Among the questions sent to me by the readers, several questions related to underground installation. This article compiles eight such questions and provides the answers to them, based on the applicable requirements of the Canadian Electrical Code, Part I (CE Code).
Are Tables 1 and 3 of the CE Code allowed to be used for the determination of ampacities of single conductor cables in underground installations?
Answer to question 1: No, they are not allowed. Although underground installations include directly buried single conductor cables, it should be noted that use of Table 1 or Table 3 for determination of their respective ampacities (for copper or aluminum single conductors or single conductor cables) is limited only to those cases, when such single conductors or single conductor cables are installed in free air.
This provision of the CE Code recognizes that heat dissipation from single conductors and operating temperature of these single conductors installed in free air is significantly different from single conductors directly buried in underground installation. As such, ampacity assigned to the same conductor size with the same insulation temperature rating, shown in Table 1 or Table 3, is different from the ampacity assigned to the identical directly buried single conductor cable of the same size and with the same insulation temperature rating under provisions of Table D8A or D8B.
Subrules 4-004(1)(a) and Subrule 4-004(2(a) state the following:
“4-004(1) The maximum current that a copper conductor of a given size and insulation is permitted to carry shall be as follows:
single-conductor and single-conductor metal-sheathed or armoured cable, in a free air run, with a cable spacing not less than 100% of the largest cable diameter, as specified in Table 1;”
“4-004(2) The maximum current that an aluminum conductor of a given size and insulation is permitted to carry shall be as follows:
single-conductor and single-conductor metal-sheathed or armoured cable, in a free air run, with a cable spacing not less than 100% of the largest cable diameter, as specified in Table 3″
This is the reason that Table 1 or Table 3 cannot be applied for determination of single conductor ampacities in underground installations.
Are Tables 2 or 4 allowed to be used to determine ampacities of conductors for underground installations?
Answer to question 2: Yes, they are allowed. Subrule 4-004(1)f) for copper conductors and Subrule 4-004(2)(f) for aluminum conductors allow ampacity of conductors with sizes smaller than No. 1/0 AWG, installed underground, to be determined from Table 2 or Table 4, as it references item (b), see below:
“f) underground configurations in conductor sizes smaller than No. 1/0 AWG, as specified in Item b) or as calculated by the IEEE 835 calculation method.”
It should be noted that item 4-004(1)(b) mandates selection of ampacities of copper conductors in accordance with Table 2, and item 4-004(2)(b) requires that the ampacity of aluminum conductors must be selected in conformance with Table 4 (see below):
“4-004 Ampacity of wires and cables (see Appendix B)
1) The maximum current that a copper conductor of a given size and insulation is permitted to carry shall be as follows:
one, two, or three conductors in a run of raceway, or 2- or 3-conductor cable, except as indicated in Item d), as specified in Table 2;
2) The maximum current that an aluminum conductor of a given size and insulation is permitted to carry shall be as follows:
b) one, two, or three conductors in a run of raceway, or 2- or 3-conductor cable, except as indicated in Item d), as specified in Table 4;”
Where in the CE Code separation requirements from underground installations of cables or conductors in raceways are mandated?
Answer to question 3: Rule 12-012 of the CE Code applies to underground installations as follows:
“12-012 Underground installations (see Appendix B)
1) Direct buried cables or raceways shall be installed to meet the minimum cover requirements of Table 53.
2) The minimum cover requirements shall be permitted to be reduced by 150 mm where mechanical protection is placed in the trench over the underground installation.
3) Mechanical protection shall consist of one of the following and, when in flat form, shall be wide enough to extend at least 50 mm beyond the cables or raceways on each side:
a) treated planking at least 38 mm thick;
b) poured concrete at least 50 mm thick;
c) concrete slabs at least 50 mm thick;
d) concrete encasement at least 50 mm thick; or
e) other suitable material.
4) Direct buried cables shall be installed so that they run adjacent to each other and do not cross over each other and with a layer of screened sand with a maximum particle size of 4.75 mm or screened earth at least 75 mm deep both above and below the conductors
5) Where cables rise for terminations or splices or where access is otherwise required, they shall be protected from mechanical damage by location or by rigid conduit terminated vertically in the
trench and including a bushing or bell end fitting, or other acceptable protection, at the bottom end from 300 mm above the bottom of the trench to at least 2 m above finished grade, and
beyond that as may be required by other Rules of the Code, and with sufficient slack provided in the cables at the bottom end of the conduit so that the cables enter the conduit from a vertical position.
6) Where a deviation has been allowed in accordance with Rule 2-030, cables buried directly in earth shall be permitted to be spliced or tapped in trenches without the use of splice boxes.
7) Raceways or cables, if located in rock, shall be permitted to be installed at a lesser depth entrenched into the rock in a trench not less than 150 mm deep and grouted with concrete to the level of the rock surface.
