generator . my dryer 240 is a 3 wire. do i have to make it a 4 wire?

Im in NJ with no power but i do have a generator. was thinking of adding a outlet to plug in my generator. The dryer outlet is a 3 wire but the new twist lock outlet is a 4 wire outlet. Do i have to run a new 4 wire line from box. if so how do i do that?

You have to install a proper transfer switch or main panel interlock system. Your dryer receptacle has no influence on a generator connection.

Joe: He is not connected to house he just wants to run a cord to the dryer.

I read it as he wants to use a suicide cord and plug his generator into the dryer receptacle to power his house.
If he actually want to run the dryer off the generator then a three prong dryer plug installed on the end of the extension cord will work. Leave the white neutral wire disconnected. Since the generator should have the neutral/ground bonded the dryer should run fine.

JoeD said:

I read it as he wants to use a suicide cord and plug his generator into the dryer receptacle to power his house.

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That's how I read it.

This same type hookup may be in my future if my PoCo doesn't improve their reliability. Generator hookup constraints & rules & Fine Print Notes are scattered all over the NEC.

The general rule seems to be sizing neutral and ground conductors properly for fault conditions and not putting heavy current into these conductors because this will cause a voltage on them substantially above ground potential.

Now that I read it again I think you are right.

nealtw said:

Now that I read it again I think you are right.

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I forgot to mention that it would not be a bad idea to run some tests on a house before hooking up a generator to make sure the wiring is what everyone thinks it is.

With a 10A, 120v load (room heater) or 240v load (wall oven) and a DVM you can check for bad neutrals, bad grounds, bad connections, whatever.

I will probably be coming in a window with the cord and tap into the nearest branch circuit with its breaker off.

BTW, a gasoline gen should be at least 15' from the house.

"suicide cord and plug his generator into the dryer receptacle to power his house."
The operable word here is suicide.
Other than that this question is not much different than hooking new four wire appliance to old three wire receptacle.
Gonna be way easier and safer to connect generator at breaker box in normal manner.
Not really sure what to do with 4 wire neutral and ground in this "backwards" 3 wire situation. Seems like you're gonna be running one wire back to panel or earth anyway. white or green/bare. Generator should have its own true "ground" to earth.

That is why I stand by my original post about a proper transfer switch or interlock system.

JoeD said:

That is why I stand by my original post about a proper transfer switch or interlock system.

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Or judicious use of overcurrent devices to prevent operator error. With low impedance systems like this the difference between normal current and current due to operator error should be considerable.

In other words, don't do it.

Wuzzat? said:

Or judicious use of overcurrent devices to prevent operator error. With low impedance systems like this the difference between normal current and current due to operator error should be considerable.

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What does this mean?

JoeD said:

What does this mean?

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It means install devices to prevent operator error or inattention from causing bad things to happen.

But, suppose the OP leaves the main breaker on and feeds 240v back into a temporarily dead PoCo line.
The danger is to lineman who are thinking the line is dead, and a warning device for the OP would be rather complicated (but someone probably does make such a device).

Did you want to know about the impedance consideration?

Wuzzat? said:

Did you want to know about the impedance consideration?

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You're an engineer, aren't you?

speedy petey said:

You're an engineer, aren't you?

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That's the guy down the hall. Should I call him?

It seems to me if you hook a generator into dead PoCo lines you will be trying to power up at least all the houses on your pole transformer, including yours. Since your neighbors are not expecting this, you could cause some harm.

If you hook a generator into live PoCo lines, you'll still get 240V but within a few milliseconds probably 5000A will flow between the gen and PoCo's transformer and your generator's breaker will trip, if it doesn't explode first because its interrupting current rating has been exceeded.

Sounds like a transfer switch is the first order of business.

I think I said that in my first post. Get a proper transfer switch or interlock device for the panel.

Keep sayin it, joe, till the OP acknowledges it. The rest of us are you're reinforcements. 'Course the OP may have done blowed his self up already.

I wasn't even thinkin about the power going out into the grid, but think of the pride you'd feel from sending your electritricity to another state. Who would have thunk that just by plugging a generator into an outlet you'd have to notify the local distribution authorities that you were a producer? I was mostly thinkin about the other outlets he might be powering up without knowing it, screwiness of the various grounding and neutral lines, and the concerns arising from having a male plug on the generator, at least I think that was what he wanted to do. Or perhaps the ever popular male-male jumper. I did use one of those in a Scout Camping eguipment/ chuck wagon trailer once, but we were dealing with 12 volts from battery for lighting.

I've never been involved in permanently installing a large back up generator. One thing I'd worry about with an automatic system is storm damaging some wiring in home, perhaps along with a gas leak. Authorities do warn us when power and gas go out to turn mains for both off so there are no surprizes when they come back on, especially if you are not home when they do.
I have built a few shelters for smaller ones for essential power, refrigerator, freezer, few lights, TV. We ran a short line to switch box near reefer to keep extension cords to minimum. Switched outlets to cut off big drains to run another one. Reefer off a few while well pump, micro wave, or small space heater or window AC is on etc.

I reckon its an individual decision how big a generator to buy, whole house or bare minimum for freezer and a light bulb, . Cost of generator compared to how long you may have to use it. Folks got along without electricity longer than we've come to rely on it. Of course some folks want it all and right now! Like that mayor who damned the Red Cross 'cause they weren't right there and fixed his town good as new within an hour of the storms passing.

I forgot to take into account the resistance of the gen. The 5000A is probably more like 500A, but even with this analysis there is too much uncertainty to suit me. There's too much riding on this if something goes wrong.

I'd do something to absolutely prevent the gen from powering PoCo's lines.

IIRC Grainger wants about $100 for a transfer switch. A DPDT or 3PDT contactor may be cheaper but if you have no power you'd need to use a battery to power the contactor coil.