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

113 views asked Jun 6, 2015

A.      Total connected load on feeder

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

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
edited Jun 6, 2015