Exercise 6: General Purpose Circuits
Objective
At the end of this exercise students should be able to construct general purpose circuits following National
Electrical Code recommendations.
Introduction
The National Electrical Code (NEC) set out recommendations to standardize the implementation of electrical
systems in order to ensure safety and protection of life and property. Among all the information included in the
NEC are guidelines for the installation of electrical branch circuits, including material selection. (See Exercise 3.)
There are three main classifications for the type of electrical branch circuits commonly found in agricultural
applications.

General purpose circuits

Special purpose circuits

Circuits for motor loads
This exercise will deal with the first of these categories: general purpose circuits. Subsequent exercises will
deal with special purpose circuits (Exercise 10) and circuits for motor loads (Exercise 11).
In very simple terms, general purpose circuits are used for loads at 120V and up to 1,800W. These include
typical lighting systems, dual convenience outlets (DCO) and other small loads. A 20A circuit breaker and 12AWG wire are recommended for these circuits. Finally, a general purpose circuit requires 3 conductors: “hot”,
neutral and ground.
Before moving on, let’s take time to briefly review and explain all the information included in the paragraph
above.
Service Entrance Panel
We will soon study the AC power system. We’ll there how electrical
energy is generated and distributed to the points of use by the local
authority.
At the point of use, each consumer receives electrical power
according to his/her own specific needs. For residential or farm use,
electrical power typically comes in the form of two “hot” wires and a
“neutral” or “return” wire. These go through a power meter, to
determine energy consumption, and reach the service entrance
panel.
The service entrance panel or distribution panel is a box connecting
the wires delivered by the authority to the individual circuits providing
power to the various loads at the farm or dwelling. This connection
happens through devices called circuit breakers which are nothing
more than specially constructed switches.
Breakers are rated to a given voltage and current. Their current carrying capacity, called ampacity, is typically
displayed on the breaker as a number. For example, the breaker in the figure shows a number 30. This means
the breaker ampacity is 30A. In other words, the breaker will let up to 30A pass through before triggering itself
off.
Circuit breakers have a dual function, they safely connect the AC power system to the individual circuits while
protecting the later from excess currents caused by short circuits or overloads.
Each “hot” wire from the AC power system carries one of the three 120V phases of the electrical power signal
from the panel to the load. The neutral wire returns the current from the load to the panel and connects to
ground at the transformer and at the service entrance panel.
Wire gage
A previous exercise covered the subject of construction materials, including wire gage. Summarizing, wire size
or gage depends of the amount of current to be delivered and the distance between the distribution panel and
the load (maximum allowable distance to carry the current to keep voltage loss under 2%).
The recommendation for general purpose circuits is to use 12-AWG wire. At 120V and 1,800W, the maximum
current would be 15A. For this current the maximum distance between the panel and the first load that would
keep the voltage losses below 2% is less than 50 feet.
When talking about wires for electrical circuits it’s important to keep in mind the color conventions. White wire is
for “neutral” conductors. Use green or bare wire for grounding. Hot conductors should have black insulation.
Yet, there are instances when several hot wires go together in a conduit or reach the same box. In order to
keep correct track of the circuits it is allowed to use other color wire) red, blue, yellow) for “hot” conductors. The
use of white wire for “hot” conductors is only allowed if the ends are properly marked with black electrical tape.
“Hot” wires carry electrical energy to loads while neutral wires take it back to the distribution panel. Grounding
conductors exits to protect life and property by removing any electrical energy from places where it’s not
supposed to be. These include conduits, boxes, metal frames and covers on machinery, and load controllers.
Currents flowing through the grounding conductor go back to the distribution panel and directed to the earth via
a grounding rod.
Load controllers are devices to turn loads on and off. The most common type is the switch.
There are hundreds of different types of switches. The preferred controller for general purpose
circuits is the single pole switch. It comes rated for 15A and 20A. The latter is recommended for
agricultural applications.
Notice that the single pole switch has three screws to secure conductors. One, marked green, is for grounding.
The ground conductor should be connected here for safety purposes. The other two are for the hot conductor
(one input from the panel and one output to the load). The neutral conductor never goes to a switch. It goes
directly from the panel to the load
Test 1: Single Pole Switch Controlling a Single Load
1) Connect the circuit as per the following diagram. Have the instructor verify your circuit before
powering it.
2) After the instructor reviews the circuit, power it up by toggling the breaker. Turn the switch ON and
OFF to verify the circuit works properly.
3) Power off the circuit and disconnect from supply.
Test 2: Single Pole Switch Controlling Two Loads
1) Connect the circuit as per the following diagram. Remember that loads connect in parallel to
ensure they get the same voltage level. Have the instructor verify your circuit before powering it.
2) After the instructor reviews the circuit, power it up by toggling the breaker. Turn the switch ON and
OFF to verify the circuit works properly.
3) Power off the circuit and disconnect from supply.
Test 3: Two Individually Controlled Loads in the Same Circuit
1) Connect the circuit as per the following diagram. Have the instructor verify your circuit before
powering it.
2) After the instructor reviews the circuit, power it up by toggling the breaker. Turn the switches ON
and OFF to verify the circuit works properly.
3) Power off the circuit and disconnect from supply.
Test 4: DCO circuits
1) Connect the circuit as per the following diagram. Have the instructor verify your circuit before
powering it.
2) After the instructor reviews the circuit, power It up by toggling the breaker. Connect a load to each
outlet to ensure it works properly
3) Power off the circuit and disconnect from supply.
Review Questions
1) What is a general purpose circuit?
2) What is a general purpose circuit used for?
3) Why do we need circuit breakers?
4) How do we know the ampacity of a circuit breaker by looking at it?
5) On which conductor do we connect switches?
6) What are pigtails?
7) Why are single pole switches called that way?
8) Prepare a diagram to show a single circuit. The circuit powers a two outlets and a lamp. The
lamp and one of the outlets are controlled by a single pole switch. The other outlet is
uncontrolled.