Understanding Electrical Voltage and Current: Exploring 120V Outlets in the US

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In summary: Earth wire, the current would flow through the Earth wire and out to the ground.In summary, a light bulb plugged into an outlet in the USA will use the maximum E.M.F. provided by the outlet, which in this case is 120 volts. If the light bulb is rated for use with a voltage higher than 120 volts, it will work, but it will draw more current than the outlet can provide. There is no maximum current for home outlets, but you should only plug in devices that are rated for use with 120 volts.
  • #1
salman213
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Hey guys I have a few simple questions:

So on a light bulb I have connected to my lamp it says "120 V" and 25 Watts. From the outlet we receive 120 V. If that is true is the light bulb using the maximum E.M.F. the outlet can provide (120V)? and therefore it is using a very low current (p=vi = 25=120i i=25/120 A)

Is that right?

if that is right, do all things made in US for example use 120 V? Or if we plug something in the max they can use is 120 V but the outlet provides anything below that as well.

What is the maximum CURRENT our home outlets can give?... (Is there a maximum)?

LASTLY

There is always a ground or neutral wire, i know it is there to prevent consequences from short circuits but when does the current flow through this wire? What causes the current to start going into the neutral wire? At what point does the current "know" to start flowing through the neutral wire? If I am not being clear please tell me.
 
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  • #2
salman213 said:
Hey guys I have a few simple questions:

So on a light bulb I have connected to my lamp it says "120 V" and 25 Watts. From the outlet we receive 120 V. If that is true is the light bulb using the maximum E.M.F. the outlet can provide (120V)? and therefore it is using a very low current (p=vi = 25=120i i=25/120 A)

Is that right?

if that is right, do all things made in US for example use 120 V? Or if we plug something in the max they can use is 120 V but the outlet provides anything below that as well.

lastly
What is the maximum CURRENT our home outlets can give?...

If I am not being clear please tell me.
Home convenience outlets in the USA are normally rated for use with devices designed for operation at 115 or 120 volts. The rating on the outlet device may be listed as 125 volts. The voltage applied to the terminals will be 120 volts nominal.

Things designed to be plugged into these outlets usually have a voltage rating of 120 volts. You will find some lamps rated at 130 volts. (Put these in a 120 volt circuit and they will last damn near forever.)

Whatever is plugged into the outlet will "pull" current according to its resistance.

A 100 watt lamp will pull .8333 amps because its resistance is about 144 ohms.

A 200 watt lamp will pull 1.66 amps because its resistance is about 72 ohms.

A 15 watt lamp will pull .125 amps because its resistance is about 960 ohms.

The lower the consumption rating of the driven device, the higher its resistance to current flow is. The higher the resistance is, the lower the current is. Thus a low wattage lamp has a higher resistance to current flow than a high wattage lamp. They are all driven by the same 120 volt supply voltage.

This is applicable to 120v outlets only. We also have incandescent lamps designed for 240 volts...and 277 volts.


You should NOT plug things into the outlet unless they are rated for 115/120 volts, especially if the rating is lower than 115 volts. Applying more voltage than the equipment is designed for will cause overcurrent which leads to overheating which leads to fire which leads to destruction of materials and possibly death.
 
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  • #3
salman213 said:
What is the maximum CURRENT our home outlets can give?... (Is there a maximum)?
It's limited by the fuse/circuit breaker to prevent too much current being drawn which would overheat the wiring.
Usually the US limit is 15Amps, but since this is at the fuse box it is shared by all the appliances on the circuit. Large appliances like ovens, washers, dryers will be on a separate 30A circuit which is often also 240V.
In the UK it is typically 32A at 220V for the circuit with a separate 13A fuse in the plug for each appliance. This is why electric kettles and electric heaters are more common in europe.

