Lesson 3



          Since branch circuits are the conductors from the utilization equipment or load back to its overcurrent protective device.  What are the conductors that feed the panel from the service entrance equipment?  Feeder conductors originate from the main Service disconnect and extend to the panelboard which contain overcurrent devices protecting each piece of utilization equipment.  So what are the Service entrance conductors?  These conductors originate outside and supply the premises to be served up to the main disconnecting means.  Many dwellings only disconnect source is in a panelboard protecting the premises wiring by a 100 a, 150 a, or 200 amp breaker.  In this situation it is not uncommon not to have any feeder conductors.  If the Service entrance conductors enter into a main disconnect or service switch with no overcurrent protection protecting any utilizations equipment and then attached to the load end of the switch it feeds a panelboard next to it, these wires described would then be classified as feeder conductors.

          Now understanding the differences of service entrance conductors, feeders and branch circuits we need to look at the maximum voltage drop permitted.  Voltage drop is affected by the:

·        Length of the feeder and the branch circuit allowing a total of 5% voltage drop between the two.  (3% and 2% or 2% and 3%)

·        Specific resistance or K of material(cu or al)

·        Size of conductor(cma)

·        Temperature factors (ambient temp. and temp. termination rating)


Volt drop = I (load) x Rw(resistance of wires)


Volt drop = 2 KIL/cma x R

2 = # of wires or for 3 phase use 1.73

K=specific resistance

I = load

L = length of 1 wire

CMA = circular mils area

R = resistance of circuit


The % volt drop is the difference or the change in voltage divided by the source voltage multiplied by 100.  Therefore if the source voltage is 120v and the load voltage is 118v then (Et – V Load / Et) x 100 = (120v – 118v / 120v) x 100 = (2v / 120v) x 100 = 1.6% volt drop.  The actual volt drop is 2v = 120v – 118v giving a percentage of 1.6% volt drop compared to the source voltage.  Remember the voltage drop of a feeder should never exceed 3% leaving 2% on the branch circuit nor should the voltage drop of a branch circuit never exceed 3% leaving 2% on the feeder.  Therefore the 1.6% is legal however even though 5% - 1.6% = 3.4% THIS DOESN’T MEAN I am allowed to have a volt drop of 3.4% on the other.  I still cannot exceed 3%.


Service Entrance conductors can further be classified as either a service drop or service lateral.  Service drop conductors are connected above ground where a service lateral is installed underground.  There are certain requirements for both; however height restrictions due to pedestrian and commercial traffic must all be followed in order to not introduce a fire to the occupancy and surrounding areas.  Even communication circuits, coax and other cable lines should not be attached to the service entrance conductors or service entrance conductors raceway due to lower height restrictions on those low voltage cables.  If those cables are permitted to be lower than our conductors and are attached to our service mast, the possibility of a truck coming in contact with those cables increases.  This contact can pull down our raceway enclosing our service entrance conductors and possibly introduce a fire to the premises.  If only the neutral or grounded conductor disconnect and the grounding electrode conductor inside the house is absent or disconnected think of the terrible possibilities. 





          All branch circuits and feeders must be protected.  The service entrance conductors are not protected however should be sized according to the loads connected as well as applying certain demand factors.  The load on a branch circuit is considered a continuous load if the load (for ex. water heater or fluorescent) is expected to operate for 3 or more hours.  If the load is continuous both the conductor size and the overcurrent device should be sized at least 125% of the load current.  Let’s say you are on a service call.  The customer had complained that there plug in heater works for a little while, then the circuit trips.  You know it’s a 15 amp separate circuit using #12 awg wires supplying the heater.  On the heater the load current that it draws is 15 amps.  So you install a 20 amp circuit breaker and it holds.  How should you have sized or made sure that the breaker will hold?


15 amps x 125% = 18.75 amps= conductors & fuses minimum ampacity




20 amp overcurrent device/80% = 18.75 amp maximum load ampacity


Remember 1/125% = 1/1.25 = .8

And .8 x 100 = 80%


          Circuit Breakers and Fuses both have advantages and disadvantages.  They are both known to provide overcurrent protection.  The current squared and the duration of time subjected to that current both have an effect on the fusible element located within the fuse (I x I x time).  Thus the fuse does not open when the load or current exceeds the fuse value nor it be used for switching a circuit.  The Circuit Breakers on the other hand will discontinue power if the load or current draw of the circuit should exceed the ampacity of the breaker and may be used for switching capability. 


Fuses come in two different types, plug fuses and the cartridge fuse.


Types of Plug fuses:    0-30a (0-125v L-L or 0-150v L-N)

·        Edison base fuses

·        Type S fuses

Types of Cartridge fuses:  0-600v

·        Ferrule             0-65a

·        Knife-Blade     70-600a

·        Bolt                601-6000a


Every switch manufactured will only accept a certain type of fuse depending on the voltage rating and ampacity of the switch.  This will reduce the chances of inserting a fuse rated for a higher voltage or ampacity than that of the switches voltage rating or ampacity thus reducing the potential of overloading the switch and its terminal ratings.  When calculating loads you must also account for the combined load of the circuit in order to determine the fuse type to be used as well as the size disconnect capable of handling that particular fuse.

 When starting a job use the NEC book to:


·        Plan your attack (service requirements) Chapter 1

·        Coordinate and Design your system.  What are the minimum code requirements of Chapter 2 and are their any other equipment to be installed of Chapter 4.  Chapter 4 and 2 must be understood before derating or applying code calculations of Chapter 2.

·        Determine the wiring methods to be used.  Chapter 3

·        Implement your Plan and Design by building the project



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