9.4 Motor Branch Circuit Protection, Overload Sizing & Single-Phasing Prevention (CEC Section 28)

9.4 Motor Branch Circuit Protection, Overload Sizing & Single-Phasing Prevention (CEC Section 28)

In Canadian industrial installations, the design, calculation, and field installation of motor circuits are governed by Section 28 of the Canadian Electrical Code (CEC CSA C22.1, Part I). Unlike standard distribution branch circuits supplying resistive heating or lighting—where a single breaker or fuse protects conductors against both overloads and short circuits—motor circuits separate protection into two distinct, specialized tiers:

1. Branch-Circuit Overcurrent Protection (Rule 28-200 to 28-210): Protects the branch conductors, motor controller, and motor against high-energy short circuits and ground faults.
2. Motor Running Overload Protection (Rule 28-300 to 28-318): Protects the motor windings, internal insulation, and branch conductors against continuous, moderate overcurrents caused by mechanical overloading, stalled rotors, or low supply voltage.

Industrial electricians must possess complete fluency in distinguishing between Table 44/45 values and actual nameplate ratings, applying Table 29 maximum overcurrent limits, sizing overload relays based on motor Service Factor, and identifying the severe hazards of single-phasing.

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1. Architecture of an Industrial Motor Branch Circuit

Under CEC Section 28, every motor branch circuit consists of four fundamental components arranged in logical electrical progression:

   Feeder Distribution Bus (e.g., 600 V Switchgear)
        │
        ▼
   ┌────────────────────────────────────────────────────────┐
   │ 1. MOTOR DISCONNECTING MEANS (Rule 28-600 to 28-604)   │
   │    - Isolates branch circuit from supply               │
   │    - Horsepower rated, load-break, lockable OFF        │
   ├────────────────────────────────────────────────────────┤
   │ 2. BRANCH-CIRCUIT OVERCURRENT PROTECTION (Rule 28-200) │
   │    - Sized per Table 29 using CEC Table 44 FLA         │
   │    - Time-delay fuses (175%) or Breakers (250%)        │
   ├────────────────────────────────────────────────────────┤
   │ 3. BRANCH-CIRCUIT CONDUCTORS (Rule 28-106)             │
   │    - Sized to minimum 125% of Table 44 FLA             │
   │    - Selected from Table 2 ampacity tables (75°C)      │
   ├────────────────────────────────────────────────────────┤
   │ 4. MOTOR CONTROLLER (Rule 28-500)                      │
   │    - Magnetic starter contactor (FVNR, FVR, RVSS)      │
   │    - Horsepower rated for motor switching duty         │
   ├────────────────────────────────────────────────────────┤
   │ 5. MOTOR OVERLOAD PROTECTION (Rule 28-300 to 28-318)   │
   │    - Sized per Rule 28-306 using MOTOR NAMEPLATE FLA   │
   │    - 115% (SF 1.0) or 125% (SF ≥ 1.15)                 │
   └────────────────────────────────────────────────────────┘
        │
        ▼
   Three-Phase Induction Motor (e.g., 600 V, Design B)

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2. Motor Branch-Circuit and Feeder Conductor Sizing

Branch Circuit Conductors (CEC Rule 28-106)

Under CEC Rule 28-106(1), branch circuit conductors supplying a single continuous-duty motor must have an ampacity of not less than 125% of the motor full-load current (FLA) as determined from CEC Table 44 (for three-phase AC motors) or Table 45 (for single-phase AC motors):

$\text{Conductor Ampacity} \ge 125\% \times I_{Table\ 44}$

[!CRITICAL] Table 44 vs. Motor Nameplate FLA for Conductors: Electricians must never use the motor nameplate current to size branch-circuit conductors or branch overcurrent devices. They must use the standardized full-load current listed in CEC Table 44 (or Table 45). Nameplates vary between manufacturers and efficiency grades. Using Table 44 ensures that if a damaged motor is replaced in the future with another standard motor of identical horsepower rating but slightly lower efficiency or power factor, the branch conductors and conduit remain code-compliant and safely sized without requiring rewiring.

