Generator back feed is a major risk to lineman during power outages. A transformer works either way it doesn't care if you put 120 in to get 12,000 or 12,000 to get 120. We always install heavy ground cables to our system before working on them. Never know when someone is going to get home from the store with their new generator and try and kill ya. We might not see ya but our grounds will destroy your new purchase. Lol

Got lucky several years ago in an ice storm. A tree had taken down a service from both the house and pole. We were putting the service back on the house and co worker said he keep getting shocked even though no hot wires were energized or even hooked up. This was in a dump of a yard with trash everywhere we finally found and extension cord buried in trash going from the neighbors basement window going to his basement window. He hooked up his furnace to the cord backwards without removing the wire to the panel and had energized the bare neutral wire trying to kill us.

Mike

We were in Texas in 2005 after Hurricane Rita blew through, and it was getting down in the short rows. We got a job to go repair a single phase tap down a street with 15-20 houses. We had one pole standing, and had to rebuild the whole street from one end to the other. Poles, primary, secondary...and most of the transformers were laying in an oily puddle on the ground. We got it all built back right before dark, and had the primary hot. We split up and started closing transformer fuses and checking voltage at the houses. I closed in a 50 kva pot, and my buddy walked around back to check the voltage. I started hearing the transformer hum a bit louder for 30 seconds or so, and about the time my buddy came around the corner yelling for the fire extinguisher, the fuse on the pot blew! Customer had one of those high buck Honda generators that you can't even hear running in the shed, and backfed to the dryer plug in the house. The 50 kva pot and the Honda were butting heads....and the Honda lost and set the shed on fire! We got the fire out....blanked the meter base and rolled! Fire dept. called the county inspector to check it out before Entergy would make it back hot. We were doing everything we were supposed to be doing with gloves and grounds, so we never noticed or felt it. It could have been a lot different if the service to the house had still been up and hooked to the transformer...and it still hooked to the primary!

Best way to do it is to run the generator through a transfer switch, the switch wires to selected breakers in your main panel and the switch is fed by a large single cord. Individual switches on the transfer switch select between your utility power and the generator power. The transfer switch wires to your ground buss and it's recommended to also ground the generator.

My installation has a 6-circuit transfer switch but you can get them with more circuits. I didn't ground the generator and that doesn't affect its operation.