For DIY electricians: reasons for rules
I’ve done a bunch of DIY wiring, always passed inspections etc, and have always blindly followed the rule of keeping your phases consistent…always using the same terminals for hot and neutral…but never really knowing why. After all, an appliance will work no matter which side is hot and which neutral, right?
I found out one good reason why this weekend. I had temporily rigged an installed sub panel to supply lights and outlets for work on a cabin addition with a circuit run via extension cord from an outside 20 amp GFI-protected outlet that supplies a sewage lift pump. The original feed to the sub-panel had been removed because the excavation for the addition required it be moved….but I wanted power out of this sub-panel until I had time to permanently re-wire the SE feed. So I just wired an extension cord to the hot, neutral, and ground lugs in the sub-panel. This extension cord was fed by two additonal extension cords. Although the lights dimmed a bit when you started a drill or circ saw, it still worked great.
SEE: http://forums.taunton.com/tp-breaktime/messages?msg=33841.1
Until I tried to extend one of the circuits inside the cabin. Double checking to make sure the power to the recepticle was off, I noticed that, while a light plugged into the outlet went off when I flipped the breaker, my handy dandy circuit tester plugged in stayed lit, indicating “Hot Neutral Reversed” …one yellow and one red.
“What the _____?” When I wired this cabin initially, I had checked all the outlets and they were good.
So I went back to the circuit feeding the lift pump…it checked out fine, two yellows, BUT the first extension cord read red…yellow. It is a cord that I had replaced the male end on a few years ago. Had always worked great. So I took the plug apart and switched the wires/terminals, plugged back in….circuits in cabin now read yellow…yellow. I had been using a HOT neutral in this panel. So the breakers wouldn’t have functioned, and I believe the GFI function also would not have worked downstream from the outlet (am I correct on this?)
Anyway, lesson learned…DIYr’s…this is why you always should double check to make sure any wires you’re working on are not hot…keep those phases consistent, and go ahead and blindly follow the rules, even if you don’t know why.
Edited 12/22/2003 8:58:20 AM ET by johnnyd
Replies
Johnny,
When it comes to electricity I have only enough knowledge to make myself dangerous.
You mention that your miswiring would prevent the breakers from tripping.
In spite of my lack of training it seems the breaker should still trip regaredless of the hot neutral. I am basing this on the idea that both wires are carrying the same current, regardless of their "phase"/color. If the current in the circuit exceeds the breakers rating it should trip.
It sounds likely that the GFI's would not function as you describe.
I will look forward to hearing the electricians give their views on this.
karl
Well, actually, I DO know that the sub-panel breakers "wouldn't" trip with a hot neutral, because I was too embarassed to tell the whole story....and too bull headed to heed the "Hot-Neutral Reversed" warning on my tester.
To extend the circuit, I needed to trim a 12 AWG cable coming from a junction box. Too ignorant to know what a hot neutral meant in my situation (feeding a panel with a hot neutral), instead of de-energizing the panel (because it was evening and I needed the lights), I took my insulated side cutters and clipped the cable. SNAP. I actually thought that this strange thing with the light going off when I flipped the breaker in the sub-panel, but the tester still showing "hot-neutral reversed" had something to do with the way I had jury-rigged the panel, and the circuit was not really hot.
But the only breaker that tripped was the one feeding the lift pump circuit way back up in the service entrance panel in the detached garage-workshop. I wonder what would have happened if my old extension cord had had the ground prong broken off, which it didn't. I'd like some insight on that.
I suppose now I'll get chastized for even doing any of this at all. But the reason I am posting this experience is because no matter how many warnings the DIY community gets about "HIRE AN ELECTRICIAN"...DIYs will be DIYs...and I'm hoping that enough will read this and finally understand the reason behind this particularily important phase rule...so that they are religious about putting the right wire under the right screw when they wire an outlet or renew the plug on an extension cord, which was actually the error I made that caused this whole senario.
Maybe I'm wrong, but I THINK a breaker is designed to trip on a dead short by disconnecting the HOT side of the circuit.
Of course, normally (i think) the only danger from mis-phased wiring is that someone might knowingly work a hot circuit and get buzzed good, or worse, by grabbing hold of an assumed non-energized white wire. Reason, I guess, why you should always mark a white wire with tape in the case of a switch loop.