Motor Feeder Conductors (CEC Rule 28-108)

Where a single feeder conductor supplies multiple motors (e.g., feeding an entire Motor Control Center or a multi-motor manufacturing cell), Rule 28-108 mandates that the feeder conductors must have an ampacity of not less than:

$\text{Feeder Ampacity} \ge 125\% \times (I_{FLA\ of\ Largest\ Motor}) + \sum (I_{FLA\ of\ All\ Other\ Motors})$

All motor currents are taken directly from Table 44. If two or more motors have identical largest ratings, only one is multiplied by 125%, while all remaining motors are added at 100%.

Conductor Termination Temperature Ratings (CEC Rule 4-006)

When selecting conductor sizes from CEC Table 2 (copper conductors in raceway), Rule 4-006 mandates that unless the equipment is marked otherwise, termination ratings for equipment operating at 100 A or less (or marked for No. 1 AWG or smaller) must be based on the 75°C column, even when 90°C rated conductors (such as RW90 or Teck90) are installed. 90°C ampacities may only be utilized for derating calculations (such as ambient temperature correction per Table 5A or conduit fill per Table 5C), provided the final corrected ampacity does not exceed the 75°C terminal rating.

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3. Motor Branch-Circuit Overcurrent Protection (Rule 28-200 & Table 29)

Because motor starting inrush currents reach 600% to 800% of FLA for several seconds, standard overcurrent protection sized at 125% would trip instantly every time the motor attempts to start. Therefore, CEC Section 28 allows branch-circuit overcurrent devices to be sized significantly higher than conductor ampacity, specifically to provide short-circuit and ground-fault protection while riding through normal starting transients.

Under CEC Rule 28-200 and Table 29, the maximum rating or setting of branch overcurrent protective devices for standard squirrel-cage induction motors (Design B, normal torque) is calculated as a percentage of the motor Table 44 FLA:

Standard Overcurrent Device Selection Procedure

1. Calculate the maximum permissible rating: $\text{Max Rating} = \text{Percentage from Table 29} \times I_{Table\ 44}$.
2. Under CEC Rule 28-200 and Rule 14-104, if the calculated value does not correspond to a standard fuse or circuit breaker rating listed in CEC Table 13, the electrician must select the next lower standard rating.
3. The Starting Inrush Exception: If the next lower standard rating trips during motor starting, the electrician is permitted by Rule 28-200(3) or 28-200(4) to step up to the next higher standard rating, provided it does not exceed the absolute Code Exception Ceiling shown in the table above.

Instantaneous-Trip Circuit Breakers (Motor Circuit Protectors - MCP)

Instantaneous-trip breakers contain no thermal overload bimetal elements; they operate solely on magnetic instantaneous trip coils designed to clear dead short circuits within 3 to 8 milliseconds. Under CEC Rule 28-210, MCP breakers:

• May never be installed as standalone circuit breakers in general panelboards.
• Are permitted only as part of an approved factory-assembled combination motor starter that incorporates matched running overload relays.
• Must be set at not more than 1300% of motor Table 44 FLA for standard Design B motors (or up to 1700% for high-efficiency Design E motors per Rule 28-210(b)).

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4. Motor Running Overload Protection (Rule 28-300 to 28-318)

While branch overcurrent devices clear high-current faults, overload relays protect the motor windings from sustained overcurrent conditions between 105% and 600% FLA. Overloads are caused by worn mechanical bearings, jammed conveyor belts, excessive pump head, low line voltage, or persistent process overfeeding.

