I didn't know you could do that. learn something new every day.
If this bothers you, a new dryer cord is as close as your local hardware store, simply replace the cord with a 4-wire cord whose plug fits your outlet and remove the bonding jumper. It's not a HUGE deal, but I believe it is technically a code violation to use a 3-wire cord where a
4-wire receptacle is available.nate
I understand, however to make it a bit clearer I used numbers that more easily translated to the OP's ability to understand. Sorry I wasn't more technical, do you think he would have understood your numbers better in relation to his dryer?
To the OP, you are exactly correct, the 3 wire system can be dangerous if the "ground neutral" wire opens becasue most dryers have a 120 volt motor so that current flows through the neutral ground. NEC code as picky as it is used to allow this. I think it is now not allowed in new homes. I have a 3 wire dryer and I added a ground wire connected from the frame of the dryer to the frame of the washer. This way, whatever else happens there can be no voltage across those two appliances and the washer has a 3 prong plug so it is grounded. You can also connect it to a cold water pipe. I would suggest you do that as an extra saftey feature. Good question Mark
To the OP, you are exactly correct, the 3 wire system can be dangerous if the "ground neutral" wire opens becasue most dryers have a 120 volt motor so that current flows through the neutral ground. NEC code as picky as it is used to allow this. I think it is now not allowed in new homes. I have a 3 wire dryer and I added a ground wire connected from the frame of the dryer to the frame of the washer. This way, whatever else happens there can be no voltage across those two appliances and the washer has a 3 prong plug so it is grounded. You can also connect it to a cold water pipe. I would suggest you do that as an extra saftey feature. Good question Mark
I would recommend using a 3 wire dryer circuit just as it was intended to be used or replace it with a proper 4 wire system. Installing a Rube Goldberg grounding addition isn't helping the situation, and could potentially cause more harm running line voltage fault current through your plumbing or other appliances
LOL, OP here. I took a couple EE courses when working on my MS aerospace engineering and did an internship where I designed one of the lightest 400Hz DC-AC inverter for aerospace use. I get the split phase, peak / RMS business quite fine. I'm just not totally up to speed with what's code and what's not -- and this rig seemed to be amiss.
I guess in the name of being pedantic, I'll point out that the two hots probably are not exactly "180 degrees" out of phase and "mirror images" of each other; close but in real life situations the reactive loads will not be [perfectly] identical across both phases, making the out-of-phase, well just a little bit different than pi radians :)
T
What you are calling two hots are comming from a center tapped transformer. There is only one phase. They can not be out of phase with each other by any ammount not counting a couple of inches of wire from the transfromer windings to the load.
Think of the extreme where one H-N has a purely resistive load, and the other H-N has a highly reactive (say inductive) load, and remember that both the transformer and the wires have an inherent inductance. The voltage peaks as measured at the load will not be exactly 180 deg out of phase. T
It depends on your point of reference. The normal point of reference (for wiring on the transformer secondary) is in the MIDDLE (center-tapped coil). Points are measured from that. That is, 2 phases.
What may be confusing to some, is these 2 phases are not 2 of the 3 phases being supplied to the transformer.
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an additional ground wire connected to the dryer frame helps the situation becasue it prevents the possiblity of line voltage appearing on the dryer frame which is far more dangerous..
Using the 3 wire system "as it was intended" can create this dangerous condition, that's why they changed it.
I agree, changing over to a 4 wire system is a better option, but if you have a 3 wire cable it is not easy to do..
To the op re being an EE and calling it 220 VAC two phases, it is common usgage but technically wrong. It is more technically correct to say two POLARITIES of one phase.
Mark
an additional ground wire connected to the dryer frame helps the situation becasue it prevents the possiblity of line voltage appearing on the dryer frame which is far more dangerous..
Using the 3 wire system "as it was intended" can create this dangerous condition, that's why they changed it.
I agree, changing over to a 4 wire system is a better option, but if you have a 3 wire cable it is not easy to do..
To the op re being an EE and calling it 220 VAC two phases, it is common usgage but technically wrong. It is more technically correct to say two POLARITIES of one phase.
