Although this subject is not new, and I wrote a few articles on it a while ago, in light of the addition of Subrule (4) to Rule 26-256 of 2018 edition of the Canadian Electrical Code, Part I (CE Code), apparently new confusion on the subject of discussion has taken place.
Let’s review Rules 26-254 and 26-256 of the CE Code in conjunction with the questions raised by the industry stakeholders.
Rules 26-254 and 26-256 state the following:
“26-254 Overcurrent protection for dry-type transformer circuits rated 750 V or less (see Appendix B)
1) Except as permitted in Subrule 2), each ungrounded conductor of the transformer feeder or branch circuit supplying the transformer shall be provided with overcurrent protection 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.
2) Notwithstanding Subrule 1), a transformer having an overcurrent device on the secondary side set at not more than 125% of the rated secondary current of the transformer shall not be required to have an individual overcurrent device on the primary side, provided that the primary feeder overcurrent device is set at not more than 300% of the rated primary current of the transformer.
3) Where a value not exceeding 125% of the rated primary current of the transformer as specified in Subrule 1) does not correspond to the standard rating of the overcurrent device, the next higher standard rating shall be permitted.
26-256 Conductor size for transformers (see Appendix B)
1) The conductors supplying transformers shall have an ampacity rating
a) not less than 125% of the rated primary current of the transformer for a single transformer; or
b) not less than the sum of the rated primary currents of all transformers plus 25% of the rated primary current of the largest transformer for a group of transformers operated in parallel or on a common feeder.
2) The secondary conductors connected to transformers shall have an ampacity rating
a) not less than 125% of the rated secondary current of the transformer for a single transformer; or
b) not less than 125% of the sum of the rated secondary currents of all the transformers operated in parallel.
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.
4) Where the transformer overcurrent protection is selected in accordance with Rule 26-250 1) or 2) or 26-254 3), the primary and secondary conductors connected to the transformer shall be protected in accordance with Rules 14-100 and 14-104.
5) Where multi-rating transformers are used, the primary and secondary conductors shall have an ampacity rating not less than 125% of the rated primary and secondary current of the transformer at the utilization voltage.”
It appears that some Code users apply different interpretation of the above-shown Rules, only because new Subrule 26-256(4) has been added to the Code as follows:
“26-256(4) Where the transformer overcurrent protection is selected in accordance with Rule 26-250 1) or 2) or 26-254 3), the primary and secondary conductors connected to the transformer shall be protected in accordance with Rules 14-100 and 14-104.”
So, what seems to be the problem?
Let’s look at the frequently asked questions:
Question 1: Is The Rating Of The Primary O/C Device Allowed To Exceed 125% Of The Primary Rated Current?
Answer 1: Subrule 26-254(1) states the following:
“1) Except as permitted in Subrule 2), each ungrounded conductor of the transformer feeder or branch circuit supplying the transformer shall be provided with overcurrent protection 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. “
Subrule 26-254(3) allows the following relaxation:
“26-254(3) Where a value not exceeding 125% of the rated primary current of the transformer as specified in Subrule 1) does not correspond to the standard rating of the overcurrent device, the next higher standard rating shall be permitted.”
Let’s use the following example:
For instance, we have a 225 KVA; 600 VAC – 120/208 VAC rated 3 phase dry-type transformer with primary current = 225A x 1000/ (600 x1.732) = 216.51 amps.
Rule 26-254 (1) requires that the primary O/C device must be rated or set at not more than 125% of the rated primary current of the transformer.
Therefore, the rating of the primary O/C shall not exceed 216.51 x 1.25 = 270.64 amps. It means that the primary O/C with the standard trip setting of 225 A or 250 A will perfectly fit the requirement of Rule 26-254(1), as either of these trip settings will not exceed 125% of the transformer primary current. If, however, the next standard value of the trip setting is selected at 300 A, as permitted by Rule 26-254(3), this selection will meet the Code requirements, provided that such O/C device will adequately protect ampacities of the primary and secondary conductors. This latter condition is specifically reflected in Subrule 26-256(4), as this Subrule reference Rules 14-100 and 14-104 of the CE Code.
