110, 208, 240, AND 480

[fletcher]

i know that 110 is used in a typical household for general lighting circuits.

however, what are some uses for 208 240 and 480??? could someone explain where each one of these is commonly used??

 

[charlie b]

Re: 110, 208, 240, AND 480

Most homes are fed by what we now call "120/240." A range or a dryer might use 240 volts. Most other household circuits use a "nominal" 120 volts. I know this used to be called a "nominal" 115 volts, but I do not know if it was ever called a "nominal" 110 volts. The word "nominal" is used in the code in the sense that this is the "name" of the voltage rating. At any given moment, the actual voltage at any given point might be slightly above or (more likely) below that value.

208 volts is part of a three phase distribution system. Phase to phase voltage is 208 volts, and the voltage between any phase and ground is 120 volts. You do not often see this in a single family unit. You might see it in a large apartment complex.

480 volts is the next step up from 208. It is used in very large apartment buildings. The 480 volt power is distributed from floor to floor, and there are step-down transformers at various locations to bring the voltage down to 120/208 or to 120/240 for use in individual units. You would also see 480 volts used for large motors. If the building has a fire pump or a complicated air conditioning and heating system, some of the components will use the 480 volt power.

Is this what you were looking for?

 

[fletcher]

Re: 110, 208, 240, AND 480

thanks for the quick reply charlie. that helps me out.

now i have a grounding question. what is the biggest reason for proper connections for an electrical grounding system???

this question is part of my homework for a intro class i am taking.

i think the reason is SAFETY for people. however, the book i have only talks about protecting people.

 

[fletcher]

Re: 110, 208, 240, AND 480

that's a type-o in the last sentence of my reply. the book only talks about protecting equipment from overcurrent via ground.

i think the reason for "proper connections for an electrical grounding system" would be safety. for example. all metal boxes must be grounded. so, in the event that a hot wire comes loose inside a metal box, the box will not be energized and be a danger to a person....not equipment.

that is the answer i am giving. anyone have a different opinion??

 

[haskindm]

Re: 110, 208, 240, AND 480

Look at Article 250.4 in the National Electric Code. It spells out what is to be accomplished by grounding. If your textbook really says that systems are grounded to protect against overcurrent, drop the course immediately and look for someone that understands a little bit about electrical wiring. Hopefully you have misunderstood something. Attend one of Mike Holt's Grounding and Bonding seminars. Grounding has NOTHING to do with protection from overcurrent. Bonding may, but not grounding.

 

[roger]

Re: 110, 208, 240, AND 480

Fletcher, grounding will do nothing for opening an OCPD and will not protect people, HOWEVER, bonding will if it is continuous and of a low impedence back to the source.

If you reworded your second sentence as follows it would be correct.

i think the reason for "proper connections for an electrical BONDING system" would be safety. for example. all metal boxes must be BONDED TOGETHER. so, in the event that a hot wire comes loose inside a metal box the OCPD will open and the box will not be energized being a danger to a person and or equipment.

Added and changed text by me is in bold


Roger

 

[pierre]

Re: 110, 208, 240, AND 480

haskindm
I think that using a blanket statement such as grounding has nothing to do with overcurrent protection needs to be much better qualified for a new student of our great industry. The NEC does state a metallic box needs to be grounded, just not in the sense that fletcher may be thinking.

 

[roger]

Re: 110, 208, 240, AND 480

Fletcher, look at this graphic,

grounding will not protect or make a metal enclosure safe in the case of a fault.

Roger

 

[fletcher]

Re: 110, 208, 240, AND 480

the exact question is:

"Describe the reason for the proper connections for an electrical grounding system."

you guys mean the reason is not safety????

 

[roger]

Re: 110, 208, 240, AND 480

Fletcher, this is 250.4(A) 1 through 5

(A) Grounded Systems

(1) Electrical System Grounding Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.


(2) Grounding of Electrical Equipment Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.


(3) Bonding of Electrical Equipment Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

(4) Bonding of Electrically Conductive Materials and Other Equipment Electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

(5) Effective Ground-Fault Current Path Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

Take special note of the distinction between Grounding and Bonding


Roger

 

[George Stolz]

Re: 110, 208, 240, AND 480

"Describe the reason for the proper connections for an electrical grounding system."

To use the NEC's definitions, as Roger posted, the answer would be:

It is to limit the voltage imposed on electrical equipment by lightning, line surges, or unintentional contact with higher-voltage lines and also to stabilize the voltage to earth during normal operation.

you guys mean the reason is not safety????

Not exactly. The goal of grounding is more to prevent lightning from striking electrical equipment, or to provide a path for the lightning should it strike.

A more compelling purpose of grounding is to provide a low resistance path to ground for line surges and unintentional contact with higher voltage lines.

For example, a typical utility voltage in residential neighborhoods is around 7200 volts.

Imagine a ground rod with 25 ohms resistance from the tip of the rod to the earth. 25 ohms is usually the benchmark for ground rod discussions, as the NEC states that a ground rod that has over 25 ohms resistance to ground be supplmented with one more ground rod (ultimate total of two rods, regardless of the resistance of the pair).