There is always a ground or neutral wire, i know it is there to prevent consequences from short circuits but when does the current flow through this wire? What causes the current to start going into the neutral wire? At what point does the current "know" to start flowing through the neutral wire?
You are confusing ground and neutral.
The neutral wire completes the circuit so the light bulb (or whatever) is connected between the incoming live wire and the outgoing neutral - exactly the same current flows in both.
The Earth wire is a separate wire which is usually connected to the neutral at the fuse box. It never normally carries any current but is connected to any external metal parts of the appliance. The idea is that if the live wire should touch the external metal (due to some fault) a large current will flow down the Earth wire which will blow the fuse and so disconnect the faulty appliance.
A more modern solution is an RCD which measures the current flowing in through the live and back through the neutral - if there is any difference it turns off the circuit. This stops any current flowing to Earth through a different path such as your body!
 
  • #4
so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.
 
  • #5
ok so the ground wires are connected to each load ?
 
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  • #6
salman213 said:
so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.
Yes - it's a little tricky because the resistance of the filament changes as it heats up and as it ages - the wattage is only an approximation.

ok so the ground wires are connected to each load ?
The ground wires are all connected together at the fuse box and then usually connected to a copper rod into the ground or a cold water pipe.
 
  • #7
I made a quick diagram if u can help me,

attached

can u give me an example of how a ground would be connected
 

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  • #8
The ground would be a separate wire from the -ve terminal of the supply to the metal case of the equipement.
At the negative terminal it would also be connected to the Earth (in the real world through a buried metal rod).
 
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  • #9
The load might be enclosed in a metal case, and the ground wire connected to that case. Somewhere else in the house, the ground wire and neutral are connected together (perhaps near the "source" in your diagram) and also (as mgb_phys mentioned in post #6) "usually connected to a copper rod into the ground or a cold water pipe."

Normally, the ground wire and metal case are not part of the circuit. But if the wiring inside the box becomes loose and touches the metal walls, that completes the circuit. Lots of current flows, trips the circuit breaker or fuse, resulting in no voltage being applied to the device.

It's a safety feature. Without that ground circuit, if the wiring in the box gets loose and touches the metal case, there is no complete circuit ... until somebody comes along and touches the box (which is in contact with the live wire) and electrocutes themself.

Edit added: just saw mgb_phys's figure. A picture is worth a thousand words!
 
  • #10
so when exactly then will current go into this ground wire?
like if there is too much current? but if there is too much current flowing into the load how does it know to start flowing in the ground wire? Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?

Thanks to everyone by the u guys are helping a lot
 
  • #11
salman213 said:
so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.
The wattage will determine the current draw at a specific voltage. For purely resistive devices (no phase shift associated with the current), the wattage will be the product of the voltage and the current.

W = E x I

We already know that E = I x R

so, in the W = E x I formula, replace E with I x R and you get

W = I x R x I = I^2 R

It is not so simple for inductive/capacitive devices because the power factor must be considered. When dealing with incandescent lamps, it is negligible.

salman213 said:
ok so the ground wires are connected to each load ?

The green ground wire is usually connected to the metal casing of the electric device, or to the mounting yoke on a receptacle. The connection to the casing of the device plugged in is through the rounded hole on the receptacle. The white neutral is connected to the lamp through the screw-shell and usually goes through the shorter of the two vertical slots on the receptacle. The hot wire is connected to the other side of the lamp filament, usually through the button on the bottom of the lamp and is wired through the longer of the two vertical slots on the receptacle.

The green ground wire is not part of the current carrying circuit as are the white neutral and the colored hot wire. The green ground wire and the white neutral are connected together at the main panelboard only.
 
  • #12
salman213 said:
so when exactly then will current go into this ground wire?

If the wiring becomes loose inside the case. It's not supposed to happen, but it can and does. Screws holding the wires in place can become loose. Soldered joints can crack. Most electric devices warm up when in use, then cool down when turned off. With heating and cooling comes thermal expansion and contraction. This places stress on wiring joints. After many times of the stress pulling on the joint or screw one way (while heating up) and then the opposite way (while cooling down), the joint can fail eventually. When that happens, you have a loose, live electrical wire inside the box.