On a side issue, what is the signifigance of the nowadays typical wider-on-one-side-prong male cord end? It's got to have something to do with keeping the appliance in phase with the rest of the system. Could it be that a "double-insulated" tool or appliance wouldn't be "double-insulated" if what was supposed to be neutral was in fact hot?
Edited 12/22/2003 10:07:51 AM ET by johnnyd
Ok, on the same subject. I have a light in the barn that when it is on and I switch on a certain other light, the first light gets twice as brite while the light I just switched on is fine.
Dave
Take a look in both light boxes and look for damaged insulation on the wires in there.
Do you have two phase power in there? That would mean two separate hots coming into the service panel. That brighter light could be getting at least part of the second phase. I think if it got a full 220, it would burn out.
I suspect we are all going to get our A S S E S kicked when the electricians start responding.
Johnny, thanks for further clarifying my misconceptions of electrical wiring. Like I said, I only know enough to be dangerous.
Karl
I have a hard time following "verbal schematics". And I have only had one cup of coffee this morning so I am not up to trying to follow exactly what happened. But I will try to make a couple of comments.
GFCI do not have any OVERLOAD PROTECTION. You can put a 30 amp load on it or you can short the hot and neutral and the GFCI will not trip. In the GFCI the Hot and the Neutral are wound around a transformer core so that the signal generated by the two currents cancel if they are identical. If there is more than 0.005 amps difference between the not and neutral then it will trip. The GFCI will work if the hot and neutral or reversed. But not if the input (LINE) and output (LOAD) are reversed.
But some of the newer GFCI's do have circuit in them to protect against line and load reversal and maybe hot and neutral reversal.
Only Breakers have over load current. Most have two parts, one for overloads and that takes 10's of seconds or minute to trip depending on the amount of overload. The second part trips on large fault currents ("shorts"). Those trip in fractions of a second. When you have a number of different CB on a circuit (main, feeder, sub-pannel main, and branch) and you have a short it is really probamatic which of those breakers will trip first.
But in some specialised case, such as power transissions lines, there is lots of work to try to make sure that farest downstream breaker trips first.
The circuit break has an in and out connections. If you have excessive current through that wire then it will trip, reguardless if it is feed by a neutral or hot or the wire inturn is connected to the hot or neutral side a receptacle. But depending on where the reversal is you can have a current carrying wire that does not do through the breaker.
The neutral is the more accurately called the Grounded conductor, because it is grounded at the main pannel. And there is no over current protection on it because there is no significant voltage difference between it and ground and if you will not have an significant fault currents if it shorts to the ground.
If the neutral and hot are reversed then can have excessive currents on the unprotected neutral.
As to the "problems with reversing the hot and neutral. First on a 240 feeder you will get 240 on one 120 volt leg which can cause lot of smoke and fire.
Another problem, as you have found, with reversing hot and neutral is the "assumption that if you open the hot the circuit is dead.
In general, on most electrical systems, it takes at least two faults to make something unsafe. While 99% of the stuff will never know the differnece if the hot and neutral are reversed, and some thing like that could exist for years without being detected, it has removed on the safety links. So a 2nd failure can cause a problem.
In general, on most electrical systems, it takes at least two faults to make something unsafe. While 99% of the stuff will never know the differnece if the hot and neutral are reversed, and some thing like that could exist for years without being detected, it has removed on the safety links. So a 2nd failure can cause a problem.
Thanks for your comments, Bill.
I think the quote generally sums up what happened in my case. That's why I wonder what WOULD have happened when I shorted the circuit IF per-chance I had used a nasty old extension cord with the EGC prong broken off AND the hot neutral reversed in relation to the rest of the system. Several of my older power tools & extension cords are in this state resulting from use in FIL's old shop which has no EGC. And I imagine there are thousand of male cord ends around, being used by the general population, in the same state.
Which brings up another question:
My electrician said: "You have to have a ground wire (what I call an EGC, or "Equipment Grounding Conductor"), so that if there is a short, the current has a safe path back to neutral".
So what really happens when there is a short, given that there is a breaker in operating condition between the short and the neutral/grounding rod?
