ELECTRICAL TIP OF THE
MONTH SEPTEMBER 2008 ISUUE # 4 CABLE SIZING
CRITERIA
Author: Ahsan Javaid Approved by: Greg Drew
Cable Sizing
Criteria behind the lection of cable type and size for an
electrical system.
Background Cables are a highly reliable and compact medium
for the transmission of power. Power at all voltages
common to the oil and gas industry can be
transmitted efficiently by using single or multi-core
cables. The lection of power cable and wire types
along with the sizing of conductors for any given
application is an esntial part of the design of a
power system.
The most common conductors ud are copper or
初中英语作文aluminum. Aluminum is infrequently ud in the
oil and gas industry becau it work hardens during
installation, has higher loss, has high voltage-
drop at rated current and requires a special
termination procedure. Copper on the other hand
provides high current capacity for a given size, is遵守游戏规则
good for terminations, has a lower coefficient of
expansion and is less susceptible to corrosion.
The cost of copper is approximately three times
that of Aluminum, therefore, Aluminum should be
considered for longer cable runs where cost savings
justify the additional engineering and installation
practices.
Cable Selection and Sizing
Oil & gas industry plants tend to be upgraded and
expanded during their lifetime. As a rule of thumb
a cable/feeder should be sized bad on all known loads at the plant design stage plus a contingency
for future expansion. Hence a contingency of
typically 15% to 25% should be added to the
conductor/feeder current.
The design and lection process is restricted to a few application tables for sizing cables that are
bad merely on experience and simplification of
the criteria involved. Therefore it is very important
to have a process for the lection and sizing of
郁郁cables that incorporates performance, regulatory, and economic factors while minimizing the engineering design cost. The following are the variables that need to be assd before lecting and sizing a cable, however only the first three will be discusd: ¾ Ampacity Considerations ¾ Volt儿媳
age Drop ¾ Short Circuit Current Rating ¾ Overcurrent Protective Devices ¾ Insulation Material ¾ Economic Consideration Ampacity Considerations The purpo of evaluating ampacity is to make sure that the operating temperature of the conductors stays below the design limit of the insulating material. Therefore, to correctly calculate the conductor size for ampacity the following should be taken into consideration: Load Characteristics ¾ Maximum continuous current ¾ Harmonic content of load current Wire/Cable Characteristics ¾ Type of wire/cable (single conductor, triplexed singles, multi-conductor cable) ¾ Insulation temperature rating ¾ Conductor material (copper or aluminum) ¾ Shield for medium voltage cables to be grounded at one or both ends Raceway Characteristics ¾ Type (conduit, tray, duck bank, etc) ¾ Number of power carrying conductors in an individual conduit ¾ Are the conduits/trays banked or thermally independent ¾ U of fire stops or other penetration barriers and any risk bad approach for cables/cable trays.
Ambient Considerations
¾Ambient air or soil temperature for the entire circuit route (including isolated “hot spots”)
¾Soil thermal resistivity (for duct banks or direct burial)
¾Exposure to heating by solar radiation (or other infra-red radiation source)
Once the above areas are evaluated, the associated ampacity tables and criteria can be applied to the conductors. Allowable ampacities (Al or Cu) for cables ud in the oil and gas industry are bad on 90o C insulation ratings.
If any part of the circuit is in an area where the ambient temperature is above 25o C, 30o C or 45o C (bad on manufacturers ambient ratings) derating factors must be applied. Derating factors (DF) are available for the following cas:
¾DF due to ambient air temperature
¾DF due to ground temperature
¾DF due to thermal resistivity of the ground
¾DF due to grouping cables together
If a cable experiences various environments along its route, then the environment that caus the most derating of the rated current should be taken and ud for the entire cable.
Voltage Drop
The voltage drop in a cable is due to its ries resistance and ries inductive reactance. The shunt capacitive reactance is usually too large to be considered for cables in a typical plant. The actual voltage received by a load at its terminals must be taken into account when lecting a suitable cable size.
The CEC recommends the voltage drop in a feeder and branch circuit combination to be 5 %, with a 3% maximum in each. For example, if there exists a power feed from a 480V unit substation to a MCC, from the MCC to a 480V power panel, and then from the power panel to the load. The allowable 5% would need to be split up among each of the three circuits. In order to determine the voltage drop the following should be taken into account:
¾Conductor material and preliminary size插画专业
¾Resistance and reactance of associated wire/cable
¾Steady state design load current and power factor ¾Design inrush (starting) current and power factor
Short Circuit Current Rating
In addition to voltage drop and ampacity considerations we need to make sure that a wire/cable is sized so that cable insulation is not damaged during a short circuit. This is an important concern for large cables that are protected by large circuit breakers which require veral cycles to operate in ca of a fault. During this time the temperature of the conductor ris significantly. To prevent insulation failure the following should be taken into consideration for a specific wire or cable lection:
¾The conductor material and size
¾The emergency temperature rating of the conductor insulation material
¾The maximum available short circuit current
¾The time require by a circuit breaker (or other protective device) to clear the fault.
Attached figures provide the clearing time
associated with maximum short circuit for
insulated aluminum and copper conductors. CEC Vs IEEE Cable Ampacity Tables
There exists a disagreement between the CEC and IEEE Standard 835 (The Okonite Company) methods of calculation for cable ampacities. The IEEE ampacities are consistently greater than tho of the CEC, and therefore, lead to the option of using smaller cable sizes for shorter cable feeds.
Generally, using IEEE ampacities allows the u of a conductor two sizes smaller than is required by CEC. This size advantage applies to installations where the desired cable length is less than approximately 100 meters (depending on the size of motor).
Where project economics dictate a large number of cables, the ur should investigate the regulatory implications and lect the appropriate ampacity tables for the project.
Applicable Standards & References:
1.IEEE Std. 399, 835青椒鸡丁
2.CEC 26-258, 28-106, 28-108, 28-110
3. A. Sheldrake, “Handbook of Electrical
Engineering - For Practitioners in the Oil,
Gas and Petrochemical Industry”, John Wiley & Sons (2003).
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Figure 1 CEC and IEEE Conductor Ampacities
Notes:
1)The values in the two columns are taken directly from CEC Tables 2 and 4.
2)The ampacities are bad on a three conductor underground direct burial configuration. The
信息安全工程师values are obtained from ampacity tables produced by The Okonite Company and are
subquently adjusted to reprent an earth ambient temperature of 30˚C.
3)The values are bad on a three conductor in a conduit (in air). The values are obtained
from ampacity tables produced by The Okonite Company and are subquently adjusted to
reprent an air ambient temperature of 30˚C.
Figure 2 Plot of CEC and IEEE Ampacities for Copper Cable
Figure 3 Plot of CEC and IEEE Ampacities for Aluminum Cable
