3 wire vs 4 wire for stove

[TimR]

A neighbor who asks my DIY advice but never takes it is getting a new stove. The old one is hardwired, she might have an electrician put in an outlet instead. I told her he'd insist on using a 4 wire outlet and probably run new wire.

But I'm not sure I know all the reasons for the 4 wire requirement, and google turns up a lot of explanations that make little sense.

Thinking out loud.

I can see if you lose the neutral on a 3 wire, AND you have a hot wire short to chassis, AND you have a good ground nearby, AND you touch stove and ground you'll likely get shocked/killed. That can also happen with a 4 wire if you lose the ground connection. But with good 4 wire, a short to chassis should trip the breaker which won't happen with bad 3 wire. That's the purpose of the ground, right, to trip the breaker?

Also, at least one of the hot is always connected to neutral through a large resistance, like the clock, stove light, etc. So with 3 wire it's also connected to the chassis. With a broken neutral, now the chassis is at 120, but through a large resistor. If you touch it AND a good ground, two resistors. Can enough current flow through both resistances to kill you? If the neutral is good, you have parallel paths, that current is flowing to the panel not through you.

I know I'm missing something because I doubt they'd have changed the code if people weren't getting killed. On the other hand I've lived with 3 wire appliances for 70 years now without a tingle.

Does a 3 wire connection require a broken or highZ neutral to be hazardous?

And if so, is a 4 wire any better, on a summer day in a drought when my ground rod is probably high Z also?

[hubops]

I have a couple of side-questions - if the existing is 3-wire < no ground wire > 1. can you keep existing cable and just run a new ground wire ? 2. if so - can you just run new ground wire to nearest box ? or does it have to go to the panel ? John T.

[Dean Hoffman]

I've never wired a stove so caveats apply. A stove with a clock or lights on a three wire circuit will have an internal jumper connecting the neutral with the frame. The neutral serves as the equipment ground and the neutral. The neutral will allow for 120 vac to make clocks and lights work. Electricity follows all available paths to its source, not just the one with least resistance. A person touching even a stove in working condition can have a minuscule amount of current flowing through him. A stove on a four wire circuit won't have that jumper. The neutral current won't be on the frame of the stove. A bad neutral connection on a three wire circuit would leave more of the current on the frame of the stove. A person touching it could provide an alternative path to the source. Older stoves without clocks or lights wouldn't need a neutral so they'd have two hots and an equipment ground.

Yes, the equipment ground normally doesn't carry current and is for safety only. It's supposed to trip the breaker if there is a short to the frame.

Ground rods are for lightning, period. There won't be enough current flowing through one rod, through the earth, then through another ground rod to trip a breaker if there's a problem. Incidentally, a bad neutral can be a bigger problem for dairy farmers. Cows are four wheel drive and it's easier for them to feel a tiny stray bit of current.

[Marilyn Manson]

Should I start by saying "local codes apply" or should I end by saying "local codes apply"?

In general, the answers to your questions are Yes & Yes.

I stole this so I don't have to re-type it:

"A #10 ground wire can be retrofit via any reasonable route, and it doesn't need to go back to the panel - it can reach any of:

- a junction box with non-bendable metal conduit back to the panel - a junction box with #10 or larger ground wire back to the panel - straight to the Grounding Electrodes, the ground wires between panel and water pipe/ground rods. (note this cannot be cut; you use a split bolt)."

Now, that stuff applies to replacing a 3-wire receptacle with a 4-wire receptacle at the same location. Apparently, if you want to extend a current 3-wire circuit and also turn it into a 4-wire circuit, things get murky. Some local inspectors will allow it, some won't.

[bud--]

To save copper during WW2 the neutral on a stove was allowed to be used as the ground. There were several necessary conditions, including the circuit had to be from the service panel.

Many years ago a ground wire was also required (like every other circuit). As g has said, the code decided the war was over. Existing 3 wire circuits are grandfathered. If using a 3 wire circuit there is a bond in the stove to be made that connects the neutral and ground.

