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

Answer 1

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%