Required Data for Cable Sizing - Intergraph Smart Electrical - Help - Hexagon

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The software performs cable sizing via a number of stages: thermal sizing, voltage drop calculation at normal running, and for a motor, voltage drop calculation at starting.

You must enter all the data required for the thermal sizing and voltage drop stages in order for the software to calculate the cable cross-section size. In addition, you can specify optionally for the software to perform short circuit sizing. For further details of the data properties, see the Cable Common Properties Dialog and refer to the appropriate dialog tab.

You can find this information in the Cable Algorithms.xlsm file:

  • In the Attachments tab of the Sidebar of the Electrical On-Line help

  • In the Smart Electrical program folder; \Program Files (x86)\SmartPlant\Electrical\bin\resdlls\0009\

  • You can create your own custom formulas for sizing cables. See Customize Cable Sizing Formulas.

  • The software calculates cable sizing by selecting a matching reference cable from the Reference Data Explorer, so only those reference cables that you created are available for selection. You should therefore ensure that you create in the Reference Data Explorer all possible cable types and sizes that you may want to use.

  • If you have changed the sizing data of a cable to such an extent that the current cable becomes unsuitable for its function, you need to manually resize the cable (select Calculate Sizing on the Sizing Data tab of the Cable Common Properties dialog). The software informs you about this by displaying an appropriate message in the Calculation information box on the Sizing Data tab of the Cable Common Properties dialog and by turning on the Sizing required flag (the software automatically selects the Sizing required check box on the General tab of the Cable Common Properties dialog). See Automatic Switch-On of the Sizing Required Flag.

Thermal Sizing

In the thermal sizing step, the software attempts to locate from the Reference Data Explorer the smallest size reference cable of the specified type and construction, with a de-rated ampacity equal to or greater than the full load current of the connected load. If the software cannot find a single cable that meets this requirement, it continues with groups of identical cables in parallel, from the minimum conductor size, up to the maximum permitted number of parallel cables. You can specify the minimum conductor size, the maximum permitted number of parallel cables, and the default suffix naming method in the Options Manager.

The software requires the following data for this stage of the calculation:

Cable specification (General tab)

Specifies the family of cables within which the software looks for a suitable reference cable.

Conductor arrangement (General tab)

Specifies the required conductor arrangement of the reference cable.

Full load current [FLA] (Sizing Data tab)

Specifies the nominal current for a load associated with the cable, or a value that you type manually.

Routing (Sizing Data tab)

Indicates the appropriate routing type for the current cable: InAir, UnderGround, or some other custom routing type that you define. The option that you select affects whether the software uses the ampacity value in air, in the ground, or custom basic ampacity when sizing a cable.

Conformity to standard

This property (set in the Properties window) determines the electrical standard, IEC or NEC that the software uses. If you select the NEC standard, the software sets the Code Factor flag to Yes . Enter a value for Code Factor and then select the Calculate Sizing button. If you do not enter any Code Factor value, the software cannot make the sizing calculation and it will display an appropriate message in the System Notes box.

De-rating factor / Utilization factor / Ambient temp. factor (Sizing Data tab)

Specifies properties that the software divides the nominal current by to obtain the de-rated cable ampacity.

Voltage Drop Calculations

The software calculates the voltage drop across the cable under full load conditions, based on the cable length and other factors and compares it with the maximum allowable voltage drop that you specify. The software attempts to locate a cable with a sufficiently large conductor size, based on the thermal sizing, which gives a voltage drop lower than the allowed maximum value. In addition, the software calculates the maximum cable length for which the voltage drop does not exceed the maximum allowable value. For a motor, the software also calculates the percentage voltage drop under starting conditions following the same methodology.

The software requires the following data for this stage of the calculation:

Cable Common Properties - General Tab

Reference cable

The reference cable you select provides, in addition to the basic ampacity values, the values of the Resistance per unit length and Reactance per unit length properties that the software requires for the voltage drop calculation.

Cable Common Properties - Design Data Tab

Estimated length / Design length

Indicates the length of the cable that the software uses to calculate the voltage drop. The software uses the design length if available. Otherwise, it uses the estimated length.