In the beginning, tools had metal chassis. A possible short in a tool was between neutral and chassis. Very important to keep your phases straight.
The purposes of the three wires commonly used today are
Hot: provide a source of current. Needs to be at a voltage different from the return line.
Neutral: The return line. Provides a path for current to return to the source after doing it's work in the load. The neutral is grounded to provide a reference voltage (zero vac) to prevent the source voltage from floating, a condition where the source voltage is not held to a ground or zero volts. A load does not care what the absolute voltage is, only the volts across the load itself. True story; a transmitter I worked on had filament voltages of 13,000 and 13,07.25. The filament operated at 7.25volts.
Ground: Equipment Grounding Conductor; a safety system in case there is a short circuit between any "hot" conductor in the equipment and the equpment chassis. The ground will hold the chassis to zero volts so no person gets electrocuted.
When people first started playing with electricity, they thought that the power flowed from the anode, so they called that pole the positive. Now we know that electrons are emitted by the negative side and attracted by the positive side.
In 110vac systems, for half the AC cycle the hot "pushes" current to the neutral. For the other half, the hot "sucks" current from the neutral.
In 240vac systems the 2 hot lines are each at 120vac, (referenced to zero by the grounded neutral at the breaker panel) but opposite in phase. They take turns, at 60 times a second, "sucking" and "pushing" current between themselves.
I hope this helps you a little in understanding electricity.
SamT
While 99% of the stuff will never know the differnece if the hot and neutral are reversed, and some thing like that could exist for years without being detected, it has removed on the safety links. So a 2nd failure can cause a problem.
'Bout 40 yrs. ago was swimming in a lake, went to climb out of water onto a floating fishing dock by grabbing the conduit there w/ an empty flood lite socket in it. Light was switched on the common leg. Fortunately, my heart was below the water line & had a knowledgeable adult (my dad) w/in eyesight. He pulled me off w/ his own hands while in water himself.
Now, I have a LOT of respect for electricity. Still cannot comprehend how buss bars in switch gear can look SOOOOO innocent when hot. (Used to run a power plant & when new generater was put on line out of phase, we had to walk INSIDE the switch gear to manually reset the reverse current relay. Still get the hebee geebees when I think about reaching out to reset the relay & lites go out! (never happened, but dont think I didnt think about it!!!)
"we are all going to get our A S S E S kicked when the electricians start r..."
It WILL be great and enlightening entertainment though. I'll be watching..
Excellence is its own reward!
Due you have a sub-pannel in that barn? If not is the barn feed with more than one circuit?
It sounds like you might have the 2 lights on oposite legs of a 240 volt circuit and the neutral is open. If that is the case that is a damagerous situation.
It is also possible that you have some kind of series parallel screw with the lights and probably another load. Are each of these lights controlled by a single switch? What happens if turn the first light off?
I do have a 220 line that runs out to the barn. I would not call it a sub panel as such. The neutral and ground are not carried separately........which as I type this makes me wonder how I have 120 out there? (My father in law built the barn 40 years ago) I have one run (300') of #10 lead covered cable with just two conductors and a ground. Me thinks it will be time to dig a trench and bury some #4 in pipe when the ground thaws. (Although the ground never really freezes where the existing cable runs under ground.
Thanks
Dave
Is the variable lamp a 240volt bulb? If not, then you don't have a 240volt situation, cuz' it would burn out in 3 or 4 seconds at 240volts.
Does it get as bright as it should when the other lamp is turned on?
I would suspect a marginal neutral, and I would inspect/verify correct wiring throughout both light systems since there must be other problems also.
Check for phase crossing at switches and lamps and any J-boxes while you're climbing around.
SamT
It depends on how bad the neutral is and what other loads are on the circuit on both legs.
Assume that there is nothing else other than the two lights, one on each leg. And that you have a high resistance neutral.
With just the one light on you might have 100 volts with 20 across the neutral. When you turn the other on and if they are the same then you will have 120 across it. If the other light is bigger than you might have 130 volts across the first one.