8) Raceways and armoured or metal-sheathed cables suitable for direct burial shall be permitted to be installed directly beneath a concrete slab at grade level, provided that the concrete slab is not
less than a nominal 100 mm in thickness, the location and depth of the underground installation is marked in a conspicuous, legible, and permanent manner, and the raceway or cable is not subject
to damage during or after installation.
9) Any form of mechanical protection that may adversely affect the cable shall not be used.
10) Backfill containing large rocks, paving materials, cinders, large or sharply angular substances, or corrosive material shall not be placed in an excavation where such materials may damage cables,
raceways, or other substructures, prevent adequate compaction of fill, or contribute to corrosion of cables, raceways, or other substructures.
11) The initial installation shall be provided with a suitable marking tape buried approximately halfway between the installation and grade level, or adequate marking in a conspicuous location to indicate the location and depth of the underground installation
12) Where underground raceways or cables are subject to movement by settlement or frost, provision shall be made for the prevention of damage to the conductors, conductor insulation, or the electrical equipment.
13) For installations not covered by the foregoing requirements of this Rule, the requirements of CSA C22.3 No. 7, or the applicable Standard, whichever is more stringent, shall apply.”
While this Rule does not specifically list clearances between various underground systems, it mandates in Subrule (13) compliance with the CSA standard C22. No. 7 “Underground Systems” for the requirements not specified in this Rule.
Section 6 of C22.3 No. 7 is dedicated to separations between underground cables and between underground cables and a wide range of underground installations, such as communication systems, pipelines, tanks, structures, ducts, etc.
General requirements in Clause 6.1.1 of C22.3 No. 7 state the following:
“6.1 Separations — Direct-buried systems
Unless the parties mutually agree otherwise, direct-buried supply and communication cables, whether individual or in combination, shall not be installed in the same vertical plane with other underground systems and shall be separated horizontally by a minimum of 300 mm. Where such separation is not possible, precautions shall be taken to ensure the maximum possible separation and to prevent future contact.
Note: Where possible, direct-buried supply or communication cables, whether individual or in combination, should have a minimum horizontal separation of 1000 mm from water and sewer lines and other deep services.”
The Code users should further review provisions of C22.3 No. 7 for separation requirements in underground installations.
If Appendix D of the CE Code is informative (non-mandatory), why are cables or conductors installed underground, must comply with Tables D8 – D11?
Answer to question 4: Appendix D of the CE Code is informative, indeed. It contains tabulated general information, including Diagrams D8 – D11 for different configurations of underground installations and Tables D8 – D11 for ampacity of conductors or cables arranged in such respective configurations.
For conductor sizes No. 1/0 AWG and larger, Subrule 4-004(1)(d) for copper conductors and Subrule 4-004(2)(d) for aluminum conductors – provides two options for calculation of conductors’ ampacities in underground installations:
To use configurations provided in Diagrams D8 – D11 and to apply Tables D8 – D11 with tabulated ampacities in accordance with the details corresponding to the selected configurations; or
To use standard IEEE 835 for computing the ampacities of copper and aluminum conductors.
Appendix B Note on Rules 4-004(1)(d) and 4-004)(2)(d) provides a comprehensive clarification in this regard. Rules 4-004(1)(d) and 4-004(2)(d) state the following:
“d) single-conductor and 2-, 3-, and 4-conductor cables and single-conductor and 2-, 3-, and 4-conductor metal-armoured and metal-sheathed cables, unshielded and rated not more than 5 kV, in conductor sizes No. 1/0 AWG and larger, installed in accordance with configurations described in Diagrams D8 to D11 in an underground run, directly buried or in a raceway, as specified in Tables D8A to D11B or as calculated by the IEEE 835 calculation method.”
So, if the Code users cannot or do not wish to use configurations, shown in Diagrams D8 – D11 of the CE Code, these users always may undertake calculations of conductors’ ampacities in underground installations by the method specified in IEEE 835 or to contact cable/conductor manufacturers’ application engineers for assistance with such calculations.
Are cables or conductors in raceways installed under a concrete slab deemed to be installed underground?
Answer to question 5: Yes, they are deemed to be installed underground. Rule 12-012 of the CE Code, shown above, provides clarifications on the extent of underground installations.
Subrule 12-012(8) clarifies the conditions under which installation of cables or raceways under a concrete slab is allowed, as follows:
“8) Raceways and armoured or metal-sheathed cables suitable for direct burial shall be permitted to be installed directly beneath a concrete slab at grade level, provided that the concrete slab is not less than a nominal 100 mm in thickness, the location and depth of the underground installation is marked in a conspicuous, legible, and permanent manner, and the raceway or cable is not subject to damage during or after installation.”
Why are ampacities in Tables D8 – D11 only listed for 90 Deg. C insulation?
