I have been wondering about the secondary protection rule for a transformer. In the NEC in section 240-21 section L subsection 1 there is reference to secondary protection being required if the length of the conductors exceed 25ft but I cannot find something like this in the Ontario or Canadian Electric Code. Does it exist? If not any idea why not?
Rule 26-256 Overcurrent Protection for Dry-Type
Transformers Rated 750 V or Less (see Appendix B)
(1) Except as permitted in Subrule (2), each transformer shall be protected by an individual overcurrent device on the primary side, rated or set at not more than 125% of the rated primary current of the transformer, and this primary overcurrent device shall be considered as protecting secondary conductors rated at 125% or more of the rated secondary current.
Note: The overcurrent device sized above will be considered as protecting the secondary conductors of the transformer that have a minimum ampacity rating of 125% of the rated secondary current of the transformer.
Now, the Tap Rules could apply if it was wired as such, and then the 7.5m would become applicable.
Rule 14-100 Overcurrent Devices Required
Each ungrounded conductor shall be protected by an overcurrent device at the point where it receives its supply of current and at each point where the size of conductor is decreased, except that such protection shall be permitted to be omitted:
(f) Where the smaller conductor supplies a transformer, and:
(i) The conductor supplying the primary of the transformer has an ampacity not less than one-third that of the larger conductor; and
(ii) The conductor supplied by the secondary of the transformer has an ampacity not less than the ampacity of the primary conductor multiplied by the ratio of the primary to the secondary voltage; and
(iii) The total length of one primary plus one secondary conductor (the longest, if more than one winding), excluding any portion of the primary conductor that is protected at its own ampacity, does not exceed 7.5 m; and
(iv) The primary and secondary conductors are protected from mechanical damage; and
(v) The secondary conductor terminates in a single overcurrent device rated or set at a value not exceeding its ampacity;
(f) In installations where the tap conductor feeds a transformer,
(1) the minimum primary conductor's ampacity is at least 1/3 of the feeder (larger) conductor it is being fed from, or the demand load (see Rule 26-258(3)), whichever is greater; and
(2) the minimum secondary conductor's ampacity is calculated by multiplying the primary conductor's ampacity (see Step 1) by the ratio of the primary to secondary voltage; and
(3) the length of the primary and secondary conductors is not more than 7.5 m; and
Note: When the primary conductor is protected at its ampacity rating, the maximum
length of the secondary conductor could be 7.5 m.
(4) the primary and secondary conductors are protected from mechanical damage; and
(5) the secondary conductors terminate in a single set of overcurrent devices having a rating not greater than the secondary conductor's ampacity.
Tony Moscioni
Electrical Inspector
Electrical Safety Authority
If I want to put a breaker panel in that is going to draw 140 amps. I would need to have a panel sized at 125% of that. 140 x 1.25=175A. There for I would require a 200A panel. If I had a 600 Volt 3phase Primary and required 120/240 single phase. I would then find my watts 175A x 240V=4200W. I would then require a 45 KVA transformer. (Can you load a transformer at 100% or would I need to de-rate this aswell?). Now for the wire size. On the secondary side I need to take 140A X 1.25 = 175A. I beleave this is 2/0 I could be wrong I dont have a code book handy. It may be 3/0. On the Primary of this transformer I will have to find the amperage. To find this I use I=KVAx1000/600=45000/600= 75A I will size my wire at 125% again. 75A x 1.25 = 93.75A I would then want to use a #3AWG.
Lastly would be fusing. I would want to size my fusing at again 125%. As done in the previous math it =93.75. I will then go with a 90A fuse. I will require one 100A disconnect. Im pretty sure this is right. What are your thoughts?