Right. And the color code is standard. In the US white is always neutral. The “hot” wire can be any color except white, however, commercially available two-strand cable has a white and a black; three strand has a white, black and red.
OK. This makes sense, I guess. If I understand correctly, the grounding of the circuit needs to be the same between the neutral path and the ground path, to eliminate a difference in potential. Which could be a safety hazard.
It wouldn’t be sufficient to ground the neutral path to the copper water pipes say, and the ground path to a different copper rod placed into the earth. The copper rod might be a “better” ground and the current would flow through it.
Is this a correct understanding?
Um, well, no. It is true there are three wires coming from the transformer. But they are labeled as such:
- Hot Leg 1
- Hot Leg 2
- center tap
Each of these carry current. There’s 120 VAC between Hot Leg 1 and the center tap. There’s 120 VAC between Hot Leg 2 and the center tap. (And these two are 180 degrees out of phase.) And there’s 240 VAC between Hot Leg 1 and Hot Leg 2. The common mode voltage is undefined; the entire secondary is floating, just like a battery. (Grounding one of the conductors “unfloats” the secondary. More on this later.)
2-wire 120 VAC outlets are connected between Hot Leg 1 and the center tap or between Hot Leg 2 and the center tap. (To keep it simple, I won’t discuss grounded or 240 VAC outlets.)
As mentioned above, the transformer’s secondary winding is “floating.” This is dangerous, as finite resistance between the transformer’s primary and secondary coils could cause the secondary to float up to the primary voltage (7000 V or so, relative to earth ground.) If this were to happen, the normal mode voltages would still be the same, i.e. there would still be 120 VAC between Hot Leg 1 and the center tap, 120 VAC between Hot Leg 2 and the center tap, and 240 VAC between Hot Leg 1 and Hot Leg 2. But the common mode voltage (the voltage between any of the secondary conductors and earth ground) could be very high.
To fix this problem we connect one of secondary conductors to earth ground. Theoretically speaking, it really doesn’t matter which conductor is grounded… you can connect Hot Leg 1 to earth ground, or Hot Leg 2 to earth ground, or the center tap to earth ground. It doesn’t matter. But in practice, we find there is a safety advantage to connecting the center tap to earth ground:
-
If you connect Hot Leg 1 to earth ground, then a person (who is grounded) touching the center tap would be shocked by 120 VAC, and a person (who is grounded) touching Hot Leg 2 would be shocked by 240 VAC.
-
If you connect Hot Leg 2 to earth ground, then a person (who is grounded) touching the center tap would be shocked by 120 VAC, and a person (who is grounded) touching Hot Leg 1 would be shocked by 240 VAC.
-
If you connect the center tap to earth ground, then a person (who is grounded) touching Hot Leg 1 would be shocked by 120 VAC, and a person (who is grounded) touching Hot Leg 2 would be shocked by 120 VAC.
Obviously, the safest configuration is #3. So we connect the center tap to earth ground. The center tap now has a new name: common, or neutral.
More…
I now understand why the transformer’s secondary needs to be tied to earth ground. I also understand why the center tap was chosen to be the “winner.” But where is tied to earth ground?
All three wires from the transformer (Hot Leg 1, Hot Leg 2, and the center tap) enter the circuit breaker box. Each connects to its own heavy bus bar. To ground the center tap, you simply connect a heavy copper wire between the center tap’s bus bar and a 4 ft. copper rod that’s been driven into the earth near the breaker box.
There’s also no harm in redundancy. I believe the transformer’s center tap may also be grounded at the utility pole. In addition, there may be more than one home getting its power from the same transformer. Each home is the same… there is a heavy copper wire between the center tap’s bus bar and a 4’copper rod that’s been driven into the earth next to the house. So as you can see, the transformer’s center tap is often connected to ground at many locations. This is a good thing… if the heavy copper wire between your center tap’s bus bar and your 4’copper rod happens to break, then all hell won’t break lose; you can usually assume the transformer’s center tap is grounded somewhere else. But because the earth has resistance, it is always safest to make sure your center tap’s bus bar is connected to your 4’copper rod.
So tell me again… what’s the deal with grounded outlets? Doesn’t the neutral and ground go to the same place in the breaker box? That doesn’t make sense.