<the first light gets twice as brite while the light I just switched on is fine.>
Dave, you have a serious problem that should be dealt with immediately! The good news is insurance will probably cover for the fire!
bake
On a side issue, what is the signifigance of the nowadays typical wider-on-one-side-prong male cord end? It's got to have something to do with keeping the appliance in phase with the rest of the system. Could it be that a "double-insulated" tool or appliance wouldn't be "double-insulated" if what was supposed to be neutral was in fact hot?
AyyyYehp. You got it. 2 in 1.
SamT
johnnyd,
First, thanks for sharing. When working with electrical wiring, there are a lot more ways to get things wrong that there are to get 'em right. And just as you discovered, a screw-up may be discovered only when there's a problem--someone gets shocked or electrocuted, or there's a fire.
About you mis-wiring the cord cap on the extension cord--mistakes happen. A properly trained electrician ought to be meticulous, and will double check their work and usually catch their mistakes. Notice I didn't just say "an electrician"; there are a lot of "ceiling fan" electricians out there who are really just installers. They've had little training and have limited abilities, and they're passing themselves off as electricians.
Anyway, with the polarity reversed in the feed to the subpanel, you had the breakers on the return leg of the circuits. Whether they'd protect the circuit wiring depends on the type of fault, where it occured, and the total impedance of the fault pathway. It sure wouldn't offer the overcurrent protection that protection on the hot (supply) conductor does. You found this out fairly painlessly. If the polarity had been reversed in the entire branch circuit, youcould have been in for some real fireworks (or more likely, a fire). The main breaker would have been your only defense and a 100 amp main breaker can pass 200-300 amps for a long time (a minute or two) before tripping. That's a lot of heat at one point of contact at one point of a fault --120V X 300 amps = a lot of watts.
In very early (early 1900s) electrical systems, both the hot and neutral legs of the main (and sometimes even branch circuits) were fused. That was soon prohibited, because if the fuse in the neutral did open the circuit, you could conclude that the circuit was dead. Big problem. In any case, having only the neutral protected as in your situation is dangerous because it won't clear a fault (short circuit for instance) in as many instances as a protected hot will.
About the GFI, it depends on the GFI. For an older GFI, the polarity of the circuit doesn't matter. The newest models have additional features and I believe one of them is a mis-wire detector, including reversed polarity. I'll check on this later and correct myself if I'm wrong.
About the slots on a receptacle outlet, yes, one is longer than the other. The hot conductor is intended to be connected to the shorter one. Thi is called "polarity", and it matters a lot for some situations. Like a light socket--if properly wired, the shell is connected to the neutral and the button at the bottom is the switched hot. If you're changing a lamp and you slip, you're more likely to touch the shell. And if the polarity is reversed and the switch is on, you're in for a shock or worse.
Take care, and work safe--
Cliff
Thanks for your thoughtful and insightful reply.
"If the polarity had been reversed in the entire branch circuit, youcould have been in for some real fireworks (or more likely, a fire). The main breaker would have been your only defense and a 100 amp main breaker can pass 200-300 amps for a long time (a minute or two) before tripping. That's a lot of heat at one point of contact at one point of a fault --120V X 300 amps = a lot of watts."
Good thing I had already checked the rest of the circuit for polarity when I installed it.
BUT:
Then what is the point of the main breaker? Besides a disconnect.
johnny,
Thanks for the feedback. And you've got a good question--what good is a breaker if it won't trip at slightly above the rated current?
Most people think that a regular circuit breaker will trip (open) instantly if the current is just higher the breaker's rating. Nope.
A regular circuit breaker is designed to open or trip based on a combination of current and time. The higher the current, the shorter the time-to-trip. This is called "inverse time" operation. The point of this design is to allow short-duration excursions above the rated circuit ampacity to accomodate things like motors starting up (some of which can draw 5-7 times the running current for a fraction of a second), or occasional short-duration overloads. It's a cost vs. convenience thing, and safety is factored in.
Every breaker manufacturer publishs graphs that show this relationship for each type of breaker. If the current is very high--as in a dead short--the breaker will trip in the blink of an eye (a tenth of a second or less, depending on how the breaker is designed). If there's a mild overload, say 30 percent to 50 percent over the breaker rating, the breaker will not trip for a couple of minutes. If the fault current is 200-300 percent of breaker rating, you might have several to 30 seconds before it opens. These time-current curves are posted at the breaker manufacturers's websites. Interesting information...