Overload Sizing Rules (CEC Rule 28-306)

Under CEC Rule 28-306, overload protection is sized strictly from the ACTUAL MOTOR NAMEPLATE FLA, never from Table 44:

1. Motors with a Service Factor (SF) of 1.15 or greater, or motors marked with a temperature rise not exceeding 40°C: $\text{Maximum Overload Trip Setting} = 125\% \times I_{Nameplate\ FLA}$
2. Motors with a Service Factor of 1.0 (or motors with no service factor marked on the nameplate): $\text{Maximum Overload Trip Setting} = 115\% \times I_{Nameplate\ FLA}$
3. Nuisance Tripping Exception (Rule 28-306(2)): If the standard overload selection trips during normal acceleration under heavy load, the setting may be increased to an absolute maximum ceiling of 140% (for motors with SF ≥ 1.15) or 130% (for motors with SF 1.0).

[!NOTE] Understanding Service Factor (SF): A motor Service Factor of 1.15 indicates that the motor has an engineered thermal safety reserve allowing it to operate continuously at 115% of its rated horsepower without catastrophic insulation failure (though at reduced insulation lifespan). A motor with SF 1.0 has zero continuous reserve capacity; any continuous operation above 100% rated current leads to rapid winding burnout. This is why the CEC restricts SF 1.0 overload sizing to 115% while permitting 125% for SF 1.15.

Overload Relay Technologies & Trip Classes

   Tripping Time (Seconds)
     ▲
 1000│         [ Class 10 Trip Curve ] (Submersible pumps, hermetic compressors)
     │         [ Class 20 Trip Curve ] (Standard general industrial motors)
  100│         [ Class 30 Trip Curve ] (High-inertia centrifuges, heavy fans)
     │
   30│───────────────────────────────────────────┐ (Class 30: ≤ 30s at 600%)
   20│─────────────────────────────┐             │ (Class 20: ≤ 20s at 600%)
   10│───────────────┐             │             │ (Class 10: ≤ 10s at 600%)
     │               │             │             │
    0└───────────────┴─────────────┴─────────────┴────────► Overcurrent (% FLA)
                    600% FLA (Locked-Rotor Point)

Overload relays are manufactured in two distinct technologies:

1. Thermal Bimetallic Overload Relays: Line current passes through resistive heater elements positioned adjacent to laminated bimetallic strips. As current heats the bimetal, the two metals expand at different rates, causing the strip to deflect mechanically. At the calibrated trip point, the deflection trips a spring-loaded snap-action NC contact, opening the 120 V control circuit to the starter coil. Ambient-compensated designs incorporate a compensating bimetal that prevents ambient MCC room temperatures from altering the trip point.
2. Solid-State Electronic Overload Relays: Utilize internal toroidal Current Transformers (CTs) to measure instantaneous line currents in all three phases. A microprocessor models motor thermal curves digitally, providing:
   • Wide adjustable current ranges (e.g., 3:1 or 5:1 ratio without changing heater elements).
   • Selectable Trip Classes (Class 10, 20, 30).
   • Integrated electronic Phase-Loss and Current Unbalance protection.

NEMA / IEC Trip Class Definitions

The trip class defines the maximum time in seconds the overload relay will take to trip when subjected to 600% of its rated current (simulating a locked-rotor condition):

• Class 10: Trips in 10 seconds or less at 600% FLA. Essential for motors with low thermal capacity, such as submersible borehole pumps (where water flow cools the motor) or hermetic refrigeration compressors.
• Class 20: Trips in 20 seconds or less at 600% FLA. The universal standard for general industrial motors, pumps, conveyors, and compressors.
• Class 30: Trips in 30 seconds or less at 600% FLA. Required for high-inertia loads with extended acceleration times—such as large industrial centrifuges, industrial wood chippers, ball mills, and heavy induced-draft fans—where normal starting inrush exceeds 15 seconds.

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5. The Single-Phasing Hazard & Phase Loss Detection

Single-phasing is the catastrophic condition that occurs when one phase of a three-phase power supply opens while a three-phase motor is running. Common field causes include:

• One blown utility pole-mounted fuse or distribution cut-out.
• One blown branch-circuit fuse in an industrial disconnect switch.
• A burned open contact blade or pitted power tip on a magnetic contactor.
• A severed or broken line conductor inside a conduit or junction box.