Mark
What you are proposing to do, routes dangerous line voltage fault current from the dryer to another circuits grounding conductor, or worse, a water pipe, which was not designed for that purpose, and is not only dangerous, but illegal as well. I suggest you get an NEC code book and research "grounding"
Correct me if I'm mistaken, but doesn't the NEC allow you, in a retrofit type situation, to replace an ungrounded receptacle with a grounded one so long as you ground the receptacle and box to any convenient point "in the grounding system" which would include a copper water pipe? I agree, would be far better to drag it back to the panel... I honestly don't know what it says re: running a ground wire from a plug-connected appliance to a water pipe. I'd sooner replace the recep. with a 4-wire and use the exception there; then replace the dryer cord with a 4-wire and remove the bonding jumper in the dryer.
Disclaimer: IANAE, I'm posting this for feedback and comment, not as advice.
nate
The wiring comming into a normal house in the US is single phase. Not two phase. True two phase power has the phases 90 deg out instead of the so called 180 deg .
Two phase powe can be made from a 3 phase circuit, but it requires more than a simple center tapped transformer the normal house has.
First of all, the code doesn't allow you to ground a receptacle to a water pipe. It allows you to ground it to any point along the "grounding electrode system", which will include the metallic water pipe feeding the building, but only the first 5 feet of it. A properly installed 3 wire dryer circuit must meet 4 conditions to be acceptable. The whole idea is to have a better quality circuit than just some old piece of Romex making the connection. Most importantly, electricity takes the path of least resistance. What happens if this guy's auxiliary ground, which he believes is to protect someone, is a better path for the normal motor circuit currents to run through? Now current that should be running through a properly sized conductor is running through what? And if it's running through some section of plumbing, and some plumbing work requires cutting that particular pipe, what happens to the plumber?
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te:
Nothing...
You have the normal neutral/ground wire in parallel with the aux ground wire. "Electricity takes the path of least resistance" is an oversimplfication. In fact the current divides inversly proportionally to the resistance in the path. Yes some of the motor current may flow through the auxiallry ground. But if that ground should be opened by the plumber then all the current will flow through the original ground. The only voltage across the water pipe will be the small drop due to the resistance of the wire.
I agree this is not an ideal situation, but the problem started becasue the NEC made a mistake in the first place and allowed 3 wire
220V circuits which are inherently dangerous for all the reasons you are pointing out. The BEST way to fix this is to replace the 3 wire system with a 4 wire system , which could requires pulling a new 4 wire cable to replace the existing 3 wire cable. Thats a big job. A second best altternative is to connect an auxiallry ground so that in case the ground/neutral in the orignal 3 wire circuit should fail OPEN which is unusal but can happen, and if it does it hapen it places a LETHAL voltage on the case of the dryer. This is unacceptable to me! But I'm not going to pull a new 4 wire cable. An auxialry ground will prevent electrocution due to a single open failure in the original 3 wire NEC approved system. In my case the auxially ground is the case of the washer right next to the dryer and the washer is plugged in via a 3 prong 120V plug. I didn't modify the outlet or cords in any way, I simply connected a wire between the metal frame of the washer and dryer. This is legal and makes it safer.Bottom line for me is.... the big green wire I added connecting the washer case and dryer case ensure that whatever else might happen, there can be no dangerous voltage across those two applicances. The orignal NEC approved 3 wire system did not ensure that.
Answer these two questions and then I am done ..
1) Does adding a big green wire between a washer and dryer case make the system more or less safe? I say more.2) Does adding a big green wire between a washer and dryer case violate any code? I say no.
Do you disagree?
thanks Mark
Nothing...
You have the normal neutral/ground wire in parallel with the aux ground wire. "Electricity takes the path of least resistance" is an oversimplfication. In fact the current divides inversly proportionally to the resistance in the path. Yes some of the motor current may flow through the auxiallry ground. But if that ground should be opened by the plumber then all the current will flow through the original ground. The only voltage across the water pipe will be the small drop due to the resistance of the wire.
I agree this is not an ideal situation, but the problem started becasue the NEC made a mistake in the first place and allowed 3 wire
220V circuits which are inherently dangerous for all the reasons you are pointing out. The BEST way to fix this is to replace the 3 wire system with a 4 wire system , which could requires pulling a new 4 wire cable to replace the existing 3 wire cable. Thats a big job. A second best altternative is to connect an auxiallry ground so that in case the ground/neutral in the orignal 3 wire circuit should fail OPEN which is unusal but can happen, and if it does it hapen it places a LETHAL voltage on the case of the dryer. This is unacceptable to me! But I'm not going to pull a new 4 wire cable. An auxialry ground will prevent electrocution due to a single open failure in the original 3 wire NEC approved system. In my case the auxially ground is the case of the washer right next to the dryer and the washer is plugged in via a 3 prong 120V plug. I didn't modify the outlet or cords in any way, I simply connected a wire between the metal frame of the washer and dryer. This is legal and makes it safer.Bottom line for me is.... the big green wire I added connecting the washer case and dryer case ensure that whatever else might happen, there can be no dangerous voltage across those two applicances. The orignal NEC approved 3 wire system did not ensure that.