Question 2: How To Apply Rules 14-100 And 14-104, When Rating Or Setting Of The Primary O/C Exceeds 125% Of The Transformer Primary Rated Current?
Answer 2: Except for a number of listed exemptions, Rule 14-100 of the CE Code requires the following:
“14-100 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.”
Rule 14-104(1)(a) mandates the following condition (with a specific exception allowed under provisions of Table 13):
“14-104(1) The rating or setting of overcurrent devices shall not exceed the allowable ampacity of the conductors that they protect, except
a) where a fuse or circuit breaker having a rating or setting of the same value as the ampacity of the conductor is not available, and the maximum calculated or known load is in accordance with the Rules of Section 8, the ratings or settings given in Table 13 shall be permitted to be used within the maximum value of 800 A;”
Therefore, newly added Subrule 26-256(4) deliberately requires compliance with Rules 14-100 and 14-104, when the setting or rating of the primary O/C exceeds 125% of the transformer primary rated current.
Let’s continue with our example above. If a primary O/C device is selected with a 300 A trip setting, then the size of primary conductors for this 225 KVA transformer would have to be at least #350 kcmil (based on the ampacity of 310 A as per 75 Deg. C column of Table 2), in order to comply with Rules 14-100 and 14-104. Of course, under provisions of Rule 14-104, this size could be further reduced to # 250 kcmil (with an ampacity of 255 A from 75 Deg. C column of Table 2) under relaxation allowed by Table 13, as the 300 A rated O/C could protect conductors with ampacity range between 251 A and 300 A.
The same approach would have to be taken for selection of the secondary conductors. In this case, a 300 A trip setting will manifest the value of 864 A on the secondary of the 225 KVA rated transformer (300 A multiplied by the transformer ratio of 2.88), and in accordance with Rule 14-104 (Table 13 would not apply in this case), installation of the secondary conductors would be quite expensive, as an electrical contractor would have to run three sets of #350 kcmil (with a total ampacity of 930 A as per 75 Deg. C column of Table 2) – to meet the requirement of Rule 14-104 of the Code.
Question 3: How Low Can The Primary Overcurrent Device Be Rated Or Set?
Answer 3: Rule 26-254 does not establish a minimum limit for rating or setting of the primary O/C device, as long as applicable provisions of Subrules 8-104(5) or 8-104(6) are met, and the primary O/C device will not trip under the transformer in-rush current. In fact, the primary O/C device could be set at the value less than the transformer primary rated current, if the conditions mentioned immediately above, are met.
In our example of using a 225 KVA rated transformer, a 200 A rated circuit breaker, marked for continuous operation at 80% will work, if such setting will not result in its trip on transformer’s inrush, and if the connected continuous load does not exceed 80% of the circuit breaker rating, as it is seen from the secondary side (i.e., through the transformer ratio). In this case, the connected continuous load must not exceed 200 A x 2.88 x 0.8 = 460.8 A.
It should be noted that Appendix B Note on Rule 26-254 provides the following clarification:
“Appendix B Note on Rule 26-254
Selection of overcurrent devices with a rating too low for the primary of a dry-type transformer can result in unintended operation when the transformer is being energized (such as might occur after a power outage). To avoid such operation, the overcurrent device should be able to carry
a) 12 times the transformer rated primary full load current for 0.1 s; and
b) 25 times the transformer rated primary full load current for 0.01 s.”
However, in a practical reality, a setting of the primary O/C with the value less than the transformer primary rated current is selected quite seldom.
Thus, if in a practical example above, a 225 trip setting is selected, it is advisable to check with the transformer manufacturer, whether such trip setting will not lead to the transformer tripping during inrush (as indicated in Appendix B Note on Rule 26-254), and a validation should be made that the selected trip setting seen through the transformer ratio (225 A X 2.88 = 648 A), is sufficient – to accommodate continuous connected load, considering that the primary O/C device is 80% rated. So, if connected load is less than 648 A X 0.8 = 518.4 A, this O/C device will allow reducing the size of the primary and secondary conductors permitted by Subrule 26-256(3), provided that these conductors are in compliance with Rule 14-100 and 14-104.
