Dry-type transformers: conductor and protection selection – Code File, December 2021
November 28, 2021 | By Anthony Capkun
When determining the installation and protection of a dry-type transformer, ask yourself: What am I trying to achieve?
November 28, 2021 – Why are the criteria for installing a transformer so complex? Why can’t it be simpler?
Because of all the rules in the CE Code, there are always other things that come into play. To be fair, those rules try to be flexible enough to allow the installation to proceed unhampered, with the end result being a functional and safe XFMR installation.
For the purpose of this column, I will use examples from one of our most common installation voltages—600V/208V—and refer to a single dry-type XFMR with a turns ratio of 2.88. Against this backdrop, I will discuss the CE Code’s rules and how they interact with each other to ensure a safe installation.
The first rule we need to look at is 26-254 “Overcurrent protection”. Subrule 1 states that, so long as the O/C protection does not exceed 125% of the rated primary overcurrent, and the secondary conductors are rated for at least 125% of the secondary current, then secondary O/C protection may be omitted.
However, if there is going to be secondary O/C protection set at not more than 125% of the secondary, and the primary is fed from a circuit protected at not more than 300% of the rated primary current, then the primary will not require an individual O/C device.
But what happens when the rating of the O/C device does not correspond with 125% of the transformer’s primary ratings?
The CE Code instructs us to go to the next higher-rated device. It does not, however, suggest you no longer need to follow the other applicable rules. So, if we increase the primary O/C device to ensure the operation of the transformer, we must then ensure the secondary remains protected at not greater than 125% of the rated secondary.
Note that Subrule 3 only provides an exemption to the primary O/C protection, not to the secondary.
This scenario is covered in greater detail in the conductor Rule 26-256, and references the protection Rules 14-100 and 14-104, so let’s have a look.
Rule 26-256 tells us conductors supplying the primary and the secondary of a XFMR shall have an ampacity of not less than 125% of the rated primary or secondary current. The conductors may also be sized according to the demand load; however, this will now require both primary and secondary conductors to be protected in accordance with the rules of Section 14.
This requirement may also bring Table 13 into play if the calculated load is also known.
Earlier I mentioned the allowance to increase the primary O/C device beyond the 125% where the value did not align with the 125% of the rated primary current. We know that the secondary protection would then need to be no greater than 125% of the rated secondary current.
This Subrule brings 14-100 to bear, which means the primary conductors must still be sized to be protected by the increased O/C protection, and the secondary conductors must also be protected by an O/C device. Now, however, we must also add the length of one conductor of the primary circuit plus the length of one conductor of the secondary circuit, and this must not exceed 7.5 m in total length.
When determining the installation and protection of a dry-type transformer, ask yourself: What am I trying to achieve? How can I best achieve it while keeping costs in check and without violating the CE Code?
David Pilon is manager, Electrical Inspections, at Technical Safety Authority, Saskatchewan (TSASK). He also serves as vice-chair of the Canadian Certified Electrical Inspector (CCEI) committee of the International Association of Electrical Inspectors (IAEI), Canadian Section. He can be reached at firstname.lastname@example.org.
This feature—plus more great content—appears in the December 2021 edition of Electrical Business Magazine. Even more back issues are located in our Digital Archive.
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