Transformer Secondary Conductors


New User
Cec Rule 26-258 (3)

Notwithstanding Subrules (1) and (2), primary and secondary conductors shall be permitted to have an ampacity rating not less than that required by the demand load, provided that they are protected in accordance with Rules 14-100 and 14-104.

Does this rule imply you don't not need to upsize your secondary conductors by 125% if secondary O/C is used ,secondary conductor wire ampacity is not less then secondary O/C and Demand load?

I realize the rule is for choking transformers/ limited amount of transformer sizes to choose from .

I am looking to install a 150KVA 600/120,208V delta to Wye Transformer .

150KVA/ (600x1.732) = 144.3 A. 144.3 x 1.25 = 180.4 - T13 ⬇️ Either going to install 150 A or 175A primary O/C

150 KVA/ (208x1.732) = 416.4 A. Going to install 400 Amp panel with main breaker , I have no drawings or prints, no idea of what the calculated demand load even is , all I am told is that need to install 400 Amp Panel .

Question I have is if parallel 3/0 secondary conductors (200A) each satisfies CEC rule 26-258 (3) , do you still have to upsize your secondary conductors when chocking a transformer with secondary O/C? I know you would with only primary O/C.

1.25 x demand load. 400x1.25 = 500 A 500\2 = 250A ➡️ T2 250 Kcmil (255A) parallel conductors
400A breaker ➡️ T2 3/0 (200A) Parallel
I don't have much information to go off , 8-104 (6) Maximum CCT Loading doesn't really apply as I am capped at 400A


If u just using a 400A panel, that means the max load is 400*208*1.73=143963w. Which means the max fuse in the primary is 143963/600/1.73=138 or smaller.

So u can just using a cable is rated equal or higher 138A for the primary and 3/0 parallel in the secondary

If u make the 150KVA TF fully load, the primary will be 150000/600/1.73*1.25=180A, so 3/0 is good for primary.
The secondary will be 150000/208/1.73*1.25=521A. that means panel for the secondary should be 600A.

Sent from my LYA-L0C using Tapatalk