Ampere rating of a circuit, feeder, or service – what is it based on?

After writing articles for quite a few years, I got used to numerous questions that the readers sent to me.

But I’m a bit surprised by a number of questions that were sent to me in relation to the fundamentals of the safety criteria prescribed by the Canadian Electrical Code, Part I (CE Code).

So, let’s try to clarify the subject indicated in this article’s title by posing five relevant questions and answers in conjunction with this subject.

Question 1. What is the rating of a branch circuit, feeder, or consumer’s service?

Answer 1. Subrule 8-104(1) of the CE Code describes a circuit (feeder or consumer’s service) rating as follows:

“8-104 Maximum circuit loading (see Appendix B)

1) The ampere rating of a consumer’s service, feeder, or branch circuit shall be the ampere rating of the overcurrent device protecting the circuit or the ampacity of the conductors, whichever is less.”

This description means that a rating of a typical branch circuit is a selected rating or setting of the overcurrent device that protects the branch circuit or ampacity of circuit conductors, whichever happened to be less. It means that (for example) if a 200 A-rated overcurrent device is selected to protect a branch circuit, and # 4/0 copper conductors intended for installation in a raceway are selected from the 75°C column of Table 2 of the CE Code, with an assigned ampacity of 230 A to such conductors, then the ampere rating of such branch circuit is 200 A.

Question 2. What is the relationship between the rating of a branch circuit and the load connected to that branch circuit?

Answer 2. Subrule 8-104(2) of the CE Code offers the following statement:

“2) The calculated load in a circuit shall not exceed the ampere rating of the circuit.”

This CE Code requirement means that under no condition, calculated load in the branch circuit (feeder or consumer’s service) is allowed to exceed the rating of that branch circuit (feeder or consumer’s service). It means that in our example above, the calculated load in a 200 A-rated branch circuit cannot exceed 200 A.

Question 3. Can the calculated load be equal to the rating of the circuit?

Answer 3. Subrules 8-104(5) and (6) of the CE Code provide a comprehensive answer to this question as follows:

“5) Where a fused switch or circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices, the continuous load as determined from the calculated load shall not exceed the continuous operation marking on the fused switch or circuit breaker and

1. 1. a) except as required by Item b), shall not exceed 100% of the allowable ampacities of conductors selected in accordance with Section 4; or
   2. b) shall not exceed 85% of the allowable ampacities of single conductors selected in accordance with Section 4.

6) Where a fused switch or circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, the continuous load as determined from the calculated load shall not exceed the continuous operation marking on the fused switch or circuit breaker and

1. 1. a) except as required by Item b), shall not exceed 80% of the allowable ampacities of conductors selected in accordance with Section 4; or
   2. b) shall not exceed 70% of the allowable ampacities of single conductors selected in accordance with Section 4.”

The above-stated Code requirements mean that if a fused switch or circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices, the continuous load as determined from the calculated load is not allowed to exceed the continuous operation marking (i.e., 100%) on the fused switch or circuit breaker. It means that if in accordance with our example above, the 200 A-rated circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices, the continuous load of such branch circuit is allowed to be as high as 200 A.

If, however, in accordance with our example above, the 200 A-rated circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, the continuous load of such branch circuit is not allowed to exceed 160 A.

The requirement also means that when multi-conductor cables or conductors in raceways are used in an electrical installation, the ampacity of such selected conductors also cannot exceed the continuous operation marking on the fused switch or circuit breaker. In our example above, selection of conductors with an assigned ampacity of 200 A would be sufficient if the 200 A-rated circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices, and the continuous load of such branch circuit is as high as 200 A. If, however, in accordance with our example above, the 200 A-rated circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, selection of conductors with an assigned ampacity of 200 A would be allowed if the continuous load of such branch circuit does not exceed 160 A.

The above-stated CE Code requirements also mean that when single-conductors are used in electrical installations, loads connected to such conductors cannot exceed:

(a)   85% of the allowable ampacities of single conductors selected in accordance with Section 4, if a fused switch or circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices; or

(b)   70% of the allowable ampacities of single conductors selected in accordance with Section 4, if a fused switch or circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices.

Thus, in our example above, if single conductor cables are utilized in an electrical installation in accordance with applicable provisions of Rule 4-004 of the CE Code, then selection of single conductors with an assigned ampacity of 235.29 A would be necessary if the 200 A-rated circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices, and the continuous load of such branch circuit is as high as 200 A. If, however, as per our example above, the 200 A-rated circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices, selection of single conductors with an assigned ampacity of at least 228.57 A would be required, if the continuous load of such branch circuit does not exceed 160 A.

Question 4: If all conductors and cables used in an electrical installation have an insulation temperature rating of 90°C, why is it required to de-rate such conductors using the 75°C temperature column, as indicated in answer to question 1?

