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.

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.

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.

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.