Answer to question 6: The ampacities shown in Tables D8A to D11B have been determined using the Neher-McGrath methodology, as applied in IEEE 835, for the cable and insulated conductor arrangements shown in Diagrams D8, D9, D10, and D11, and the notes under each such Table in Appendix D, provide following clarification in this regard:
1) This Table provides the allowable ampacities for 90 °C rated single copper conductor cables with spacings directly buried in earth.
2) Underground ampacities for an insulated conductor or cable temperature of 75 °C may be obtained by multiplying the appropriate allowable ampacity at 90 °C conductor insulation temperature by the derating factor 0.886.
3) See Rule 4-006 for equipment termination temperature requirements.”
It should be noted as well that ampacities of conductors or cables in underground runs, selected in accordance with Tables D8 – D11, could be based on ampacities values for 90 Deg. C insulation, if the conductors and cables have insulation, rated at 90 Deg. C. When these conductors and cables transition from the underground portion of installation to the installation above the ground for termination at electrical equipment, the ampacity of these conductors and cables would have to be determined based on 75 Deg. C columns of Tables 1 – 4 – to consider requirements of Rule 4-006 of the CE Code. The Code users should be also aware that provisions of Rule 4-006 apply only up to 1.2 m from the point of termination of conductors and cables on electrical equipment (see the answer to question 8 below).
Why the depth requirements are different for conductors in underground installations between values listed in Table 53 and values specified in Diagrams D8 – D11?
Answer to question 7: As it was indicated earlier, the ampacities specified in Tables D8A to D11B have been calculated in accordance with IEEE 835, and these calculations are based on a range of technical characteristics such as types of cables or insulated conductors, their voltage rating, maximum conductor temperature rating, maximum continuous current rating, short-circuit rating, mutual capacitance, and reactance of conductors, the voltage of the system, etc. As the result, spacing between underground conductors and the depth of their installation, depicted in Diagrams D8 – D11, have become fundamental conditions for the calculation of ampacities shown in Tables D8 – D11. It should be noted that the depth indicated in Diagrams D8 – D11, is the maximum allowed depth for the purpose of using applicable values in Tables D8 – D11. The lesser selected depth than the values depicted in Diagrams D8 – D11, will not adversely impact ampacities tabulated in Tables D8 – D11, as heat from the underground conductors will dissipate faster. However, if the depth of installation would have to be increased – to meet protection requirements mandated by Table 53, then use of Tables D8 – D11 would not be appropriate, and in this case, ampacity calculations would have to be done in accordance with IEEE 835 in conjunction with the selected depth, instead of relying on Tables D8 – D11.
Criteria specified in Table 53 are based on the minimum mechanical protection (and minimum cover requirements) for direct buried cables or insulated conductors in raceways, depending on conditions listed in Table 53.
So, if the minimum cover requirement mandated by Table 53 for an installation of 600 V directly buried single conductor armoured cable is only 450 mm, but the selected ampacity of this cable is based on the value of Table D8 in accordance with Diagram D8, then the minimum depth of installation of such armoured cable could be reduced to 450 mm, but it could not exceed 915 mm, as specified in Diagram D8, if ampacity values from Table D8 are to be used.
What rules apply to the selection of conductor ampacities, when such conductors transition from an underground portion to a portion above ground for termination at electrical equipment?
Answer to question 8: Subrules 4-004(16) and (17) of the CE Code apply to such types of installations, and these Subrules provide the following requirement:
“16) Where more than one ampacity could apply for a given circuit of single-conductor or multi-conductor cables as a consequence of a transition from an underground portion to a portion above ground, the lower value shall apply except as permitted in Subrule 17).
17) Where the lower ampacity portion of a cable installation consisting of not more than four
conductors in total does not exceed 10% of the circuit length or 3 m, whichever is less, the higher ampacity shall be permitted.”
Perhaps the answer to question 8 could be illustrated by a couple of the following examples:
Example 1. Let’s assume that an electric vehicle charging station with a continuous load of 265 A is intended to be supplied by a branch circuit consisting of RW90 XLPE 600 Volts 90°C aluminum conductors in a raceway installed underground in accordance with Detail 1 of Diagram D11, where a single set of conductors per phase is utilized, and the portion of installation above the ground is limited to a minimum length for the sole purpose of termination of these conductors at the electrical equipment.
Let’s also consider that the circuit breaker selected to protect these aluminum conductors, is marked for continuous operation at 80% of the ampere rating of its overcurrent devices.
In this case, in accordance with provisions of Rule 8-104(6) of the CE Code, ampacity of the selected conductors and the rating of the O/C device must be not less than 265 A: 0.8 = 331.25 A.
Let’s select the 400 A-rated circuit breaker with the trip setting at 350 A and see later on whether this trip setting will adequately protect selected conductors in accordance with Rule 14-104 of the CE Code.