If the loose, live electrical wire makes contact with the walls of the box, it's a safety hazard. A person who touches the box while standing on the ground (or floor) completes a circuit, and becomes electrocuted.

By making a path from the box to ground (via the ground wire), the circuit breaker is tripped and power is shut off. No more safety hazard.
 
  • #13
salman213 said:
so when exactly then will current go into this ground wire?
Only if there is a fault in the equiepment and the live wire touches the metal case.
A light bulb isn't a good example since there isn't ussually anything metal to connect the ground wire to. Modern light fittings often have a screw terminal in the plastic base to connect the ground wire to. This is completely pointless but is needed to meet wiring codes - this might be what is confusing you!
If you look inside a metal desk lamp or a toaster you will see a green/yellow (at least in europe) wire connected to the case.

like if there is too much current?
No, the fuse is there to blow if there is to much current. The Earth wire is needed to blow the fuse.
As Redbelly said, suppose there was no Earth and the live wire came loose and touched the case. There would be no path for the current and so the fuse wouldn't blow - but if you come along and touch the metal case the current can flow through you (bad!). The Earth wire is there so that if the live does touch the case it will flow down the Earth wire - this short circuit will pull a lot of current and blow the fuse.

Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?
there is less resitance directly down the Earth wire than through the load. The load must have some resistance to do work. The Earth wire is designed to have much lower resistance.

edit - don't know why the picture doesn't show up properly.
 
  • #14
salman213 said:
so when exactly then will current go into this ground wire?
like if there is too much current? but if there is too much current flowing into the load how does it know to start flowing in the ground wire? Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?




Thanks to everyone by the u guys are helping a lot
The circuit through the device (lamp, motor, lamp ballast, solenoid, whatever) is normally carried through insulated wires. They are insulated from contact with the supporting case either by insulation material or by physical separation. When the insulation breaks down due to age and excess heat, the circuit can conduct current through the metal case. It is at this stage (failure of containment of the current to its intended path) that the green ground wire becomes helpful. It carries the current to the grounding electrode outside the house and keeps it from going through people that may be touching the metal case. When the current through the multi-breaker (or fuse) becomes too high, the breaker will open (or the fuse will melt) causes the current flow to stop completely.
 
  • #15
OOOkkkk i never knew that. THANKS! So basically this ground we are talking about protects from US completing the circuit due to some reason as stated in ur postss ... I was always confused about thattttttt that's coooooool...

BUT in an outlet the "third prong," what is that?
 
  • #16
salman213 said:
BUT in an outlet the "third prong," what is that?

That is for the ground (or "Earth") wire which is connected to the metal case.
 
  • #17
salman213 said:
So basically this ground we are talking about protects from US completing the circuit due to some reason as stated in ur postss
Exactly!

BUT in an outlet the "third prong," what is that?
That's the ground, on the equipment (plug) side it just goes to the metal case, on the supply (socket) side it is the Earth wire going back to the fuse box and then into the Ground.

In most countries the earht prong is sligghtly longer than the other two, to guarrantee that it is connected first - so if the plug is only partly in your are sure the case is earthed BEFORE any power can be supplied.
 
  • #18
I SEEEEEEE I THINK lol...

I hope this is right to say... CHECK PIC :)??
I HOPE THAT IS RIGHT: IF IT IS

IS THE BLACK THE LIVE WIRE
GREEN THE NEUTRAL WIRE
THE RED THE GROUND WIRE:s
 

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  • #19
Yes, that's it. You also got the narrow & wide slots correct!
 
  • #20
AWESOME!

thanks to everyone who helped me, i was very confused about that for a while now.

THANKS A LOT!
 
  • #21
salman213 said:
I SEEEEEEE I THINK lol...

I hope this is right to say... CHECK PIC :)??