So this 25 ohm rod is driven in the earth, and you, being reckless and curious, connect it to an "ungrounded" (which is NEC parlance for what we commonly call "hot") source. Say it's a 20 amp breaker, with a 12 AWG wire running to the rod. What happens? Does it kick the breaker?

Electricity seeks any path back to it's source. (It doesn't seek the easiest path to ground, that's a myth.)

Since the transformer supplying the house is grounded, and the substation is grounded, and the power station is grounded, and the generator at the power plant is grounded, the earth is a path back to the source. In this case, our source is the transformer just outside the house.

So when you hook up this 12 AWG wire to the ground rod, turn the breaker on, current is travelling from the breaker, through the wire, into the ground, and into the ground rod at the transformer. (Magically, we'll assume this rod has no resistance at all, for this example.)

Will the breaker kick?

No.

Ohm's law states that Volts = Amps x Resistance. When you solve for Amps (because we're wanting to know if enough amps are being drawn to kick our breaker), then it paints the picture:

Amps = Volts / Resistance
I = 120 / 25
I = 4.8

There are under 5 amps flowing on that wire, and through the rod. The breaker isn't even breaking a sweat, much less breaking the circuit because of the problem.

Now, from his truck, the utility guy is eating his lunch, musing over why this putz in the house is trying to kill himself by such a bizarre method, and decides to spice things up a bit. He walks over to the transformer, whips open the lid, and sends 7200 volts into the secondary side of the transformer, which was 120/240.

Now, how many amps are flowing through that 12 AWG wire and your rod?

7200 / 25 = 288 amps.

That ground rod is much more effective at a higher voltage. Add to that the other grounding electrodes that are present at the house, and all the grounding electrodes connected to that transformer, and you'll have a lower resistance still.

So now, with all these electrodes pitching in, there's a shot that something the utility company owns is going to be receiving a massive amount of fault current , and hopefully the event ends quickly.

That is grounding.

Bonding is a totally different concept.

Right now, the NEC unfortunately uses the term loosely, so you have to be careful of what you're dealing with when you read it and practice it. For example, an "Equipment Grounding Conductor" has absolutely nothing in the world to do with grounding at all, not one little bit.

Does this make sense to you? As kaskindm said, if the class you are taking does not adhere to this philosophy, drop it like a shot, they do not understand the difference. Mike Holt's Grounding vs. Bonding material is very informative, and as excellent for the beginner as for the seasoned. I'd highly recommend it.

And stick around here, there's several guys around here who're way sharper than I am on all this stuff.

 

[charlie b]

Re: 110, 208, 240, AND 480

Originally posted by fletcher: the exact question is: "Describe the reason for the proper connections for an electrical grounding system."

It's a badly worded question. If you are simply looking to get the "book's right answer," and if the multiple choice answers include "safety," that would not be a bad guess.

But if you want to know how things work and why things are done the way they are, please be patient. This is not an easy topic to learn.

But if I may be allowed to offer a bit of advice, and this applies to anyone in the Home Inspection business, it would be this: Learn to recognize hazards. You can't be expected to become experts on what does or does not comply with code. But you can do your clients a great service if you can see when something represents a danger to life or property.

Sometimes this is simple. If a receptacle has signs of scorching, then something had happened to the system. If you push the test button on a GFCI and it doesn't trip, then something is broken.

Sometimes it is less easy. If you don't see a GFCI receptacle in a bathroom or in the kitchen, that might be a problem. But you can put a GFCI receptacle in one bathroom and have it feed a receptacle in a second bathroom, and you would not know that by only looking at the second bathroom.

Probably the single most significant hazard that homeowners might create, in the mistaken impression that they are solving a problem, has to do with grounded receptacles. If they test a receptacle with one of those 3-light testers and see that it is not grounded, or if their computer's power supply beeps at them to inform them that the power supply is not grounded, they will often "fix" the problem by connecting the neutral and ground pins in the outlet box. You can take the cover off a couple of receptacle outlet boxes and just take a look. This "fix" is relatively easy to see. If you do see it, then write it up as a safety hazard, and recommend that the owner call an electrician.

 

[haskindm]

Re: 110, 208, 240, AND 480

Pierre,
Your comment is well taken, but I do not want to encourage mis-information. There are sections of the code that do not conform to the definitions established in the code itself. In Article 100 the term grounded is defined as "connected to earth..." As you correctly state, article 314.4 states that metallic boxes shall be "grounded". A strict interpretation of this would mean that we should drive a ground rod for each metallic box! What the code is referring to here is "bonding". We in the industry, including those who write the code, need to be more careful about how we use words. The purpose of grounding is NOT to facilitate the operation of overcurrent devices, or provide safety from short circuits and ground faults, that is the purpose of bonding and installing grounding conductors. The terms grounding and bonding are not interchangeable, even though we often treat them as if the were.

 

[pierre]

Re: 110, 208, 240, AND 480

From George

"For example, an "Equipment Grounding Conductor" has absolutely nothing in the world to do with grounding at all, not one little bit."

Not really 100% true...but close.

The equipment grounding conductor that has a part of its existance as related to grounding, is the part of a zero reference to equipment.

I think (correct me if I am wrong) George was trying to say that the operation of an overcurrent device does not rely on the current that travels along the GEC to the grounding electrodes. It depends very much on the "effective ground fault current path".