So far, so good – all applicable provisions of Rule 8-104(6) appear to be met.
Let’s select the suitable conductor’s size.
As insulation temperature rating of these aluminum conductors is 90 Deg. C, and if Detail 1 of Diagram D11 is selected for underground installation of aluminum conductors in raceways, (1 conductor per phase), then in accordance with Table D11B, any conductor with ampacity, not less than 331.25 A will meet provisions of Rule 8-104(6) of the CE Code. Thus, a single set of 400 kcmil conductors per phase with an ampacity of 337 A for conductors with insulation rated at 90 Deg. C in accordance with Table D11B, would appear to meet Rule 8-104(6). It should be noted that the selected underground conductors with ampacity based on 90 Deg. C insulation, will not be terminated on the equipment below ground, as such equipment is located above the ground. Let’s also assume that a portion of above the ground installation after the transition from below ground run, will be limited to 1.5 m at one end of this above-ground installation and to 2 m – at another end before these conductors are terminated on the electrical equipment.
Now, we need to evaluate a small portion of these conductors above the ground for the determination of their ampacity.
Therefore, after the transition from the underground run into installation above ground for the purpose of termination at the piece of electrical equipment, the ampacity of the selected 400 kcmil underground conductors would have to be evaluated for the assigned ampacity to such 400 kcmil conductors based on 75 Deg. C column of Table 4, as required by Rule 4-006 of the CE Code.
Table 4 of the Code clearly indicates the ampacity of these originally selected 400 kcmil aluminum conductors in 75 Deg. C column is 270 A, which is less than 331.25 A, required by Rule 8-104(6) for our example. As such, the next standard ampacity exceeding the required 331.25 A, would have to be selected, and this ampacity is shown in a 75 Deg. C column of Table 4 as 340 A for the 600 kcmil conductors.
By reviewing Table D11B again, we’ll find that Table D11B assigns 424 A to a single set of 600 kcmil aluminum conductors at 90 Deg, C, which is higher than the value based on 75 Deg. C column of Table 4.
Thus, in accordance with Subrule 4-004(16), a lower ampacity value at 340 A is considered for the entire set (underground and above the ground portion) of three current carrying conductors in a raceway. Considering that voltage drop in these conductors meets requirements of Rule 8-102, selection of the conductors’ size in this example, complies with the requirements of Sections 4 and 8 of the CE Code.
Now, it is time to correlate the selected ampacity of conductors for this circuit (340 A) with the selected trip setting of the O/C device (350 A), protecting these conductors – for compliance with Rule 14-104(1) of the CE Code. For the purpose of this Rule, Table 13 could be used, and in accordance with Table 13, 350 A-rated O/C device perfectly protects conductors with ampacity at 340 A.
Example 2 Let’s assume that a distribution panelboard supplying 580 A of the continuous load is installed in a building, power to which is provided from another building containing the main distribution centre., and the feeder supplying this distribution panelboard from the main distribution centre, runs above and below ground, and consists of a 3 phase, TECK 90 cable containing XLPE copper conductors.
Let’s also assume that this feeder is protected by the 600 A-rated circuit breaker, and this circuit breaker is marked for continuous operation at 100% of the ampere rating of its overcurrent devices. Let’s also assume that the distribution panelboard supplied by this feeder is rated at 600 A.
Let’s review conditions for the size selection of multi-conductor cables in this feeder for its above the ground and underground portions.
Evaluation of cable ampacity will be based on criteria listed in Rule 8-104(5) of the CE Code (based on the condition that cable ampacity must be not less than 580 A of continuous load).
75 Deg. C column of Table 2 will be used for evaluation of cable ampacity for installation above ground.
Let’s select 3 sets of cables per phase consisting of #3/0 copper conductors with a total ampacity of 600 A.
For installation of these cables below the ground, let’s use Detail 3 of Diagram D10 (3 cables per phase) and review the ampacity of these 3 sets of #3/0 cables in accordance with Table D10A, Detail 3.
The total ampacity of such 3 sets of cables in the underground run will be 744 A at 90 Deg. C. There is no need to evaluate the ampacity of these cables at 75 Deg. C, as these cables, will be terminated at the equipment located above the ground.
Considering that voltage drop in these conductors meets requirements of Rule 8-102, selection of the size of the cable in this example (3 sets of #3/0 cables per phase), complies with the requirements of Sections 4 and 8 of the CE Code.
In accordance with Rule 4-004(16), the lower ampacity value of 3 sets per phase for #3/0 TECK 90 cables would be 600 A, and cables with this ampacity could be perfectly protected by the 600 A-rated circuit breaker. Therefore, provisions of Rule 14-104(1) of the CE Code are also met in this example.
Hopefully, answers to the posted questions, have clarified the subject of our discussion on underground installations. And as usual, local AHJ’s enforcing the CE Code, should be consulted in respect of each specific installation.