Let’s say you have a 2-wire 120 VAC outlet (just like the good ol’ days) and a 2-wire 120 VAC appliance. We’ll assume the outlet is connected between Hot Leg 2 and the transformer’s center tap (a.k.a. common, neutral, etc.) And as we just learned, the neutral is connected to earth ground somewhere. Finally, we’ll also assume the appliance has a metal chassis. Again, just like the good ol’ days.
The appliance’s metal chassis is not connected to anything; it’s not connected to Hot Leg 2, it’s not connected to neutral – nothing.
Now let’s say a wire breaks inside the appliance, and the wire happens to touch the chassis. Let’s also pretend the wire is at 120 VAC. Guess what? The chassis is now at 120 VAC. And this is with respect to earth ground! (As we learned earlier, Hot Leg 1 and Hot Leg 2 are each at 120 VAC with respect to earth ground. This is because we ground the transformer’s center tap, a.k.a. neutral/common.)
This is obviously a dangerous situation. If you’re grounded and you happen to touch the chassis you’ll receive a shock.
Yea, you’re right. But I’ve got an idea… let’s connect the neutral to the chassis. This sounds safe. After all, the neutral is tied to earth ground somewhere, so no one should get shocked by touching the chassis. And while there is current on this wire, the voltage (due to V = IR) isn’t usually dangerous; a few volts at the most. But the good news is that, if a hot wire inside the appliance happens to break and touch the chassis, it will cause a short and trip the circuit breaker. Furthermore, we’ll use a “polarized plug.” Sounds like a good idea to me…
Hmmm. While it does sound like a good idea, let’s think this through.
Let’s say the appliance is in good working order. We’ll also assume it doesn’t have a power switch. Or the power switch is “on.” Let’s also say you’re grounded. (Either directly or with some resistance. Both cases are dangerous. And it’s very common for you to be grounded.) Finally, we’ll assume you’re holding or touching the appliance, which means you’re touching the chassis. (Which, again, is internally connected to neutral.)
You insert the plug into the outlet.
What are the chances both of the plug’s prongs will connect at exactly the same time as you’re pushing the plug into the outlet? Nil. This means one of two things:
-
As you’re pushing the plug into the outlet, the neutral will connect before the hot. Only the neutral (and not the hot) will be connected for (let’s say) 20 milliseconds. After 20 milliseconds, both the hot and the neutral will be connected.
-
As you’re pushing the plug into the outlet, the hot will connect before the neutral. Only the hot (and not the neutral) will be connected for (let’s say) 20 milliseconds. After 20 milliseconds, both the hot and the neutral will be connected.
There’s a 50/50 chance #1 will occur. There’s a 50/50 chance #2 will occur. #1 is safe. #2 is dangerous. Why? Because during those 20 milliseconds, only the hot is connected. And guess what? It forms a closed circuit, and you’re in the loop! During those 20 milliseconds current will flow out of the transformer’s hot conductor, to the outlet, through the circuitry inside the appliance, into the neutral, to the chassis (because the neutral is connected to the chassis), into your hand, through your arm, out of your body (because you’re grounded), through the dirt (or whatever), into the copper rod, through the heavy copper wire, into the breaker box, to the neutral’s bus bar, and to the transformer’s center tap. This is not good.
O.K, I have an idea. On the plug, let’s make the neutral prong longer! That way it will always connect first.
Good idea, but it still has problems. First of all, it is difficult to design a 2-conductor plug with one prong longer than the other. Because it’s a thin, flat blade, the neutral prong will bend easily during the insertion/removal process. Secondly, remember what I said about the voltage that appears on the neutral due to V = IR? I said that the voltage wasn’t very high, and thus not very dangerous. But this isn’t always true. If there’s a lot of current on the neutral, and/or if the distance between the outlet and breaker box is very long (using a long extension cord, for example), it is possible to develop a sizable voltage on the neutral strictly due to V = IR. So even if you could guarantee the neutral always connected before the hot, it certainly wouldn’t be an ideal situation.
I’m out of ideas.
