Dakota Transformer Conductor Guide for Red Seal Exam (CEC 2024)
Red Seal Exam Question Guides
When feeding power to and from a transformer, we need to make sure the wires are beefy enough to handle the load safely. This is called conductor sizing, and the rules are in CEC Rule 26-256.
RULE #1: 125% Rule for Single Transformers
For one transformer, both the primary (input) and secondary (output) wires must be sized for:
PRIMARY: Not less than125% of the transformer’s rated primary current (RPC)
SECONDARY: Not less than 125% of the transformer’s rated secondary current (RSC)
Guess what? Rule 4-006 applies!
HOW TO FIND CURRENT (Primary or Secondary)
For 3-phase transformers:
I =
VA
V × √3
For Single-phase transformers:
I =
VA
V
Then multiply that current by 1.25 (125%), and match it to a wire size from CEC Table(s) 1-4.
Example: 100 kVA 3Ø 600V :120/208V
Primary Current:
I = 100,000 / (600 x √3) = 96.225A
96.225 * 125%= 120.28A
Table 2 (75C) Use #1 AWG copper (good for 130A)
Secondary Current:
I = 100,000 / (208 x √3) = 277.57A
277.57 x 125%= 346.97A
Table 2 (75 C) → Use 500 kcmil (Good for 380A)
Parallel Transformers = Bigger Feast
For parallel transformers, the rules say:
Add up the rated currents of all transformers
Then take 125% of the largest one
Add it all together for total required ampacity
Example:
60kVA + 40kVA + 20 kVA
Single-Phase
600V Primary Side:
60kVA = 100A
40kVA = 66.7A
20kVA = 33.3A
125% of biggest = 100 × 1.25 = 125A
Add them all: 125 + 66.7 + 33.3 = 225A → Use #4/0 AWG (good for 230A)
Example:
480V Secondary Side
Secondary current = VA / V
Add all secondaries, then ×1.25
Let’s say the secondary currents are 125A + 83.3A + 41.7A = 250A 250A × 1.25 = 312.5A
-> Use 400 kcmil
When It Gets Too Big... Split It!
If the current gets too high for one wire:
Divide by 2 (or 3, etc.)
Run parallel conductors!
Example:
624.5A → too big for one cable
Split: 624.5 ÷ 2 = 312.25A
Use two runs of 400 kcmil
TL;DR – Transformer Conductor Sizing Made Simple
Step 1: Find transformer VA rating
Step 2: Calculate current (use VA ÷ Voltage × √3 for 3Ø)
Step 3: Multiply by 125%
Step 4: Use appropriate Table(s) 1-4 to size conductor
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