An interlock is legal, you working on someone else's house is what is illegal.
The fact that you are asking some of these questions indicates you are ill-equipped to do this even safely, let alone legally.

The big advantage of interlockkit is they sell kits for a large variety of panels. You're out of luck on home depot other than for Square D HOM or GE panels.

Backfed breakers must have some mechanical means to keep them from being removed from the panel (over and above the press in nature of them). Manufacturers typically provide some feature to install a screw or holddown to accomplish this.

It's rare for breakers (main or otherwise) to fail. Why do you ask?

The downside of using the main breaker as your disconnect for the interlock means that it is hard to do detect power on the Utility side. Generally you just use cues like the neighbors lights or the street lights come back.

You compute the voltage drop by knowing three things:
1. The length of the cord ( you told us this)
2. The size of the conductors in the cord (you didn't tell us this)
3. The amount of current being drawn (again you didn't tell us).

It is important to understand how insurance companies view the world.
If you are the homeowner and you do a bit of work in your own place and later a claim to your insurer results from it, there are just two parties involved which keeps it simple.
When you get into helping others without having "the papers" and your own appropriate insurance to back you up, and a claim results, the helped homeowner's insurance company will get very interested in who did what and when and can they stick you for part or all of it.
You would be wise to offer advice only to you wife's co-worker.

laat2, you sound like my brother-in-law! He's a defense attorney and you're right.

ronnatalie, you said, "Backfed breakers must have some mechanical means to keep them from being removed from the panel...". Wouldn't it be locked down by the main cover anyway (just like all of the other breakers)? Sure, you could take the cover off, then just snap out the 30 amp breaker. But it would seem to me the only difference between a backfeed breaker and a regular breaker is that you have one more screw to remove.

Regular breaker = held in place by main cover, which is screwed down and snapped into the panel
Backfeed breaker = held in place by main cover, which is screwed down and snapped into the panel and screwed into the panel with an additional fastener

I mean, you're going to make sure the power is dead before taking off the cover anyway and if your goal is to cause havoc, what's one extra fastener? If I'm missing something, please let me know. I mean, it's a moot point anyway. My guess is it would be needed to pass an inspection, but was just curious.

The only reason I asked about the main breaker failing is that this whole setup hinges on that breaker not failing. Otherwise, you could go on happily backfeeding your panel, not realizing you're also energizing the outside line. So much hinges on that one breaker doing it's job!

Ok, so I guess I was right. You'll just have to get word from your neighbor to know when you've got power back. That kind of sucks, but ok.

The size of the conductors in the cord? I guess I'm showing my ignorance again. Do you mean the gauge? If you do, it's #10 wire.

Thanks in advance for your help.

Back-feed breaker require something besides a trim panel.

A strap and screw inside the panel are the typical solution.

The breaker must NOT simply freely unplug any more.

The additional screw prevents this.

Not understanding this means you should not be doing this work, even on your own house.

Hey, brickeyee. Thanks for responding. You said

"Not understanding this means you should not be doing this work, even on your own house."

I understand that it needs to be done. What I don't understand is why. You said

"Back-feed breaker require something besides a trim panel."

And again I ask, why? Are breakers known to dislodge even with the cover on? Again, if the answer is just, "That's code and just do it", that's fine. But so far I'm essentially hearing, "Backfeed breakers need to be held down with two fasteners (the cover and a strap) and not just one thing" without an explanation as to the reason. Why aren't all breakers held down with an extra fastener?

I mean, you're going to make sure the power is dead before taking off the cover anyway Why?

The extra holddown has nothing to do with the breaker dislodging. It tells whoever that this breaker is hot even if the rest of the panel is off.

WHY does every lay person need explanation of a code that has been evolving for 125 years????
"Because it's code" SHOULD be enough for goodness sakes.

WHY do you want to know? So you can second guess that as well? I'm sure as a lay person you have the background and experience to know better than those that have been writing the codes for over 100 years. Most of us have studied and worked in the field for decades to learn this stuff. It is frustrating when someone completely clueless wants things explained to them because they don't like what they are hearing.

The reason IS because breakers HAVE been know to pop off the buss. Now you have a breaker that is live feeding the buss, and popping off as the cover is removed. This HAS and does happen.
Satisfied?

ISTM that the underlying reason for the additional hold-down for a back-fed breaker, or main breaker for that matter, is that if the breaker is removed, or pops off', the exposed contacts on its underside, some brands protruding 3/8" or more from the breaker body, could be live, thus creating a highly hazardous situation to the unwary.

I haven't responded to this one in a while, but thanks, randy for your explanation.

Sorry if I created any anger. I wasn't looking to do so. Thanks for all of your help, guys.