For more common times, earthing electrodes hold the system ground and voltages at a reasonable potential with respect to earth. The NEC does not allow the earthing system to be used to provide the current to trip a breaker (as you said). If you have a code compliant ground rod with 20 ohms resistance to earth and connect a 120V hot to it what is the current?

Ground fault current goes through a neutral-ground bond at the service and back to the transformer on the service neutral.

Stray currents may seriously affect milk production and animals may not want to enter barns. Livestock buildings are built kinda like swimming pools (lots of bonding in and near the floor).

Conversations from electricians that have worked on stray current say a major problem is that the transformer primary neutral, secondary neutral and transformer can are connected together and earthed at the utility pole. If the utility neutral is not too good the service neutral and then farm ground (eartbing) system are a parallel path to the service neutral, resulting in earth currents and stray voltage. One fix would be to keep the secondary neutral separate from the rest, but that may allow damaging voltages for the secondary with surges or lightning, so utilities don't like it. An isolation transformer at the farm with separation between the primary and secondary presumably fixes the problem. (Not the only fix.)

[Jim Joyce]

Nothing to do with the current conversation, but the statement about ground rods being only for lightning reminds of a youtube video I saw not too long ago. It was posted by an electrician named Ben Sahlstrom. He drove two ground rods, quite a distance apart, then he ran a single copper conductor across the same distance and connected a light fixture at the far end. He wanted to demonstrate that current will find its way through the earth to complete the circuit. His experiment was successful.

[trader_4]

Others have addressed the other issues, so I'll just address the last sentence here. The purpose of a ground is not to just to trip the breaker, though it will do that if there is a short and there is sufficient current to exceed the breaker. But the main purpose is to keep the metal case at about the same potential as earth. Without the ground, a partial short could easily put lethal voltage on the metal case, without tripping the breaker. With the ground, it still won't trip the breaker, but you can't get electrocuted by touching the case.

That would depend on the appliance. Old appliances with clock motors, incandescent bulbs, I would expect so. Worst case it only takes 30ma, how lucky do you feel?

If the neutral is good, you have parallel paths, that current is flowing to the panel not through you.

The neutral and ground are bonded together at the panel, so yes. Modern earthing is designed to avoid a high resistance. Even old earthing that just used the incoming metal water line should be fine.

[Scott Lurndal]

WARNING - DO NOT TAKE YOUR ELECTRICAL ADVICE FROM HOFFMAN.

[Dean Hoffman]

Suppose, just for discussion, there's a 120 volt short to the metal of a disconnect switch in a building. The ground rod is right below and correctly connected. The equipment ground doesn't exist. The electricity would try to flow back to its source at the meter. It's available path would be through the ground rod then back to the meter. The resistance between the ground rod and actual earth is 20 ohms. Ohm's Law would say 6 amps flows to the earth.

e T It would have to go through the ground rod at the meter since the equipment ground is gone. That would be a series circuit so let's say that ground rod also has 20 ohms resistance for a total of 40. Only 3 amps would flow from the shorted disconnect switch back to its source at the meter. That wouldn't blow a fuse and stop the short circuit. What would you change?

[Scott Lurndal]

Try your experiment with a loose grounded (neutral) conductor.

[bruce bowser]

A loose connection meaning ... ? No cad-weld?

[TimR]

Left turn Clyde.

Code question: apparently (I haven't seen this myself) there are two stove breakers, one for each oven in the existing double oven stove, and the wires come out of a "pipe" through the floor into some kind of metal box and then into the stove.

If so then at least 5 and possibly 6 wires are in one conduit. Would it meet code to use one breaker for the two legs and neutral, remove the other breaker, and use one of the now spare wires in the conduit for the ground? That ground wire would then be larger than required, but that shouldn't matter as long as the conduit is not overfilled?

Can you leave unused wires in a working conduit? If they have to pull them out, then silly not to just run a new circuit I would think. I guess if I'm a professional electrician and worried about liability I'm not trusting old wires anyway. A DIYer would do it in a heartbeat though.