Cable Common Properties - Sizing Data Tab

Supply

Allows you to select the type of supply: alternating current (AC) or direct current (DC). If the cable is connected to a load for which you select the Consumer check box on the General tab of the appropriate load Common Properties dialog, the software copies the value from the load to this box. Note that the value of this property is automatically inherited from the load, converting equipment, or any other electrical equipment that this power cable is connected to.

The software calculates the cable cross-section size using different voltage drop formulas for AC and DC supplies.

Number of Phases

Select the number of phases in the power supply to the load: 1, 2, or 3. If the cable is connected to a load for which you select the Consumer check box, the software uses the value from the load. This property applies for an AC supply only. Note that the value of this property is automatically inherited from the load, converting equipment, or any other electrical equipment that this power cable is connected to.

Rated voltage

Allows you to select an appropriate operating voltage for a load associated with the cable. If the cable is connected to a load for which you select the Consumer check box, the software uses the value from the load.

Starting current

(This is a required property for motors and some other loads) If a motor is connected to the cable, the software enters the value of the locked rotor current (LRC). For a motor, the starting current value can typically be 400 - 600% larger than the full load current and is therefore important when evaluating the current requirements used to perform sizing. Note that this option is available for all power cables whether they are connected to motors or not.

Power factor at full load

Allows you to enter the power factor of the equipment that the cable feeds under normal full load running conditions. The power factor is the ratio of active power to apparent power and can have a value between 0 and 1. If the cable is connected to a load for which you select the Consumer check box, the software uses the value from the load. Note that if the power supply is DC, the power factor at full load and at starting is always 1 (one) and it cannot be changed.

Power factor at starting

Allows you to enter the power factor of the equipment that the cable feeds under starting conditions. If the cable is connected to a motor for which you select the Consumer check box, the software uses the value from the motor. This property is not available for loads other than motors. Note that if the power supply is DC, the power factor at full load and at starting is always 1 (one) and it cannot be changed.

Allowable at full load

Allows you to enter the maximum allowable voltage drop percentage when the connected load is running under normal full load conditions. The allowable voltage drop must have a value greater than zero. To ignore the voltage drop in the sizing calculation, type a value of 100.

Allowable at starting

Allows you to enter the maximum allowable percentage voltage drop when starting the equipment. The allowable voltage drop must have a value greater than zero. To ignore the voltage drop in the sizing calculation, type a value of 100.

Short Circuit Sizing

You can specify that you want the software to perform short circuit sizing by selecting the Use short circuit cable sizing check box on the Sizing Data tab. The software examines each reference cable that already meets the sizing requirements for the thermal and voltage drop criteria. Based on the short circuit parameters and conductor short circuit coefficients, the software calculates the minimum required conductor cross-section area for the cable in cmils and displays the value in the Required area (cmils) box. The algorithm compares the results of this calculation with the value in the Conductor Area cmils lookup table for the reference cable in question to determine whether the reference cable meets the short circuit sizing criteria. The software checks each reference cable in turn until it finds the smallest size cable that meets the short circuit sizing criteria. It is important that the lookup tables contain suitable values to enable the software to perform the calculation successfully.

Note that after changing the short circuit fault values, the software does not re-calculate the voltage drop automatically. This is a manual process that you have to start yourself.

The software requires the following data for this stage of the calculation:

Material (General tab)

Select the required material for the current-carrying conductors.

Insulation (General tab)

Select the required material used for insulating the current-carrying conductors.

Use short circuit cable sizing (Sizing Data tab)

You must select this check box to enable the software to perform the short circuit sizing calculation.

Duration (Sizing Data tab)

Type the maximum time for which the cable is expected to withstand a fault short circuit.

Fault current (Sizing Data tab)

Type the maximum current that the cable is expected to withstand in the event of a short circuit fault current.

  • The software displays in the Calculation information box the message 'Not found' under the following conditions: when no reference cable exists corresponding to the cable specification or arrangement, when there are missing values for ampacity, resistance, or reactance, or when the number of parallel cables exceeds the specified limit in Options Manager.

  • After selecting the Calculate Sizing button on the Sizing Data tab, the formulas and values used in the cable sizing calculations can be viewed on the Sizing Report tab.