Mike Dolt Enterprises
Mike Dolt Enterprises
User: frank_inks, Location: Olathe, Kansas
Asked on: Saturday, April 30, 2011 at 4:30 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:49 am EDT
Question: helo guyzi just want to ask for opinions about wiring a feeder circuit from the main electrical room to the buildings...uhmm on what type of conduit, conduit fill, method of embedding conduits underground through a manhole, and also on how to compute its voltage drop, with all possible factors to be considered...thanks in advance for ur ideas..
## Wiring a Feeder Circuit for Underground Distribution### Conduit Selection PVC or HDPE conduit
Durable, corrosion-resistant, and suitable for underground burial.
Diameter
Based on the number and size of conductors, conduit fill requirements, and bends.### Conduit Fill
NEC Article 310.15(B)(1): Maximum cable fill for any raceway is 40%. Adjust for ambient temperature
Increase fill percentage for higher temperatures.
Consider derating
Reduce allowable fill percentage for continuous load conditions.### Underground Conduit Installation
Trenching
Excavate trenches at least 24 inches deep and maintain proper slope for drainage.
Manholes
Install manholes at intervals for access, splicing, and pulling cables.
Conduits through manholes
Protect conduits with sleeves or other approved methods.
Bending
Bend conduits smoothly using approved methods to avoid kinks or damage.### Voltage Drop Calculation
Factors to Consider
Conductor length Conductor size Load current Power factor Ambient temperature Soil resistivity Conduit length factor
FormulaVoltage Drop = (2 × Conductor Length) × (Load Current) × (Conductor Resistance) × (Length Factor)
Conductor ResistanceConductor Resistance = (Resistivity × Conductor Length) / Conductor Area
Length Factor
Based on conduit type, soil conditions, and conductor spacing.
Corrections for Temperature and Power Factor
Temperature
Adjust voltage drop by a factor from NEC Table B.310(A)(1)
Power Factor
Multiply voltage drop by the inverse of the power factor (e.g., for a power factor of 0.8, multiply by 1.25)
Example
Calculate the voltage drop for a 100-amp feeder circuit from the main electrical room to a building 100 feet away using #4 AWG conductors in a PVC conduit underground.
Data
Conductor length = 100 feet (x2 = 200 feet) Load current = 100 amps Conductor resistance (from wire table) = 0.25 ohms/1000 feet Length factor = 0.9 (from NEC Table B.310(A)(1)) Ambient temperature = 30°C Temperature correction factor = 1.11 (from NEC Table B.310(A)(1))
Voltage DropVoltage Drop = (2 × 200 feet) × 100 amps × 0.25 ohms/1000 feet × 0.9 × 1.11= 9.89 volts
Conclusion:
The calculated voltage drop is 9.89 volts. This value is within acceptable limits for a 100-amp feeder circuit. It is important to adhere to NEC requirements and industry best practices to ensure a safe and reliable electrical distribution system. Flag for review