[TimR]

I'm assuming EMT. That affects the 3-wire grandfather? I didn't know that. I thought of using the EMT as ground too, but when I worked in a factory EMT didn't seem to be reliably continuous. I think if it were my house and I wanted an outlet I'd just start over and run the new circuit correctly. Hopefully she gets a good electrician.

It appears now there are not two circuits. Only three wires come out of the EMT. She didn't know a 240 breaker would take two slots so she assumed 2 breakers and 2 circuits. If EMT is not grandfathered then this is simple, the electrician has to run a new circuit.

[bud--]

If the supply is a "pipe" it wouldn't be the 3-wire grandfathered exception.

If the "pipe" is metal (probably EMT) and continuous box to panel it is a ground - no reason not to use it.

If flex it should have a ground wire if installed now, but may not have been required when installed. (You probably don't know if there is a ground wire installed.)

If it is a one-unit stove with 2 ovens I would be extremely surprised if there were 2 supply circuits.

Is a cook-top part of that stove? If it is separate it could be a separate ckt, breaker.

I would have to look up the specific for 2 separate adjacent ovens on one ckt (and would need ratings). But the breaker and probably the wires would have to be larger.

Unused wires - no problem.

[TimR]

Ah, maybe I understand. I found this:

- 250.140 Frames of Ranges and Clothes Dryers.

- - Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be connected to the equipment grounding conductor in the manner specified by 250.134 or 250.138.

- - - Exception: For existing branch-circuit installations only where an equipment grounding conductor is not present in the outlet or junction box, the frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are part of the circuit for these appliances shall be permitted to be connected to the grounded circuit conductor if all the following conditions are met.

- - - - (1) The supply circuit is 120/240-volt, single-phase, 3-wire; or 208Y/120-volt derived from a 3-phase, 4-wire, wye-connected system.

- - - - (2) The grounded conductor is not smaller than 10 AWG copper or 8 AWG aluminum.

- - - - (3) The grounded conductor is insulated, or the grounded conductor is uninsulated and part of a Type SE service-entrance cable and the branch circuit originates at the service equipment.

- - - - (4) Grounding contacts of receptacles furnished as part of the equipment are bonded to the equipment.

The exception requires: equipment grounding conductor is not present in the outlet or junction box

But if the wires are in conduit, then an equipment grounding conductor IS present and the stove cannot be connected as a 3 wire. But you could add a grounded 4 wire outlet to the junction box and use a 4 wire plug on the stove, and everything should meet code. Or, and I'm not 100% on this, you could direct wire it to the two hots and neutral wires coming out of that box, and jumper the conduit to the stove using a ground clamp attachment and short length of bare 10AWG.

[Scott Lurndal]

A loose connection. Improperly torqued or not tightened at all. Either at the device or the panel. I've seen both.

It's called a safety ground for a reason.

[Dean Hoffman]

A loose neutral would make lights blink, vacuum cleaners cut out off and on, and make the tv cut out just before the deciding play in a college football game. My experiment puts power on the disconnect switch enclosure without a good return back to its source since the equipment ground is disconnected. Ground rods don't cut it for safety. It would be a little like an electric livestock fence. The fencer puts power on the wire intermittently. One can hear the clicking sound as it cycles. A critter touching the wire completes the path from the wire back to the source. It doesn't matter if the critter is 100' or a 1000' feet away from the fencer. The electricity wants to go back to its source.

[bud--]

The original ckt may be heavy enough for the new stove. Circuits are commonly 40 or 50A. If the existing ckt is not heavy enough for the new stove larger wires can be fished into the existing EMT - could be pulled in using the existing wires.

New wiring would be in the existing EMT.

Boxes for a long time have a tapped 10-32 hole for a ground screw. I would use a lug on the ground wire, attached by the ground screw. If there wasn't a tapped hole I would make one.

If the box is 4" x 4" a cover with a hole for a 40 or 50A grounded receptacle can be used. May need an extension ring for a 1.5" deep 4x4.

Should be a disconnect. Common is a removable drawer in the bottom of the stove where you can reach through to the back and unplug the stove.

If hardwired you would use a 4 conductor cord.