120V 240V Electricity explained – Split phase 3 wire


– Hey there guys. Paul here from theengineeringmindset.com. In this video we’re going to be learning how three wire split
phase electricity supplies work to get 120 and 240 volts. We’ll look at how the electricity gets from the power station
over to the property and then how it’s connected
around the property and each of the main components. Now, this system is used in North America so we’ll be using their
terminology and colour coding. If you’re from outside this region, then you can still follow along but your electrical system
will work differently. We’ve covered that in a separate video. Do check that out. Links are in the video
description down below. Remember electricity is
dangerous and can be fatal. You should be qualified and competent to carry out any electrical work. So, electricity is generated
at the power station which is usually located far away. The power station generates
AC alternating current and is connected to a step-up transformer. This transformer increases
the voltage to reduce losses and is connected to the grid. The grid carries high-voltage electricity over long distances, over
to the towns and cities. Once it reaches the towns and cities, it will enter a step-down transformer which will decrease the
voltage to a safer level. From here it will be distributed locally into smaller circuits on different streets or groups of properties. Connected to these distribution cables will be smaller transformers, usually pole-mounted which
again reduce the voltage down even further to a level safe
enough for residential use. On the property will
be an electricity meter which will quantify how much electricity has been used and the
electricity company will use this to invoice the property. The transformer will be connected
to the electricity metre by some cables which will either run above ground or underground. These cables will be two hot
wires and a neutral wire. Inside the transformer we
have two coils of wire. The primary coil is connected
to the power station and the second coil will be
connected to the property. The two hot wires are
connected to each end of the secondary coil and the neutral is connected
to the centre of the coil. Now, don’t worry about
that too much for now. We’re going to look at this
again later in the video to understand it. So, if we zoom into the property, we find a main service panel
which is sometimes called a load centre or breaker box. If we remove the cover and look inside, we first find the main breaker. This is usually at the top of the panel but it might be at the bottom. The two hot wires from
the electricity metre will connect directly to the
lugs on the main breaker. Coming out of the main breaker
will be two main bus bars. These are basically exposed metal sheets which carry electricity
to the circuit breakers. Notice I’ve shown the current flowing backwards and forwards. That’s because this is AC
or alternating current. These bus bars as well as
the lugs are not insulated, they are live or hot. The main breaker can be manually
flipped to cut the power to everything downstream
of the main breaker. The main breaker will also
provide over current protection to the property. It is rated to handle a certain
amount of electrical current passing through it, typically
between 100 and 200 amps. If this value is exceeded then
it will trip automatically to try and protect the property
and its electrical circuits. Inside the panel, we also have
a neutral and ground bus bar. This is basically a strip
of metal with lots of holes and screws in it. The neutral and ground
wires will sit in the holes and the screws will lock them in place. In this example, we have a block on either side of the panel. As this is a main panel, the two bus bars can be joined together so we have a connector bar between them. That way we have a shared
neutral ground bus bar. Sub panels must have their bar separated but that’s a topic for a separate video. From the electricity metre, we’ll have the neutral
wire connected to the lug on the top of the neutral ground bar. Notice the green screw. This is bonding the neutral
bar to the metal casing of the service panel. The purpose of the
neutral bar is to return the used electricity
back to the transformer. It does actually get a little
bit more advanced than that but we’re gonna look at that in a more advanced complex video, this is just covering the basics. So, the two hot wires will
provide the electricity and once it is used it will
return to the transformer via the neutral bar. It is still AC alternating current but to make it easier to visualize, I’ve only animated the current
flowing in one direction so you can see the path it will take. Now, if we were to take our multimeter and connect one lead to the bus bar and the other lead to the neutral bar, we would get the reading
of around 120 volts. If you don’t already have a multimeter then I highly encourage you
to get one for your toolkit. It’s essential for any electrical
testing and fault-finding. Links down below for which
one to get and from where. If we connect the multimeter
leads to the other bus bar and the neutral bar, we would again get a reading of around 120 volts but if we connect the multimeter
leads to the two bus bars then we get a reading of double
that at around 240 volts. So, why is that? What’s happening here? So, when we look at how the transformer is connected to the main panel, we have the two hot bus
bars connected to either end of the secondary coil in the transformer and then we take the neutral bus bar connected to the centre
of the secondary coil. So basically, when we
connect across the bus bar and the neutral bar, we’re only using half of the coil. So, we are only picking up
half the electrical voltage the transformer can provide. So, that way we get 120 volts. When we connect to the two bus bars, we’re connecting to the
full length of the coil. So, we’re picking up the full voltage which the transformer can provide. Therefore, we get 240 volts. If you want to learn
how transformers work, then check out our video
on transformer basics. Links for that down in the
video description below. Now, coming back to the panel
connected to the bus bar, we’ll have our circuit breakers. These will look something like this with is black plastic casing
and a toggle switch on top. The circuit breaker controls
the flow of electricity into individual circuits in the property. It can be manually
tripped to cut the power but it also has two important features. The first feature is overload protection. The circuit breaker is rated to handle a set amount of electrical current. When appliances or lights
are connected to the circuit they will each increase
