Transformer Overcurrent Protection: CEC Rule 26-256 Solved Step by Step

Every dry-type transformer in a commercial building needs three things sized correctly: the primary overcurrent device, the secondary panelboard rating, and the conductor sizes on both sides. Get any of these wrong and you'll either trip nuisance breakers on energization or fail ESA inspection. Here's the complete CEC calculation method with worked examples.

The Three Rules You Need to Know

Transformer overcurrent protection in Canada is governed by three interconnected CEC rules. Understanding how they work together is the key to sizing everything correctly:

• Rule 26-256: Sizing the primary overcurrent protection device (OCP) for the transformer
• Rule 14-606: Determining the minimum panelboard rating based on primary OCP
• Rule 26-258: Sizing primary and secondary conductors for the transformer

Rule 26-256: Primary Overcurrent Protection

For dry-type transformers rated 750V or less, the primary overcurrent device must be rated at not more than 125% of the rated primary current.

CEC Rule 26-256(1): Each ungrounded conductor of the transformer feeder shall be provided with overcurrent protection rated or set at not more than 125% of the rated primary current.

If the calculated value doesn't match a standard breaker size, the next higher standard rating is permitted (Rule 26-256(3)).

Solved Example 1: 15 kVA Transformer

Size the primary OCP and determine the minimum panelboard rating for a 15 kVA, 600V primary / 120/208V secondary dry-type transformer.

Step 1: Calculate Primary Full Load Current

Step 2: Calculate Secondary Full Load Current

Step 3: Determine Minimum Panelboard Rating (Rule 14-606)

Minimum panelboard rating = Primary OCP × (Primary Voltage / Secondary Voltage)
= 20A × (600 / 208) = 57.7A
Next standard panelboard size: 60A panelboard

Complete Quick-Reference Table

The following table provides pre-calculated values for standard 600V to 120/208V dry-type transformers:

Transformer kVA	Primary FLC	Primary OCP	Secondary FLC	Min. Panelboard	Primary Conductors	Secondary Conductors
15	14.43A	20A-3P	41.64A	60A	3 #10 + #10 GND	4 #6 + #10 GND
30	28.9A	40A-3P	83.4A	125A	3 #8 + #6 GND	4 #2 + #6 GND
45	43.3A	60A-3P	124.9A	225A	3 #6 + #6 GND	4 #1/0 + #6 GND
75	72.1A	100A-3P	208.4A	400A	3 #3 + #4 GND	4 #250mcm + #4 GND
112.5	108.4A	150A-3P	312.6A	600A	3 #1/0 + #3 GND	2×(4 #250 + #8 GND)
150	144.5A	200A-3P	416.9A	600A	3 #3/0 + #1 GND	2×(4 #300 + #1 GND)
225	216.8A	300A-3P	625.1A	1200A	3 #300 + #1/0 GND	2×(4 #500 + #1/0 GND)

Solved Example 2: Reverse Calculation — Sizing OCP from Panelboard

What is the maximum rating of the primary overcurrent device for a transformer supplying a 200A, 120/208V, 3-phase, 4-wire panelboard from a 600V primary?

Primary OCP = Panelboard Rating / (Primary Voltage / Secondary Voltage)
= 200A / (600 / 208)
= 200 / 2.885 = 69.4A

The maximum rating is 60A — the next-lower standard rating (per Table 13), because when working backwards from the panelboard, you must round down.

Rule 26-258: Conductor Sizing for Transformers

The conductors on both sides of the transformer must meet specific minimum ampacity requirements:

Side	Scenario	Minimum Ampacity
Primary	Single transformer	125% of rated primary current
Primary	Multiple transformers on common feeder	125% of largest + 100% of all others
Secondary	Single transformer	125% of rated secondary current
Secondary	Transformers in parallel	125% of sum of all secondary currents

Transformer Energization: Why Breakers Trip on Power-Up

One of the most frustrating field problems is a transformer tripping its primary breaker the moment it's energized — even with no load connected. This happens because of magnetizing inrush current.

CEC Appendix B, Rule 26-256: To avoid nuisance tripping during energization, the overcurrent device should be able to carry:
• 12× rated primary FLC for 0.1 seconds
• 25× rated primary FLC for 0.01 seconds

This is why the code allows rounding up to the next standard breaker size — an undersized breaker will trip every time the transformer is re-energized after a power outage.

Common Mistakes

• Rounding the wrong direction — when sizing from kVA, round UP to next standard OCP. When sizing from panelboard rating (reverse), round DOWN
• Forgetting the voltage ratio for panelboard sizing — the panelboard minimum is NOT the secondary FLC, it's the primary OCP × (V_pri / V_sec)
• Sizing conductors for load instead of transformer rating — conductor ampacity must be based on 125% of the transformer's rated current, not the actual connected load
• Using 120V instead of 208V for three-phase calculations — the secondary voltage for a 3-phase, 4-wire system is 208V line-to-line, not 120V line-to-neutral