On Thu, 4 Jan 2018 15:23:28 -0500, "Mr. Luddite"
wrote:


I definitely agree that not everyone should do this. The best way is to
have an automatic transfer box installed to code by a licensed electrician.

That said, I've done this several times, including the house we had in
Florida after Wilma. I have sufficient knowledge of what I am doing but
even so, I stop, check and re-check before firing up the generator and
throwing the breaker that it backfeeds through.

It's illegal, but safe if you pay attention and know something about
house wiring.

Here's a little test that I've even tried on licensed electricians to
see how knowledgeable they a

Your house service is usually 240vac, split phase. Split phase means
two "hot" leads, a neutral (used with both hot legs) and a ground (which
really is tied to the neutral leg. Across the two "hot" leads you have
240 volts which is used for stoves, large AC units, etc. Between either
hot lead and neutral you have 120 volts which is used for your outlets,
lighting, refrig and small appliances. Your panel box is supposed to be
wired to balance the loads as best as possible. So, here's the
question. Let's say:

Leg "A" of the 120 volt supply is drawing 40 amps.
Leg "B" of the 120 volt supply is drawing 30 amps.

How much current is flowing through the common neutral leg that is used
for both legs?

The answer is 10 amps.

Many people assume it is the sum of both current draws or 70 amps in
this example and it's amazing how many "electricians" don't know that.
They don't realize that the two "hot" legs are 180 degrees out of phase,
so the current in the neutral adds algebraically.

That's why the neutral feed from the street to your power panel is the
same size wire as the two hot leads. It will never carry more than what
one hot leg is rated to draw.

I can't imagine any real electrician does not know that.
In fact with the right engineering, the neutral really only needs to
be sized to the maximum unbalanced load.
For example
You are allowed to use 70% for ranges and dryers in dwelling calcs.
OTOH on 3 phase, you may end up actually using a neutral of close to
200% of the ungrounded conductor load because of triplin harmonics.
This became an issue with electronic ballasts and switcher power
supplies.