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’?

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.

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

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

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

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

  • 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

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!

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! Yay!

Q: 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?

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!

Q: 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?

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

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

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!)