Answer these two questions and then I am done ..
1) Does adding a big green wire between a washer and dryer case make the system more or less safe? I say more.2) Does adding a big green wire between a washer and dryer case violate any code? I say no.
Do you disagree?
thanks Mark
I disagree on both counts. The first for the reasons already stated, and no,the NEC does not approve your method
m...
Out of curiousity, what exactly in the NEC says you can't bond the metal cases of two appliances together? In some cases, SPAs for example, code specifically requires that all metal within a certain distance be bonded together. So, if that washer or dryer happened to be within the specified distance from a spa, it would have to be bonded to the spa pump, heater, etc.
I see both sides to this discussion. I'm generally not in favor of rube goldberg adaptions. But on the other hand, not sure what says you can't bond two metal objects together. Also, in the case of the washer/dryer, bonding would seem to be a good idea, because the washer is a very likely object to come in contact with simultaneously with the dryer, so having them at equi potential seems like a good idea.
Out of curiousity, what exactly in the NEC says you can't bond the metal cases of two appliances together? In some cases, SPAs for example, code specifically requires that all metal within a certain distance be bonded together. So, if that washer or dryer happened to be within the specified distance from a spa, it would have to be bonded to the spa pump, heater, etc.
I see both sides to this discussion. I'm generally not in favor of rube goldberg adaptions. But on the other hand, not sure what says you can't bond two metal objects together. Also, in the case of the washer/dryer, bonding would seem to be a good idea, because the washer is a very likely object to come in contact with simultaneously with the dryer, so having them at equi potential seems like a good idea.
With a hot tub, you want anything within reach, likely to become energized, to be bonded to the very substantial grounding system of the tub
Three wire electric dryers are more of an isolated application, so typical laws or rules that normally make sense, don't apply. The body of the machine is part of the live circuit, not just fault currents. It requires a special cable (SE) or insulated ground/neutral, originating only from a main service panel. Under no circumstances would you want the grounding system of another circuit to become or act as a live neutral conductor from a clothes dryer, or potentially more dangerous would be using an internal plumbing system. In the event you lose your neutral/ ground on a three wire dryer circuit, the motor won't turn and although the chassis of the machine would be live, you'd still know that a problem existed, and would have it fixed. If you add a supplemental ground to the neighboring washer, you'd have no way of even knowing that dangerous neutral currents were running through the washer outlet's grounding system
Are you sure you mean POLARITY there? The polarity is reversing 120 times per second. That sounds like "phase".
The problem with that is that "phase" already means something. Something that applies equally well to your (I mean the 90-deg apart one) 2-phase system, 3-phase power, and the normal system used in houses. Measurements are made in reference to a common point (ground) which is in the middle of that (residential) transformer. Measuring the ends of the transformer with a dual-trace scope will show TWO PHASES. In this case, one leg will show 90 degrees (most positive) while the other leg will show 270 degrees (most negative) at the same time. That's two different phases.
Applies if there is not a grounding conductor at the receptacle. In this case there is.
Don't think there is a prohibition against a redundant ground, except in this case it becomes a parallel neutral conductor.
The NEC allowed this "dangerous" method for about 30 years. Apparently the NEC didn't think it was particularly "dangerous". And since the best way to get the code changed is to show dead bodies, apparently it was not a "dangerous" method.
If I had an electric dryer with a 3-wire circuit I might replace the wiring if it was easy. If not I would use the existing 3-wire circuit. As Roy pointed out, there were major limitations on how these circuits were wired. Like for instance you couldn't use Romex.
It would be nice to edit these things out.
The argument that can be made in this case is that the alternate ground path also becomes an alternate neutral return. That puts some normal neutral current on the grounding conductor which is verbotten. Just like bonding neutral and ground at a subpanel.
In some cases, SPAs for
Without looking it up, I believe it is isolated metal that might become energized.
I believe the specified distance is 5 feet (so you can be in contact with the spa and the other metal). You can't have receptacles in that area. You wouldn't want a hypothetical dryer either.
One can certainly argue both sides. I agree with Roy.
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