Puddin' Man wrote:

On Thu, 21 Dec 2006 18:38:30 GMT, "Pete C." wrote:

Puddin' Man wrote:

On Thu, 21 Dec 2006 09:48:39 -0500, "The Streets" wrote:

"terry" wrote in message
oups.com...
HeyBub wrote:

There are two phases dropped to the house, 120 each. If you lose one, you
still have the other. ....................... snip ...................

Also: While agreeing that the term 'phase' is sometimes used
........................

More correctly there are two 'legs' which are the opposite ends of the
same centre tapped 230 volt single phase!

Thus one can be considered (sort of, although incorrectly as far as AC
is concerned) as being plus 120 volts and the other as neg. 120 volts
and therefore about 240 volts between the two of them; which is used to
power heavy y items such as water heaters, electric cooking stoves
etc.

If the the two wires were really different phases they would have
voltages 120 degrees apart and there would be a peculiar voltage
between the two. Not the 230/240 volts (180 degrees with the two ends
always of opposite polarity) expected.

Same thing happened to my neighbours garage; they lost one side (or
leg) of the 230 volt supply. The 'electrician' they hired came over to
my house to borrow a multimeter and I went over and checked it for
them! Utility company fixed a defective over head drop later that day.

Losing one leg is not all that uncommon, at least around here. I've seen
this at least three time in the past year -- all problems that the local
utility
had to fix. As another poster has mentioned - the tip off is if every other
circuit is off. Also, be careful of back feeds through 220v appliances like
ranges, water heaters and dryers.

I'll bite.

Suppose I've got a (residential) 200A main breaker panel, and the
street power fails.

I switch off the main breaker and use a gas powered 40A generator
to backfeed the 30A (220v) circuit for the elec. dryer.

What can I expect? Live circuits, dead circuits? Overload
conditions??

Elec. power around here (MO) seems to get less and less reliable
by the month, else I wouldn't ask.

Thx,
Puddin'

Pease pudding hot,
Pease pudding cold,
Pease pudding in the pot
Nine days old ...

That's not the same kind of back feed.

The back feed referenced is the potential for the leg that is "dead" due
to a failed utility connection to become energized from the remaining
live leg when a 240V appliance is switched on. The 240V appliance
providing a connection between the two legs. The appliance will of
course not operate since one of the hot legs it relies on is "dead", but
it will provide a conduction path from the one that is live to the other
presenting a hazard if you are carelessly working on that "dead" leg.

Well, that one got by po' me. Shoulda thought about it longer.

Back feeding your house from a generator through a breaker other than
the main in the panel works perfectly fine. The situation you indicated
with the dryer outlet is not legal due to the lack of a safety interlock
to insure the main is indeed off when power is being back fed through
the dryer outlet. The electrically identical scenario is legal when the
main breaker and the dedicated back feed breaker are interlocked with an
approved device to insure the main is off when the back feed breaker is
on.

So, for the safety interlock, I'd need to replace or rewire
the existing main breaker to a "safety interlock" box or
somesuch?

For a code approved (NEC and others) generator connection you need
either an approved transfer switch or the more recent option of an
approved interlock kit that is installed on your electrical panel.

One example of the interlock kits is one available for Square D panels
which provides a mechanical interlock between the main breaker and a
dedicated back feed breaker in the 2/4 position of the panel.

If you want an automatic generator system you will need an automatic
transfer switch. For manual systems there are a number of options with
the interlock kits being the most economical option in most cases.


When you say "not legal", you refer to common bldg. codes?


National electrical code (NEC), other codes that reference it and
similar codes.

Pete C.