Therefore, under this condition, two sets of #400 kcmil copper conductors would be permitted to be selected as per 75 Deg. C Column of Table 2 (with a combined ampacity of 670 A), as such ampacity meets provisions of Rule 8-104(6) and provisions of Rules 14-100 and 14-104. If there is a desire to select two sets of smaller conductors (let say # 350 kcmil with a total ampacity of 610 A), such selection would be prohibited, as in this case, although provisions of Table 13 could be met, the fundamental condition of Rule 8-104(6) would be violated (i.e., the 610 A rating of the circuit will be less than 125% of the continuous connected load).
Of course, the size of primary conductors could be also reduced based on permission granted by Rule 26-256(3) and #4/0 primary copper conductors with an ampacity of 230 A in accordance with 75 Deg. C column of Table 2 would do the trick.
Question 4: Can A Primary O/C Device Also Protect The Panelboard Located On The Transformer Secondary If This Panelboard Is Not Equipped With The Main O/C Device?
Answer 4: Rule 26-254(1) specifically states that the primary O/C device set in conformance with this Rule, is deemed to protect conductors connected to the transformer secondary, if ampacity of these conductors is not less than 125% of the rated secondary current (see below):
“1) Except as permitted in Subrule 2), each ungrounded conductor of the transformer feeder or branch circuit supplying the transformer shall be provided with overcurrent protection 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.”
Nevertheless, Rule 26-254 is silent on the adequacy of protection by the primary O/C for the panelboard connected to the transformer secondary.
However, Subrule 14-606(2) offers the following clarification in this regard:
“14-606 Panelboard overcurrent protection
1) Except for panelboards where more than 90% of the overcurrent devices supply feeders or motor branch circuits, every panelboard shall be protected on the supply side by overcurrent devices having a rating not greater than that of the panelboard.
2) The overcurrent protection required by Subrule 1) shall be permitted to be in the primary of a transformer supplying the panelboard, provided that the panelboard rating in amperes is not less than the overcurrent rating in amperes multiplied by the ratio of the primary to the secondary voltage.”
This means that a 225 A rated O/C on the primary of the 225 KVA rated transformer in our example (225 A X 2.88 = 648 A) will adequately protect 800 A rated panelboard. However, if this primary O/C is marked for continuous operation of 80%, then the continuous load connected to this panelboard, cannot exceed 648 A X 0.8 = 518.4 A.
In order to increase the load allowed to be connected to this 800 A rated panelboard to 648 A, then the primary O/C must be rated for 100% of its continuous operation.
Question 5: Can A Primary O/C Device Be Used To Trip The Ground Fault Protection Required By Rule 14-102?
Answer 5: Rule 14-102(1) of the CE Code mandates the installation of a ground fault protection under one of the following conditions:
“14-102 Ground fault protection (see Appendix B)
1) Ground fault protection shall be provided to de-energize all normally ungrounded conductors of a faulted circuit that are downstream from the point or points marked with an asterisk in Diagram 3 in the event of a ground fault in those conductors as follows:
a) for circuits of solidly grounded systems rated more than 150 volts-to-ground, less than 750 V phase-to-phase, and 1000 A or more; and
b) for circuits of solidly grounded systems rated 150 V or less to ground and 2000 A or more.”
Let’s consider a 750 KVA; 600 VAC – 120/208 VAC rated 3 phase dry-type transformer with a primary current of 722.54 A. This transformer could be perfectly protected by the primary O/C device set at 800 A.
The rating of this O/C device from the secondary side (via transformer ratio) will exceed 2000 A at 120/208 V.
It means that the ground fault protection would be required in the transformer secondary feeder in accordance with paragraph 14-102(1)(b) of the CE Code.
In this case, ground fault relays (or sensors), which are used to sense low magnitude ground faults, would have to be installed in the secondary feeder of the newly established 120/208 V solidly grounded system. When the ground fault current magnitude and time reach the ground fault relay/sensors pick up setting, the control scheme signals the feeder O/C device to open, and these signals would be sent to the O/C device located in the transformer primary.
Hopefully, the discussion above helps clarify this subject. However, as usual, the local AHJ’s enforcing the legally adopted CE Code in each specific jurisdiction should always be consulted regarding specific questions related to each individual installation.
