Dakota Transformer Overcurrent Guide for Red Seal Exam (CEC 2024)
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We’re protecting transformers from too much current (aka: "electrical stress"). To do that, we install fuses or circuit breakers on the primary and/or secondary sides.
PRIMARY
Firstly:
Pay very close attention to which Over-Current the exam is asking for!
Secondly:
When it comes to sizing over-current:
Sometimes it depends on the TYPE of over-current, sometimes not
Sometimes we size up, sometimes we size down
Basically, Transformer Over-Current code is an absolute mess.
Transformers over 750V
CEC Rule 26-250
Primary Protection:
FUSE: at not more than 150% of the Rated Primary Current
BREAKER: at not more than 300% of the Rated Primary Current
If exact match isn’t in Table 13:
Go DOWN for breakers, UP for fuses
Example:
200 kVA, 3Ø, 4160:600V Transformer 5% Z
Primary Current:
I = 200,000 / (4160 x √3)
I = 27.75A
Fuse: 27.75 × 1.5 = 41.6A → Go UP → 45A fuse
Breaker: 27.75 × 3 = 83.25A → Go DOWN → 80A breaker
BUT WAIT!!! You might not need primary protection if...
According to Rule 26-250(4):
If you’ve got:
A secondary O/C device (like a breaker)
The Secondary O/C device AND the Primary Feeder O/C device follow the multipliers of Table 50.
Then you can skip the dedicated primary over-current!
Transformers 750V or Less (Dry-type)
CEC Rule 26-254
Primary O/C must be NOT MORE THAN 125% of the rated primary current
That breaker also protects the Secondary (if sized 125% OR MORE of secondary current)
Example:
50 kVA, 600V:120/240V, 3-phase Transformer
Primary current:
I = 50,000 / (600 x √3)
I = 48.1A
Primary O/C=48.1A* 1.25= 60.125A
Use Table 13
GO UP = 70A
Ok. So.
As long as you have the PRIMARY Over-Current- YOU DO NOT NEED A SECONDARY.
But, if you choose to have a Secondary:
You must have a PRIMARY FEEDER O/C as long as it is set at not more than 300%.
Secondary current:
I = 50,000 / (240 x √3)
I = 138.8A
Secondary O/C = 138.8A x 1.25 = 173.5A
GO DOWN = 150A
Primary current:
I = 50,000 / (600 x √3)
I = 48.1A
Primary O/C = 48.1A x 300% = 144.3A
Use Table 13
GO DOWN = 125A
Transformers 750V or Less (Other types)
CEC Rule 26-252
This one is Extra Weird.
Either the Primary Feeder or Primary = Rated Primary Current (RPC) * 150%.
But…..
If the RPC is 9A or more, and it doesn’t match anything on Table 13, GO UP.
If the RPC is less than 9A, then use 167% instead of 150%. And then go to Table 13 and GO DOWN.
But…..
If the RPC is less than 2A, use 300% instead of 150% and go to Table 13 and GO DOWN.
But…..if you choose to have a Secondary:
You must have a PRIMARY FEEDER O/C as long as it is set at not more than 300%.
But…..
If the RSC is 9A or more, you multiply by 125% and it doesn’t match anything on Table 13 GO UP.
If the RSC is less than 9A, then use 167%. And then go to Table 13 and GO DOWN.
Here's an example:
Secondary current:
I = 2000 / 120
I = 16.66A
Secondary O/C: 16.66A * 125%= 20.833A
Use Table 13
GO UP -> 25A O/C
Primary current:
I = 2000 / 240
I = 8.33A
Primary O/C = 8.33A x 300% = 24.99A
Use Table 13
GO DOWN=20A
BONUS: Thermal Overload Protection!
CEC 26-252 (6)
If the transformer has built-in smart thermal protection, you can skip the primary O/C if:
not more than 7.5% impedance → O/C can be 6× primary current
7.5–10% impedance → O/C can be 4× primary current
This only applies if the transformer can cut itself off when overheating.
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