Try this one on for size: Let’s design outlets and plugs with a third conductor. We’ll call it “ground.” This will be a separate wire that connects an appliance’s chassis to earth ground. Because the chassis is not connected to hot or neutral inside the appliance, there will not be any current on this wire (under normal conditions), thus there will not be any concern about voltage developing on the chassis due to V = IR. Furthermore, we’ll design the plug so that the grounded prong always connects first. In other words, we’ll make it longer than the hot and neutral prongs. And to enhance mechanical integrity we’ll design the prong to have a circular cross section.
Actually there might not be any theoretical reason why a longer and thicker (to prevent breaking) ground prong wouldn’t be just as good as the present system. However, a system has to be picked from among all the possibilities and the one we now use grew naturally out of the system that was already in place, I.e. all outlet recepticals, the thing on the wall you plug into, already fit the two equal-prong plugs; the white neutral was already grounded to the earth at the breaker panel and so on. So just adding a green safety ground resulted in less retrofit. In fact no retrofit except a 25 cent adapter to plug new appliances into the recepticals in an old house.
Actually, that is our present system. On three-wire plugs, the ground prong is longer and more mechanically robust than the hot and neutral prongs.
Anyway, I was just trying to explain the general layout of the system from a layman’s point of view. I left a lot of details out, obviously. These include 240 VAC outlets, circuit breakers, shared neutrals, etc. Also, there are certain appliances that require an earth ground connection for a purpose other than safety. There was a thread not long ago where we listed a bunch. Maybe I’ll dig it up.
More:
Crafter_Man: I understand what you’re saying now. But to my mind, grounding the transformer’s secondary (in one sense) kinda sucks. I mean, does it not guarantee that Hot Leg 1 and Hot Leg 2 are always at 120 VAC with respect to earth ground? That crap can kill you if you’re grounded!
Yea, you’re right. It does kinda suck. But it is “the lesser of two evils.” Let’s look at both options:
-
If we do not ground the HV transformer’s secondary, then there will not be a (low impedance) 120 VAC between Hot Leg 1 and earth ground (good!), and there will not be a (low impedance) 120 VAC between Hot Leg 2 and earth ground (good!). And there will still be 120 VAC between Hot Leg 1 and the center tap (good!), there will still be 120 VAC between Hot Leg 2 and the center tap (good!), and there will still be 120 VAC between Hot Leg 1 and Hot Leg 2 (good!). Sounds great, right? But as mentioned in a previous post, the entire secondary winding (meaning Hot Leg 1, Hot Leg 2, and the center tap) could “float up” to the primary voltage, which is around 7000 V or so with respect to earth ground. The floating action is due to a finite amount of resistance between the primary and secondary windings. So not grounding the HV transformer’s secondary is a bad idea.
-
If we do ground the transformer’s secondary, then we guarantee that Hot Leg 1 and Hot Leg 2 are always at 120 VAC with respect to earth ground. This sucks. If you’re grounded, and you come in contact with Hot Leg 1 or Hot Leg 2, you’ll get zapped.
Both options suck to a certain extent. So which is less evil? Option #2. It is “better” to get hit with 120 VAC than 7000 V. (Even though the 7000 V would have a fairly high source impedance, there’s no guarantee it will be high. Even it were high, I still wouldn’t want to come in contact with it.)
But isn’t there a way to design a system so that hot and neutral are not referenced to earth ground, yet the same system guarantees they won’t float up to a very high voltage, either?
Yes. First of all, keep in mind that the system only needs to guarantee three things: 120 VAC between Hot Leg 1 neutral, 120 VAC between Hot Leg 2 neutral, and 240 VAC between Hot Leg 1 and Hot Leg 2. From the point of view of almost all appliances, there is no requirement for there to be 120 VAC between Hot Leg 1 and earth ground, and 120 VAC between Hot Leg 2 and earth ground! The fact that there is 120 VAC between Hot Leg 1 (and Hot Leg 2) vs. earth ground is simply a fallout of the “necessary evil” described above. It offers no benefit to most appliances.
So how would you do this? You could install a “whole house transformer.” Here’s how it would work: The secondary on the HV transformer out on the pole would still be grounded as usual. Hot Leg 1 and Hot Leg 2 would then go to the primary of a 1:1 “whole house transformer.” On the secondary of this new transformer would be three taps:
(New) Hot Leg 1
(New) Hot Leg 2
(New) center tap
The house would then be wired as usual with these taps.