Answer 4. When 90°C rated conductors are used in electrical installations and are terminated at the electrical equipment marked with the maximum termination temperature, their selected ampacity must ensure that at the termination point, temperature generated by such selected conductors does not exceed temperature limitations marked on the equipment. Rule 4-006 of the CE

Code elaborates on this condition as follows:

“4-006 Temperature limitations (see Appendix B)

1) Where equipment is marked with a maximum conductor termination temperature, the minimum size of conductor used shall be based on the allowable ampacity in the temperature column in Table 1, 2, 3, or 4, with all relevant correction factors being applied as required by Rule 4-004, corresponding to the maximum termination temperature marked on the equipment.

2) For the purpose of Subrule 1), and except as provided for by other Rules of this Code, where the maximum conductor termination temperature for equipment is not marked, the maximum conductor termination temperature shall be considered to be

1. 1. a) 60°C for equipment
   2. i) rated not more than 100 A; or
   3. ii) marked for use with No. 1 AWG or smaller conductors; and
   4. b) 75°C for equipment
   5. i) rated more than 100 A; or
   6. ii) marked for use with conductors larger than No. 1 AWG.

3) Notwithstanding Subrule 2), for high-voltage equipment where conductor termination temperatures are not marked, it shall be permitted to consult the manufacturer to establish the permitted termination temperature.

4) Subrules 1) and 2) shall apply only to the first 1.2 m of conductor length measured from the point of termination on the equipment.

5) Where a cable transition is made to meet the requirements of Subrule 1) or 2), the length of a  conductor terminating on equipment shall be not less than 1.2 m.

6) Where the conductor ampacity is selected from Tables D8A to D11B, Tables D17A to D17N, or Table 12E, Subrules 1) and 2) shall apply.”

Therefore, if in our example above, as indicated in answer 1, a 200 A-rated circuit breaker is marked for maximum termination temperature at 75°C ampacity of such 90° rated conductors intended for installation in a raceway, would have to be selected not from 90° Column of Table 2 of the CE Code, but from 75°C column – to comply with temperature limitation requirements of Rule 4-006 of the CE Code.

Question 5: Are there any other potential requirements in the CE Code, which would “force” reduction of the ampacity of the selected conductors and “force” upsizing of such conductors for the purpose of accommodating calculated load?

Answer 5: Subrule 4-004(7) of the CE Code mandates the use of correction factors, which might further reduce the ampacity of selected conductors under the following conditions:

“7) The correction factors specified in this Rule

1. 1. a) shall not apply to conductors installed in auxiliary gutters containing 30 conductors or less; and
   2. b) shall apply only to power and lighting conductors as follows:
   3. i) the ampacity correction factors of Table 5A, where conductors are installed in an ambient temperature exceeding or anticipated to exceed 30 °C;
   4. ii) the ampacity correction factors of Table 5B, where single-conductor cables are installed in free air in accordance with Subrule 9);

iii) the ampacity correction factors of Table 5C, where single-conductor cables are installed in free air, single conductors are installed in totally enclosed non-ventilated raceways, or multi-conductor cables are installed in ventilated and ladder-type cable trays or in nonventilated cable trays in accordance with Subrule 1) c), 2) c), 11), 12), 13), 23), or 25), as applicable; and

1. 1. iv) the ampacity correction factors of Table 5D, where single-conductor cables are installed in free air or single-conductor or multi-conductor cables are installed in ventilated and ladder-type cable trays in accordance with Subrule 8) or 24), as applicable.”

As it could be seen from the referenced Subrule 4-004(7) above, specific conditions of the installation may trigger additional requirements for ampacity reduction of selected conductors. For example, if selected conductors are intended to be installed in an ambient temperature exceeding 30°C, then correction factors required by Table 5A would have to apply.

Where single conductor cables are intended for installation in free air in accordance with Table 1 or table 3 of the CE Code, and there are from two to four single conductor cables present and spaced less than 25% of the largest cable diameter, then provisions of Table 5B, would be warranted for application.

If more than three conductors are installed in accordance with Tables 2 and 4 of the CE Code, then correction factors mandated by Table 5C would have to be applied – to further reduce ampacity (and to further increase size) of already selected conductors.

If conductors are intended for installation in ventilated and ladder-type cable trays, then additional current rating correction factors could be necessary in accordance with Table 5D.

And finally, the size of conductors might have also to be increased – to comply with voltage drop requirements of Rule 8-102 of the CE Code.

Hopefully, this brief illustration of the CE Code requirements is helpful in the determination of the ampere rating of a branch circuit, feeder, or service.

And as always, local electrical safety regulators should be consulted regarding specific conditions of the intended installation.