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NEC Six Duct Configuration Example, (Page 7 of 8)

Voltage Drop, Load Flow, Duct Fill and Jam Calculations

To assist in underground cable system circuit design, in addition to the ampacity calculations, AmpCalc offers voltage drop, load flow duct fill and jam calculations. The former calculations are provided in each position data entry form, once the user has entered sufficient information to make the calculation.

Load the ‘ex1a.ugs’ file back into AmpCalc, go to the position data entry form for position 1 and note the voltage drop calculation result duplicated below.

Position Data Entry Form for ‘ex1a.ugs’

The voltage calculation remains at zero because a value greater than zero for ‘Specified Amps’ must be entered. This is the current that is assumed to be in the circuit for voltage drop and load flow calculation purposes.

Load file ‘ex1b.ugs’ file back into AmpCalc, go to the position data entry form for position 1 and note that a non-zero 3.9% voltage drop calculation is now depicted, since a non-zero entry for ‘Specified Amps’ now exists. This voltage drop is also based on a 1000 foot cable distance and a load power factor of 80% lagging.

Note that the duct fill and jam calculations are satisfactory. Change the duct size to 3.5" to obtain the following jam warning although the duct fill is still acceptable.

 

Voltage drop, load flow and Duct fill and jam calcs with 3-1/c per position and 271.6 amps per conductor.

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