the current in the circuit. If too many things are
plugged in and turned on then eventually the current will be more than the breaker can
handle and the breaker will automatically trip to cut
the power off to the circuit and protect the property. The second feature is
short-circuit protection. When the hot and neutral
come into direct contact with each other, the current will dramatically
increase almost instantly. When this occurs, it
creates a magnetic field which will trip the breaker and
cut the power automatically. Let’s have a look at
how the circuit breaker is connected to the electrical circuit. In this example, we will connect
to a simple light fitting which is controlled by a switch. We take the hot wire
from the circuit breaker and run this to the switch. We then run another wire from the switch and over to the light fitting. From the light fitting
we have a neutral wire which carries the return current back to the neutral bus bar. We take the ground wire
from the metal casing of the sealing box and the switch and we also join this
to the neutral bus bar as in this case it’s shared. The purpose of the hot wire is to carry the electrical current
over to the light fitting. The purpose of the
neutral wire is to carry the used electrical current
back to the main panel and then back to the transformer. The purpose of the ground
wire is to provide protection for a fault current. If for example, the hot wire came loose and touched the metal
casing of the light fitting, the ground wire provides
a low resistance path back to the panel. Without this path electricity
could flow through you if you touch the metal box. As the current flows
through the ground wire, it would increase the current and that will trip the
breaker automatically. So, the electricity flows
through the hot wire, through the main breaker,
down the main bus bar and into the circuit breaker. From there it flows along the hot wire, across the switch and light then back along the neutral wire and into the neutral bus bar
along the neutral bus bus wire and back to the transformer. I’ve animated this using
AC alternating current but to make it easier
to understand the path, I’ve shown it flowing in
a single direction now. We’ve covered lighting circuits in detail in a separate video. Do check that out. Links in the video description below. Okay, so what else might we find here? Well, we might find a
double pole circuit breaker which will let us connect to
both bus bars to get 240 volts which we can use the
power larger appliances like dryers, ranges and
air conditioning units. If we look at the dryer circuit example, we run the red hot wire
from the circuit breaker which is connected to the
main bus bar number two and we run this to the receptacle. Then we run our black hot
wire from the other terminal of the circuit breaker which
is connected to bus bar one and we connect that to
the receptacle also. In this case, we have the neutral wire connected between the neutral
bus bar and the receptacle which will allow us to get
either 120 or 240 volts from the outlet. Then we have a ground wire
to provide a safe route for any fault current. Now, we can either connect
across the two hot wires for our 240 volt connection or between the hot wire
and the neutral wire to get 120 volt connection. We’ll also very likely
find a GFCI circuit breaker which stands for ground
fault circuit interrupter. This will look something like this and depending on the model you buy, you will usually have a
pigtailed neutral wire connected to it. GFCIs are required on certain circuits where outlets are used for
places like kitchens, bathrooms, hot tubs et cetera. You should check with the
National Electric Code for exact details. The GFCI breaker has both
the hot and the neutral flowing through it. This way you can measure the
current flowing from both wires and ensure they are equal. If we took a standard outlet, we would take a hot wire from the breaker and connect this to the outlet terminal. Then we take the neutral wire
and run this back directly to the circuit breaker to a
specific neutral terminal. We then connect the pigtail
wire into the neutral bus bar. This will provide the return path. And of course, we run the
ground wire from the outlet back to the neutral ground bar. If we look at this example,
the current is flowing normally until the guy sticks a
screwdriver into the socket. The electricity then flows through him instead of the neutral wire. The GFCI is measuring the
current in the two wires and notices that these are not equal. It then automatically trips
the breaker to cut the power and save the man’s life. We might also come across
an AFCI circuit breaker. This stands for arc fault
circuit interrupter. These are required for
circuits feeding bedrooms, hallways, kitchens et cetera. Again, check with the
National Electric Code for exact details. AFCIs work also by being connected to both the hot and neutral wires. Inside this circuit
breaker is a circuit board which is measuring the circuit and monitoring for patterns which indicate an arc fault is occurring. These are installed
pretty much identically to how we saw the GFCI breaker. Under normal conditions, the
current flows through the hot back through the neutral into the breaker and through the pigtail and
back through the neutral bar. But if for example, a screw
was accidentally inserted very close to the cable
and removed the insulation to expose the copper wires, the electricity could now potentially jump across or arc from the hot
wire and into the neutral. The arc is incredibly hot and causes most residential
electrical fires. As the arc occurs, it
creates a unique signal in the electrical cable. The circuit breaker can detect this and will automatically
trip to cut the power. Connected to the neutral ground bar will be a thick uninsulated copper wire which runs out of the bottom of the panel and off to the ground rod
which is pushed into the earth near the property. Under normal circumstances,
no electrical current will flow through this wire. Its purpose is to dissipate
high static voltages from things like lightning. This way the electrical
systems and equipment is protected from damage. Additionally, we will
also find a bonding wire to metal pipe work in the property. This is to provide a safe
route for electricity to flow should a hot wire come into
contact with a metal pipe and this way will prevent
a person being electrocuted if they were to touch the pipe work. Okay guys, that’s it for this video but if you want to continue your learning then check out one of
the videos on screen now and I’ll catch you there
for the next lesson. Don’t forget to follow us on
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