The beauty of this system is that we would still have 120 VAC between Hot Leg 1 neutral, 120 VAC between Hot Leg 2 neutral, and 240 VAC between Hot Leg 1 and Hot Leg 2. But it would be floating, and thus there would not be a (low impedance) 120 VAC between Hot Leg 1 and earth ground, and Hot Leg 2 and earth ground. It would be much safer.
- But wouldn’t it float up to the primary, which is at 240 VAC?*
Yes, it might. But because the source impedance would be very high (many megaohms), it would still be a lot safer that what we currently have. Besides, if we think it will be a problem, we could “ground” the secondary winding using a 1 megaohm resistor.
Would an earth ground be required with such a system?
Yes, but its “importance” is decreased. You would still want a ground wire for the appliances that need an earth ground other than for safety reasons (EMI shielding, etc.)
Sounds good. So why isn’t this done?
Two reasons:
-
Expense. Such a transformer would be very expensive. And bulky. And inefficient. (Transformers are not 100% efficient. Most are between 85% and 95% efficient.)
-
I could be wrong, but there might be some appliances that actually require a ground-referenced hot leg. Though I can’t thnk of any off the top of my head.
Crafter_Man: I’m a little worried about you Michael. This recent tendency to talk to yourself is a little disturbing. Looks like you have a multiple personality thing going on here. 
All kidding aside, your explanations are interesting reading. Thanks.
I just got a refill of Haldol. I’m better now. 
Disagreement, here. The statement above does not account for switching legs, which render the white every bit as hot as the black or red, thinking of 3 way switching.
A few other observations: Everyone seems to be aware of the need to connect to a driven ground rod, yet all metallic piping systems must be connected to equalize potential. The cold water piping must be connected to the neutral bus, the point of connection being not greater than 5’ from where the pipe enters the dwelling.
When doing a service or replacement/upgrade, I make it a point to correct grounding/bonding deficiencies, among them being bonding natural or propane gas lines, jumping the water heater inlet and outlet (nonmetallic dip tubes), and bonding past a water meter in case dielectric unions were used.
The inexpensive outlet testers are nifty but don’t indicate the quality of a ground. In the past, grounding of a device box was accomplished by wrapping the bare conductor around the cable clamping screw. In theory it was fine, but depended upon a series of properly made connections in daisy-chain arrangement to clear a fault. Tests by UL and others have shown that even wrench-tight conduit fittings are not foolproof for clearing of fault conditions.
My first home was in Levittown, PA. Branch feeders were two conductor. To ground boxes, a separate aluminum conductor was run, again in daisy-chain fashion to land on each device box. In the process of remodeling, I found instances where corrosion or work by others had rendered these grounds useless.
With the number of sensitive devices being purchased by the average consumer, the place for surge suppresson is in the panelboard, IMHO-an option I offer to customers.
If you really want to learn all about grounding, the book to get is The IAEI Soares Book of Grounding published by the International Association of Electrical Inspectors, in Dallas, Tx. Eustace Soares, P.E. was a visionary man who authored texts on electrical safety and was considered an expert on the National Electric Code.
Excellent point, Dances.
Regarding the use of one of those $3 outlet testers… if it tells you there’s a problem, then there’s likely a problem. But if it indicates “everything’s O.K.” then you really don’t know. Those neon bulbs don’t need much excitation current, and they will not tell you if you have a conductor with too much resistance. They’re just dumb neon bulbs.
And when the white is "hot,"as in a switch loop, you are required to paint the wire insulation with a distinctive color at a point close to the switch so that it is not confused with the white neutral.
Not to be contrary, could you offer an article and section of the 2002 NEC to substantiate that claim? Many thanks.
No but when I wired my house in San Bernardino County, CA I was required to mark the white wire in a switch loop with a “distinctive color painted on the insulation.” Maybe the inspector was exceeding his authority but I think the county or state code requires it and if not it makes so much sense to do so that I didn’t question him.
In all fairness, since my last posting I’ve had discussion with other electrical friends and your claim is correct. Color coding of switch legs was an exception which was converted to rule in the 1999 Code, so shame on me. 
I hate missing those changes!