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What is the principal of feeder and branch circuit design in electrical system?

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113 views asked Jun 6, 2015 by Devan Talebi

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A.      Total connected load on feeder

  1. Calculate load for Convenience power, Mechanical system, Elevators, Escalators, Lab   equipment, Kitchen equipment, Appliances and lighting.


B.      Demand loads

  1. General floor power: per local code.
  2. Computer equipment :100%
  3. Motors: 100% + 25% of largest motor.   
  4. Elevators: per local code
  5. Laboratories: 75% of connected loads or 20 watts/square ft (if allowed by local code).
  6. X-Ray equipment: per local code.
  7. Appliances: Per local code.
  8. Lighting : 100%


C.      Spare Percent loads

  1. General: 25% spare for future loads.


D.      Cable size selection

  1. Cable size to be based on the following:
  • Maximum Voltage Drop of 2.5% from service to last distribution point. Maximum of 5% from service to load device.
  • Feeder serving continuous loads must have a cable ampacity equal to or greater than the feeder overcurrent protective device.
  • The ampacity of the feeder must equal or exceed the continuos load times 1.25 plus the non- continuous load. For feeders serving multiple motors size feeder on demands amps + 25% of largest motor.
  • Feeder serving elevator load to be sized on the connected load times the diversity factor per local code plus 40% of the largest elevator.


E.       Conduit size

  1. Conduit size shall be based on cable size and insulation type. Maximum 40% of conduit cross section fills. Refer to local Codes.


F.       Calculations and formulas

  1. Feeder Demand Amps= Connected Amps x Demand + Spare
  2. Voltage Drop= Impedence x Feeder demand Amps x Length
  3. V = 2 x R x L x I /CIRC.MILLS x 2

G.     Lighting Branch Circuiting

    a.  15A,120V circuits

1)   General: 1100 watts

2)   Maximum: 1300 watts

    b.   20A,120V circuits

1)   General: 1500 watts

2)    Maximum: 1800 watts

    c.    20A, 277V circuits

1)    General: 3500 watts

2)      Maximum: 4500 watts

Spare circuits: Approximately 25%     


H.     Receptacles Branch Circuiting

                                    1. Convenience Receptacle- 180 watts per receptacle per NEC:

    a.   20A,120V circuits per NEC

    b.   General: 6 outlets

    c.   Maximum: 10 outlet

2. Appliance Receptacles - check local electrical load for circuit limitations.

Spare circuits: Approximately 25%

answered Jun 6, 2015 by Devan Talebi
edited Jun 6, 2015 by Devan Talebi

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