn’t tell us anything about termination temps, Rule 4-006 kicks in.
• That means we use the 75°C column in Table 4
• Result: You need a #1/0 AWG aluminum conductor for the Controller Supply. Boom!

Resistor Supply:

• Still a Secondary Circuit → 90A
• Follow Rule 28-112(b) and use Table 28, which says: Multiply by 110%
• 90A × 1.10 = 99A
• Still aluminum. Still no termination rating → Rule 4-006
• Use the 60°C column in Table 4
• Result: You need a #1/0 AWG aluminum conductor for the Resistor Supply.

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Let’s try another one, but more tricky!

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Question: We have a 5-minute Intermittent Duty 208V 3ø 20HP Wound Rotor Motor with a secondary current of 90A. What is the size of Teck90 Aluminum conductor required for the Controller Supply and Resistor Supply with a Heavy Starting Duty?

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It’s asking for Controller Supply, which makes this a Secondary Circuit, so we use the Secondary Current.

Controller Supply:

• 28-112 (b) says to head to Table 27
• 90A × 85% = 76.2A
• Aluminum cable → Rule 4-006
• Use the 60°C column in Table 4
• Result: You need a #1 AWG aluminum conductor for the Controller Supply. Boom!

Resistor Supply:

• Still a Secondary Circuit → 90A
• Follow Rule 28-112(b) and use Table 28, which says: Multiply by 45%
• 90A × 0.45 = 40.5A
• Aluminum → Rule 4-006
• Use the 60°C column in Table 4
• Result: You need a #4 AWG aluminum conductor for the Resistor Supply.

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Keeping it Classy

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So we see in some questions that a CLASS of Motor has been mentioned. What exactly does this mean?

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Class A

• a squirrel cage induction motor that:
  • Has normal starting torque
  • Has low starting current
  • Runs at constant speed
  • Is used when no big jolts or heavy loads happen at startup

Class B

• the most common squirrel cage induction motor. It:
  • Has good starting torque (better than Class A)
  • Has low starting current
  • Runs at pretty constant speed
  • Is great for general-purpose use — fans, pumps, conveyors, etc.

Class F

• has special insulation that lets it handle higher heat — up to 155°C total (including room temp + motor heat)
  • It won’t overheat easily
  • It lasts longer in hot or heavy-duty jobs
  • It's tougher than motors with Class A or B insulation

Class H

• has even stronger insulation than Class F — it can handle up to 180°C total temperature (ambient + heat from the motor)
  • It’s built for super hot, tough environments
  • Can run safely in high-temp areas, like near furnaces or in industrial plants
  • It’s one of the most heat-resistant motor types

💡 Table 37 Gives you the options of: Class A, B, F and H motors, with 2 categories of Totally Enclosed Non-Ventilated (TENV) and All Except TENV.

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This table has nothing to do with termination temperatures! Its purpose is to tell you what KIND of conductor to use for each motor.

• Example: If it is a Class B TENV motor, you must use a conductor with a minimum insulation rating of 90°C such as Teck90.
• Example: If it is a Class H motor, you must use a conductor with a minimum insulation rating of 110°C, such as Teck90 XLPE (The XLPE is a special type of jacket that allows a higher insulation rating!)

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