With the high-quality insulation on modern wire, this approach is O.K. In an overload or high impedance short circuit situation the wires heat up, and the concern is this heat degrading the insulation, quite possibly leading to a dead short or ground fault. The worst situation that can occur then is an arcing fault, which sputters and throws sparks. And a regular breaker will not trip in many of there arcing fault cases, because the current is high but intermittent. Arc-fault circuit interruptors are designed to detect some types of arc-faults and trip.
As far as the effect of heat on a wire's insulation, the heat in an attic in summer adds to the heat generated in the conductor by current, and that's why the Code requires the apmpacity of conductors to be adjusted for ambient temperature. This isn't an issue for most modern wire and cable in residential branch circuits, because 12 gage and 14 gage conductors have a bit of "headroom" in their ampacity ratings.
Anyway, back to breakers, I've seen situations where there's one-half to one amp of ground fault current, and the breaker is closed and holding. It really made me realize that you need to check for voltage on the equipment grounding conductor before you break any splice or connection. It's the unusual situations that'll get ya.
Take care,
Cliff
> Then what is the point of the main breaker?
The point is that you're allowed to have branch circuit breakers in a panel that add up to more amps than the panel is rated for.
In actual use, very rarely would you have a large majority of the branch circuits loaded to or near their maximums. So it wouldn't be cost effective to require that the panel be rated for more than the branch breakers add up to. But in that rare case when somebody turns everything on at once, the main breaker should protect the box and feeder.
In the old days when people had 60 amp or even 30 amp services, maxing out the main was less rare than it is now with 100 and 200 amp services being common.
-- J.S.
When I started to work on my house before the service panel was upgraded, I plugged all the tools into the bathroom GFI outlet since it "should have been a dedicated 20A circuit, right?" All the other outlets in the house were older ungrounded 2-wire on knob and tube. Well, later when the other side of the bathroom wall was opened up, we discovered that the bathroom GFI was actually just tapped into the 2-wire cable which ran up the back wall to supply all the first floor ceiling fixtures. The ground from the GFI was a separate wire screwed into the metal box and connected to a nearby cold water pipe. I would say it was 50-50 what the polarity was on that device. I'm glad to report that since then everything has been verified correct and/or fixed.
If you really want to show off you can demonstrate that cords, mostly applicable for cords that have no cord caps on it yet or during a repair, have a male and female end. Round cords have a set sequence of conductors.
Female end of the cord, looking into the end of the cord and moving clockwise, being White- Black- Green (W-B-G).
Male end being B-W-G.
If you try try to install a male cord cap on the female end of the cord you have to cross two of the conductors. If you go male cord end to male cord cap, or female to female, there is no need to cross wires. I often win a friendly bet or two, on each job site, with this factoid. often electricians who know it all and are cocksure of themselves swear that cords are not polarized, sexed, end for end.
As with so many other things with electrical work these things are not random. They are are, in fact, set in industry and trade standards. many of these have been hammered out over decades between installers, inspectors, designers, manufacturers and various other code writing and enforcement bodies.
All of them have some basis. Most are for mechanical, electrical or safety reasons. A few, like 120v receptacles being installed with the ground, mouth, on the bottom are based in tradition and the general tenancy of people to see three holes in a triangular pattern as a face. Anthropomorphism.
Nothing in the NEC specifies this orientation. In fact the article that specifies orientation, under installation in hospitals, specifies that the ground must be on top. This would be the preferred placement as it would tend to ground any conductive object falling between the plug and the receptacle. This would be safer.
If you look closely at the devices the writing is only right-side-up with the ground on top. Some even have a sign stamped in specifying 'TOP' this is inside electrical humor as, other than in hospitals, this is inevitably installed up-side-down to meet customers expectations. So it goes.
its really not hard.
there are only two main wires, a black and a white.
thats it.
what?
a green - nope i don't think so and
what?
a red
what are you talking about.
always went by RH and LH vs male/female, the RH as you read the cord type printing right side up has the BWG to your right, plug (male) goes to RH side.
Lots of the cords I get surplus (usually for the Hubbel plug) seem to be randomly connected, as the better plugs are not that particular about crossovers.