               Normal Three-Phase Run               Single-Phasing (Phase L1 Open)

                 L1 (100% FLA) ──► Stator               L1 (BLOWN FUSE: 0 A)
                 L2 (100% FLA) ──► Stator               L2 (SURGES TO 173% FLA) ──► Stator
                 L3 (100% FLA) ──► Stator               L3 (SURGES TO 173% FLA) ──► Stator

             Forward Rotating Stator Field        Pulsating Field + Counter-Rotating Field
             Synchronous Speed (e.g. 1800 RPM)    Severe 120 Hz Negative-Sequence Rotor Currents
             Zero Rotor Negative Sequence         Rotor Core Temperatures > 250°C in Minutes

The Electrical & Magnetic Physics of Single-Phasing

1. Starting vs. Running: A three-phase motor at rest cannot start on single phase. It produces zero starting torque, hums loudly, and draws locked-rotor current until it trips or burns out. However, if a motor is already running at rated speed when one phase opens, rotor inertia and single-phase field pulsation allow the motor to continue turning, albeit with severe mechanical vibration and audible groaning.
2. Line Current Surge: To maintain the required mechanical shaft horsepower output ($P_{mech} = \sqrt{3} \times V_{L-L} \times I_{line} \times \text{PF} \times \eta$), the remaining two active phase conductors must supply all the energy. The line current in the two intact phases immediately surges to approximately $\sqrt{3} \times \text{FLA} \approx 173%$ of normal full-load running current:

$I_{remaining\ lines} = \sqrt{3} \times I_{FLA} \approx 1.732 \times I_{FLA}$

1. The Delta Winding Trap: In a Delta-connected stator, the internal phase winding connected across the two remaining live phases carries an astonishing 230% of its normal full-load phase current, leading to rapid localized winding burnout while the other two windings carry less current.
2. Negative-Sequence Heating: The unbalanced single-phase supply creates a forward-rotating magnetic field and an equal-magnitude reverse-rotating (negative-sequence) magnetic field. This reverse field cuts the rotor bars at nearly double synchronous frequency (approximately 120 Hz). These high-frequency induced currents generate immense $I^2R$ resistive heating and eddy-current losses in the rotor core. Rotor temperatures can exceed 250°C within two minutes, destroying rotor bar brazing and melting internal insulation long before heat transfers to the external stator frame.

Why Thermal Overloads Fail to Protect Partially Loaded Motors

Consider a motor operating at 60% load when an upstream fuse blows:

$I_{single-phase} = 0.60 \times 1.732 \times I_{FLA} \approx 104\% \text{ of rated motor FLA}$

Because 104% FLA is well below the standard 115% or 125% thermal overload trip setting, a standard thermal overload relay will never trip! The motor will continue running for hours while its rotor and internal Delta windings suffer severe, irreversible thermal degradation.

Protective Solutions

To eliminate this vulnerability, modern Canadian industrial facilities install:

1. Solid-State Overload Relays with Phase-Loss Detection: Continuously monitor line current balance across all three poles. If current in any phase drops below 15% to 20% of the remaining lines, the microprocessor initiates a trip within 1 to 3 seconds, regardless of motor load.
2. Phase-Monitoring Relays: Connected to the three-phase line supply ahead of the contactor to monitor phase voltage balance, phase loss, and reverse phase rotation. If any phase voltage unbalance exceeds 2% to 4%, the relay opens the starter control circuit instantly.

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6. Comprehensive Industrial Calculation Walkthrough: Sizing a 50 HP Motor Branch Circuit

An industrial electrician is assigned to install a continuous-duty, three-phase squirrel-cage induction motor driving an industrial sawdust exhaust cyclone in a British Columbia manufacturing plant. The installation parameters are:

• Nameplate Data: 50 HP, 600 VAC, 3-Phase, 60 Hz, Nameplate FLA = 48 A, Service Factor = 1.15, Design B, Code Letter G, 75°C terminal ratings.
• Conductors: Copper single conductors installed in rigid PVC conduit in a 30°C ambient environment.
• Protection Type: Dual-element time-delay fuses and a separate inverse-time circuit breaker alternative.

