Electrical - 3 vs 4 Wire 240v

[Russ Filtz]

Saw a thread on another forum where someone is trying to use a 4-wire dryer with 3-wire service. Sure this can be made to work by mixing neutral/ground, but I would think it's not safe to do (or per code). Any thoughts?

I recommended trying to convert the socket to a 4-wire system if there are 4-wires into the outlet box (separate neutral & ground) and use the dryer as is. Others are telling him to use a 3-wire cord and just wire it up, ignoring the ground?

Playing with fire (literally!) or is this standard practice?

[Dan Friedrichs]

Not standard practice at all - very bad, very dangerous. As you said, he ought to have 4 wires in the box, so may be able to change the outlet. If he doesn't, he needs to pull a 4th wire. No two ways about it - running the neutral on the ground (or running without a ground) is absolutely unacceptable.

[Joe Chritz]

I'm sure one of the electrical guys will chime in with the official answer but the way I understand it is that if the appliance is designed to have a separate ground then it is unsafe to not use it.

Something about grounding back through the appliance and a short possibly causing a fault / shock through the unit.

Not all 3 wire 220 circuits are unsafe but a clothes drier requires one.

Assuming three wires (red, black, white) couldn't you run a bare ground to any other outlet and get the ground from there?

Joe

[John Coloccia]

One problem is if you loose your neutral. If chassis ground and neutral are the same, and you loose the neutral, the chassis becomes hot. The alternative is to not have a chassis ground at all, in which case a short to the chassis will make it hot. Both cases are bad.

[Rod Sheridan]

Russ, as others have indicated, the correct way to handle that is to change the wiring and the receptacle to a 4 wire system.

The issue with using the neutral as the ground is that if it fails, the frame of the dryer will be electrically energized through the 120 volt loads in the dryer (motor or controls/electronics).

This can be an electrocution hazard.

Regards, Rod.

[Pat Germain]

So, every single 220V machine with three wires, which is almost all of them, are just an electrocution waiting to happen?

[Alan Trout]

As an inspector I will not tell you if its ok or not but I will tell you the reasoning. As most have indicated the bare wire is a chassis ground or bond. Today they want the appliance chassis bonded to the grounding system of the house. Dryer's ranges etc. This is to help keep the metal components that you touch from ever becoming electrified.

While not an expert at electrical I deal and check systems everyday. I have had this argument several times with Double E's and master electricians that at the main service panel the neutrals and grounds share the same buss and are not isolated. Only at sub-panels are the neutrals and grounds isolated so IMHO in the really big picture there is only millisecond difference in time it takes to trip a circuit and there is still the potential for the chassis that is bonded to become hot, because if there is significant imbalance in the system electricity as I see it could still feed back through the shared buss at the main service panel.

I agree that bonding of the chassis is the ideal situation and should always be employed when possible. I will also say that most of us survived close to a century without chassis bonding of appliances.

Good Luck

Alan

[Mike Cutler]

So, every single 220V machine with three wires, which is almost all of them, are just an electrocution waiting to happen?

Absolutely not!
In a standard three wire configuration red and black are either pole and white is the neutral return for red and black. Ground is ground, the bare copper conductor. The ground and neutral return are bonded only at the main service panel. Drop the neutral, on a properly designed 3 wire, 240 appliance and you have no return current path. The motor and timer are isolated from the appliance chassis.
In a four wire configuration the internal 120 circuits are provided with a seperate return. In abscense of that return they will use ground if available.

[Dan Friedrichs]

So, every single 220V machine with three wires, which is almost all of them, are just an electrocution waiting to happen?

Motors use 2 hot legs for power, and a bare ground connected to the chassis for safety (in case one of the hots contacts the chassis, the current has a safe path to ground). Thus, 3 wires for most motors.

Certain appliances (take a dryer, for instance), use 2 hot legs to power the heating element, and 1 hot leg and a neutral to power a lightbulb (gives you 120V for the lightbulb). In this case, the lightbulb's return current is through the neutral, so a 4th conductor is needed to provide a safety ground to the chassis. Thus, 4 wires for electric ranges, dryers, etc.

[Matt Hutchinson]

So if I interpret this correctly, you can use a 4 wire circuit to power 3 wire loads, cuz you can simply cap the extra wire in the box?

[Russ Filtz]

Here is a pic of the setup in question. Is what is depicted an actual 4-wire appliance, or did someone just connect the ground for extra measure? Seems to me the cord's green wire should be bonded to where the striped green wire is to be extremely "proper"? The 3-wire cord is what they want to hook up since the outlet is 3-prong now.




Also, on a true 4-wire appliance (one with 120v internal circuit) you actually do have return current going through the neutral? Just like any 120v circuit? This is what I see as the problem, you don't want a neutral feeding current constantly through a ground on a 3-wire plug.

[John Coloccia]

Unplug the dryer, take a multimeter and check for continuity between the neutral (white) and the ground (green). If there is continuity there, that dryer is an "old" style dryer designed for 3 wires, or it was modified for three wire usage. Yes, the neutral is a current carrying conductor, usually there to run a 120V light, timer, computer, whatever. This is what makes it dangerous. If you loose the neutral, and touch something on the dryer that's "grounded", you become the next best thing to a neutral and...well, it'll get you're attention for sure.

On a modern 4 wire appliance, there won't be continuity between the neutral and ground, which is actually the point of the 4 wire setup.

I hope I'm explaining that well. The old trick was to simply jump between neutral and ground on the appliance and use 3 wires but as far as I know you are not allowed to modify a 4 wire system into a 3 wire system such as the JERK that wired my washer in California did (which is partly why I watch everyone like a hawk these days to make sure whatever they're doing they do it right).

[John Coloccia]

By the way, it's easy to have misunderstandings on forums. Getting a pro is always a good choice since most of us are not licensed electricians (myself included).

[Jim Koepke]

In my experience, some appliances are designed to have a separate ground wire connected outside of the power cord.

This looks like there is a cable with 4 wires already attached and it is in need of a strain relief where it enters the appliance.

The striped green wire is to supply a ground to another part of the chassis. Depending on how the cover for the power connection is made, I would say you may be correct. The wiring in place does not look to be a heavy enough gauge for a dryer.

The neutral and ground should not be tied together at the appliance.

A dryer is wired to use one line to neutral for the motor and the other line to neutral for the heating element. The heating element is not always on.

Tying the neutral and ground together at any place other than the main electrical service box can be hazardous and is illegal as far as I know.

If the neutral wire breaks, every circuit on the same ground wire could become electrified.

The downfall could be that if there ever was a cause for an insurance claim this might become a reason for denial of payment.

jim

[Dan Friedrichs]

So if I interpret this correctly, you can use a 4 wire circuit to power 3 wire loads, cuz you can simply cap the extra wire in the box?

Yes. You simply don't use the neutral.