Transformer Conductor Sizing

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.

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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!

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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)

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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)

A diagram of a power supply system

AI-generated content may be incorrect.

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

A diagram of a voltage

AI-generated content may be incorrect.

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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!