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#101787 - 04/17/03 12:43 PM Underground cable ampacity
Ares Offline
Junior Member

Registered: 09/26/02
Posts: 3
The CEC is not specific what thermal resistance values of soil, concrete, wire insulation was used when computing the tables D8A to D15B.

Is that information available anywhere ?

What if the actual soil thermal resistance is 110?

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2014 / 2011 NEC & Related Books and Study Guides
#101788 - 04/17/03 01:36 PM Re: Underground cable ampacity
Tony Moscioni Offline
Moderator

Registered: 05/15/01
Posts: 144
Loc: CANADA
Ares,

These tables were computed based on the IEEE 835.

To determine underground cable ampacities, refer to the IEEE Standard #835, "Standard Power Cable Ampacities".

If the installation is exactly as illustrated in Diagrams B4-1 to B4-4 of Appendix B, use Tables D8A - D11B and D13A - D16B of Appendix D.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority

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#101789 - 04/21/03 10:37 AM Re: Underground cable ampacity
Ares Offline
Junior Member

Registered: 09/26/02
Posts: 3
Yes
Because these tables were COMPUTED based on the IEEE 835. some assumption about thermal resistance must have been made, otherwise one would not be able to compute anything.

The question is what was it?
was it 90°C-cm/W?

Can I use the tabels if thermal resistance for soil in my area is 120°C-cm/W

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#101790 - 04/22/03 06:46 AM Re: Underground cable ampacity
Tony Moscioni Offline
Moderator

Registered: 05/15/01
Posts: 144
Loc: CANADA
Ares,

Section 4
4-004


Underground ampacities for conductor temperatures of 75°C and 60°C respectively may be obtained by multiplying the appropriate ampacity at 90°C conductor temperature from Tables D8A to D11B and D13A to D16B by the derating factor 0.886 (for 75°C) or 0.756 (for 60°C).*

Ampacities for underground installations at ambient earth temperatures other than the ASSUMED VALUES of 20°C may be obtained by multiplying the appropriate underground ampacity obtained from Tables D8A to D11B and D13A to D16B by the factor:

SQRT [(90-Tae)/70]

Where

Tae = the new ambient temperature.*


Ampacities of underground installations based on conditions of use not as set out in the foregoing notes or the defined assumptions preceding them should either be justified by precise calculation according to the method of Paragraph (1)(d) or (2)(d), or derived in accordance with Paragraph (1)(b) or (2)(b).

*Where precisely calculated values are not available.


36-304(3)
The procedure required by Rule 36-304(3), Station Ground Electrode Design, may be found in CEA Report 249 D541, Simplified Rules For Grounding Customer-Owned High Voltage Substations.

NOTES:
(1) Table values calculated in accordance with IEEE Standard No. 80.

(2) A typical substation installation is designed for 0.5 s fault duration and the entire ground surface inside the fence is covered with 150 mm of crushed stone having a resistivity of 3000 W·m.

4-004(1) and (2)
IEEE Standard 835, Standard Power Cable Ampacity

Tables, and Tables 1 and 2

As an alternative to Rule 4-004(1) and (2) and related Tables 1, 2, 3, and 4, IEEE Tables 1 and 2 may be used for computing the ampacities of copper and aluminum conductors if the electrical inspection department is agreeable and the data submitted as stipulated below is satisfactory to the department.


The following data should be supplied, in writing, where it is desired to use IEEE Tables 1 and 2:

(e) Direct Burial Data
(i) Length and profile of route;
(ii) Depth of burial;
(iii) Spacing of cables;
(iv) Thermal resistivity and kinds of soil along the route and whether data are based on measurement and inspection or only assumed;
(v) Maximum ground temperature if in excess of 20°C at burial depth;
(vi) Proximity of other current-carrying cables or other heat sources with details;


Tony Moscioni
Electrical Inspector
Electrical Safety Authority

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