   600V Bus ───[ Fuses: 90A TD ]───[ Disconnect: 60A/50HP ]───[ #4 AWG Cu ]───[ Overload: 60A ]───► 50HP Motor

Step 1: Determine Full-Load Current from CEC Table 44

Refer to CEC Table 44 (Three-phase AC motors):

• Locate 50 HP under the "Horsepower" column.
• Read across to the 600 V column: Table 44 FLA = 52 A.
• (Notice that Table 44 FLA is 52 A, whereas the nameplate FLA is 48 A. We must use 52 A for conductors and overcurrent devices, and 48 A for overloads!)

Step 2: Size Branch Circuit Conductors (Rule 28-106)

Per Rule 28-106(1), conductors must be rated for at least 125% of Table 44 FLA:

$\text{Minimum Ampacity} = 1.25 \times 52\text{ A} = 65.0\text{ A}$

Refer to CEC Table 2 (Copper conductors):

• Look under the 75°C column (mandated by equipment termination Rule 4-006):
  • No. 6 AWG copper has an ampacity of 65 A.
  • (Note: While No. 6 AWG is exactly 65 A, engineering best practice for heavy industrial starting duty frequently selects No. 4 AWG rated at 85 A to mitigate starting voltage drop across long conduit runs). Per minimum code: No. 6 AWG copper (75°C) is code compliant.

Step 3: Size Branch-Circuit Overcurrent Protection (Rule 28-200 & Table 29)

Option A: Time-Delay (Dual-Element) Fuses

Per CEC Table 29, standard maximum for time-delay fuses on Design B motors is 175%:

$\text{Max Fuse Rating} = 1.75 \times 52\text{ A} = 91.0\text{ A}$

Refer to CEC Table 13 (Standard Overcurrent Ratings):

• The standard fuse sizes near 91 A are 80 A, 90 A, and 100 A.
• Per Rule 28-200, select the next lower standard rating: 90 A.
• (If the 90 A time-delay fuse blows during high-inertia cyclone acceleration, Rule 28-200(3) permits stepping up to the next higher standard rating up to an absolute ceiling of 225% ($2.25 \times 52\text{ A} = 117\text{ A}$), permitting a 100 A or 110 A fuse). Standard selection: 90 A Time-Delay Fuses.

Option B: Inverse-Time Circuit Breaker

Per CEC Table 29, standard maximum for inverse-time breakers on Design B motors is 250%:

$\text{Max Breaker Rating} = 2.50 \times 52\text{ A} = 130.0\text{ A}$

Refer to CEC Table 13:

• The standard breaker sizes near 130 A are 125 A and 150 A.
• Select the next lower standard rating: 125 A.
• (If 125 A trips on start, Rule 28-200(4) allows increasing to next higher standard up to 300% ($3.00 \times 52\text{ A} = 156\text{ A}$), which would permit a 150 A breaker). Standard selection: 125 A Inverse-Time Circuit Breaker.

Step 4: Size Motor Running Overload Protection (Rule 28-306)

Per Rule 28-306, overload sizing is calculated strictly from the Nameplate FLA (48 A). Because the motor has a marked Service Factor of 1.15, the maximum setting is 125%:

$\text{Max Overload Trip Setting} = 1.25 \times 48\text{ A} = 60.0\text{ A}$

• The solid-state or thermal overload relay dial is set to 60.0 A (or lower if operating strictly at rated nameplate 48 A without utilizing the service factor reserve).

Step 5: Size the Disconnecting Means (Rule 28-600 & 28-604)

Per Rule 28-604, the disconnect must have an ampere rating not less than 115% of Table 44 FLA ($1.15 \times 52\text{ A} = 59.8\text{ A}$) and possess a horsepower rating not less than the motor rating: Select a 60 A or 100 A, 600 VAC, 50 HP rated heavy-duty disconnect switch.