I HOPE THAT IS RIGHT: IF IT IS

IS THE BLACK THE LIVE WIRE
GREEN THE NEUTRAL WIRE
THE RED THE GROUND WIRE:s
In your outlet box,

Black is the hot wire (will knock you down if you touch it while you are grounded)
White is the current-carrying neutral wire (is grounded at the panel and will not knock you down if touched separately)
Green is the ground wire (is also grounded and will not knock you down if touched)
 
  • #22
Thats in real life right? those were actually referring to my diagram, i just picked random colours.but wait I see if you touch the black wire you complete the circuit since the "white wire is grounded).

But if you touch the neutral wire how does that create any potential difference. Same with green :s...
 
  • #23
salman213 said:
Thats in real life right? those were actually referring to my diagram, i just picked random colours.
At least in Isly's country.
Brown=live,blue=neutral, green/yellow=earth is standard for europe

but wait I see if you touch the black wire you complete the circuit since the "white wire is grounded).
If you touch the black wire you create the circuit through your feet and the ground - you don't need to touch any other wire.

But if you touch the neutral wire how does that create any potential difference. Same with green :s...
Correct, if the device is correctly wired both the neutral and Earth wires are at (almost) 0 volts and are safe.
But since you never know if a house has been wired corectly it's safest not to touch any of them unless the fuse is removed!
 
  • #24
mgb_phys said:
Correct, if the device is correctly wired both the neutral and Earth wires are at (almost) 0 volts and are safe.
But since you never know if a house has been wired corectly it's safest not to touch any of them unless the fuse is removed!
if the fuse is removed there is no other grounding going on anywhere else that would cause u to be electrocuted if you touched the black wire?
 
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  • #25
Correct, if the fuse is removed the black (live) wire isn't connected to anything and so is safe
 
  • #26
isly ilwott said:
In your outlet box,

Black is the hot wire (will knock you down if you touch it while you are grounded)
White is the current-carrying neutral wire (is grounded at the panel and will not knock you down if touched separately)
Green is the ground wire (is also grounded and will not knock you down if touched)


so u cannot touch any ONE wire at all ..safely?
 
  • #27
mgb_phys said:
Correct, if the fuse is removed the black (live) wire isn't connected to anything and so is safe

at each transformer isn't it grounded before coming to your home so isn't it always going to be grounded even if u remove the fuse?
 
  • #28
at each transformer isn't it grounded before coming to your home so isn't it always going to be grounded even if u remove the fuse?
The neutral wire is grounded at the transformer. The live wire comes from the power line, through the fuse and to your appliance. If you remove the fuse it's like disconnecting the live wire from the power line.

so u cannot touch any ONE wire at all ..safely?
In theory only the black wire is live, the neutral and ground are both at (or near) 0 volts.

But since you never know if the socket has been wired correctly it's always best to treat all the wires as potentially dangerous, remove the fuse and check them with a meter before touching anything.
 
  • #30
salman213 said:
in between the source and outlet on the live wire there should be a fuse right?
Yes

and the source is acting like the transformer basically... is that right?
Well the power station + powerline + transformer - yes.
 
  • #31
Ok Thank You very much to every individual who helped me. I appreciate it greatly. THANK YOU!

If I have anymore questions I won't hesitate to ask :D.
 
  • #32
One thing I don't get...

If Ground is connected to the Neutral Bar at the Breaker Box, which it is.
(in residential)...what is stopping the current of that branch circuit on the neutral side from ground out instead of returning to the source?

The Ground is being "spurred" off of the neutral bar...and providing the path of least resistance...right?
 
  • #33
XPTPCREWX said:
One thing I don't get...

If Ground is connected to the Neutral Bar at the Breaker Box, which it is.
(in residential)...what is stopping the current of that branch circuit on the neutral side from ground out instead of returning to the source?

The Ground is being "spurred" off of the neutral bar...and providing the path of least resistance...right?

It'd only return through ground (the wire or physical ground) if the ground were actually in the path of the current (as is the case when you have a wiring fault or a case short). Think of it as being two paths to ground, one lower resistance than the other, but in which only one (the higher resistance "neutral") is usually connected.

