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Why do I need all 4 wires in my generator twistlock?
The generator neutral and ground are bonded. I have installed a Siemens 3-pole transfer switch, but would prefer to use 3 wire extension from the generator to the transfer switch. (mainly cause the 4 wire is so big and bulky)
This depends on whether your neutral-to-ground bonding is before or after the transfer switch. If the transfer switch is your service entrance, you can bond neutral to ground there (on the common side, after the switch, inside the transfer switch enclosure), and do switching only for the hot conductors. If neutral is bonded to ground before the transfer switch, then all points beyond the point of bonding are now 4-wire, and a transfer switch must switch both hots and also the neutral (not the ground wire).
You can't mix by having the utility power be one way (for example 4-wire) and the generator be another way (3-wire) because you can't switch the bonding. It has to be either both 3-wire or both 4-wire coming up to the transfer switch.
Edit: since you have bonding at the entrance and at the generator, you need to switch the neutral in the transfer switch. Because of that you need 4 wires from both sources in order to keep the neutral and ground separated so the two bondings are never connected together by more than one wire (it will be the ground wire).
I assume you are referring to a home installation, as it would be a somewhat different answer if this was in an industrial situation.
I believe you will find that neutral and ground are not bonded within the generator, therefore the 4 lines. The only place it should be bonded is at the Main Service Panel. The neutral is a current carrying conductor, ground is not! Lets say you have a high load (or short) that the Main panel sees. The neutral potential to ground can be as high as the Main voltage (120V or 220V)! If it were bonded at the generator and you were touching it at that instant, you're dead!
The ground is never switched by the transfer switch, only the main conductors (& sometimes neutral). I suggest you check the NEC and local codes before you proceed. Also do not bond neutral and ground at the transfer panel, or any sub panels!
This link shows the transfer switch wiring. It also says that some Transfer switches do not switch the neutral leg. Yours is three pole so it would.
EDIT: Please read Since yours has bonding at the Gen, and you are switching the neutral you're okay. This article expresses the same concerns I listed above.
It's done just like your house. You only have 3 wires coming into house. The receptacle is probably a NEMA L14-30R our maybe 50 amp. At work we have a designated ground rod for our's which we attach to the frame. If it set up correctly coming in it will be set up correctly going out. Also you can't switch your neutral with the xfer switch.