The whole ground wire thing is used as primarily as a safety feature, and as a means of having the same "ground" everywhere in your wiring (which might not be the case from the top of your house to the bottom--potentially, a lot of potential between these points). Some companies cheap out and use only two prongs, connecting neutral to case resulting in an extremely dangerous situation if the wiring is reversed at an outlet, and the whole case is live instead of neutral/ground. MORAL OF THE STORY: AC is AC, and reversed hot and neutral may allow what you're using to work, but stay safe and make sure hot and neutral aren't reversed and that ground is actually connected (it's a $10 plug-in tester at any hardware store).

To keep from fear-mongering that all two-prong==bad, two prong live-and-neutral is okay if the case isn't metallic (and the electrical bits well protected) or if there's an isolation transformer being used which keeps the entire unit isolated (no external paths to ground).
 
  • #34
Ok you that's actually somewhat my question now too..

how is the ground less resistive than a load? wouldn't the ground have a HIGHER resistance than any load.

can someone answer my diagram
 

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  • #35
I think you may be confusing "ground" (source of electrons) with the ground wire. Ground is not less resistive than a load because ground is where the current goes to through the neutral wire in the end (if you were to cut the wire at any point, the resistance to ground of the ground wire will be similar to the resistance of the neutral wire). You're right, there may still be some current that goes through the load, but this will be negligible compared to the current going through the ground wire to ground.

You can see this if you go back to my earlier analogy about having two resistors (one large: the load, and one much smaller: the resistance of the ground wire) side by side. If both have one end connected to ground, normal operation is represented by the connection of the live wire to the higher resistance. A case fault would be represented by having the small resistance connect in parallel to the high resistance. In this case, the vast majority of the current will go through the small resistance (the ground wire) rather than through the load. Hopefully, a breaker or fuse pops at this point and turns off the live wire.
 
<h2>1. What is the difference between voltage and current?</h2><p>Voltage is the measure of electrical potential difference between two points in a circuit, while current is the measure of the flow of electric charge through a circuit.</p><h2>2. Why is the standard voltage for outlets in the US 120V?</h2><p>The standard voltage for outlets in the US is 120V because it was determined to be the most efficient and safe voltage for household appliances and electronics.</p><h2>3. How does the voltage of an outlet affect the devices plugged into it?</h2><p>The voltage of an outlet determines the amount of electrical energy that can flow through a device. If the voltage is too high, it can damage the device, and if it is too low, the device may not function properly.</p><h2>4. What are the safety precautions for working with 120V outlets?</h2><p>Some safety precautions for working with 120V outlets include turning off the power before working on the outlet, using insulated tools, and wearing protective gear such as gloves and goggles.</p><h2>5. Can I use a device with a different voltage in a 120V outlet?</h2><p>It is not recommended to use a device with a different voltage in a 120V outlet as it can damage the device and potentially cause a fire. If necessary, a voltage converter should be used to safely adapt the device to the outlet's voltage.</p>

1. What is the difference between voltage and current?

Voltage is the measure of electrical potential difference between two points in a circuit, while current is the measure of the flow of electric charge through a circuit.

2. Why is the standard voltage for outlets in the US 120V?

The standard voltage for outlets in the US is 120V because it was determined to be the most efficient and safe voltage for household appliances and electronics.

3. How does the voltage of an outlet affect the devices plugged into it?

The voltage of an outlet determines the amount of electrical energy that can flow through a device. If the voltage is too high, it can damage the device, and if it is too low, the device may not function properly.

4. What are the safety precautions for working with 120V outlets?

Some safety precautions for working with 120V outlets include turning off the power before working on the outlet, using insulated tools, and wearing protective gear such as gloves and goggles.

5. Can I use a device with a different voltage in a 120V outlet?

It is not recommended to use a device with a different voltage in a 120V outlet as it can damage the device and potentially cause a fire. If necessary, a voltage converter should be used to safely adapt the device to the outlet's voltage.

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